Quantum Cascade Structures Research Articles

Charge Transport in Interband Cascade Lasers: An Ab‐Initio Self‐Consistent Model

AbstractInterband cascade lasers (ICLs) stand out due to their low threshold current and minimal power consumption, rendering them viable sources for compact and mobile devices in the mid‐infrared. Since their first demonstration, they experienced major performance improvements. Mostly they originate from either improved material quality or the outcomes of numerical analysis of secluded parts. Encouraged by the impact of secluded models, an ICL‐specific simulation tool can lead to performance breakthroughs and a better comprehension of governing mechanisms. Drawing from an evaluation of existing tools designed for quantum cascade structures, a self‐consistent density matrix rate equation model is implemented to simulate the transport in both conduction and valence band heterostructures. Albeit the extensive inclusion of the quantum effects, special care was taken to maintain a high numerical efficiency. The charge transport model additionally considers optical field calculations, allowing for predictive calculations of light–current–voltage curves. The model is benchmarked against well‐established ICL designs and demonstrate reliable performance predictability. Additionally, detailed insights into device characteristics extracted from the model are provided. This ultimately allows to deepen the understanding of ICL and not only refine existing ones but also generate novel optimized designs.

Novel coding schemes for dual wavelength quantum cascade laser‐based free space optical links

AbstractDual wavelength quantum cascade lasers (DW‐QCLs) are a type of semiconductor laser that can emit two different wavelengths of light simultaneously. They are constructed using quantum cascade structures, consisting of multiple layers of semiconductor materials carefully engineered to produce the desired wavelengths of light. This research article reports the numerical simulation and evaluation of the maximum capacity of free space optical (FSO) link employing novel coding schemes for DW‐QCL‐based pulsed optical transmitters. A 48‐stage DW‐QCL emitting at 10.5 and 8.9 μm wavelengths and a Quantum‐Well based Infrared Photodetector based receiver are considered for the study. The performance of the link is analyzed under short pulse transmission from the device at both wavelengths for various scenarios. Optical pulses are obtained either at a single wavelength or two different wavelengths by proper choice of electrical input current amplitude. This feature enables the implementation of four different coding schemes. Among the various coding approaches proposed, the scheme, which operates on symbols, performs the best, providing a maximum link length of 1700m, for a BER of 10−9.

Inversionless terahertz lasing in a four-level Raman-type scheme within mid-infrared quantum cascade structures

This paper applies the density matrix formalism to investigate a feasibility of inversionless terahertz (THz) lasing within a mid-infrared (mid-IR) quantum cascade laser (QCL). The proposed structure aims to use a mid-IR QCL as an efficient pump source that integrated with a four-level Raman-type scheme to generate THz radiation through strong coherence excited in the system. A key aspect of the design is that the THz generation is achieved by interplay between inversion-based lasing and coherent effects due to quantum interference between the microscopic polarizations of the dipole-allowed transitions in the four-level system. THz intensity gain arising from both the inversion-based lasing and coherent effects is derived from the off-diagonal density matrix elements in the four-level system. The simulation results indicate that the proposed design demonstrates the potential to achieve THz lasing with good temperature characteristics and without the need for population inversion.

Near-infrared (λ ∼ 1.2 μm) intersubband electroluminescence in Si/CaF2 quantum cascade structures

A clear electroluminescence (EL) from a Si/CaF2 quantum cascade structure has been successfully observed. The structure was equipped with 15 periods of an active region comprising of Si/CaF2 multi-quantum wells and a waveguide grown on the silicon-on-insulator substrate by MBE-based technique. As a result of an optical spectrum measurement by Fourier transform IR spectroscopy, a clear EL spectrum with a peak at λ ∼ 1.2 μm was observed. The EL spectrum is reasonably explained by fitting it with a Lorentzian model that considers the thickness fluctuation of a single monoatomic layer of a Si quantum well, the intra- and inter-subband scattering times, and the carrier escape time. These results indicate that the EL was generated by intersubband transitions in the Si quantum well.

