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The ultrafast variation of optical bandgap, due to the presence of photocarriers, was investigated in exfoliated InSe. The thickness and surface condition play an important role in the carrier lifetime.

One of the challenges in integrating nanomechanical resonators made from van der Waals materials in optoelectromechanical technologies is characterizing their dynamic properties from vibrational displacement. Multiple calibration schemes using optical interferometry have tackled this challenge. However, these techniques are limited only to optically thin resonators with an optimal vacuum gap height and substrate for interferometric detection. Here, we address this limitation by implementing a modeling-based approach

Single-atom tips (SATs) have crucial scientific and technological applications, such as in scanning probe microscopy and charged particle beam technology. We reported a reliable method of preparing and regenerating noble metal-covered W(111) SATs through laser annealing at approximately 1000 K under ultrahigh vacuum. The field emission patterns obtained during laser heating revealed the self-assembly process of a pyramidal tip. The SATs can be regenerated through laser annealing tens of times with little change in sharpness, indicating a long lifetime. Various pyramidal SATs can be generated and regenerated using visible-light, near-infrared, mode-locked, and continuous-wave lasers at different polarizations relative to the tip axis. The generation of well-defined pyramidal SATs through laser annealing can facilitate various applications of SATs.

Silicon photonics have attracted significant interest because of their potential in integrated photonics components and all-dielectric meta-optics elements. One major challenge is to achieve active control via strong photon–photon interactions, i.e. optical nonlinearity, which is intrinsically weak in silicon. To boost the nonlinear response, practical applications rely on resonant structures such as microring resonators or photonic crystals. Nevertheless, their typical footprints are larger than 10 μm. Here, we show that 100 nm silicon nano-resonators exhibit a giant photothermal nonlinearity, yielding 90% reversible and repeatable modulation from linear scattering response at low excitation intensities. The equivalent nonlinear index is five-orders larger compared with bulk, based on Mie resonance enhanced absorption and high-efficiency heating in thermally isolated nanostructures. Furthermore, the nanoscale thermal relaxation time reaches nanosecond. This large and fast nonlinearity leads to potential applications for GHz all-optical control at the nanoscale and super-resolution imaging of silicon., Resonant structures are a promising method to enhance the nonlinear response of Si, however they are typically larger than 10 μm. Here, the authors present Si nano-resonators that amplify photothermal nonlinearity, resulting in reversible and repeatable modulation.

Multifunctional lanthanide-doped upconversion nanoparticles (UCNPs) have spread their wings in the fields of flexible optoelectronics and biomedical applications. One of the ongoing challenges lies in achieving UCNP-based nanocomposites, which enable a continuous-wave (CW) laser action at ultralow thresholds. Here, gold sandwich UCNP nanocomposites [gold (Au

Studies involving nanomechanical motion have evolved from its detection and understanding of its fundamental aspects to its promising practical utility as an integral component of hybrid systems. Nanomechanical resonators’ indispensable role as transducers between optical and microwave fields in hybrid systems, such as quantum communications interface, have elevated their importance in recent years. It is therefore crucial to determine which among the family of nanomechanical resonators is more suitable for this role. Most of the studies revolve around nanomechanical resonators of ultrathin structures because of their inherently large mechanical amplitude due to their very low mass. Here, we argue that the underutilized nanomechanical resonators made from multilayered two-dimensional (2D) materials are the better fit for this role because of their comparable electrostatic tunability and larger optomechanical responsivity. To show this, we first demonstrate the electrostatic tunability of mechanical modes of a multilayered nanomechanical resonator made from graphite. We also show that the optomechanical responsivity of multilayered devices will always be superior as compared to the few-layer devices. Finally, by using the multilayered model and comparing this device with the reported ones, we find that the electrostatic tunability of devices of intermediate thickness is not significantly lower than that of ultrathin ones. Together with the practicality in terms of fabrication ease and design predictability, we contend that multilayered 2D nanomechanical resonators are the optimal choice for the electromagnetic interface in integrated quantum systems.

