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Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

The nonlinear optical response of organic polaritonic matter has received increasing attention due to their enhanced and controllable nonlinear response and their potential for novel optical devices such as compact photon sources and optical and quantum information devices. Using $z$ scans at different wavelengths and incident powers we have studied the nonlinear optical dispersion of ultrastrongly coupled organic cavity polaritons near the lower polariton band. We show that the up to 150-fold enhancement of the nonlinear response compared to a cavityless organic film arises from an intensity-dependent polaritonic resonant frequency shift (``blueshift''). Consequently, we find that these $z$-scan data can only be described by several terms of a power series expansion in intensity whose respective contributions depend on power broadening and detuning from the lower polariton band. We further show that the nonlinear response can be quantitatively described by a semiclassical three-level molecular model coupled to the cavity in which saturation reduces the Rabi splitting, thus accounting for the lower polariton band's observed blueshift.

We studied theoretically optical spectra when circularly polarized light obliquely impinges on a slab of a chiral photonic structure with linearly and uniformly spatial varying pitch. The material that we considered locally has a $\overline{4}2m$ point-group symmetry and presents Pockels effect, hence, we controlled optical spectra by a low frequency (DC) electric field applied along the nonhomogeneity axis. The spectra display a Bragg-type broadband, where the edge wavelengths of the photonic band are expressed in terms of the medium parameters and the external electric-field magnitude. This allows us to select the region of the electromagnetic spectrum, where the band could become as broad as we choose. We studied three samples with iridescent, silver, and golden colors due to their reflection properties. We observed an enhancement as well as a broadening of the optical band by the application of the electric field. We found that, under certain conditions, when the optical band is practically absent, a very broad band could be created (comparable to the whole visible spectrum) when a DC electric field is applied. This is the result of electro-optic contributions in the permittivity tensor elements, which give rise to a tremendous increase in both the rotatory power and dichroism. Therefore, these media could be used as electrically controlled broadband frequency and polarization filters. Moreover, we observed the usual blueshift of the band as the incidence angle of light increases and an asymmetry in the reflection band amplitude as the incidence angle of the light increases, depending on whether the pitch increases or decreases and the pitch gradient; which endows the sample with different reflection colors for different gradients and incidence angles of the light. The asymmetry in the reflection band vanishes as the magnitude of the electric field increases.

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in ${\mathrm{Cu}}_{2\ensuremath{-}x}\mathrm{S}@\mathrm{Au}$ core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

Monolayer transition-metal dichalcogenide (TMDC) semiconductors host strongly bound two-dimensional excitonic complexes, and form an excellent platform for probing many-body physics through manipulation of Coulomb interaction. The quantum confined Stark effect is one of the routes to dynamically tune the emission line of these excitonic complexes. In this paper, using a high-quality graphene/hexagonal boron nitride $(\mathrm{hBN})/{\mathrm{WS}}_{2}/\mathrm{hBN}/\mathrm{Au}$ vertical heterojunction, we demonstrate an out-of-plane electric-field driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. Such universal nonmonotonic Stark shift with electric field arises from a competition between the conventional quantum confined Stark effect driven redshift and a suppressed binding-energy driven anomalous blueshift of the emission lines, with the latter dominating in the low-field regime. We also find that the encapsulating environment of the monolayer TMDC plays an important role in wave-function spreading, and hence in determining the magnitude of the blue Stark shift. The results for neutral and charged excitonic species are in excellent agreement with calculations from the Bethe-Salpeter equation that use a seven-band per spin tight-binding Hamiltonian. The findings have important implications in probing many-body interaction in two dimensions as well as in developing layered semiconductor based tunable optoelectronic devices.

We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near-unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell coupling is negligible. However, for sufficiently small interlayer spacing ($\stackrel{l}{\ensuremath{\sim}\phantom{{}_{x}}}20\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}\mathrm{m}$), coupling between the metamaterial layer and the ground plane is non-negligible. Therefore, near-field interactions must be taken into account for a full understanding of the electromagnetic response (EMR). Interference theory is often used to model the EMR of MMPAs analytically, in which interlayer coupling between the metamaterial and ground plane is usually neglected, resulting in a predicted blueshift of the absorption peaks in comparison to experiment. To account for near-field coupling, we incorporate correction terms into the analytical interference model by taking into account the effective interlayer capacitance and inductance. This results in good agreement between interference theory and experiment (and full-wave numerical simulations). Our findings demonstrate that interlayer coupling is an important design parameter for ultrathin MMPAs.

We develop an analytical framework to study the influence of a weakly intercoupled inplane spherical metal nanoparticle (MNP) assembly on a coherently illuminated quantum emitter (QE). We reduce the analytical expressions derived for the aforementioned generic planar setup into simple and concise expressions representing a QE mediated by a symmetric MNP constellation, by exploiting the symmetry. We use the recently introduced generalized nonlocal optical response (GNOR) theory that has successfully explained plasmonic experiments to model the MNPs in our system. Due to the use of GNOR theory, and our analytical approach, the procedure we suggest is extremely computationally efficient. Using the derived model, we analyse the absorption rate, resultant Rabi frequency, effective excitonic energy shift and dephasing rate shift spectra of an exciton bearing QE at the centre of a symmetric MNP setup. We observe that the QE experiences plasmon induced absorption rate spectral linewidth variations that increase in magnitude with decreasing MNP-QE centre separation and increasing number of MNPs. Our results also suggest that, parameter regions where the QE exhibits trends of decreasing linewidth against decreasing MNP-QE centre separation are likely to be associated with plasmon induced excitonic energy redshifts. Similarly, regions where the QE absorption rate linewidth tends to increase against decreasing MNP QE centre separation are likely to be accompanied by plasmon induced excitonic energy blueshifts. In both these cases, the magnitude of the observed red/blueshift was seen to increase with the number of MNPs in the constellation, due to enhancement of the plasmonic influence.

