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Polar ve semipolar yönlerde büyütülen BeMgZnO bariyer tabakaları üzerindeki ZnO kuyu katmanlarının polarizasyon özellikleri araştırıldı. Gevşemiş ve gerilmiş bariyer katmanların durumları göz önünde bulunduruldu. Arayüzlerdeki polarizasyon farkının, kuyu tabakası içinde 8 MV cm^(-1) büyüklüğünde yerleşik bir elektrik alanına yol açtığı bulundu. Polar olmayan ZnO/BeMgZnO kuantum kuyuları, altbantlar arası geçişler açısından incelendi. Hesaplamalar Be ve Mg konsantrasyonlarını sırasıyla 0.18 ve 0.5'e kadar kapsamaktadır. 50 ila 700 meV arasında değişen altbantlar arası geçiş (ISBT) enerjilerinin mümkün olduğu bulundu. Bariyer kalınlığının ISBT enerjileri üzerindeki etkisi incelendi. Sonuçlar, enerjilere kıyasla ISBT enerjilerinde önemsiz değişiklikler olduğunu göstermektedir. Polarizaton properties of ZnO well layers on BeMgZnO barrier layers grown in polar and semipolar orientations have been investigated. Cases of relaxed and strained barrier layers are considered. It is found that the polarizaton difference at the interfaces leads to a built-in electric field inside the well layer as much as 8 MV cm^(-1) in magnitude. Nonpolar ZnO/BeMgZnO quantum wells have been studied in terms of intersubband transitions. The calculations have covered Be and Mg concentrations up 0.18 and 0.5, respectively. It has been found that intersubband transition (ISBT) energies ranging from 50 to 700 meV are possible. The effect of barrier thickness on the ISBT energies has been studied. The results indicate insignificant changes in ISBT energies compared to the energies.

International audience; The present work focuses on the modeling and simulation of a new photovoltaic solar cell based on a CIGS/Si nanowire. The study of the effect of inserting quantum wells in a nanowire structure for different characteristic parameters has been done to evaluate the potential of the confinement phenomenon. The best results of the solar cell are obtained with a number of 14 quantum wells integrated in a Si nanowire-based solar cell with an efficiency of 3.16%, FF of 73.50%, Voc equal to 0.41 V and Jsc equal to 10.50 mA/cm2.

The use of strained substrates may overcome indium incorporation limits without inducing plastic relaxation in InGaN quantum wells, and this is particularly important for short-period InGaN/GaN superlattices. By incorporating elastic strain into these heterostructures, their optoelectronic behavior is modified. Our study employed density functional theory calculations to investigate the variation in the band-gap energy of short-period InGaN/GaN superlattices that comprise pseudomorphic quantum wells with a thickness of just one monolayer. Heterostructures with equibiaxially strained GaN barriers were compared with respective ones with relaxed barriers. The findings reveal a reduction of the band gap for lower indium contents, which is attributed to the influence of the highly strained nitrogen sublattice. However, above mid-range indium compositions, the situation is reversed, and the band gap increases with the indium content. This phenomenon is attributed to the reduction of the compressive strain in the quantum wells caused by the tensile strain of the barriers. Our study also considered local indium clustering induced by phase separation as another possible modifier of the band gap. However, unlike the substrate-controlled strain, this was not found to exert a significant influence on the band gap. Overall, this study provides important insights into the behavior of the band-gap energy of strained superlattices toward optimizing the performance of optoelectronic devices based on InGaN/GaN heterostructures.

International audience; Photon bunching in incoherent cathodoluminescence (CL) spectroscopy originates from the fact that a single high-energy electron can generate multiple photons when interacting with a material, thus, revealing key properties of electron−matter excitation. Contrary to previous works based on Monte Carlo modeling, here we present a fully analytical model describing the amplitude and shape of the second order autocorrelation function (g (2) (τ)) for continuous and pulsed electron beams. Moreover, we extend the analysis of photon bunching to ultrashort electron pulses, in which up to 500 electrons per pulse excite the sample within a few picoseconds. We obtain a simple equation relating the bunching strength (g (2) (0)) to the electron beam current, emitter decay lifetime, pulse duration, in the case of pulsed electron beams, and electron excitation efficiency (γ), defined as the probability that an electron creates at least one interaction with the emitter. The analytical model shows good agreement with the experimental data obtained on InGaN/GaN quantum wells using continuous, ns-pulsed (using beam blanker) and ultrashort ps-pulsed (using photoemission) electron beams. We extract excitation efficiencies of 0.13 and 0.05 for 10 and 8 keV electron beams, respectively, and we observe that nonlinear effects play no compelling role, even after excitation with ultrashort and dense electron cascades in the quantum wells.