We demonstrate that the upconversion nanoparticles (UCNPs) fluoresce 50 times more on a gold (Au) coated Cicada wing. UCNPs are attractive bioimaging, and therapeutic materials as it is excited in the infrared, limited only by the low fluorescence quantum yield. Here, a plasmonic effect, coupled with an anti-reflecting (AR) Cicada wing substrate coated with Au is demonstrated to enhance the fluorescence of the UCNPs. Silica (SiO2) coated Erbium doped green emitting core-shell UCNPs (NaYF4: Yb3+, Er3+@SiO2) show conventional metal enhanced fluorescence. The AR property of the Cicada wing (R ~0.2% @ 1000 nm) contributes >6-fold enhancement as compared to flat (silicon, and quartz) substrates (R~10–30% @ 1000 nm). Upon plasmon coupling, with an optimally sputtered Au coating, an unprecedented enhancement of >50-fold for the 520, and 655 nm emission was obtained on the Au coated Cicada wings, vis-a-vis planar uncoated (silicon, and quartz) substrates. The enhancement was also confirmed by direct fluorescence imaging of the photonic substrates used. The fluorescence lifetime of the core, and the core-shell UCNPs (~300 μs) decreased by ~30–40%, and 10–30%, respectively, when placed on Au coated substrates.

Two-dimensional ternary materials are attracting widespread interest because of the additional degree of freedom available to tailor the material property for a specific application. An In1-xSnxSe phototransistor possessing tunable ultrahigh mobility by Sn-doping engineering is demonstrated in this study. A striking feature of In1-xSnxSe flakes is the reduction in the oxide phase compared to undoped InSe, which is validated by spectroscopic analyses. Moreover, first-principles density functional calculations performed for the In1-xSnxSe crystal system reveal the same effective mass when doped with Sn atoms. Hence, because of an increased lifetime owing to the enhanced crystal quality, the carriers in In1-xSnxSe have higher mobility than in InSe. The internally boosted electrical properties of In1-xSnxSe exhibit ultrahigh mobility of 2560 ± 240 cm2 V-1 s-1 by suppressing the interfacial traps with substrate modification and channel encapsulation. As a phototransistor, the ultrathin In1-xSnxSe flakes are highly sensitive with a detectivity of 1014 Jones. It possesses a large photoresponsivity and photogain (Vg = 40 V) as high as 3 × 105 A W-1 and 0.5 × 106, respectively. The obtained results outperform all previously reported performances of InSe-based devices. Thus, the doping-engineered In1-xSnxSe-layered semiconductor finds a potential application in optoelectronics and meets the demand for faster electronic technology.

In this study, we found giant photothermal nonlinearity with 𝑛2 = 10-1𝜇𝑚2/𝑚𝑊 in ~100𝑛𝑚 silicon nanoblocks, based on Mie-resonance enhanced absorption and efficient temperature increase via thermal insulation. Through a continuouswave pump-probe setup, we demonstrated an ultrasmall high-contrast all-optical switch with 90% modulation depth. Due to the 0.001𝜇𝑚3 small geometrical size, thermal dissipation is as fast as nanosecond, leading to modulation speed at GHz, which is much faster than other thermal optic switches. The large and fast all-optical switching could open the possibility toward high-density integrated photonic nanocircuits based entirely on silicon.

