Terahertz Frequency Quantum Cascade Lasers Research Articles

Effects of background doping, interdiffusion and layer thickness fluctuation on the transport characteristics of THz quantum cascade lasers

In this work, we investigate the effects of n and p-type background doping, interface composition diffusion (interdiffusion) of the barrier material and layer thickness variation during molecular beam epitaxy (MBE) growth on transport characteristics of terahertz-frequency quantum cascade lasers (THz QCLs). We analysed four exemplary structures: a bound-to-continuum design, hybrid design, LO-phonon design and a two-well high-temperature performance LO-phonon design. The exemplary bound-to-continuum design has shown to be the most sensitive to the background doping as it stops lasing for concentrations around 1.0·1015\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1.0\\cdot 10^{15}$$\\end{document}–2.0·1015\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.0\\cdot 10^{15}$$\\end{document} cm-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document}. The LO-phonon design is the most sensitive to growth fluctuations during MBE and this is critical for novel LO-phonon structures optimised for high-temperature performance. We predict that interdiffusion mostly affects current density for designs with narrow barrier layers and higher Al\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{Al}$$\\end{document} composition. We show that layer thickness variation leads to significant changes in material gain and current density, and in some cases to the growth of nonfunctional devices. These effects serve as a beacon of fundamental calibration steps required for successful realisation of THz QCLs.

Harmonic quantum cascade laser terahertz frequency combs enabled by multilayer graphene top-cavity scatters

Abstract Optical frequency comb synthesizers, operating in the harmonic regime, are metrological sources in which the emitted optical power is concentrated in a few modes, spaced by several multiples of the cavity free spectral range (FSR). This behavior reflects in a large correlation degree and, in principle, in an increased optical power per mode. In miniaturized quantum cascade lasers (QCLs), harmonic frequency combs (HFCs) are hence particularly attracting to explore quantum correlation effects between adjacent harmonic modes, enabled by the inherently large gain media third-order χ (3) Kerr nonlinearity, even if controlled generation of stable HFCs of predefined order, is typically demanding in such electrically pumped sources. Here, we demonstrate stable 2nd order and 3rd order HFC emission in terahertz frequency QCLs by respectively patterning an individual or a couple of equally spaced distributed multilayer graphene absorbers on the top metallic waveguides.

Terahertz quantum cascade laser frequency combs with engineered operation frequency around 4.0 THz

Freely engineering the operation frequency of frequency comb sources is crucial for various applications, e.g., high-precision spectroscopy, ranging, communications, and so on. Here, by employing band structure simulations, group velocity dispersion (GVD) analysis, and experimental verifications, we demonstrate that the operation frequency of terahertz (THz) quantum cascade laser frequency combs can be engineered from 4.2 to 4.0 THz. First of all, from the viewpoint of the band structure engineering, we shift the frequency corresponding to the optical transitions in the active region from 4.2 to 4.0 THz by slightly altering the thicknesses of quantum wells. Meanwhile, a GVD analysis is applied to evaluate the potential comb performance. Finally, experimental characterizations, e.g., emission spectra, inter-mode beatnote, dual-comb operation, are performed to validate the exceptional comb operation at 4.0 THz. The advancement in simulations and experimental results present a comprehensive method to customize the desired THz radiative frequency for comb generation, which facilitates the practical development of broadband, high-precision THz comb sources.

Acoustic band engineering in terahertz quantum-cascade lasers and arbitrary superlattices

We present theoretical methods for the analysis of acoustic phonon modes in superlattice structures, and terahertz-frequency quantum-cascade lasers (THz QCLs). Our generalized numerical solution of the acoustic-wave equation provides good agreement with experimental pump-probe measurements of the acoustic resonances in a THz QCL. We predict that the detailed layer structure in THz QCLs imprints up to $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$ detuning of the acoustic mode spacing, which cannot be seen in analytical models. This effect is strongest in devices with large and abrupt acoustic mismatch between layers. We use an acoustic deformation potential within a density-matrix approach to analyze electron transport induced in a range of the most common THz QCL active-region design schemes. We conclude that acoustic modes up to $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$ are capable of significantly perturbing QCL transport, highlighting their potential for ultrafast modulation of laser emission.

Near-Full Current Dynamic Range THz Quantum Cascade Laser Frequency Comb.

The present study proposes a terahertz quantum cascade laser frequency comb (THz QCL FC) with a semi-insulated surface plasma waveguide characterized by a low threshold current density, high power and a wide current dynamic range. The gain dispersion value and the nonlinear susceptibility were optimized based on the combination of a hybrid bound-to-continuum active region with a semi-insulated surface plasmon waveguide. Without any extra dispersion compensator, stable frequency comb operation within a current dynamic range of more than 97% of the total was revealed by the intermode beat note map. Additionally, a total comb spectral emission of about 300 GHz centered around 4.6 THz was achieved for a 3 mm long and 150 µm wide device. At 10 K, a maximum output power of 22 mW was obtained with an ultra-low threshold current density of 64.4 A·cm-2.