A monolithic two-section InGaAs/GaAs double quantum wells (DQWs) mode-locked laser (MLL) emitting at 1.06~\mu \text{m} is demonstrated. Stable mode locking operation is achieved up to 80 °C. The fundamental repetition rate is at 9.51 GHz with a signal-to-noise ratio (SNR) of more than 55 dB, and up to the fourth harmonic at 38.04 GHz is observed. The characteristic temperature ( T_{0} ) of the laser and the influences of absorber bias voltage on T_{0} have been systematically investigated. From our findings, T_{0} shows a two-segment feature, and is slightly affected by the absorber bias voltage for photon saturation. National Research Foundation (NRF) Published version This work was supported in part by Finance science and technology project of Hainan Province under Grant ZDYF2020036, in part by the National Natural Science Foundation of China under Grant 61964007, Grant 61774024, Grant 61864002, and Grant 62064004; in part by the Hainan Provincial Natural Science Foundation of China under Grant 2018CXTD336; in part by the National Research Foundation of Singapore under Grant NRF-CRP12-2013-04; in part by the National Key Research and Development Project under Grant 2017YFB0405100; in part by the Jilin Province Science and Technology Development Plan under Grant 20190302007GX; and in part by the Young Talents Science and Technology Innovation Project of Hainan Association for Science and Technology under Grant QCXM201810.

This work received financial support from the Leverhulme Trust (RPG-2017-231), European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC grant agreement no. 640012 (ABLASE), EPSRC (EP/P030017/1), and the Humboldt Foundation (Alexander von Humboldt professorship). MS acknowledges funding by the Royal Society (Dorothy Hodgkin Fellowship, DH160102; Research Grant, RGF\R1\180070; Enhancement Award, RGF\EA\180051). ADF acknowledges support from European Research Council (ERC) under the European Union Horizon 2020 research and innovation program (Grant Agreement No. 819346). Biointegrated intracellular microlasers have emerged as an attractive and versatile tool in biophotonics. Different inorganic semiconductor materials have been used for the fabrication of such biocompatible microlasers, but often operate at visible wavelengths ill-suited for imaging through tissue. Here, we report on whispering gallery mode microdisk lasers made from a range of GaInP/AlGaInP multi-quantum well structures with compositions tailored to red-shifted excitation and emission. The selected semiconductor alloys show minimal toxicity and allow fabrication of lasers with stable single-mode emission in the NIR (675 – 720 nm) and sub-pJ thresholds. The microlasers operate in the first therapeutic window under direct excitation by a conventional diode laser and can also be pumped in the second therapeutic window using two-photon excitation at pulse energies compatible with standard multiphoton microscopy. Stable performance is observed under cell culturing conditions for five days without any device encapsulation. With their bio-optimized spectral characteristics, low lasing threshold and compatibility with two-photon pumping, AlGaInP-based microlasers are ideally suited for novel cell tagging and in vivo sensing applications. Publisher PDF

espanolLa segregacion superficial de atomos en las aleaciones de semiconductores III-V produce interfaces abruptas y modifica los perfiles del potencial, alternando los estados electronicos en el pozo cuantico y la energia de emision en el espectro de fotoluminiscencia. En este trabajo se resuelve mediante serie de potencias la ecuacion de Schrodinger considerando un potencial simetrico tipo Cauchy, el cual es suave y decreciente al infinito. Se propone dicho potencial debido a los cambios en el perfil del potencial del pozo cuantico por la segregacion de atomos durante el proceso de crecimiento. Se determino la energia del estado base en funcion de los parametros que caracterizan este potencial. Este modelo fue aplicado al caso particular de la segregacion de indio en el sistema InGaAs/GaAs. La energia de transicion del estado base se calcula a partir de las diferencias de energia entre el electron y el hueco en funcion del ancho del pozo. Dichos calculos estan de acuerdo con los picos de energia de fotoluminiscencia reportados. Adicionalmente, la influencia del campo electrico debido al efecto piezoelectrico en la emision de fotoluminiscencia es estudiada. Para esto se considero una funcion de onda variacional de electrones y se calculo la transicion de energia del estado base en la region activa de la heteroestructura a partir de las diferencias de energia de electrones y huecos en funcion del ancho del pozo y del campo electrico. Para pozos cuanticos de InGaAs/GaAs la energia base es ajustada dentro de este modelo coincidiendo nuestros calculos teoricos con la parte experimental. EnglishThe surface segregation in III-V semiconductor alloys produce abrupt interfaces, and modifies the potential profiles, alternating the electronic states in the quantum well and the emission energy in the photoluminescence spectrum. In this work, the Schrodinger equation is solved by means of a power series considering a Cauchy type symmetrical potential, which is soft and decreasing to infinity. This potential is proposed due to the changes in the potential profile from quantum well by the segregation of atoms during the growth process. The ground state energy was determined according to the parameters that characterize this potential. This model was applied to the particular case of indium segregation in the InGaAs/GaAs system. The ground state energy transition is calculated from the difference in energy between the electron and hole in function of well width. These calculations are in agreement with the reported photoluminescence peak energies. In addition, the influence of the electrical field due to the piezoelectric effect on the photoluminescence emission is studied. For this purpose, an electron variational wavefunction was considered and the ground state energy transition in the active region of the heterostructure was calculated from the difference in energy between the electron and hole in function of well width and the electric field. For InGaAs/GaAs quantum wells, the ground energy is adjusted within this model coinciding our theoretical calculations with the experimental part.