Proposal for Deep-UV Emission from a Near-Infrared AlN/GaN-Based Quantum Cascade Device Using Multiple Photon Up-Conversion

We propose the use of an n-doped periodic AlN/GaN quantum cascade structure for the optical up-conversion of multiple near-infrared (near-IR) photons into deep-ultraviolet (deep-UV) radiation. Without applying an external bias voltage, the active region of such a device will (similar to an un-biased quantum cascade laser) resemble a sawtooth-shaped inter-subband structure. A carefully adjusted bias voltage then converts this sawtooth pattern into a ‘quantum-stair’. Illumination with λ = 1.55 µm radiation results in photon absorption thereby lifting electrons from the ground state of each main well into the first excited state. Three additional GaN quantum wells per period then provide by LO-phonon-assisted tunneling a diagonal transfer of these electrons towards the ground level of the neighboring period. From there, the next near-infrared (near-IR) photon absorption, electron excitation, and partial relaxation takes place. After 12 such absorption, transfer, and relaxation processes, the excited electrons have gained a sufficiently high amount of energy to undergo in the final AlN-based p-type contact layer an electron-hole band-to-band recombination. By employing this procedure, multiple near-IR photons will be up-converted to produce deep-UV radiation. Since for a wavelength of 1.55 µm very powerful near-IR pump lasers are readily available, such an up-conversion device will (even at a moderate overall conversion efficiency) potentially result in an equal or even higher output power than the one of an AlN-based p-n-junction light-emitting diode. The proposed structures are therefore very interesting for applications such as ultra-high-resolution photolithography or printing, water purification, medical equipment disinfection, white light generation, or the automotive industry.

Gain saturation effects in THz quantum cascade lasers

The effect of gain saturation in quantum-cascade structures with 2–4 quantum wells per period is herein analyzed on the basis of a system of balance equations. It is shown that the nonlinearity parameter decreases with an increase in the relaxation rate of laser levels, but the total current through the structure also increases. The use of the proposed multiphoton designs leads to a decrease in the non-linearity parameter without increasing the operating current. For example, in a two-photon scheme of laser transitions with the same transition probabilities and differential gains, two times slower saturation of the gain with an increase in the photon density is achieved, which leads to a high generation efficiency than in single-photon schemes.

Terahertz generation in quantum cascade structures by two-color deriving fields: An approach to reach inversion population via optical pumping

In this paper, a quantum cascade laser (QCL) design is proposed based on GaAs/AlGaAs material system, which simultaneously operates at three widely separated wavelengths (λ1=11.1μm,λ2=14.1μm and λTHz=60μm). In the design, all the wavelength radiations are achieved by the engineering of the electronic spectrum via the quantum-well widths and the applied electric field in a single active region within a same waveguide. The mid-infrared (mid-IR) wavelengths are obtained by adoption a dual-upper-state active region, and the proposed design aims to use both the mid-IR radiations as the coherent deriving fields to populate the upper THz lasing state to aid the THz-laser population inversion via optical pumping instead of direct electrical injection. A detailed analysis of electronic transport in the structure is carried out using a multi-level rate-equation model. The results show that the proposed structure offers an alternative approach to room temperature THz generation in QCLs.

Double longitudinal-optical phonon intrawell depopulated terahertz quantum cascade structures: Electron transport modeling using a density matrix method

Terahertz quantum cascade structures using double longitudinal-optical phonon intrawell scattering for depopulation are theoretically studied. A density matrix Monte Carlo method is used to calculate the temperature dependent optical power, in double phonon structures with diagonal optical transitions. It is shown that using depopulation transitions greater than the resonant longitudinal-optical phonon energy ΔE > ℏωLO reduces the phonon absorption thermal backscatter, allowing for higher operating temperatures, with prospects for 300 K room temperature and beyond. Furthermore, results indicate that the temperature limit may also be improved in single phonon structures, by similarly increasing the depopulation transition.

Data Link with a High-Power Pulsed Quantum Cascade Laser Operating at the Wavelength of 4.5 µm.

