Terahertz Quantum Cascade Lasers Research Articles

Leakages suppression by isolating the desired quantum levels for high-temperature terahertz quantum cascade lasers

The key challenge for terahertz quantum cascade lasers (THz-QCLs) is to make it operating at room-temperature. The suppression of thermally activated leakages via high lying quantum levels is emphasized recently. In this study, we employ the advanced self-consistent method of non-equilibrium Green’s function, aiming to reveal those kinds of leakages in the commonly used THz-QCL designs based on 2-, 3- and 4-quantum well. At the high temperature of 300 K, if all the confined high lying quantum levels and also the continuums are included within three neighboring periods, leakages indeed possess high fraction of the total current (21%, 30%, 50% for 2-, 3- and 4-quantum well designs, respectively). Ministep concept is introduced to weaken those leakage channels by isolating the desired levels from high lying ones, thus the leakages are well suppressed, with corresponding fractions less than 5% for all three designs.

Field-resolved high-order sub-cycle nonlinearities in a terahertz semiconductor laser

The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.

Optical pump assisted broadband terahertz frequency comb

A broadband terahertz (THz) frequency comb assisted by an optical pump in THz quantum cascade lasers (QCLs) is investigated theoretically and numerically through a Maxwell–Bloch model combined with the coupled wave theory. When an optical pump is injected into the laser cavity with dispersion, the intrinsic four-wave-mixing nonlinear process becomes not only an important elementary phase-locking mechanism during the mode proliferating process, but also the bandwidth of the frequency comb is increased and the power is amplified through the nonlinear parametric process. The relative shift between the frequency of the optical pump and the zero-dispersion frequency of THz QCLs tremendously affects the conversion efficiency of the nonlinear parametric process. The simulation results show that appropriately optical pumping could assist in generating the broadband THz frequency comb with over 1 THz and more than 80 lines, which may open many potential applications in designing and optimizing high resolution THz spectroscopy sources.

Optimization of THz quantum cascade lasers with an active module based on two-quantum wells for high-temperature operation

Over the past two decades, the operation temperature of terahertz quantum cascade lasers (THz QCLs) has continuously increased from cryogenic level to the current record value of 250 K (about -23°C) [1]. Here we review the state-of-the-art and future prospects of high-temperature THz QCL designs with two-quantum wells in active module based on conventional heterojunction GaAs/AlGaAs and alternative material system HgCdTe. We have analyzed the temperature dependence of the peak gain and predicted the maximum operation temperatures of the given designs.

Thin THz QCL active regions for improved continuous-wave operating temperature

We compare the performance of 10 and 5 μm thick metal–metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.

Terahertz quantum cascade laser under optical feedback: effects of laser self-pulsations on self-mixing signals.

In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal - well known in interband diode lasers - was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 < C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.

Terahertz Quantum Cascade Lasers as Enabling Quantum Technology

AbstractQuantum cascade lasers (QCLs) represent the most fascinating achievement of quantum engineering, showing how artificial materials can be generated through quantum design, with tailor‐made properties. Their inherent quantum nature deeply affects their core physical parameters. QCLs indeed display intrinsic linewidths approaching the quantum limit, and show spontaneous phase‐locking of their emitted modes via intracavity four‐wave‐mixing, meaning that they can naturally operate as miniaturized metrological frequency rulers, also in frontier frequency domains, as the far‐infrared, yet unexplored in quantum science. Here, the authors discuss the fundamental quantum properties of QCLs operating at terahertz frequencies and their key technological performances, highlighting future perspectives of this frontier research field in disruptive areas of quantum technologies such as quantum sensing, quantum metrology, quantum imaging, and photonic‐based quantum computation.

Barrier height study of two-well resonant-phonon terahertz quantum cascade lasers. I. The third-order tunneling current theory

The third-order tunneling current model, reflecting the effect of carrier leakage from bound to continuum states, is modeled based on the second-order tunneling current and semi-classical leakage current theories and is incorporated in a simulation based on the rate equation. GaAs/AlGaAs-based two-well resonant-phonon terahertz quantum cascade lasers, published in the past, are theoretically examined by this method. Based on the latest recorded device, the influence of barrier height is investigated by changing the Al-composition from 12.5% to 45% with a fixed-layer thickness. As a result, there are carrier dynamics that diminish the carrier density of lasing subbands in both high and low barrier conditions. On the one hand, carrier leakage into continuum states increases drastically below 25% Al-composition. On the other hand, in high barrier conditions, substantial carriers accumulate in the ground state due to a strong confinement effect in addition to oscillator strength lowering.

Model of a Terahertz Quantum-Cascade Laser Based on Two-Dimensional Plasmons

Model of a Terahertz Quantum-Cascade Laser Based on Two-Dimensional Plasmons

Engineering of electron–longitudinal optical phonon coupling strength in m-plane GaN terahertz quantum cascade lasers

In this work nonpolar (m-plane) GaN terahertz quantum cascade lasers are designed based on the non-equilibrium Green’s function method. The lasing quantum-level broadening that arises from electron–longitudinal optical (LO) phonon coupling is studied by tuning the coupling strength, and the changes of optical gain due to this broadening are demonstrated in a self-consistent way. It is found this coupling process largely widens the linewidth of intersubband transition and then quenches the lasing even at a low temperature of 10 K. The electron–LO phonon coupling strength, however, can be manually engineered by tailoring the quantum well structure. As a result, the optical gain will be recovered above the cavity threshold, indicating a potential for terahertz lasing from GaN semiconductors.