We briefly review the features of interband cascade lasers (ICLs) and discuss rationales for using an InAs-based plasmon waveguide configuration for extending efficient operation of ICLs to long wavelengths. We then present our recent progress in InAs-based ICLs based on an advanced waveguide configuration. We demonstrated room temperature continuous wave operation of ICLs at wavelengths longer than 6 μm.

Spin-orbit splitting of conduction band in HgTe quantum wells was studied experimentally. In order to recognize the role of different mechanisms, we carried out detailed measurements of the Shubnikov-de Haas oscillations in gated structures with a quantum well widths from $8$ to $18$ nm over a wide range of electron density. With increasing electron density controlled by the gate voltage, splitting of the maximum of the Fourier spectrum $f_0$ into two components $f_1$ and $f_2$ and the appearance of the low-frequency component $f_3$ was observed. Analysis of these results shows that the components $f_1$ and $f_2$ give the electron densities $n_1$ and $n_2$ in spin-orbit split subbands while the $f_3$ component results from magneto-intersubband oscillations so that $f_3=f_1 - f_2$. Comparison of these data with results of self-consistent calculations carried out within the framework of four-band \emph{kP}-model shows that a main contribution to spin-orbit splitting comes from the Bychkov-Rashba effect. Contribution of the interface inversion asymmetry to the splitting of the conduction band turns out to be four-to-five times less than that for the valence band in the same structures., 6 pages, 6 figures

In this study optical properties of InGaN/GaN/Al2O3 multi-quantum well (MQW) structures are investigated in detail. Three samples containing InGaN/GaN/Al2O3 MQWs are grown by using metal organic chemical vapor deposition technique. Sapphire (6H-Al2O3) is used as the substrate. Forbidden energy band gaps (E-g) of these three samples are determined from photoluminescence and absorption spectra. Results gained from these two spectra are compared with each other. It is found that E-g values are between 2 and 3eV. For determining refraction index, absorption coefficients, extinction coefficients and thickness of the films a rare method called Swanepoel envelope method is used. It is seen that results gained from this method are consistent with those in literature.

Mercury telluride (HgTe) thin films with a critical thickness of 6.5 nm are predicted to possess a gapless Dirac-like band structure. We report a comprehensive study on gated and optically doped samples by magnetooptical spectroscopy in the THz range. The quasi-classical analysis of the cyclotron resonance allowed the mapping of the band dispersion of Dirac charge carriers in a broad range of electron and hole doping. A smooth transition through the charge neutrality point between Dirac holes and electrons was observed. An additional peak coming from a second type of holes with an almost density-independent mass of around 0.04m0 was detected in the hole-doping range and attributed to an asymmetric spin splitting of the Dirac cone. Spectroscopic evidence for disorder-induced band energy fluctuations could not be detected in present cyclotron resonance experiments.

Metallic nanocavity lasers provide important technological advancement towards even smaller integrable light sources. They give access to widely unexplored lasing physics in which the distinction between different operational regimes, like those of thermal or a coherent light emission, becomes increasingly challenging upon approaching a device with a near-perfect spontaneous-emission coupling factor $\beta$. In fact, quantum-optical studies have to be employed to reveal a transition to coherent emission in the intensity fluctuation behavior of nanolasers when the input-output characteristic appears thresholdless for $\beta = 1$ nanolasers. Here, we identify a new indicator for lasing operation in high-$\beta$ lasers by showing that stimulated emission can give rise to a lineshape anomaly manifesting as a transition from a Lorentzian to a Gaussian component in the emission linewidth that dominates the spectrum above the lasing threshold.

In this paper we investigate strain and local polarization field effects in zinc-blende indium gallium nitride (InGaN) alloys and quantum wells. To do so we parametrize and establish a Stillinger-Weber potential with parameters fitted to hybrid functional density functional theory data. The developed model gives very good agreement with quantities to which it has not been fitted, such as Kleinman parameters of cubic III-N materials or the composition dependence of the lattice constant in InGaN alloys. Equipped with this model, we extract the composition dependence of elastic constants C11 and C12 in InGaN alloys, including bowing parameters for these quantities, which may form input for continuum-based calculations. Furthermore, applying this model to InGaN alloys and wells reveals that random alloy fluctuations can lead to strong local strain field fluctuations. Building on this information, we present a model that allows for the calculation of connected local built-in field fluctuations at the microscopic level, accounting for first- and second-order piezoelectric effects. The approach is general and can be applied to any zinc-blende III–V alloy or heterostructure investigated in the frame of semiempirical models (e.g., valence force field models) targeting strain fields on an atomistic level. Here, building on our Stillinger-Weber potential we show that local strain fluctuations in zinc-blende InGaN quantum wells can lead to strong piezoelectric built-in field fluctuations. This contribution has been widely overlooked in previous theoretical studies of these systems. Finally, we briefly discuss the impact of these polarization field fluctuations on carrier localization effects in such quantum well systems.