This article is a short study of the application of high-power quantum cascade lasers and photodetectors in medium-infrared optical wireless communications (OWC). The link range is mainly determined by the transmitted beam parameters and the performance of the light sensor. The light power and the photodetector noise directly determine the signal-to-noise power ratio. This ratio could be maximized in the case of minimizing the radiation losses caused by atmospheric attenuation. It can be obtained by applying both radiation sources and sensors operated in the medium infrared range decreasing the effects of absorption, scattering or scintillation, beam spreading, and beam wandering. The development of a new class of laser sources based on quantum cascade structures becomes a prospective alternative. Regarding the literature, there are descriptions of some preliminary research applying these lasers in data transmission. To provide a high data transfer rate, continuous wave (cw) lasers are commonly used. However, they are characterized by low power (a few tens of mWatts) limiting their link range. Also, only a few high-power pulsed lasers (a few hundreds of mWatts) were tested. Due to their limited pulse duty cycle, the obtained modulation bandwidth was lower than 1 MHz. The main goal of this study is to experimentally determine the capabilities of the currently developed state-of-the-art high-power pulsed quantum cascade (QC) lasers and photodetectors in OWC systems. Finally, the data link range using optical pulses of a QC laser of ~2 W, operated at the wavelength of ~4.5 µm, is discussed.

THz intersubband electroluminescence from n-type Ge/SiGe quantum cascade structures

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δf/f≈0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency.

Feasibility of lasing in the GaAs Reststrahlen band with HgTe multiple quantum well laser diodes

Operation of semiconductor lasers in the 20–50 µm wavelength range is hindered by strong non-radiative recombination in the interband laser diodes, and strong lattice absorption in GaAs-based quantum cascade structures. Here, we propose an electrically pumped laser diode based on multiple HgTe quantum wells with band structure engineered for Auger recombination suppression. Using a comprehensive model accounting for carrier drift and diffusion, electron and hole capture in quantum wells, Auger recombination, and heating effects, we show the feasibility of lasing at λ = 26, …, 30 µm at temperatures up to 90 K. The output power in the pulse can reach up to 8 mW for microsecond-duration pulses.

Модель терагерцового квантово-каскадного лазера на основе двумерного плазмона

In this work, we calculated the characteristics of a two-dimensional plasmon amplified by an active medium based on a terahertz quantum-cascade structure. It is shown that for realistic parameters of GaAs/AlGaAs structures with quantum wells (mobility 2×105 cm2V-1s-1 at an electron concentration of 5×1011 cm-2 and at temperatures up to 77 K), the gain of a two-dimensional plasmon can reach 1500 cm-1 for frequency 2.3 THz. In addition, due to the strong localization of the plasmon electric field near the quantum well, only a few cascades of the active medium are required for amplification.

Terahertz Intersubband Electroluminescence from Nonpolar m-Plane ZnO Quantum Cascade Structures

The ZnO-based heterostructures are predicted to be promising candidates for optoelectronic devices in the infrared and terahertz (THz) spectral domains owing to their intrinsic material properties. Specifically, the large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, which is predicted to greatly improve the temperature performance of THz quantum cascade lasers. However, to date, no experimental observation of intersubband emission from ZnO optoelectronic devices has been reported. Here, we report the observation of THz intersubband electroluminescence from ZnO/MgxZn1–xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence peak shows a line width of ∼20 meV at a center frequency of ∼8.5 THz at 110 K, which is not accessible for GaAs-based quantum cascade structures because of the reststrahlen band absorption from 8 to 9 THz. This result is an important step toward the realization of ZnO-based THz quantum cascade lasers.

Longitudinal-optical phonon absorption and dephasing in three-level terahertz quantum cascade structures with different injector anticrossings

The effects of longitudinal-optical phonon scattering and dephasing in quantum cascade structures are studied. Three-level longitudinal-optical phonon depopulated terahertz structures are investigated using a density matrix Monte Carlo method. Ideally, these structures do not contain energy states above the upper lasing state, which in principle, can reduce parasitic leakage. The light output and current density as a function of lattice temperature are calculated and shown to be consistent with experiment for a recently reported structure that is confirmed to be a good approximation to a three-level structure. The pure dephasing time is self-consistently found to be relatively constant over the temperature range, which differs from the previous analysis of other structures. At higher lattice temperatures, particularly at elevated temperatures beyond the lasing point, the reduction in current density is rather due largely to rapid longitudinal-optical phonon absorption, which reduces the lifetime of the ground state. It is shown that the operating temperature limit may further be improved by adequately increasing the injector anticrossing.