The polarized components of transmission spectrum of a multilayered photonic structure containing a nematic liquid crystal under the magnetic-field-induced torsion deformation have been investigated. Since the planarly twisted director structure of the nematic is unrelated to a decrease in its optical anisotropy, the control of the polarization and spectral characteristics of the eigenmodes is based on the occurrence of a geometric phase, whose contribution gradually increases upon nematic deformation. Along with the previously observed blue shift of the cavity o modes, the anomalous shift of the cavity e modes to the long-wavelength region (red shift) of the transmission spectrum has been detected. It has been shown that the countershift of the modes narrows the intermode interval. Remarkably, the extremal approaching of e and o modes in magnetic field H_{ex} occurs below the avoided crossing phenomenon observed in the stronger equalization field H_{eq} (H_{ex}

Although hybrid halide perovskites $(\text{MAPb}{X}_{3},$ $\mathrm{MA}={\mathrm{CH}}_{3}{\mathrm{NH}}_{3} \mathrm{and} X=\mathrm{I}, \mathrm{Br}, \mathrm{Cl})$ have been ubiquitously explored from the photovoltaic perspective, there are still a few unanswered questions which require a more fundamental understanding. One such unsettled issue is the puzzling behavior of the band gap. Unlike conventional semiconductors, $\text{MAPb}{X}_{3}$ $(X=\mathrm{I}, \mathrm{Br})$ is found to show a blueshift (increase) in the band gap $({E}_{g})$ with increasing temperature $(T)$, while ${\mathrm{MAPbCl}}_{3}$ shows an unusual redshift (decrease). In order to understand this, we performed a detailed $T$-dependent study of electronic, optical, and structural properties of $\text{MAPb}{X}_{3}$ combined with the state-of-the-art first-principles calculations. With increasing $T$, two dominant mechanisms which come into play are lattice dilation and electron-phonon coupling (EPC). The former (latter) is responsible for an increase (decrease) in ${E}_{g}$. We found that lattice dilation effect dominates in $\text{MAPb}{X}_{3}$ $(X=\mathrm{I}, \mathrm{Br})$, causing an enhancement in ${E}_{g}$. EPC involves various contributions, of which the interaction of charge carriers with the longitudinal optical phonon mode via Fr\"ohlich interaction is the most dominant one at room temperature. We quantify this contribution using Fr\"ohlich's theory of large polarons and show that the ${E}_{g}$ correction due to this effect in ${\mathrm{MAPbCl}}_{3}$ is almost double as compared to ${\mathrm{MAPbBr}}_{3}$ and thus explain the reduction in ${E}_{g}$ for the former.

Cubic ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ perovskite is considered to be an ideal lead-free and air-stable alternative to traditional $A\mathrm{Pb}{X}_{3}$ [A = $\mathrm{Cs}$, $\mathrm{K}$, ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$, $\mathrm{CH}({\mathrm{NH}}_{2}{)}_{2}$] perovskites for optoelectronic applications, such as solar cells and photodetectors. Herein, both experiments and theoretical calculations are carried out to study the lattice dynamics and temperature dependences of band gaps. Using Raman spectroscopy, we show that the obtained ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ film maintains a long-range crystalline structure of the cubic phase within 77--300 K, which is further confirmed via lattice dynamics calculations based on density functional theory (DFT). An ultrasmall net blueshift of 0.035 eV deduced from its temperature-dependent photoluminescence (PL) spectrum is observed as the temperature increases from 103 to 300 K, making it an ideal material for optoelectronic devices. Notably, this blueshift of the ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ film is significantly smaller than that of the DFT-calculated band-gap shift (\ensuremath{\sim}0.085 eV) based on thermal expansion theory. This deviation is attributed to the strong phonon-electron (ph-e) interaction inherently occurring in ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ material, which counterbalances the thermal expansion effects. We further evaluate the magnitude of the ph-e interaction by fitting the full width at half maximum (FWHM) of the PL spectrum. The fitted Huang-Rhys factor (S) of 4.9 is consistent with reduced electronic dimensionality and increased vibrational degree of freedom, indicating stronger ph-e coupling strength in the ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ film compared with regular ${A\mathrm{Pb}X}_{3}$-type perovskites (S 3.3). Adopting both thermal expansion and ph-e coupling effects, the ultrasmall band-gap shift is theoretically calculated and is in good agreement with the experimental blueshift of the band gap. In summary, our work illustrates the solid structural stability of ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ in both lattice and electronic structures, paving the way for the next generation of optoelectronic devices.

Nanostructured lead halide perovskites have received extensive attention due to their potential applications in integrated photonics. Despite many successful realizations of nanoscale coherent-light sources from lead halide perovskites, the role and interplay between excitonic and electron-hole plasma (EHP) states in the stimulated-emission process are not demonstrated yet. In this work, the carrier behavior and dynamics of the stimulated emission in ${\mathrm{Cs}\mathrm{Pb}\mathrm{Br}}_{3}$ single nanowires are studied with a streak camera under power-dependent one-photon and two-photon excitation at room temperature. A redshift of the lasing gain profile with increasing excitation fluence is observed, suggesting the transition from the excitonic state to EHP state that is responsible for the lasing. Whereas, a blueshift of the gain profile with time decay represents the opposite direction of the former process. Moreover, the individual lasing modes show a blueshift as excitation fluence increases and a redshift as time elapses, due to the carrier-density dependence of the refractive index. This study could provide a comprehensive and deeper understanding of the carrier-density-driven stimulated-emission dynamics in lead halide nanowires and extend their applications in integrated photonics.

