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Enhancement of the surface photoemission from metal into semiconductor by resonance tunneling of photoexcited electrons through (quasi-) discrete level in quantum well, located within Schottky barrier of the metal-semiconductor interface, is studied theoretically taking into account the difference between the electron masses in metal and semiconductor. It is shown, in particular, that resonance tunneling through the discrete level can lead to the redshift of the threshold wavelength of surface photoeffect, higher slope linear growth in photocurrent near the threshold (in contrast to quadratic growth, i.e., Fowler's law), and the possibility to increase substantially the photoemission efficiency similarly to recent experimental results on hot carrier generation in plasmonic structures with a discrete energy level at metal interface. The difference in the effective masses is shown to significantly affect the results. Double-barrier tunneling structures with resonant tunneling may become attractive for applications in photochemistry and in plasmonic photodetectors in near IR and middle IR regions of the spectrum.

An analytical expression for the maximum potential bandwidth of the frequency modulated comb naturally occurring in free running quantum cascade lasers (QCLs) has been derived. The maximum bandwidth is not as wide as the one attainable in passively mode locked lasers, but wider than that in the actively mode locked ones. The bandwidth is shown to critically depend on the flatness of the gain spectrum and the cavity length and less so on pump current. The results firmly establish that the performance of QCL frequency combs can be on par with combs generated by other means.

We propose a graphene-based terahertz detection scheme capable of measuring not only pulse energy but also electric field shape. The scheme leverages strong nonlinear velocity saturation characteristics of graphene in combination with envelope-carrier phase offset imposed by propagation of the pulses through the dispersive medium to produce shape-dependent electric charge. These charges can then be easily detected by conventional electronics, and as numerical calculations show, the original pulse shape can be recovered with the help of deep neural networks.We propose a graphene-based terahertz detection scheme capable of measuring not only pulse energy but also electric field shape. The scheme leverages strong nonlinear velocity saturation characteristics of graphene in combination with envelope-carrier phase offset imposed by propagation of the pulses through the dispersive medium to produce shape-dependent electric charge. These charges can then be easily detected by conventional electronics, and as numerical calculations show, the original pulse shape can be recovered with the help of deep neural networks.

Chip-scale frequency combs such as those based on quantum cascade lasers (QCLs) or microresonators are attracting tremendous attention because of their potential to solve key challenges in sensing and metrology. Though nonlinearity and proper dispersion engineering can create a comb—light whose lines are perfectly evenly spaced—these devices can enter into different states depending on their history, a critical problem that can necessitate slow and manual intervention. Moreover, their large repetition rates are problematic for applications such as dual comb molecular spectroscopy, requiring gapless tuning of the offset. Here, we show that by blending midinfrared QCL combs with microelectromechanical comb drives, one can directly manipulate the dynamics of the comb and identify new physical effects. Not only do the resulting devices remain on a chip-scale and are able to stably tune over large frequency ranges, but they can also switch between different comb states at extremely high speeds. We use these devices to probe hysteresis in comb formation and develop a protocol for achieving a particular comb state regardless of its initial state.Chip-scale frequency combs such as those based on quantum cascade lasers (QCLs) or microresonators are attracting tremendous attention because of their potential to solve key challenges in sensing and metrology. Though nonlinearity and proper dispersion engineering can create a comb—light whose lines are perfectly evenly spaced—these devices can enter into different states depending on their history, a critical problem that can necessitate slow and manual intervention. Moreover, their large repetition rates are problematic for applications such as dual comb molecular spectroscopy, requiring gapless tuning of the offset. Here, we show that by blending midinfrared QCL combs with microelectromechanical comb drives, one can directly manipulate the dynamics of the comb and identify new physical effects. Not only do the resulting devices remain on a chip-scale and are able to stably tune over large frequency ranges, but they can also switch between different comb states at extremely high speeds. We use these de...

Graphene may be a fascinating material but at this time there exists no experimental or theoretical evidence that it can improve performance of practical nonlinear optical devices.