Phase change of Ge2Sb2Te5 under terahertz laser illumination

Ge2Sb2Te5 (GST) is the typical phase change material (PCM) that can reversibly transform between the amorphous (a) and crystalline (c) states. Because the optical properties are phase-dependent, GST has been widely used in various photonic applications, such as optical switches and non-volatile memories. Currently, the photonic applications of the GST PCM have been demonstrated by employing lasers in visible and infrared wavelengths. Extending the photonic applications into other wavelengths is much demanded. Here, we investigate the phase change of the GST material illuminated by using a terahertz quantum cascade laser emitting around 2.5 THz. A finite-element simulation is employed to study the temperature and transmission changes induced by terahertz irradiation. It reveals that the phase change can be achieved and the transmission is reduced by 30% when the GST material is irradiated by the 2.5 THz laser light. Furthermore, a z-scan technique experimentally presents the phase change behaviors of GST illuminated by the terahertz light, which is visually proved by transmission electron microscopy. Our work paves a way for the applications of GST materials in the terahertz optical components, e.g., modulators and switches.

Continuous wave vertical emission from terahertz microcavity lasers with a dual injection scheme.

Quantum cascade lasers (QCLs) represent a most promising compact source at terahertz (THz) frequencies, but efficiency of their continuous wave (CW) operation still needs to be improved to achieve large-scale exploitation. Here, we demonstrate highly efficient operation of a subwavelength microcavity laser consisting of two evanescently coupled whispering gallery microdisk resonators. Exploiting a dual injection scheme for the laser cavity, single mode CW vertical emission at 3.3 THz is obtained at 10 K with 6.4 mA threshold current and 145 mW/A slope efficiency up to 320 μW emitted power measured in quasi-CW mode. The tuning of the laser emission directionality is also obtained by independently varying the pumping strength between the microdisks. By connecting the resonators through a suspended gold bridge, the laser out-coupling efficiency in the vertical direction is strongly enhanced. Owing to the high brightness, low-power consumption and CW operation, the proposed microcavity laser design could allow the realization of high-performance CW THz QCLs ready for massive parallelization.

Self-Mixing Signal Characteristics of Complex-Coupled Distributed-Feedback Terahertz Quantum-Cascade Lasers

Self-mixing interference (SMI) in terahertz quantum cascade lasers (THz QCLs) is one of the significant approaches for coherent THz imaging and sensing techniques. Here, the output characteristics of SMI in distributed feedback (DFB) THz QCLs from the index-to the gain-coupling regimes are studied using the coupled wave theory and the multi-mode rate equation method. A mode hopping phenomenon is found to occur when the DFB coupling factor changes from index-coupling to gain-coupling, and the characteristics of the self-mixing signals of DFB-QCLs change greatly with this mode hopping. With the modulus of the coupling factor fixed and its argument varied from 0 to π/2, an extreme point of the self-mixing frequency and power signals of DFB-QCLs is found at π/9 due to the mode hopping. For index-coupling dominated DFB-QCLs, both the varying ranges of the self-mixing frequency signals and amplitudes of power signals increase with increasing DFB coupling factor argument. For gain-coupling dominated DFB-QCLs, with increasing argument value, the amplitude of the self-mixing power signal increases, but the varying range of the self-mixing frequency signal decreases. With the argument of the coupling factor fixed, we also found that the varying ranges of the self-mixing frequency signals decrease with increasing modulus for both index-coupling dominated and gain-coupling dominated DFB-QCLs. For index-coupling dominated DFB-QCLs, the amplitudes of the self-mixing power signals decrease with increasing modulus; however, the amplitudes of the self-mixing power signals of gain-coupling dominated DFB-QCLs increase. With the argument of the coupling factor fixed, for index-coupling dominated DFB-QCLs, we found that the varying ranges of the self-mixing frequency signals and amplitudes of power signals decrease with the increasing modulus. For gain-coupling dominated DFB-QCLs, with the coupling factor modulus increasing, the varying ranges of the self-mixing frequency signals decrease, however, the amplitudes of the self-mixing power signals increase. These results may help with the application of DFB-QCLs to self-mixing interferometers.

Terahertz imaging with self-pulsations in quantum cascade lasers under optical feedback

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.

Improved comb and dual-comb operation of terahertz quantum cascade lasers utilizing a symmetric thermal dissipation.