We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar projected bulk band structures and the ensemble interpretation of quantum mechanics, according to which all resonant electrons are governed by a single wave function. Ag(100) has an energy gap above its vacuum level, whereas Ag(111) does not. Consequently, the dissipation rate of the wave function, which is relevant to the FER linewidth, on Ag(111) was almost stable, whereas that on Ag(100) fluctuated. The fluctuation revealed that the quantum trapping effect and surface dipole layer on Ag(100) surface can be investigated through FER., 16 pages, 4 figures

Photocatalytic water splitting is a key technology for long-term hydrogen evolution with low environmental impact. In this work, an environment-friendly photocatalyst consisting of FeS2–TiO2 heterostructured nanocrystals has been successfully prepared by wet chemical synthesis for enhancing the photocatalytic hydrogen production rate over a wide range of absorption wavelengths. Its absorption range has been extended from the ultraviolet–visible to near-infrared (NIR) spectrum. The results of ultrafast optical spectroscopy suggest that charge transfer occurs between the FeS2 and TiO2 interface under NIR pulsed laser irradiation (800 nm). The highest photocatalytic hydrogen production rate of the FeS2–TiO2 heterostructure has been obtained in comparison with FeS2 nanocrystals (NCs) and TiO2 nanoparticles (NPs) under irradiation by a mercury arc lamp or a xenon lamp. In the FeS2–TiO2 heterostructure, the FeS2 NCs as a light harvester can efficiently absorb visible and NIR light to generate much more photoelectrons than TiO2 NPs only for hydrogen production. For long-term photocatalytic hydrogen production, the FeS2–TiO2 heterostructures also reveal an outstanding durability over 40 h. Overall, the FeS2–TiO2 heterostructure shows potential to be a TiO2-based photocatalyst for clean hydrogen production in practical applications.

We report electrostatically conjugated core- silica (SiO2) shell upconversion nanoparticle (UCNPs@SiO2), and gold nanorod (AuNR) nanocomposite (NC) combined with graphene to demonstrate > 200-fold UC fluorescence enhancement. Plasmonic AuNR and graphene, resulted in enhanced fluorescence in UCNP with a maximum for the SiO2 shell thickness of 7 nm supported by finite difference time domain simulation of electric field distribution. In addition to the conventionally reported spectroscopic evidence, the plasmon aided UC fluorescence enhancement was demonstrated by direct confocal fluorescence imaging also, which was corroborated by a ~40% decrease in fluorescence lifetime. Finally, we have fabricated a NC/graphene hybrid photodetector (PD) that showed broadband (455–980 nm) photoresponse, with photoresponsivity of ~5000 AW−1, and response times of 80–200 ms under 980 nm illumination. The multiphoton infrared (IR, ~980 nm) absorbing UCNPs also show interesting high energy (blue (B), green (G), and red (R)) photoresponse which is now conclusively attributed to one-photon absorption in the UCNPs. We report, probably for the first time, the performance of the hybrid PD under monochromatic and polychromatic illumination of B, G, R, B + G, B + R, G + R, and B + G + R, among others. The photocurrent under polychromatic conditions is dominated by the strongest monochromatic response, and does not scale with net power of the illumination. The loss in photocurrent is attributed to saturation in absorption, and photothermal heating. The fast response of the PD device has been demonstrated while detecting high frequency modulated AC remote controller signals, and attributed to the fast charge sweeping by the AuNRs.

Two-dimensional ternary materials are attracting widespread interest because of the additional degree of freedom available to tailor the material property for a specific application. An In

Observation of resonance modes is the most straightforward way of studying mechanical oscillations because these modes have maximum response to stimuli. However, a deeper understanding of mechanical motion can be obtained by also looking at modal responses at frequencies in between resonances. Here, an imaging of the modal responses for a nanomechanical drum driven off resonance is presented. By using the frequency modal analysis, these shapes are described as a superposition of resonance modes. It is found that the spatial distribution of the oscillating component of the driving force, which is affected by both the shape of the actuating electrode and inherent device properties such as asymmetry and initial slack, greatly influences the modal weight or participation. This modal superposition analysis elucidates the dynamics of any nanomechanical system through modal weights. This aids in optimizing mode‐specific designs for force sensing and integration with other systems., Nanomechanical motion in between resonances are imaged and then described as superposition of resonance modes. The modal weight behavior allows for the elucidation of the dynamics of any nanomechanical system such as the spatial distribution of the oscillating driving force. This analysis demonstrates that spatial imaging of mechanical motion is more than just a visualization tool.