We propose the idea of making quantum dot lasers by embedding direct-band gap quantum dots in a short period superlattice whose band gap is indirect. This technique reduces the threshold current and its temperature dependence. We show that a higher characteristic-temperature T0 can be achieved in a quantum dot laser with indirect GaAs/AlAs superlattice barriers compared to that with direct GaAs barriers.

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.

Longitudinal optical (LO) phonons in GaN generated in the channel of high electron mobility transistors (HEMT) are shown to undergo nearly elastic scattering via collisions with hot electrons. The net result of these collisions is the diffusion of LO phonons in the Brillouin zone causing reduction of phonon and electron temperatures. This previously unexplored diffusion mechanism explicates how an increase in electron density causes reduction of the apparent lifetime of LO phonons, obtained from the time resolved Raman studies and microwave noise measurements, while the actual decay rate of the LO phonons remains unaffected by the carrier density. Therefore, the saturation velocity in GaN HEMT steadily declines with increased carrier density, in a qualitative agreement with experimental results.

Coupling between the transverse electromagnetic mode of the coplanar strips and transverse acoustic phonon mode in the multilayer piezoelectric structures is considered. It is shown that it leads to the formation of a coupled entity—acoustic cavity polariton, whose frequency and velocity can be adjusted within a very broad range by the proper design of the piezoelectric cavity. The acoustic cavity polaritons can be used in filters, delay lines, and solid-state traveling wave devices for frequencies of up to 100 GHz.

The polarization and power dependencies of cascaded Raman self-frequency shifted solitons generation in a polarization maintaining high power Er/Yb fiber amplifier have been experimentally investigated. The experimental results show the number and amount of cascaded Raman solitons generations and frequency shifts, respectively, dependent on the input polarization state and the gain of the all-PM fiber amplifier. The numerical modeling was also conducted based on the experimental parameters and the experimental results are shown to be in good agreement with the theory.

We explore the feasibility of reducing the lifetime of longitudinal optical phonons in InP by injecting coherent longitudinal acoustic modes of frequency given by the difference between those of the longitudinal and transverse optical phonons. Calculations show that a ten-fold reduction in the lifetime can be attained using a 40 mW acoustic source. The increase in the phonon decay should reduce the scattering rate of hot electrons in fast devices.

We propose a semiconductor laser (λ≈1.25 μm) whose active region is a type-II multiple-quantum-well structure (AlGaInAs/AlGaAsSb/AlInAs on InP) to lengthen the free-carrier lifetime. Simulations project an order-of-magnitude enhancement of the Q-switched energy per pulse without increasing the pump current.

We propose a method of heterodyne detection of far-infrared signals using generation of directional current in semiconductors via a coherent interaction of one- and two-photon absorption, and evaluate the sensitivity and detectivity of this method.

Exchange interactions between the holes confined in arrays of strained quantum dots subject to compressive and tensile strain are shown to lead to ferrimagnetic arrangement of magnetic moments. Using example of strained In1−xGaxAs quantum dots on InP substrate, it is shown how magnetic properties of a semiconductor material can be engineered without resorting to doping with magnetic ions, and the Curie temperature is estimated to be of the order of 25 K. Potential applications in information storage and processing are considered.

Quantum-confined structures such as multiple quantum wells (MQWs) exhibit room temperature exciton resonances, and it is well known that these resonances may enhance the nonlinear optical properties of an MQW compared with that of a bulk material. Although there have been some studies on the nonlinear optical properties of MQWs near half the bandgap, they an failed to show clearly both the magnitude and the position of the two photon excitonic resonance. In this work, we show detailed two-photon absorption (TPA) studies of an MQW near half the bandgap where we have observed the excitons with reduced binding energy due to the two-photon transitions between the bound and continuum states. For the experiments, we used a l-cm-long MQW channel waveguide with 7-nm-thick GaAs wells.

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

We report on Gallium Nitride-based tunneling hot electron transistor amplifier with common-emitter current gain greater than 1. Small signal current gain up to 5 and dc current gain of 1.3 were attained in common-emitter configuration with collector current density in excess of 50 kA/cm2. The use of a combination of 1 nm GaN/3 nm AlN layers as an emitter tunneling barrier was found to improve the energy collimation of the injected electrons. These results represent demonstration of unipolar vertical transistors in the III-nitride system that can potentially lead to higher frequency and power microwave devices.