Orientador: Maria José Santos Pompeu Brasil Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin" Resumo: Nesta dissertação investigamos os efeitos dos íons de Mn na dinâmica do spin de elétron em poços quânticos de InGaAs/GaAs. Os poços têm um gás de buracos gerado por dopagens em suas barreiras, sendo uma dopagem tipo delta de Mn numa das barreiras e uma dopagem tipo delta de C, na outra. A densidade de buracos foi determinada mediante medidas de transporte e são consistentes com as densidades obtidas das energias de Stokeshift. Utilizamos diversas técnicas ópticas, como a fotoluminescência no modo contínuo (PL-CW) e resolvida no tempo (PL-RT), a fotoluminescência de excitação (PLE-CW), e o efeito Hanle óptico, sempre usando luz circularmente polarizada para excitação e analisando a polarização circular da luz emitida. Comparamos os tempos de vida ( ? ) e de relaxação do spin ( ? s) dos elétrons obtidos através destas técnicas e discutimos as diferenças intrínsecas destes métodos e o significado físico dos parâmetros fornecidos por eles. Analisamos também o efeito da presença dos íons de Mn, que são íons magnéticos, sobre os tempos vida e de spin dos elétrons em uma série de amostras com diferentes quantidades de Mn incluindo a amostra de referencia sem Mn. Os resultados encontrados revelam um limite para a concentração de Mn, para a qual ambos, ? e ? s, apresentam uma queda abruta. Surpreendentemente, esta queda não afeita o grau de polarização CW, pois a razão ?/? s que determina este parâmetro permanece basicamente constante para todas as amostras Abstract: We have studied the effect of Mn ions on the spin-relaxation of electrons in a InGaAs/GaAs quantum well (QW). The QW has a two-dimensional hole-gas generated by doping the barriers, whereas one of the barriers has a Mn-planar layer and the other one, a C planarlayer. The hole densities were determined by Shubnikov-de-Haas oscillations and are consistent with the Stokes-shift energies obtained by optical measurements. We have performed continuouswave photoluminescence measurements (CW-PL), excitation photoluminescence (CW-PLE), timeresolved (TR-PL), and Hanle effect with circularly polarized excitation and detection. We compare the lifetime ( ? ) and the spin relaxation time ( ? s) obtained using those techniques and we discuss the differences between the various techniques and the physical meaning of those parameters. We also analyze the effect of Mn ions on ? s and ? for the series of samples with different Mn concentrations, including a reference sample with no Mn doping. The results revealed a threshold of Mn concentration at which both, and ? s, show a strong and abrupt fall. Surprisingly, this fall does not affect the CW effective polarization degree, since the ratio s that determines this parameter remains basically constant for all samples Mestrado Física da Matéria Condensada Mestre em Física

Strain-based band structure engineering is a powerful tool to tune the optical and electronic properties of semiconductor nanostructures. We show that we can tune the band structure of InGaAs semiconductor quantum wells and modify the helicity of the emitted light by integrating them into rolled-up heterostructures and changing their geometrical configuration. Experimental results from photoluminescence and photoluminescence excitation spectroscopy demonstrate a strong energy shift of the valence-band states in comparison to flat structures, as a consequence of an inversion of the heavy-hole with the light-hole states in a rolled-up InGaAs quantum well. The inversion and mixing of the band states lead to a strong change in the optical selection rules for the rolled-up quantum wells, which show vanishing spin polarization in the conduction band even under near-resonant excitation conditions. Band structure calculations are carried out to understand the changes in the electronic transitions and to predict the emission and absorption spectra for a given geometrical configuration. Comparison between experiment and theory shows an excellent agreement. These observed profound changes in the fundamental properties can be applied as a strategic route to develop novel optical devices for quantum information technology FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

The spin-orbit interaction permits to control the state of a spin qubit via electric fields. For holes it is particularly strong, allowing for fast all electrical qubit manipulation, and yet an in-depth understanding of this interaction in hole systems is missing. Here we investigate, experimentally and theoretically, the effect of the cubic Rashba spin-orbit interaction on the mixing of the spin states by studying singlet-triplet oscillations in a planar Ge hole double quantum dot. Landau-Zener sweeps at different magnetic field directions allow us to disentangle the effects of the spin-orbit induced spin-flip term from those caused by strongly site-dependent and anisotropic quantum dot g tensors. Our work, therefore, provides new insights into the hole spin-orbit interaction, necessary for optimizing future qubit experiments. published

High-quality n-type continuously graded Ge-rich Si1−xGex parabolic quantum wells with different doping level were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1÷6)×1011 cm−2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line-shape is independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step-forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role.

We present a free-running 80-MHz dual-comb polarization-multiplexed solid-state laser which delivers 1.8 W of average power with 110-fs pulse duration per comb. With a high-sensitivity pump-probe setup, we apply this free-running dual-comb laser to picosecond ultrasonic measurements. The ultrasonic signatures in a semiconductor multi-quantum-well structure originating from the quantum wells and superlattice regions are revealed and discussed. We further demonstrate ultrasonic measurements on a thin-film metalized sample and compare these measurements to ones obtained with a pair of locked femtosecond lasers. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers a significant reduction in complexity and cost for this widely adopted non-destructive testing technique.