Two-dimensional spectroscopy on a THz quantum cascade structure

AbstractUnderstanding and controlling the nonlinear optical properties and coherent quantum evolution of complex multilevel systems out of equilibrium is essential for the new semiconductor device generation. In this work, we investigate the nonlinear system properties of an unbiased quantum cascade structure by performing two-dimensional THz spectroscopy. We study the time-resolved coherent quantum evolution after it is driven far from equilibrium by strong THz pulses and demonstrate the existence of multiple nonlinear signals originating from the engineered subbands and find the lifetimes of those states to be in the order of 4–8 ps. Moreover, we observe a coherent population exchange among the first four intersubband levels during the relaxation, which have been confirmed with our simulation. We model the experimental results with a time-resolved density matrix based on the master equation in Lindblad form, including both coherent and incoherent transitions between all density matrix elements. This allows us to replicate qualitatively the experimental observations and provides access to their microscopic origin.

Domino Effect of Thickness Fluctuation on Subband Structure and Electron Transport within Semiconductor Cascade Structures.

Effectively restraining random fluctuation of layer thickness (RFT) during the thin-film epitaxy plays an essential part in improving the quality of low-dimensional materials for device application. While it is already challenging to obtain an ideal growth condition for thickness control, the tangle of RFT with interfacial problems makes it even more difficult to guarantee the properties of heterostructures and the performance of devices. In our research, the RFT of potential barriers and wells within a semiconductor multilayer is demonstrated to correlate with the interfacial grading effect (IFG) and to affect the band offset strongly. Then, the synergetic effect of RFT and IFG that serves as the first domino is shown to impact the subband structure and the electron transport successively. On the basis of an investigation of a quantum cascade structure, statistical results indicate a normal distribution of RFT with a standard deviation of about 1 Å and an extreme value of 3 Å (about one monolayer) for all the layers within 38 cascade periods. The "seemingly negligible" RFT could actually reduce the conduction band offset for tens to hundreds of meV and alter the subband gaps at a rate of 40 meV/monolayer at most. Furthermore, the dependence of different subband gaps on the barrier/well thickness differs from one another. In addition, the distribution of wave function could also be regulated dramatically by RFT to change the type of electron transition and thus the carrier lifetime. Further impacts of RFT and the RFT-modulated subband alignment on electron transport result in two different mechanisms (injection-dominant and extraction-dominant) of electron population inversion (PI), which is manifested by comparatively discussing the results of in situ electron holography and macro performances.

The quantum confined Stark effect in N-doped ZnO/ZnO/N-doped ZnO nanostructures for infrared and terahertz applications

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications.

Quantum cascade superluminescent light emitters with high power and compact structure

Quantum cascade (QC) superluminescent light emitters (SLEs) have emerged as desirable broadband mid-infrared (MIR) light sources for growing number of applications in areas like medical imaging, gas sensing and national defense. However, it is challenging to obtain a practical high-power device due to the very low efficiency of spontaneous emission in the intersubband transitions in QC structures. Herein a design of ~5 μm SLEs is demonstrated with a two-phonon resonance-based QC active structure coupled with a compact combinatorial waveguide structure which comprises a short straight part adjacent to a tilted stripe and to a J-shaped waveguide. The as-fabricated SLEs achieve a high output power of 1.8 mW, exhibiting the potential to be integrated into array devices without taking up too much chip space. These results may facilitate the realization of SLE arrays to attain larger output power and pave the pathway towards the practical applications of broadband MIR light sources.

Comparison of quantum cascade structures for detection of nitric oxide at ~\xa05.2 \u03bcm

Nonequilibrium Green’s function method is used to calculate electronic and optical characteristics of various quantum cascade structures emitting light at ~ 5.2 μm wavelength. Basing on these simulations, the choice of optimal design can be done.

An electrically pumped phonon-polariton laser.

We report a device that provides coherent emission of phonon polaritons, a mixed state between photons and optical phonons in an ionic crystal. An electrically pumped GaInAs/AlInAs quantum cascade structure provides intersubband gain into the polariton mode at λ = 26.3 μm, allowing self-oscillations close to the longitudinal optical phonon energy of AlAs. Because of the large computed phonon fraction of the polariton of 65%, the emission appears directly on a Raman spectrum measurement, exhibiting a Stokes and anti-Stokes component with the expected shift of 48 meV.