We utilized transmission-type pump–probe technique to investigate coherent acoustic phonons in the superlattices of perovskite oxides such as SrIrO3/SrTiO3 and SrRuO3/SrTiO3. Because the films in the superlattices are of high-quality and their thicknesses are only several monolayers, quasi-monochromatic acoustic phonons with THz frequency have been achieved. By investigating the propagation of coherent acoustic phonons in the superlattices with different epitaxial periods, the phonon mean free paths of SrTiO3 were studied in the frequency range between 0.5 THz and 1 THz. We further demonstrated coherent control to amplify or cease the THz coherent acoustic phonon oscillations in the superlattices. By controlling the delay of two pulses for shining the superlattices, the amplitude and phase of the THz coherent acoustic phonons were manipulated. According to the measurements of time-domain Brillouin scatterings, we found the optoacoustic conversion efficiency of SrIrO3 outperforms that of SrRuO3 for generating acoustic phonons.

Photolithography has been widely implemented with a photomask in contact or in close proximity to the photoresist layer. The flatness of the substrates is a crucial factor to guarantee the quality of the entire patterned photoresist (PR) layer especially for large-area photolithography. However, some substrates, such as sapphire wafers, do not possess highly uniform thickness as silicon wafer does. In this work, we demonstrate that a flexible polydimethylsiloxane (PDMS) photomask with optical total-internal-reflection structure can effectively circumvent this problem for mass production. Different from conventional photomask that the light is blocked by the patterned reflective/absorbing materials, the distributions of light intensity on the PR is engineered by the geometric structure of the transparent PDMS photomask. We demonstrate that 4 in. patterned sapphire wafers can be successfully fabricated by using this PDMS photomask, which can be easily integrated into the present techniques in industry for mass production of substrates for GaN-based optoelectronic devices.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

This study used laser multiphoton absorption (MPA) to crosslink and immobilize protein in 3D microfluidics. MPA can be used to crosslink proteins to form smaller crosslinked-protein structures than those formed through single-photon absorption. However, studies on MPA protein crosslinking in 3D microfluidics are lacking. In this study, the MPA was implemented using pulse laser to induce bovine serum albumin (BSA) crosslinking. We also applied (3-glycidyloxypropyl)trimethoxysilane to increase the adhesion of crosslinked BSA. Also, the width of crosslinked-BSA structure was measured, and then curve-fitting and optical equation were used to predict heights of the crosslinked-BSA structures formed under various laser powers. At last, BSA structures were effectively crosslinked in the three-layered glass or polydimethylsiloxane microchannels. In contrast to studies on single-photon absorption in 3D microfluidics or MPA protein crosslinking in 2D microfluidics, we obtained relatively small crosslinked-BSA structures through MPA and immobilize BSA in 3D microfluidics. In the future, this method can be applied in monolithic 3D microfluidics for protein crosslinking to avoid misalignment and leakage in layer-by-layer 3D microchannels and form more compact 3D microfluidics. These features are conducive to enhance the diversity and convenience of microfluidics in biomedical testing.

Production of multicolor or multiple wavelength lasers over the full visible-color spectrum from a single chip device has widespread applications, such as superbright solid-state lighting, color laser displays, light-based version of Wi-Fi (Li-Fi), and bioimaging, etc. However, designing such lasing devices remains a challenging issue owing to the material requirements for producing multicolor emissions and sophisticated design for producing laser action. Here we demonstrate a simple design and highly efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core–shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multicolor lasing emitted from core–shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables decreasing the energy consumption of photon propagation. As a result, the energy upconversion...