Agradecimentos: This work was supported by the 5-100 Competitiveness Enhancement Program. The work of D.V.K. has been supported by the Ministry of Science and Higher Education of Russian Federation under the State Assignment No. 0729- 2020-0058. The work of K.S.K. has been supported by the President of Russian Federation grant for young researchers Grant No. MK-2740.2021.1.2. F.I. acknowledges the CNPq (Grant No. 432882/2018-9) for financial support. M.A.G.B. acknowledges FAPEMIG Grant No. CEX-APQ-00753-18. M.V.D. and M.V.V. acknowledge the grant of the President of the Russian Federation, Grant No. MD-1708.2019.2. The authors are grateful to Dr. V. Ya. Aleshkin (Institute for Physics of Microstructures RAS) for providing the software package. The authors declare no conflict of interest Abstract: The GaAs/InGaAs quantum wells with a ferromagnetic delta(Mn) layer in GaAs barrier demonstrate a set of interesting spin-related phenomena originating from Mn-hole interaction. One of such phenomena is a spin-memory effect which consists of Mn spin polarization induced by interaction with vicinity spin-polarized holes generated under the exposure by short circularly polarized light pulses. Here long Mn spin relaxation time (similar to 5 ns) allows preserving the spin polarization of the entire system. In the present paper the spin-memory effect investigation was carried out by analyzing the polarization kinetics of quantum-well photoluminescence in the pump-probe technique. It was shown that the photoluminescence circular polarization degree is strongly affected by the magnetic interaction of holes with Mn atoms prepolarized by the pump pulse. In the case of antiparallel Mn and hole polarizations, magnetic interaction leads to decrease of circular polarization degree as compared with single-pulse excitation (so called Delta P effect). Interestingly, the amplitude of hole-mediated Delta P effect is strongly affected by the concentration of resident electrons in the quantum well. The latter was shown to be caused by the specific compliance with selection rules for optical transitions with the participation of unpolarized resident electrons and spin-polarized holes affected by Mn-hole interaction CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAIS - FAPEMIG Fechado

In quantum wells (QWs) formed in HgCdTe/CdHgTe heterosystems with a variable composition of Cd(Hg), Shubnikov-de-Haas (SdH) oscillations are investigated to characterize the Rashba-type spin-orbit coupling in QWs with both a normal and inverted band structure. Several methods of extracting the Rashba spin-splitting at zero magnetic field and their magnetic field dependences from the beatings of SdH oscillations are used for greater reliability. The large and similar Rashba splitting (25–27 meV) is found for different kinds of spectrum, explained by a significant fraction of the p-type wave functions, in both the E1 subband of the sample with a normal spectrum and the H1 subband for the sample with an inverted one.

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron&ndash, phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1&rarr, 3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

Agradecimentos: This work was carried out with the support of the Russian Foundation for Basic Research (grant no. 18-29-19137_mk) and with the partial support of grant no. 19-19-00545 of the Russian Science Foundation (fabrication of structures using pulsed laser deposition) Abstract: A significant (almost two orders of magnitude) increase in the intensity of photo- and electroluminescence of a diode structure with an InGaAs/GaAsSb/GaAs quantum well, GaMnAs layer as a spin injector, and contact coating of a multilayer graphene film has been experimentally detected. The result has been explained by the possible formation of a hybrid system of multilayer graphene and GaAs semiconductor under the influence of He-Ne laser radiation, which leads to a change in the band diagram of the heterostructure Fechado

We investigate the optical properties of strain-free mesoscopic GaAs/AlxGa1 - xAs structures (MGS) coupled to thin GaAs/AlxGa1 - xAs quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs (X-point) and GaAs (Gamma-point) gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x >= 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x >= 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Gamma bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X - Gamma mixing of the MGS states and can be increased from 0.3 to 2.5 ns by changing the Al content. The slower component, however, depends on the X - Gamma mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X - Gamma mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ Fechado

Agradecimentos: The study was supported by the President of the Russian Federation, grant no. MD-1708.2019.2, the Russian Foundation for Basic Research, project no. 20-38-70063, and by the FAPEMIG grants CEX-APQ-00753-18 Abstract: The results of studies of the time-resolved photoluminescence in semiconductor heterostructures containing two noninteracting InGaAs quantum wells in a GaAs matrix are reported. One of the quantum wells was undoped, and the other was uniformly doped with chromium atoms (InGaAs:Cr). It was shown that the introduction of Cr had a profound effect on the recombination lifetime of charge carriers in quantum wells. The change in the photoluminescence intensity after excitation cannot be described by a monoexponential decay function, which is attributed to a change in the built-in electric field of the surface barrier in the quantum wells because of screening by photoexcited charge carriers FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAIS - FAPEMIG Fechado