Alongside a vast number of two-dimensional (2D) materials, $g\text{\ensuremath{-}}{\mathrm{SiC}}_{7}$ siligraphene has already been introduced to the 2D nanomaterial family with extraordinary sunlight absorption. In this work, we exploit the concept of the in-plane biaxial strain-induced electro-optical engineering in siligraphene from ab initio many-body perturbation theory calculations (GW + Bethe-Salpeter equation). While the semiconductor nature of the system is robust for both tensile and compressive strains, the direct GW band gap of 1.35 eV is decreased (increased) linearly with tensile (compressive) strain. This is accompanied by a variation in the optical transitions. The tensile strain in $g\text{\ensuremath{-}}{\mathrm{SiC}}_{7}$ shifts the optical absorption peaks to the lower photon energies, the so-called red shift, whereas the compressive strain causes a shift to the higher frequencies, the so-called blue shift. Eventually, the exciton binding energy trend and the real-space exciton distribution for the bright bound state is studied with strain, resulting in a alteration of the spatial distribution radius of exciton wave functions from tensile to compressive. Our findings extend the logic applications of siligraphene in photovoltaic devices, especially in tailoring the power conversion efficiency of excitonic solar cells.

The impact of single-axis lattice expansion on the optical response of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films is examined. Low-energy He implantation is used to tailor morphotropic phases of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like $\mathrm{BiFe}{\mathrm{O}}_{3}$ films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of $\mathrm{BiFe}{\mathrm{O}}_{3}$ films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe $3d\phantom{\rule{0.28em}{0ex}}{t}_{2g}$ states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. The results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.

Near-field radiative heat transfer (NFRHT) is strongly related with many applications such as near-field imaging, thermos-photovoltaics and thermal circuit devices. The active control of NFRHT is of great interest since it provides a degree of tunability by external means. In this work, a magnetically tunable multi-band NFRHT is revealed in a system of two suspended graphene sheets at room temperature. It is found that the single-band spectra for B=0 split into multi-band spectra under an external magnetic field. Dual-band spectra can be realized for a modest magnetic field (e.g., B=4 T). One band is determined by intra-band transitions in the classical regime, which undergoes a blue shift as the chemical potential increases. Meanwhile, the other band is contributed by inter-Landau-level transitions in the quantum regime, which is robust against the change of chemical potentials. For a strong magnetic field (e.g., B=15 T), there is an additional band with the resonant peak appearing at near-zero frequency (microwave regime), stemming from the magneto-plasmon zero modes. The great enhancement of NFRHT at such low frequency has not been found in any previous systems yet. This work may pave a way for multi-band thermal information transfer based on atomically thin graphene sheets., Comment: 8 pages, 7 figures

We studied the effect of external pressure on the electrodynamic properties of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ single crystals in the frequency range of the phonon modes and the Ir $d\ensuremath{-}d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure ${P}_{c}=3.8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. With increasing pressure an overall decrease in spectral weight of the Ir $d\ensuremath{-}d$ transitions is found up to ${P}_{c}$. Above ${P}_{c}$, the local (on-site) $d\ensuremath{-}d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The nonlocal (intersite) Ir $d\ensuremath{-}d$ transitions show a monotonic blueshift and decrease in spectral weight. The changes observed for the nonlocal excitations are most prominent well above ${P}_{c}$, namely, for pressures $\ensuremath{\ge}12$ GPa, and only small changes occur for pressures close to ${P}_{c}$. The profile of the optical conductivity at high pressures ($\ensuremath{\sim}20$ GPa) appears to be indicative of the dimerized state in iridates.

Herein, we report on the growth of single crystals of various ${\left({\text{Mo}}_{2/3}{\mathrm{RE}}_{1/3}\right)}_{2}\text{Al}\text{C} (\mathrm{RE}=\mathrm{Nd},\phantom{\rule{0.16em}{0ex}}\mathrm{Gd},\phantom{\rule{0.16em}{0ex}}\mathrm{Dy},\phantom{\rule{0.16em}{0ex}}\mathrm{Ho},\phantom{\rule{0.16em}{0ex}}\mathrm{Er}) i$-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing $i$-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.

We study the nature of photo-excited charge carriers in CsPbBr3 nanocrystal thin films by ultrafast optical pump - THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

Optical excitation of stripe-ordered ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ has been shown to transiently enhance superconducting tunneling between the ${\mathrm{CuO}}_{2}$ planes. This effect was revealed by a blueshift, or by the appearance of a Josephson plasma resonance in the terahertz-frequency optical properties. Here, we show that this photoinduced state can be strengthened by the application of high external magnetic fields oriented along the $c$ axis. For a 7 T field, we observe up to a tenfold enhancement in the transient interlayer phase correlation length, accompanied by a twofold increase in the relaxation time of the photoinduced state. These observations are highly surprising, since static magnetic fields suppress interlayer Josephson tunneling and stabilize stripe order at equilibrium. We interpret our data as an indication that optically enhanced interlayer coupling in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ does not originate from a simple optical melting of stripes, as previously hypothesized. Rather, we speculate that the photoinduced state may emerge from activated tunneling between optically excited stripes in adjacent planes.

Transmission and correlation properties of a two-photon pulse are studied in a one-dimensional waveguide (1DW) in the presence of three types of quantum emitters: two-level atom (TLA), side optical cavity (SOC), and Jaynes-Cummings model (JCM). Since there are many plane-wave components for a two-photon pulse, a nonlinear waveguide dispersion is used instead of the linearized one. The two-photon transmission spectra become flatter with decreasing the pulse width. With respect to the $\ensuremath{\delta}$ coupling between the 1DW and quantum emitter the transmission dips show a blueshift for the non-$\ensuremath{\delta}$ one and the blueshift first increases and then decreases with increasing the width of the coupling. The TLA and JCM can induce an effective photon-photon interaction that depends on the distance between the two photons, while the SOC cannot. We show that the 1DW coupled with the TLA or JCM is able to evaluate the overlap of the two photons and that the non-$\ensuremath{\delta}$ coupling has potential for controlling the two-photon correlation.