A multiple quantum dot structure consisting of alternating strained and unstrained quantum dots and having nonzero net dipole moment, i.e., possessing ferroelectric properties, is proposed. Curie temperature and spontaneous polarization are estimated to be of the order of 20 K and 10−5 C/m2, respectively, for the case of InGaAs quantum dots grown on InP substrate and are shown to be insensitive to the lateral arrangement of the dots. Practical applications in information processing and storage are considered.

We have investigated second-harmonic generation in short-period periodically-poled bulk and waveguide potassium titanyl phosphate crystals to generate blue light using subpicosecond laser pulses. For the bulk, the highest conversion efficiency is ≈5.5%, which is about two orders of magnitude larger than that achieved previously. For the waveguide, the highest conversion efficiency is ≈32%, which is about a factor of 4 higher than that obtained before. These measured values are in good agreement with our theoretical results. We have observed saturation of conversion efficiency, which sets a limit to the maximum efficiency that can be achieved.

We propose a fundamentally new method of heterodyne detection of FIR signals using quantum interference between one- and two-photon absorption. We suggest different implementations of the scheme using multiple QW detectors and evaluate its sensitivity.

We propose a novel intersubband laser based on transition between the ground-state heavy-hole subband (HH1) and light-hole subband (LH1) in a k-space region where the light-hole effective mass is inverted. The laser structure can be electrically pumped with a simple quantum cascade scheme. Our calculation shows that with only a small fraction of the carrier population in the upper subband (LH1), it is possible to achieve population inversion between the two subbands locally in K space where the light-hole effective mass is inverted. Optical gain in excess of 150/cm can be achieved with a pumping current density on the order to 100 A/cm2 at the temperature of liquid nitrogen.

We have observed an anomalously large blueshift of the apparent donor–acceptor pair transition peak as the laser intensity increases in compensation-doped coupled quantum wells. The mechanism for the blueshift is the change of the Coulomb interaction energy between the recombined donors and acceptors due to the decrease in their separation as the laser intensity increases. The blueshift is accompanied by the saturation of the relative donor–acceptor pair transition peak intensity due to filling of the impurity states.

We have observed saturation of photoluminescence peak at extremely low pump intensities in growth-interrupted and undoped asymmetric-coupled quantum-well structures. We believe the saturation is due to filling of the exciton states localized at the interface islands. The exciton density required to completely fill the localized exciton states at the interface islands for one of the samples is two orders of magnitude lower than that for the previous sample. We have demonstrated that even under the same growth condition, two samples with the same structure exhibit distinctly different photoluminescence spectra and saturation characteristics.

We show that the autocorrelation function of mode-locked laser pulses can be measured by interfering a pulse with its time-delayed replica in a conventional photoconductor to generate a pulsating grating that can be represented as a superposition of moving gratings. The resonance current peak resulting from matching the carrier drift velocity to the velocity of one of the grating components is proportional to the autocorrelation function. Unlike conventional autocorrelators, the proposed method uses no nonlinear effects and thus can be utilized for measuring weak pulses.

A novel sensor, based on the intervalley transfer of electrons in AlxGa1−xAs (x∼0.3) subjected to hydrostatic pressure and/or strain is proposed. The operation characteristics are evaluated, optimized theoretically, and demonstrated experimentally.

A systematic study of the size dependence of the photoluminescence spectra of Si quantum nanocrystals in the SiO2 matrix has been performed. The results have been fitted to a quantum‐confinement model that includes the nanocrystal size dispersion rather than a specific size of the nanocrystal. This, in conjunction with the results [Z. H. Lu, D. J. Lockwood, and J.‐M. Baribeau, Nature 378, 258 (1995)] for amorphous Si layers serves as a strong confirmation of the confinement‐induced nature of the photoluminescence.