Agradecimentos: G.N., T.A.C.F., and L.A.P. acknowledge the support from Sao Paulo Research Foundation (FAPESP), under grant numbers 2013/16911-2, 2018/15574-6, and 2018/25339-4. G.N. acknowledges Brazilian National Council for Scientific and Technological Development (CNPq) and T.A.C.F. acknowledges FAPESP (Project No. 2019/22565-6) for their scholarships. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government t(MSIT). (No. 2020R1A2C2011478) Abstract: Due to the strong confinement of carriers, electronic interactions play a major role in the performance of colloidal nanocrystals (NCs) as materials for optical gain. Therefore, understanding how to manipulate this phenomenon is essential to achieve feasible devices in the future. In this context, plane colloidal quantum wells (PQWs) have emerged as one of the most promising classes of NCs to be applied as a lasing material, mainly due to the way that carriers and light interact in these structures. Here, a new NC geometry is introduced, called a spherical quantum well (SQW), composed of a CdS/CdSe/CdS (core/well/shell) heterostructure. Similar to PQWs, in this structure it is possible to combine a long biexciton Auger recombination lifetime (>1 ns) with a large absorption cross section and near-unity photoluminescence quantum yield. Also, due to the quantum well structure, this material enables enhanced biexciton interactions even for large volumes. This boosts the gain performance, decreasing the gain threshold to an average number of excitons of (N) T = 0.61 and enhancing the intrinsic gain coefficient. Additionally, by growing thick-shell SQWs it is possible to enhance the absorption cross section, reaching an amplified spontaneous emission (ASE) threshold of 10.6 mu J/cm(2) . Finally, ASE tests demonstrated that this geometry enables enhanced stability, showing no signs of degradation after 287 days of exposure to regular atmospheric conditions and sustained ASE for more than 13 h under femtosecond laser irradiation. These results demonstrate that the SQW structure combines important characteristics of both PQW and core/shell geometries, representing an important step toward a solution-processed quantum dot laser in the future FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ fechado

The lateral bipolar electric transport has been investigated for multi-period n-In 0.08 Ga 0.92 As/GaAs heterostructures with tunnel-coupled double-quantum wells for various carrier mobilities at temperatures 4.2...160 K. The presence of two types of charge carriers – electrons and holes – is identified by observation of interband electrolumine- scence (EL). We found that the current-voltage characteristic has a complex nonlinear shape and changes with current, which is accompanied by modification of the EL intensity and spectrum. We observed oscillations of the current and EL intensity with the frequency of tens MHz, both arise at the electric fields well below the threshold field of the Gunn instability. Oscillations of the EL intensity occur in the opposite phase to the current. The electric and EL measurements have shown that the minority carriers, the holes, are supplied from the anode side of the sample. Spatial separation of electrons and holes in the double- quantum well structures provides abnormally large both electron-hole recombination time and drift length of the holes. Studied behavior of the current and EL can be interpreted as a combined effect of the spatial separation of the electrons and holes and dynamics of their transfer between undoped and doped quantum wells. We suggest that observed real-space transfer effects in high-field bipolar electric transport and, particularly, highly intensive interband electroluminescence from macroscopically large areas may be used in a number of optoelectronic applications.

The carrier recombination dynamics of InGaP/InGaAsP quantum wells are reported for the first time. By studying the photoluminescence (PL) and time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well(MQW) heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very low non-radiative recombination rate and high radiative efficiency compared to the control InGaP sample. Along with the analyses of PL emission spectrum and external quantum efficiencies, it suggests that this is due to small confinement potentials in the conduction band but high confinement potentials in the valence band. These results explain several features found in InGaP/InGaAsP MQW solar cells previously., Comment: 6 pages, 5 figures, 2 tables

We present a solution of the quantum mechanics problem of the allowable energy levels of a bound particle in a one-dimensional finite square well. The method is a geometric-analytic technique utilizing the conformal mapping w → z = wew between two complex domains. The solution of the finite square well problem can be seen to be described by the images of simple geometric shapes, lines, and circles, under this map and its inverse image. The technique can also be described using the Lambert W function. One can work in either of the complex domains, thereby obtaining additional insight into the finite square well problem and its bound energy states. This suggests interesting possibilities for the design of materials that are sensitive to minute changes in their environment such as nanostructures and the quantum well infrared photodetector.

International audience; In single junction solar cells a large part of the incident energy ends up as heat which limits their maximum achievable efficiency. Thus the achievement of maximum power conversion efficiencies relies on complex multijunction devices. Here we show the possibility to harvest the available solar energy using hot carrier devices and evidence a positive contribution of the hot carrier effect on photovoltaic performances. We investigated a semiconductor heterostructure based on a single InGaAsP quantum well using quantitative optoelectrical characterization, especially luminance measurements. The quantitative thermodynamic study of the hot carrier population allows us to discuss the hot carrier contribution to the solar cell performance. We demonstrate that voltage and current are enhanced due to the presence of the hot carrier population in the quantum well. These experimental results substantiate the potential of increasing photovoltaic performances in the hot carrier regime. Moreover, by developing a suitable analytic theoretical framework, we show how to obtain separate (hot) temperatures of electrons and holes from photoluminescence spectra analysis. The individual thermalization coefficients of each carrier type are also discussed. The method developed in this article paves the way towards the design of new energy harvesting devices and to the development of advanced characterization tools. Finaly, to increase the PV performance enhancement and reduce the concentration factor, an optimize design is investigated.