We studied GHz hypersound in heterostructures of ${\mathrm{SrRuO}}_{3}$ (SRO) and ${\mathrm{SrTiO}}_{3}$ (STO) by using femtosecond pulses with fluence as low as several tens of $\ensuremath{\mu}\mathrm{J}/\mathrm{c}{\mathrm{m}}^{2}$. We found that the efficiency of generating hypersound in SRO, through thermal expansion, is linear up to optical fluence of a few $\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$. We further investigated the propagation of acoustic pulses in the STO substrate. The acoustic properties of STO dramatically changed around the temperature of structural transition (105 K) because the generated hypersound was coupled to the softening mode. Nevertheless, our experimental results revealed that the quadratic frequency dependence of acoustic attenuation in STO, predicted by the mode-coupling theory, breaks down at tens of GHz. Finally, the refractive index of STO was experimentally obtained between 365 and 400 nm, for which there was disagreement in previous results.

An acoustic mismatch model is used to describe the reflection and transmission of acoustic waves at interfaces of two materials with mismatched impedances. Acoustic attenuation is typically ignored for calculating acoustic impedance, and the parameters in the acoustic mismatch model are thus all real numbers instead of complex numbers. However, acoustic attenuation up to the THz regime is significant. The validity and potential applications of the complex acoustic mismatch model have not been investigated in the sub-THz regime. We report the verification experiments of this model in a complex form. We experimentally measured the amplitude and phase of subpicosecond acoustic pulses, reflected from the interface of GaN and silica. Based on the acoustic mismatch model, the frequency dependence of complex acoustic impedance of silica, which includes the information of sound velocity and attenuation, was experimentally obtained in the sub-THz regime. The acoustic properties of silica were compared with the conventional method, which analyzes the acoustic pulses traveling inside the same silica film. Agreement of sound velocity from two methods was confirmed. The attenuations of the silica film, obtained through using the acoustic mismatch model to analyze the reflected acoustic pulses from silica, were validated by a typical method up to 0.3 THz. This work demonstrates potential applications for characterizing the sub-THz acoustic properties of thick and highly damped materials, which could be challenging by using conventional methods.An acoustic mismatch model is used to describe the reflection and transmission of acoustic waves at interfaces of two materials with mismatched impedances. Acoustic attenuation is typically ignored for calculating acoustic impedance, and the parameters in the acoustic mismatch model are thus all real numbers instead of complex numbers. However, acoustic attenuation up to the THz regime is significant. The validity and potential applications of the complex acoustic mismatch model have not been investigated in the sub-THz regime. We report the verification experiments of this model in a complex form. We experimentally measured the amplitude and phase of subpicosecond acoustic pulses, reflected from the interface of GaN and silica. Based on the acoustic mismatch model, the frequency dependence of complex acoustic impedance of silica, which includes the information of sound velocity and attenuation, was experimentally obtained in the sub-THz regime. The acoustic properties of silica were compared with the con...

GaN is a pivotal material for acoustic transducers and acoustic spectroscopy in the THz regime, but its THz phonon properties have not been experimentally and comprehensively studied. In this report, we demonstrate how to use double quantum wells as a THz acoustic transducer for measuring generated acoustic phonons and deriving a broadband acoustic spectrum with continuous frequencies. We experimentally investigated the sub-THz frequency dependence of acoustic attenuation (i.e., phonon mean-free paths) in GaN, in addition to its physical origins such as anharmonic scattering, defect scattering, and boundary scattering. A new upper limit of attenuation caused by anharmonic scattering, which is lower than previously reported values, was obtained. Our results should be noteworthy for THz acoustic spectroscopy and for gaining a fundamental understanding of heat conduction.