We use frequency-resolved x-ray microscopy to fully characterize the potential well of a magnetic vortex in a soft ferromagnetic permalloy square. The vortex core is excited with magnetic broadband pulses and simultaneously displaced with a static magnetic field. We observe a frequency increase (blueshift) in the gyrotropic mode of the vortex core with increasing bias field. Supported by micromagnetic simulations, we show that this frequency increase is accompanied by internal deformation of the vortex core. The ability to modify the inner structure of the vortex core provides a mechanism to control the dynamics of magnetic vortices.

A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a FeCo/MgO spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of ∼69 A/cm^{2} in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of ∼25% is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of ∼7.2 kA/cm^{2}. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Photoinduced absorption in rutile ${\mathrm{TiO}}_{2}$ has been studied by means of Fourier transform infrared spectroscopy. It is shown that near-band-gap illumination results in a charge-state transition of a hydrogen defect with an O--H stretch mode at 3500 ${\mathrm{cm}}^{\ensuremath{-}1}$, as evidenced by the appearance of an anticorrelated vibrational mode with a blueshift of $\ensuremath{\sim}1.3\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The charge-state transition is accompanied by a broad near-infrared absorption band with maximum intensity at $\ensuremath{\sim}7000\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our data on both the photoinduced vibrational mode and the near-infrared absorption can be conclusively explained in the framework of a model of small electron polarons bound to a Ti atom adjacent to the O--H group of the hydrogen defect.

Elucidating the details of electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile ${\mathrm{TiO}}_{2}$, revealing a strong anisotropic polarization dependence with increasing temperature, namely, an anomalous blue shift for light polarized along the $a$ axis and a conventional red shift for light polarized along the $c$ axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blue shift of the $a$-axis exciton. Our approach paves the way to a complete ab initio treatment of the electron-phonon interaction and of its influence on the optical spectra of polar materials.

The group-velocity-led optical event horizon (OEH) in optical fibers provides a convenient way to efficiently control a weak dispersive pulse by a comparatively strong solitonic pulse. State-of-the-art experiments demonstrating OEH make use of two different light sources, which increases the system complexity and cost. Here, we propose an elegant and cost-effective approach to observe the OEH through intermodal nonlinear interaction between azimuthally symmetric ${\text{LP}}_{01}$ and ${\text{LP}}_{02}$ modes of a specially designed triple-clad multimode fiber through only a single pump. The simultaneous control over dispersion and the walk off of ${\text{LP}}_{01}$ and ${\text{LP}}_{02}$ modes of the designed fiber pave the way for the OEH interaction. The ${\text{LP}}_{02}$ mode possesses anomalous dispersion, while the ${\text{LP}}_{01}$ mode exhibits normal dispersion over the wavelength range of interest. Depending upon the location of the input pump wavelength, the ${\text{LP}}_{02}$ soliton can reflect a weak copropagating ${\text{LP}}_{01}$ dispersive pulse, which possesses the same carrier wavelength as the soliton. The findings of this work might be useful for all-optical switching and also for the optical transistor action, where the increased feasibility is obtained by the fact that the proposed approach does not require separate input wavelengths for the soliton and the dispersive pulse. Moreover, a tunable pump source can be used to observe either a redshift or a blueshift in the dispersive pulse.

Semiconductor photocatalysts that can produce hydrogen from water splitting are of great interest. Among the various possibilities, nitrogen (N)-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ is a promising candidate for hydrogen evolution under visible-light irradiation. In this study, hybrid density-functional calculations are employed to investigate the stability and impact of nitrogen impurities on the electronic and optical properties in $\mathrm{SrTi}{\mathrm{O}}_{3}$. We find that the substitutional N on O site (${\mathrm{N}}_{\mathrm{O}}$) is a deep acceptor in $\mathrm{SrTi}{\mathrm{O}}_{3}$. Moreover, ${\mathrm{N}}_{\mathrm{O}}$ predominates over other N-related defects under equilibrium growth conditions in $n$-type $\mathrm{SrTi}{\mathrm{O}}_{3}$. Our results reveal that ${\mathrm{N}}_{\mathrm{O}}$ gives rise to visible-light absorption in agreement with experimental observations. In addition, we find that hydrogen can bind to ${\mathrm{N}}_{\mathrm{O}}$, forming ${\mathrm{N}}_{\mathrm{O}}\ensuremath{-}{\mathrm{H}}_{i}$ complex which leads to a blueshift in the optical absorption. Other N configurations can also contribute to optical absorption in the visible range. The vibration frequencies of different N configurations are provided to support identification using vibrational spectroscopy techniques.

We address the theory of the coupled lattice and magnetization dynamics of freely suspended single-domain nanoparticles. Magnetic anisotropy generates low-frequency satellite peaks in the microwave absorption spectrum and a blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency resonances are very sharp with maxima exceeding that of the FMR, because their magnetic and mechanical precessions are locked, thereby suppressing Gilbert damping. Magnetic nanoparticles can operate as nearly ideal motors that convert electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is significant even in the absence of a net rotation., Comment: 11 pages, 5 figures

Optical excitation of apical oxygen vibrations in $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$ has been shown to enhance its $c$ axis superconducting-phase rigidity, as evidenced by a transient blueshift of the equilibrium interbilayer Josephson plasma resonance. Surprisingly, a transient $c$ axis plasma mode could also be induced above ${T}_{c}$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$. However, despite the similarities between the transient plasma mode above ${T}_{c}$ and the equilibrium Josephson plasmon, alternative explanations involving high-mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport and rather support transient superfluid tunneling as the origin of the light induced interlayer coupling in $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$.