In the terahertz frequency range, the quantum cascade laser (QCL) is a suitable platform for the frequency comb and dual-comb operation. Improved comb performances have been always much in demand. In this work, by employing a symmetric thermal dissipation scheme, we report an improved frequency comb and dual-comb operation of terahertz QCLs. Two configurations of cold fingers, i.e., type A and B with asymmetric and symmetric thermal dissipation schemes, respectively, are investigated here. A finite-element thermal analysis is carried out to study the parametric effects on the thermal management of the terahertz QCL. The modeling reveals that the symmetric thermal dissipation (type B) results in a more uniform thermal conduction and lower maximum temperature in the active region of the laser, compared to the traditional asymmetric thermal dissipation scheme (type A). To verify the simulation, experiments are further performed by measuring laser performance and comb characteristics of terahertz QCLs emitting around 4.2 THz mounted on type A and type B cold fingers. The experimental results show that the symmetric thermal dissipation approach (type B) is effective for improving the comb and dual-comb operation of terahertz QCLs, which can be further widely adopted for spectroscopy, imaging, and near-field applications.

Traveling Wave Effects in Microwave Quantum Photonics

Traveling wave effects are increasingly exploited in microwave and terahertz quantum devices to implement novel functionalities and obtain improved performance. A suitable quantum circuit modeling approach is discussed for Josephson and bulk-material kinetic inductance traveling-wave parametric amplifiers enabling broadband, quantum-limited amplification. As another example, a multi-domain simulation approach for novel terahertz quantum cascade laser structures exploiting microwave and optical co-propagation effects is presented.

Terahertz quantum cascade laser array with spatially-separated beams

A lens-coupled terahertz (THz) quantum cascade laser (QCL) array with multiple spatially-separated beams is presented. The beam divergence and focusing efficiency after lens are both investigated by simulations and experiments. By integrating an appropriate hyper-hemispherical silicon lens with a well-designed six-element distributed feedback (DFB) QCL array inside the cryostat window, excellent emission characteristics of small divergence angles from 4.1°×3.5° to 7.2°×5.8° and multi-wavelength spectra are realized at 77 K. The deflection angle between adjacent beams is ~ 10°. The output intensity after lens is 113 to 25 times that of original beams with a focusing efficiency ranging from ~ 58% to ~ 37% for lasers at different positions. This method provides a simple solution to form a multi-beam THz source with high intensity and low far-field divergence.

Frequency tuning behaviour of terahertz quantum cascade lasers revealed by a laser beating scheme.

In the terahertz frequency range, the commercialized spectrometers, such as the Fourier transform infrared and time domain spectroscopies, show spectral resolutions between a hundred megahertz and a few gigahertz. Therefore, the high precision frequency tuning ability of terahertz lasers cannot be revealed by these traditional spectroscopic techniques. In this work, we demonstrate a laser beating experiment to investigate the frequency tuning characteristics of terahertz quantum cascade lasers (QCLs) induced by temperature or drive current. Two terahertz QCLs emitting around 4.2 THz with identical active regions and laser dimensions (150 µm wide and 6 mm long) are employed in the beating experiment. One laser is operated as a frequency comb and the other one is driven at a lower current to emit a single frequency. To measure the beating signal, the single mode laser is used as a fast detector (laser self-detection). The laser beating scheme allows the high precision measurement of the frequency tuning of the single mode terahertz QCL. The experimental results show that in the investigated temperature and current ranges, the frequency tuning coefficients of the terahertz QCL are 6.1 MHz/0.1 K (temperature tuning) and 2.7 MHz/mA (current tuning) that cannot be revealed by a traditional terahertz spectrometer. The laser beating technique shows potential abilities in high precision linewidth measurements of narrow absorption lines and multi-channel terahertz communications.

Terahertz Frequency Combs: Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching (Laser Photonics Rev. 15(6)/2021)

In article number 2000575, Miriam S. Vitiello and co-workers report on an integrated architecture for the generation of high power terahertz (THz) frequency combs (FCs) comprising an ultrafast THz polaritonic reflector, exploiting intersubband cavity polaritons, and a broad bandwidth heterogeneous THz quantum cascade lasers (QCL). It is shown that the polaritonic reflector induces spectral reshaping of the QCL emission and stable FC operation over 38% of the laser operational range.

The Effect of Doping in Split-Well Direct-Phonon THz Quantum-Cascade Laser Structures

We have studied the effect of doping on the temperature performance of a split-well (SW) direct-phonon (DP) terahertz (THz) quantum-cascade laser (QCL) scheme supporting a clean three-level system. Achieving a system that is as close as possible to a clean n-level system proved to be the strategy that led to the best temperature performance in THz-QCLs. We expected to obtain a similar improvement to that observed in resonant-phonon (RP) schemes after increasing the carrier concentration from 3 × 1010 cm−2 to 6 × 1010 cm−2. Our goal was to improve the temperature performance by increasing the doping, ideally the results should have improved. To our surprise, in the devices we checked, the results show the contrary. Although an increase in doping had previously shown a positive effect in RP schemes, our results indicated that this does not happen with SW–DP devices. However, we observed a significant increase in gain broadening and a reduction in the dephasing time as the doping and temperature increased. We attribute these effects to enhanced ionized-impurity scattering (IIS). The observation and study of effects related to dephasing included in our experimental work have previously only been possible via simulation.