Because of their exceptional local-field enhancement and ultrasmall mode volume, plasmonic components can integrate photonics and electronics at nanoscale and active control of plasmons is the key. However, all-optical modulation of plasmonic response with nanometer mode volume and unity modulation depth is still lacking. Here we show that scattering from a plasmonic nanoparticle, whose volume is smaller than 0.001 μm3, can be optically switched off with less than 100 μW power. Over 80% modulation depth is observed and shows no degradation after repetitive switching. The spectral bandwidth approaches 100 nm. The underlying mechanism is suggested to be photothermal effects and the effective single-particle nonlinearity reaches nearly 10−9 m2/W, which is to our knowledge the largest record of metallic materials to date. As a novel application, the non-bleaching and unlimitedly switchable scattering is used to enhance optical resolution to λ/5 (λ/9 after deconvolution), with 100-fold less intensity requirement compared to similar superresolution techniques. Our work not only opens up a new field of ultrasmall all-optical control based on scattering from a single nanoparticle, but also facilitates superresolution imaging for long-term observation.

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2.

The nanopatterned c -plane sapphire substrate with different morphologies can be successfully used to grow polar and nonpolar GaN epilayers. The nanopattern of sapphire substrate was prepared with natural lithography and dry-etching methods and the following GaN epilayer was grown by MOCVD. The GaN crystal orientation and crystalline quality were then characterized by high resolution X-ray diffraction, which reveals the GaN epilayer can be c -plane (0001) or m -plane (1010) orientation depending on the surface morphology of nanopatterned sapphire substrate. The corresponding full width at half maximum of the rocking curves for c - and m -plane GaN are 211 and 316 arcsec, respectively. The rms surface roughness was measured to be 0.3 nm by atomic force microscopy. The atomic structure of the sapphire-GaN heterointerface was studied by high resolution transmission electron microscope to reveal the growth mechanism. Furthermore, photoluminescence, time-resolved photoluminescence, and polarization Raman spectroscopy were employed to realize the optical and structural properties of these GaN epilayers.

Nonpolar ( 10 1 ¯ 0 ) m-plane GaN epilayer was grown on nanoscale patterned c-plane sapphire substrates by MOCVD. The nanoscale patterned sapphire substrates were fabricated by natural lithography and dry etching methods. The nanopatterns are defined particularly with no c-plane surfaces exposed. The ( 10 1 ¯ 0 ) m-plane GaN epilayer was characterized by X-ray diffraction and atomic force microscope, which indicated the FWHM of the rocking curve is 316 arcsec and the rms surface roughness is 0.3 nm, respectively. The SEM and TEM studies reveal that the m-plane GaN grains nucleate on the exposed n-plane facets of the etched c-plane sapphire substrates.

One of the key issues affecting the performance of solar cells is the behavior of carrier transfer. In this work, the time-resolved photoluminescence (TRPL) technique was utilized to investigate th...

Polycrystalline diamond films were demonstrated as good candidates for electron field emitters, and their mechanical/thermal properties should thus be considered for real devices. We utilized ultrafast optical techniques to investigate the phonon dynamics of several polycrystalline diamond films, prepared by microwave plasma enhanced chemical vapor deposition. The mechanical properties (longitudinal acoustic velocity) and thermal conductivities of diamond films were evaluated from the coherent and incoherent phonon dynamics, respectively. Ultrananocrystalline diamond films were grown using a CH4 (2%)/Ar plasma, while microcrystalline diamond films were grown using a CH4 (2%)/H2 plasma. The ultrananocrystalline diamond film (with a grain size of several nanometers) possesses low acoustic velocity (14.5 nm/ps) and low thermal conductivity (3.17 W/m K) compared with other kinds of diamond films. The acoustic velocity of diamond films increased abruptly to nearly the same as that of natural diamond and remained there when the rod-shaped diamond grains were induced due to the incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivities of the materials increased monotonously with increasing incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivity of 25.6 W/m K was attained for nanocrystalline diamond films containing spherical diamond grains (with a size of several tens of nanometers). Compared with single crystalline diamond, the low thermal conductivity of polycrystalline films results from phonon scattering at the interfaces of grains and amorphous carbon in the boundary phases.