We use photoluminescence (PL) microscopy to measure the interaction between polyethylene-glycol-coated (PEGylated) silicon nanocrystals (SiNCs) and two model surfaces: lipid bilayers and surfactant interfaces. By characterizing the photostability, transport, and size-dependent emission of the PEGylated nanocrystal clusters, we demonstrate the retention of red PL suitable for detection and tracking with minimal blueshift after a year in an aqueous environment. The predominant interaction measured for both interfaces is short-range repulsion, consistent with the ideal behavior anticipated for PEGylated phospholipid coatings. However, we also observe unanticipated attractive behavior in a small number of scenarios for both interfaces. We attribute this anomaly to defective PEG coverage on a subset of the clusters, suggesting a possible strategy for enhancing cellular uptake by controlling the homogeneity of the PEG corona. In both scenarios, the shape of the apparent potential is modeled through the free or bound diffusion of the clusters near the confining interface.

We investigate the optical properties of type-${\mathrm{I}}_{1}$ basal-plane stacking faults (BSFs) in ZnO and ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ by cathodoluminescence spectroscopy supported by envelope function calculations. We report on a quantum-well-like band alignment of the ${\mathrm{I}}_{1}$ BSFs in ZnO taking into account the spontaneous polarization as well as an intrinsic self-screening effect on the polarization-related electric field. We present a systematic investigation of the luminescence properties associated with ${\mathrm{I}}_{1}$ BSFs in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ for varying Mg content $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ using spatially and spectrally resolved cathodoluminescence spectroscopy. Both the near-band-edge emission and the luminescence line related to the ${\mathrm{I}}_{1}$ BSF exhibit the expected blueshift and line broadening with increasing Mg content. We propose a band diagram to describe the recombination mechanism of excitons in a ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ film containing ${\mathrm{I}}_{1}$ BSFs. Based on a statistical analysis, we compile the experimentally obtained ${\mathrm{I}}_{1}$ BSF emission energies of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ samples and establish a linear dependence of the ${\mathrm{I}}_{1}$ BSF-related emission energy on the Mg content. This correlation provides an alternative way to identify the presence of ${\mathrm{I}}_{1}$ BSFs in ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}\mathrm{O}$ without the necessity of sophisticated transmission electron microscopy investigations.

In Lema\^��tre -- Tolman (L--T) models, light rays emitted \textit{radially} at the Big Bang (BB) at such radial coordinates $r$ where the bang-time function $t_B(r)$ has $\dril {t_B} r \neq 0$ reach every observer with \textit{infinite blueshift}, $z = -1$. Consequently, there exist rays, emitted soon after the BB, that will reach later observers with finite blueshift ($-1 < z < 0$). But in spacetimes without symmetry there are no radial directions. The question thus arises whether blueshifts can exist at all in the Szekeres models that contain L--T as a limit, but in general have no symmetry. The aim of the present paper is to show that strong blueshifts can be generated in quasi-spherical Szekeres (QSS) models. It is shown that in an axially symmetric QSS model, infinite blueshift can appear only on axial rays, which intersect every space orthogonal to the dust flow on the symmetry axis. In an exemplary QSS model it is numerically shown that if such a ray is emitted from the Big Bang where $\dril {t_B} r \neq 0$, then indeed observers see it with $z \approx -1$. Rays emitted shortly after the BB and running close to the symmetry axis will reach the observers with a strong blueshift, too. Then, in a toy QSS model that has no symmetry, it was shown by numerical calculations that two null lines exist such that rays in their vicinity have redshift profiles similar to those in a vicinity of the axial rays in the axially symmetric case. This indicates that rays generating infinite blueshifts exist in general QSS spacetimes and are concentrated around two directions., 20 pages, 19 figures. All references to gamma-ray bursts removed at the insistence of the referees; otherwise this version is the same as version 1. The text matches the one published in Phys. Rev. D

It is shown that the basic observed properties of the gamma-ray bursts (GRBs) are accounted for if one assumes that the GRBs arise by blueshifting the emission radiation of hydrogen and helium generated during the last scattering epoch. The blueshift generator for a single GRB is a region with a nonconstant bang-time function $t_B(r)$ (described by a Lema\^{\i}tre -- Tolman (L--T) exact solution of Einstein's equations) matched into a homogeneous and isotropic (Friedmann) background. Blueshift visible to the present observer arises \textit{only on those rays that are emitted radially in an L--T region}. The paper presents three L--T models with different Big Bang profiles, adapted for the highest and the lowest end of the GRB frequency range. The models account for: (1) The observed frequency range of the GRBs; (2) Their limited duration; (3) The afterglows; (4) Their hypothetical collimation into narrow jets; (5) The large distances to their sources; (6) The multitude of the observed GRBs. Properties (2), (3) and (6) are accounted for only qualitatively. With a small correction of the parameters of the model, the implied perturbations of the CMB radiation will be consistent with those actually caused by the GRBs. A complete model of the Universe would consist of many L--T regions with different $t_B(r)$ profiles, matched into the same Friedmann background. This paper is meant to be an initial exploration of the possibilities offered by models of this kind; the actual fitting of all parameters to observational results requires fine-tuning of several interconnected variables and is left for a separate study., Comment: 23 pages, 18 figures. Typos corrected, title and two sentences in the abstract modified, bibliography updated. The present text matches the published version