We consider theoretically a novel lasing scheme in semiconductor quantum boxes. Ultralow threshold pumping power can be obtained due to the development of images charges in surrounding layers. The impact on the intersubband lasing is stressed.

Radiative recombination of the excitonic states in semiconductor superlattices with an applied electric field is studied theoretically. It is shown that when the electron‐hole Coulomb interaction energy exceeds the miniband width, a coherent excitonic state is created whose oscillator strength surpasses the oscillator strength of a single quantum well by orders of magnitude. It is also demonstrated that a small external field can split the coherent state into isolated well states and thus severely deplete the oscillator strength of the exciton. This opens the possibility of modulating and switching of superradiance in semiconductor devices.

A novel effect, consisting of optically induced effective mass change due to the change in the degree of localization in the semiconductor superlattices is theoretically investigated. Possible application as a nonabsorbing differential light detector/switch is considered.

It is shown that in three‐dimensional semiconductor superlattices, the effective mass near the Fermi level can be made substantially smaller than in bulk materials. This results in a higher carrier mobility. A novel high transconductance field‐effect transistor is proposed and modeled based on this higher mobility.

The possibility of the surface‐emitting second‐harmonic generation (SHG) based on intersubband transitions in multiple quantum‐well (QW) structures is examined theoretically. The critical role of valence band mixing is demonstrated. The off‐diagonal SHG coefficients, necessary for the surface‐emitting SHG, are evaluated for the transitions between the valence subbands in GaAs/AlAs QW structures are found to be comparable in magnitude to the diagonal SHG coefficients reported in literature.

The strengths of the intersubband and band‐to‐band transitions in quantum wells are compared using simple k⋅p analysis, and contrary to the prevalent opinion, found to be nearly equal. The implications for the design of detectors and nonlinear optical devices are considered.

A simple theory for the two‐photon intersubband absorption and, associated with it, nonlinear refraction in multiple quantum wells (QWs) is developed. Using it, the third‐order nonlinear susceptibility is evaluated for QWs of different shapes. It is shown that χ(3) of as much as 10−7 esu can be achieved below the two‐photon resonance where the absorption is small. Thus, it is preferable to use multiple QWs in nonlinear devices at frequencies less than half of the intersubband frequency.

We report the realization of light emission from a Fe2+ doped ZnSe polycrystal with efficient pumping from a quantum cascade (QC) laser. The QC laser photon energy is near the absorption edge of the Fe2+:ZnSe with less than ∼60 meV Stokes' shift. The Fe2+:ZnSe polycrystal shows an absorption band of ∼2.2 – 5 μm in the mid-infrared (mid-IR) at room temperature, which narrows down to ∼2.4–4.2 μm at 80 K. Clear photoluminescence (PL) from 4.5 μm to 6 μm are observed throughout the temperature range of 300 K to 80 K. At room temperature, the luminescence lifetime is about 0.38 ± 0.1 μs which increases up to 101 ± 2 μs at 118 K. This system transfers energy stored in the upper state of a QC laser with ∼ps lifetime to the Fe2+:ZnSe with 105 to 108 times longer lifetime, at the cost of minute photon energy losses.

In space communications, atmospheric absorption and Rayleigh scattering are the dominant channel impairments. Transmission using mid-infrared (MIR) wavelengths offers the benefits of lower loss and less scintillation effects. In this work, we report the telecom wavelengths (1.55 μm and 1.3 μm) induced optical quenching effects on MIR quantum cascade lasers (QCLs), when QCLs are operated well above their thresholds. The QCL output power can be near 100% quenched using 20 mW of near-infrared (NIR) power, and the quenching effect depends on the input NIR intensity as well as wavelength. Time resolved measurement was conducted to explore the quenching mechanism. The measured recovery time is around 14 ns, which indicates that NIR generated electron-hole pairs may play a key role in the quenching process. The photocarrier created local field and band bending can effectively deteriorate the dipole transition matrix element and quench the QCL. As a result, MIR QCLs can be used as an optical modulator and switch controlled by NIR lasers. They can also be used as “converters” to convert telecom optical signals into MIR optical signals.