n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III-V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement via multiple barrier heights., 5 pages, 5 figures

Selective area epitaxy (SAE) is widely used in photonic integrated circuits, but there is little information on the use of this technique for the growth of heterostructures in ultra-wide windows. Samples of heterostructures with InGaAs quantum wells (QWs) on GaAs (100) substrates with a pattern of alternating stripes (100-&mu, m-wide SiO2 mask/100-&mu, m-wide window) were grown using metalorganic chemical vapour deposition (MOCVD). It was found that due to a local change in the growth rate of InGaAs QW in the window, the photoluminescence (PL) spectra measured from the edge to the center of the window exhibited maximum blueshifts of 14 and 19 meV at temperatures of 80 K and 300 K, respectively. Using atomic force microscopy, we have demonstrated that the surface morphologies of structures grown using standard epitaxy or SAE under identical MOCVD growth conditions correspond to a step flow growth with a step height of ~1.5 ML or a step bunching growth mode, respectively. In the structures grown with the use of SAE, a strong variation in the surface morphology in an ultra-wide window from its center to the edge was revealed, which is explained by a change in the local misorientation of the layer due to a local change in the growth rate over the width of the window.

Reliability investigations were conducted after GaN-based LEDs were stressed in seawater vapour. Multiple electrical, optical, and material analyses on the fine nanostructures of the LED were examined. Results indicate that dark spots on the surface and etching trenches observed on the cross section might damage the quantum well and degrade LED performance. Dissolved sodium ions might diffuse and punch through the quantum well and be responsible for these spots and trenches.

The electronic and optical properties of indium-graded semipolar quantum well (QW) structures with different indium variation schemes and well widths have been investigated by 6 $\times$ 6 $k\cdot p$ calculations. Both increasing the indium composition difference between the maximum and the minimum points in the well layer and moving the location of the maximum indium composition in the opposite direction of the built-in field existing in the well layer of indium constant semipolar QW can improve the overlap of electron and hole wave functions, as well as the intensity of spontaneous emission rate spectra for $y^{\prime}$ - polarization of the indium-graded semipolar QW. With the increasing well width, the overlaps of optimized indium-graded semipolar QWs decrease more slowly than those of the indium constant QW, and the optical polarization ratios $\rho_{y^{\prime}x^{\prime}}$ of the semipolar QWs increase more slowly than those of the indium constant one.

Electron cyclotron resonance in InAs/AlSb heterostructures with quantum wells of various widths in pulsed magnetic fields up to 45 T are investigated. Our experimental cyclotron energies are in satisfactory agreement with the results of theoretical calculations performed using the eight-band kp Hamiltonian. The shift of the cyclotron resonance (CR) line, which corresponds to the transition from the lowest Landau Level to the low magnetic-field region, is found upon varying the electron concentration due to the negative persistent photoconductivity effect. It is shown that the observed shift of the CR lines is associated with the finite width of the density of states at the Landau levels.

The ongoing miniaturization of semiconductor lasers has enabled ultra‐low threshold devices and even provided a path to approach thresholdless lasing with linear input-output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator, and mode‐density enhancement. Here, high‐β metal‐clad coaxial nanolasers, which facilitate thresholdless lasing are investigated. Both the conventional lasing characteristics, as well as the photon statistics of the emitted light at 10 K under continuous wave excitation are experimentally and theoretically investigated. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. The work confirms an important aspect of high‐β lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero‐threshold laser. Moreover, the results reveal an excitation power dependent β‐factor which needs to be taken into account to correctly describe the experimental data.

Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one-dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments, these structures emit visible light, which is optically polarized up to 86% at 10 K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2 nm to 8 nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarized along the length of these nanostructures.

Using methods related to the Lambert W function, we present solutions of two apparently different problems: (1) The one-dimensional double square well potential in quantum mechanics and (2) The transverse electric and magnetic modes for a step-index electromagnetic waveguide. The solution techniques provide insight into the bound energy states for the single and double square well problems and the allowed modes of propagation in a waveguide with varying refractive indices. The solutions can be viewed in either of two related complex plane representations. Comparison of the solution geometries suggests that interesting applications may be possible in nanostructures and devices which are designed to be sensitive to small changes in their environment.

This work suggests new alternative explanation why a single InGaN quantum well (QW) or the deepest QWs in the multiple quantum well (MQW) structures suffer with a high non-radiative recombination rate. According to SIMS results, positron annihilation spectroscopy and photoluminescence measurements we suggest that vacancy of Ga in complex with hydrogen atoms can play a dominant role in non-radiative Shockley-Read-Hall recombination of the deepest QWs in InGaN/GaN MQW structures. Vacancy of gallium originate dominantly in GaN buffer layers grown at higher temperatures in H2 atmosphere and are transported to the InGaN/GaN MQW region by diffusion, where they are very effectively trapped in InGaN layers and form complex defects with hydrogen atoms during epitaxy of InGaN layers. Trapping of gallium vacancies is another suggested mechanism explaining why the widely used In containing prelayers help to increase the luminescence efficiency of the InGaN/GaN MQW active region grown above them. Understanding the mechanism why the luminescence efficiency is suppressed in deeper QWs may be very important for LED community and can help to develop new improved technologies for the growth of InGaN/GaN MQW active region.