We theoretically investigate gap plasmons for two silver nanocubes coupled through a molecular tunnel junction. In absence of tunneling, the red-shift of the bonding mode saturates with decreasing gap distance. Tunneling at small gap distances leads to a damping and slight blue-shift of the bonding mode, but no low-energy charge transfer plasmon mode appears in the spectra. This finding is in stark contrast to recent work of Tan et al. [Science 343, 1496 (2014)]., Comment: 5 pages, 4 figures

The ternary compounds $\mathrm{CsPb}{X}_{3}$ ($X=\mathrm{Br}$ or Cl) have perovskite structures that are being considered for optical and electronic applications such as lasing and gamma-ray detection. An above-band-gap excitonic photoluminescence (PL) band is seen in both $\mathrm{CsPb}{X}_{3}$ compounds. An excitonic emission peak centered at 2.98 eV, $\ensuremath{\sim}0.1$ eV above the room-temperature band gap, is observed for $\mathrm{CsPbC}{\mathrm{l}}_{3}$. The thermal quenching of the excitonic luminescence is well described by a two-step quenching model, yielding activation energies of 0.057 and 0.0076 eV for high- and low-temperature regimes, respectively. $\mathrm{CsPbB}{\mathrm{r}}_{3}$ exhibits bound excitonic luminescence peaks located at 2.29 and 2.33 eV that are attributed to recombination involving Br vacancy centers. Activation energies for thermal quenching of the excitonic luminescence of 0.017 and 0.0007 eV were calculated for $\mathrm{CsPbB}{\mathrm{r}}_{3}$. Temperature-dependent PL experiments reveal unexpected blueshifts for all excitonic emission peaks in $\mathrm{CsPb}{X}_{3}$ compounds. A phonon-assisted step-up process leads to the blueshift in $\mathrm{CsPbB}{\mathrm{r}}_{3}$ emission, while there is a contribution from band-gap widening in $\mathrm{CsPbC}{\mathrm{l}}_{3}$. The absence of significant deep level defect luminescence in these compounds makes them attractive candidates for high-resolution, room-temperature radiation detection.

We report on the nonlinear optical response of a monochiral sample of (6,5) single-wall carbon nanotubes by means of broadband two-color pump-probe spectroscopy with selective excitation of the ${S}_{11}$ excitons. By using a moment analysis of the transient spectra, we show that all the nonlinear features can be accurately accounted for by elementary deformations of the linear absorption spectrum. The photogeneration of ${S}_{11}$ excitons induces a broadening and a blueshift of both the ${S}_{11}$ and ${S}_{22}$ excitonic transitions. In contrast, only the ${S}_{11}$ transition shows a reduction of oscillator strength, ruling out population up-conversion. These nonlinear signatures result from many-body effects, including phase-space filling, wave-function renormalization, and exciton collisions. This framework is sufficient to interpret the magnitude of the observed nonlinearities and stress the importance of intersubband exciton interactions. Remarkably, we show that these intersubband interactions have the same magnitude as the intraband ones and bring the major contribution to the photobleaching of the ${S}_{22}$ excitonic transition upon ${S}_{11}$ excitation through energy shift and broadening.

Antimony sulphide ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ has emerged as a promising material for a variety of energy applications ranging from solar cells to thermoelectrics and solid-state batteries. The most distinctive feature of ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ is its crystal structure, which consists of parallel 1-nm-wide ribbons held together by weak van der Waals forces. This structure clearly suggests that it should be possible to isolate individual ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ ribbons using micromechanical or liquid-phase exfoliation techniques. However, it is not clear yet how to identify the ribbons postexfoliation using standard optical probes. Using state-of-the-art first-principles calculations based on many-body perturbation theory, here we show that individual ribbons of ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$ carry optical signatures clearly distinct from those of bulk ${\mathrm{Sb}}_{2}{\mathrm{S}}_{3}$. In particular, we find a large blueshift of the optical absorption edge (from 1.38 to 2.30 eV) resulting from the interplay between a reduced screening and the formation of bound excitons. In addition, we observe a transition from an indirect band gap to a direct gap, suggesting an enhanced photoluminescence in the green. These unique fingerprints will enable extending the research on van der Waals materials to the case of one-dimensional chalchogenides.

We study both theoretically and experimentally the effects of introducing deliberate disorder in a slow-light photonic crystal waveguide on the photon density of states. We first introduce a theoretical model that includes both deliberate disorder through statistically moving the hole centres in the photonic crystal lattice and intrinsic disorder caused by manufacturing imperfections. We demonstrate a disorder-induced mean blueshift and an overall broadening of the photonic density of states for various amounts of deliberate disorder. By comparing with measurements from a GaAs photonic crystal waveguide, we find good qualitative agreement between theory and experiment which highlights the importance of carefully including local field effects for modelling high-index contrast perturbations. Our work also demonstrates the importance of using asymmetric dielectric polarizabilities for modelling positive and negative dielectric perturbations when modelling a perturbed dielectric interface in photonic crystal platforms., Comment: 10 pages, 4 figures

We present an approach for controlling quantum coherences in condensed matter by interaction with a nonresonant optical control field. Coherent intersubband (IS) excitations of electrons in GaAs/AlGaAs quantum wells are manipulated by a strong nonresonant terahertz (THz) field as demonstrated by phase-resolved two-color two-dimensional spectroscopy. In the linear regime of IS response, we observe a THz-induced enhancement of the midinfrared (MIR) IS absorption and a dispersive perturbed free induction decay caused by a THz-induced blueshift of the IS polarization. In the regime of IS Rabi oscillations, the THz field causes pronounced phase shifts of the coherently emitted MIR field, while the IS Rabi frequency remains unaffected. Such behavior is accounted for by a full solution of the Maxwell-Bloch equations, treating the THz and MIR fields without approximations. Our control scheme paves the way for THz control of IS emitters and holds potential for an extension to other systems.

We theoretically investigate the nonadiabatic spectral redshift of high-order harmonics with the help of a VUV pulse. It is found that the nonadiabatic spectral redshift of high-order harmonics can be observed when a weak VUV pulse is properly added in the falling part of the fundamental laser due to the nonadiabatic response of the dipole to rapid change of laser intensity. Further time-frequency analysis shows that the high-order harmonics are mainly generated in the falling part of the fundamental pulse. This is because the VUV pulse enhances the ionization in the falling part of the fundamental pulse by the 1s-2p transition of $\text{H}{e}^{+}$. In addition, this scheme is also used to observe the nonadiabatic spectral blueshift of high-order harmonics by changing the time delay between the fundamental laser and the VUV pulse.

We theoretically study the central wavelength tuning of a laser pulse propagating in a hollow-core fiber filled with prealigned nitrogen gas. The simulation results show that the double pump pulses, suitable pump pulse energy, gas pressure, and temperature can extend the wavelength shift range, and the spectral blueshift or redshift can be controlled by tuning the time delay between the pump and probe pulses. It is also shown that several hundred nanometers wavelength tuning is obtained by the double pump pulses with appropriate parameters.

The effect of K, Co, and P dopings on the lattice dynamics in the ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ system is studied by infrared spectroscopy. We focus on the phonon at $\ensuremath{\sim}253\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, the highest energy in-plane infrared-active Fe-As mode in ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$. Our studies show that Co and P dopings lead to a blueshift of this phonon in frequency, which can be simply interpreted by the change in lattice parameters induced by doping. In sharp contrast, an unusual redshift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped ${{\mathrm{BaFe}}_{2}\mathrm{As}}_{2}$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.

We have calculated the transition energies of neutral and charged excitons in ${\mathrm{In}}_{0.5}\mathrm{Ga}{\phantom{\rule{0.16em}{0ex}}}_{0.5}\mathrm{As}$/GaAs quantum dots (QDs) under a lateral electric field up to 40 kV/cm. First, the single-particle electron and hole states under the lateral field are calculated using the 8-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory. The linear and quadratic piezoelectricity is included. Next, the transition energies are calculated from the electron-hole, electron-electron, and hole-hole Coulomb energies. For a QD with 15-nm base length under a lateral electric field along the $[100]$ direction, the transition energy of the positively charged exciton exhibits a blueshift with increasing field up to 28 kV/cm, followed by a redshift under higher field. In contrast, those of the neutral and negatively charged excitons exhibit only redshifts accompanied by a crossing of the two exciton levels. The calculated result for the positively charged exciton reproduces the unconventional ``M''-shaped exciton energy shift observed in our experiment by Nakaoka et al. [Appl. Phys. Lett. 99, 181109 (2011)]. The origin of the blueshift calculated for the positively charged exciton is the enhanced hole density in the QD base corner due to the modification of the piezoelectric potential by the lateral electric field, which causes an increase in the hole-hole Coulomb energy. We found that the amount of the blueshift increases with the QD size. In order to understand the effect of the lateral field direction, we calculate the transition energies under a lateral electric field of 20 kV/cm along the $[1\overline{1}0],[100]$, and $[110]$ directions. For the positively charged exciton, the transition energy exhibits a redshift for the $[1\overline{1}0]$ direction, and blueshifts of different amounts for the $[100]$ and $[110]$ directions, indicating that the effects of the lateral field are not equivalent for the $[1\overline{1}0]$ and $[110]$ directions. It is demonstrated that the direction dependence of the transition energy reflects the symmetry of the confinement potential in the QDs due to piezoelectricity.

High-energy collective electronic excitations (plasmons) in freestanding multilayer graphene are studied by momentum-resolved electron energy-loss spectroscopy (EELS). For normal incidence, only the high-energy plasmon band is excited and we measure a blueshift of the π -plasmon dispersion with increasing thickness. The observed transition between two-dimensional and three-dimensional behavior is explained using a layeredelectron-gas (LEG) model. We propose a method to measure all individual plasmon bands by tilting the sample with respect to the electron beam. As a proof of concept, EELS experiments for three-layer graphene are compared with predictions from the LEG model.

In the Lema\^{\i}tre -- Tolman (L--T) models that have nonconstant bang-time function $t_B(r)$, light emitted close to those points of the Big Bang where $\dril {t_B} r \neq 0$ is blueshifted at the receiver position. The blueshifted rays are expected to perturb the temperature of the cosmic microwave background (CMB) radiation along the lines of sight of the present central observer. It is shown here that, in a general L--T model, the $t_B(r)$ can be chosen so that the blueshift-generating region is hidden before the recombination time, where the L--T model does not apply. The rest of the paper is devoted to investigating blueshifts in one specific L--T model, called L--T$(t_B)$ -- the one that duplicates the luminosity distance vs. redshift relation of the $\Lambda$CDM model using nonconstant $t_B(r)$ alone. The location of the blueshift-generating region in the L--T$(t_B)$ spacetime is determined. Profiles of redshift/blueshift along several rays intersecting the past light cone of the present central observer are calculated. The L--T$(t_B)$ model matched to Friedmann is considered, and profiles of redshift/blueshift in such a composite model are calculated. The requirement of invisibility of blueshifts makes the L--T$(t_B)$ model astrophysically unacceptable if it should apply back to the recombination time, but does not "rule out" a general L--T model -- it only puts limits on $\dril {t_B} r$., Comment: 13 pages, 12 figures; axis labels added in all figures, new paragraph added after (2.16), Ref. 22 updated; no substantial changes