We experimentally demonstrate a novel approach of using coupling between a leaky mode resonance and intersubband transitions in semiconductor quantum wells to realize a hybrid dielectric-semiconductor metasurface with high second-harmonic conversion efficiency and increased bandwidth.

International audience; The science and applications of electronics and optoelectronics have been driven for decades by progress in the growth of semiconducting heterostructures. Many applications in the infrared and terahertz frequency range exploit transitions between quantized states in semiconductor quantum wells (intersubband transitions). However, current quantum well devices are limited in functionality and versatility by diffusive interfaces and the requirement of lattice-matched growth conditions. Here, we introduce the concept of intersubband transitions in van der Waals quantum wells and report their first experimental observation. Van der Waals quantum wells are naturally formed by two-dimensional materials and hold unexplored potential to overcome the aforementioned limitations—they form atomically sharp interfaces and can easily be combined into heterostructures without lattice-matching restrictions. We employ near-field local probing to spectrally resolve intersubband transitions with a nanometre-scale spatial resolution and electrostatically control the absorption. This work enables the exploitation of intersubband transitions with unmatched design freedom and individual electronic and optical control suitable for photodetectors, light-emitting diodes and lasers.

We study analytically and numerically the condensation of a driven-dissipative exciton-polariton system using symmetric nonresonant pumping geometries. We show that the lowest condensation threshold solution carries a definite parity as a consequence of the symmetric excitation profile. At higher pump intensities competition between the two parities can result in critical quenching of one and saturation of the other. Using long pump channels, we show that the competition of the condensate parities gives rise to a different type of topologically stable defect propagating indefinitely along the condensate. The defects display repulsive interactions and are characterized by a sustained wave packet carrying a pair of opposite parity domain walls in the condensate channel. MOE (Min. of Education, S’pore) Published version

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer. (ET) efficiencies of up to similar to 25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d-4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Frster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to similar to 21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

S.K. acknowledges the European Commission for the H2020 Marie Sklodowska-Curie Actions (MSCA) fellowship (Topopolis). The Wurzburg group acknowledges the financial support by the state of Bavaria and the Deutsche Forschungsgemeinschaft (DFG) within the project Schn1376-3.1. We study the condensation of exciton-polaritons in a two-dimensional Lieb lattice of micropillars. We show selective polariton condensation into the flatbands formed by S and Px,y orbital modes of the micropillars under non-resonant laser excitation.The real space mode patterns of these condensates are accurately reproduced bythe calculation of related Bloch modes of S- and P-flatbands. Our work emphasizes the potential of exciton-polariton lattices to emulate Hamiltonians of advanced potential landscapes. Furthermore, the obtained results provide a deeper inside into the physics of flatbands known mostly within the tight-binding limit. Postprint Postprint

International audience; Layered hybrid organic perovskites (HOPs) structures are a class of low-cost two-dimensional materials that exhibit out- standing optical properties, related to dielectric and quantum confinement effects. Whereas modeling and understanding of quantum confinement are well developed for conventional semiconductors, such knowledge is still lacking for 2D HOPs. In this work, concepts of effective mass and quantum well are carefully investigated and their applicability to 2D HOPs is dis- cussed. For ultrathin layers, the effective-mass model fails. Ab- sence of superlattice coupling and importance of non-parabo- licity effects prevents the use of simple empirical models based on effective masses and envelope function approxima- tions. An alternative method is suggested in which 2D HOPs are treated as composite materials, and a first-principles ap- proach to the calculation of band offsets is introduced. These findings might also be relevant for other classes of layered 2D functional materials.

A method is proposed for the optimization of structural parameters of GaN/AlGaN quantum wells and Bragg-confined structures, with respect to peak intersubband absorption from the ground to the first excited electronic state in the mid and near infrared spectral range. It is based on the application of the Genetic Algorithm and delivers globally optimal structures with a preset number of embedded layers. Simple rectangular quantum well profile is investigated for the maximal Stark effect and applications to tunable mid-infrared photodetectors. In case of Bragg-confined structures, an above the barrier bound state is used to extend the range of transition energies above the values available in conventional quantum wells. The effects of band nonparabolicity are taken into account. (C) 2013 Elsevier Ltd. All rights reserved

SrCuO2/Sr0.9La0.1CuO2/SrCuO2 trilayers were grown by oxide-molecular beam epitaxy. The thicknesses of the top and bottom SrCuO2 layers were fixed, while the thickness of the infinite-layer electron-doped cuprate Sr0.9La0.1CuO2 central layer was systematically changed. Transmission electron microscopy, x-ray reflectivity and x-ray diffraction measurements were performed to assess the sample quality and the abruptness of the interfaces. Electrical transport measurements as a function of temperature and as a function of central layer thickness, confirm that the normal state properties of the trilayers are altered by the confinement of the charge carriers in the central layer.