In this paper, we critically review the usual explanation of the blue shift in the temperature dependence of the light emission, which is commonly observed for polar GaInN/GaN quantum wells at intermediate temperatures. We demonstrate that this blue shift is not necessarily caused by a thermally induced change of occupation in an inhomogeneously broadened density of states. Instead, different energy dependencies of radiative and nonradiative lifetimes may lead to an energy dependence of the internal quantum efficiency strongly influencing the peak position of the luminescence: the piezoelectric fields within the quantum well induce an approximately exponential relationship between the electron-hole overlap matrix element for the radiative transition between the lowest quantized states and the respective transition energy. Via time-resolved photoluminescence spectroscopy, we observe a corresponding exponential energy dependence of radiative lifetimes at low temperatures and almost energy independent nonradiative lifetimes toward room temperature throughout the emission of polar single quantum wells. An analytical model is demonstrated, predicting a significant blue shift of several 10 meV for polar and a negligible shift for nonpolar GaInN/GaN quantum wells at the transition from high to low internal quantum efficiency due to the energy-dependent competition between radiative and nonradiative recombination processes. This model consistently explains the lack of a blue shift in nonpolar quantum wells grown on $m$-plane bulk GaN as well as a reduced characteristic temperature for the onset of the blue shift after artificial defect generation via argon ion implantation.

We present an experimental study on the ignition and decay of a polariton optical parametric oscillator (OPO) in a semiconductor microcavity pillar. The combination of a continuous wave laser pump, under quasi-phase matching conditions, and a non-resonant, 2 ps-long pulse probe allows us to obtain the full dynamics of the system. The arrival of the probe induces a blue-shift in the polariton emission, bringing the OPO process into resonance with the pump, which triggers the OPO-process. We time-resolve the polariton OPO signal emission for more than 1 nanosecond in both real and momentum-space. We fully characterize the emission of the OPO signal with spectral tomography techniques. Our interpretations are backed up by theoretical simulations based on the 2D coupled Gross-Pitaevskii equation for excitons and photons., 8 pages, 8 figures

Room temperature electrically pumped inversionless polariton lasing is observed from a bulk GaN-based microcavity diode. The low nonlinear threshold for polariton lasing occurs at 169 A/cm(2) in the light-current characteristics, accompanied by a collapse of the emission linewidth and small blueshift of the emission peak. Measurement of angle-resolved luminescence, polariton condensation and occupation in momentum space, and output spatial coherence and polarization have also been made. A second threshold, due to conventional photon lasing, is observed at an injection of 44 kA/cm(2).

We have investigated a large set of individual wurtzite/zinc-blende GaAs heterojunction nanowires using transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence (CL). In several cases we have combined PL and TEM on the same wire. We find a fairly deep emission that shows a strong blueshift with increasing excitation power density. By using a variety of experiments we show that this emission is due to recombination across a heterojunction with a type II band alignment. The data give a valence band offset of about 100 meV, in excellent agreement with theoretical predictions. Spatially resolved CL data on heterojunction nanowires and PL data on pure wurtzite nanowires show that the band gap of wurtzite GaAs is very close to the band gap of zinc-blende GaAs. (Less)

Starting from the extended Su-Schrieffer-Heeger model, multiband semiconductor Bloch equations are formulated in momentum space and applied to the analysis of the linear optical response of semiconducting carbon nanotubes (SCNTs). This formalism includes the coupling of electron-hole pair excitations between different valence and conduction bands, originating from the electron-hole Coulomb attraction. The influence of these couplings, which are referred to as nondiagonal interband Coulomb interaction (NDI-CI), on the linear excitonic absorption spectra is investigated and discussed for light fields polarized parallel to the tube direction. The results show that the intervalley NDI-CI leads to a significant increase of the band gap and a decrease of the exciton binding energy that results in a blueshift of the lowest-frequency excitonic absorption peak. The strength of these effects depends on the symmetry of the SCNT. Furthermore, for zigzag SCNTs with higher symmetry other nonintervalley NDI-CI terms also affect the spectral positions of excitonic absorption peaks.

The excitation, emission, and Fourier transform infrared (FTIR) spectra of freshly prepared and aged porous silicon (PS) are examined. The effects of aging and temperature changing on the emission and excitation spectra of fresh and aged PS samples is also studied. Aging in air is found to result in a redshift in the emission spectrum, but a blueshift in the excitation spectrum. Increasing the temperatures leads to a general decrease in the emission intensities of both samples: however, the green emission of fresh sample shows an unusually large and sudden increase in its intensity at the same temperature at which the proton conductivity in the silicon wafer increases. The temperature dependence of the O-H vibration intensity of silicon wafers shows a sudden decrease at that temperature. These results are consistent with a model in which the excitation leads to the formation of exciton in the confined nanostructure, and the formation of electron and hole that are trapped by surface traps, whose nature is dependent on the chemical composition of the surface (and thus its age). These conclusions are confirmed by the results of the FTIR spectra of the two samples, the temperature dependence of the emission, and its correlation with the proton conductivity in the fresh PS.

Direct application of biaxial strain to GaN and InGaN/GaN multiple quantum-well (MQW) structures is achieved through the use of a specially designed pressure cell. The photoluminescence of the samples is measured as a function of the applied biaxial strain. The luminescence of the GaN sample redshifts with tensile strain in a manner that agrees quantitatively with the expected shrinkage of the energy gap. The luminescence of the InGaN MQW structures shows a smaller than expected redshift for one sample, and a blueshift for another. This blueshift agrees with calculations based on the built-in electric field and the piezoelectric effect in a quantum well in which the radiative recombination is dominated by the quantum-confined Stark effect.