Directed delivery of terahertz frequency radiation from quantum cascade lasers within a dry 3He dilution refrigerator.

We present a scheme for the full integration of terahertz (THz) frequency quantum cascade lasers (QCLs) within a dilution refrigerator in order to provide a directed delivery of THz power into the sample space. We describe a successful operation of a 2.68THz QCL located on the pulse tube cooler stage of the refrigerator, with its output coupled onto a two-dimensional electron gas (2DEG) located on a milli-kelvin sample stage via hollow metal waveguides and Hysol thermal isolators, achieving a total loss from QCL to the sample of ∼-9 dB. The thermal isolators limit heat leaks to the sample space, with a base temperature of ∼210 mK being achieved. We observe cyclotron resonance (CR) induced in the 2DEG by the QCL and explore the heating impact of the QCL on all stages of the refrigerator. The CR effect induced by the THz QCL is observable at electron temperatures as low as ∼430 mK. The results show a viable route for the exploitation of THz QCLs within the environment of a dilution refrigerator and for the THz power delivery in very low-temperature (<0.5K) condensed matter experiments.

Terahertz quantum cascade laser frequency combs with optical feedback.

Optical feedback exists in most laser configurations and strongly affects laser performances depending on the feedback strength, length, and phase. In this paper, we investigate the frequency comb behaviour of a semiconductor quantum cascade laser emitting around 4.2 THz with external optical feedback. A periodic evolution of the laser inter-mode beatnote from single-line to multiple-line structures is experimentally observed with a minor change of optical feedback length (phase) on the wavelength scale. The comb stability of the laser with feedback is also measured and compared with the same laser without feedback. Furthermore, our simulations reveal that the dynamical oscillations invoked by optical feedback are responsible for the measured multiple-line beatnotes. It is found that the characteristic feedback period is determined by the half wavelength of the laser, while the comb operation is maintained at most feedback length positions. Therefore, terahertz quantum cascade laser combs are robust against the minor position vibration of the feedback mirror in practice, owing to the much smaller feedback phase change than that of common near-infrared laser diodes.

All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

AbstractLayered 2D materials display unique optical and electrical properties that can enable manipulation, propagation, and detection of electromagnetic waves over a broad spectral range, with a high level of control, offering the potential to activate different functionalities, by optical or electrical means, in a single chip. Here, a compact optoelectronic device behaving as an amplitude modulator, saturable absorber mirror (SA mirror), and frequency‐tuner is conceived at terahertz (THz) frequencies. It comprises a gate‐tunable single layer graphene (SLG), embedded in a quarter‐wave cavity, operating in the 1–5 THz range. The use of electrolyte ionic liquid gate ensures 40% optical amplitude modulation depth. Z‐scan self‐mixing interferometry reveals 60% reflectivity modulation, with ≈4.5 W cm−2 saturation intensity. By integrating the modulator/SA mirror with a heterogeneous THz quantum cascade laser frequency comb, in an external cavity configuration, fine‐tuning of the intermode beatnote frequency is also demonstrated. This opens intriguing perspectives for short pulse generation, phase‐locking, frequency tuning/chirping, phase modulation, and metrological referencing, inter alia.

Resonant two-photon terahertz quantum cascade laser.

Lasers that can emit two photons from a single electron relaxation between two states of the same parity have been discussed since the early days of the laser era. However, such lasers have seen only limited success, mainly due to a lack of suitable gain medium. We propose that terahertz (THz) frequency quantum cascade lasers (QCLs) are an ideal semiconductor structure to realize such two-photon emissions. In this work, we present a THz QCL heterostructure designed to emit two resonant photons from each electronic relaxation between two same-parity states in the active region. We present coupled Maxwell-Bloch equations that describe the dynamics of such a two-photon laser and find analytical solutions for the steady-state light intensity, the steady-state energy-resolved carrier densities, and the total threshold carrier density. Due to the two-photon emission from each excited state relaxation and an increased photon-driven carrier transport rate, our simulations predict a significant enhancement of light intensity in our designed resonant two-photon THz QCL when compared to an exemplar conventional THz QCL structure.

High-Power Terahertz Quantum Cascade Lasers Based on High-Al-Composition Four Quantum Wells

High-power terahertz frequency quantum cascade lasers operating in both pulsed and continuous wave (CW) modes are demonstrated. High Al composition four quantum wells active region based on diagonal structure are employed to suppress the escape of electrons into continuum and thermally activated LO-phonon scattering, as evidenced by the activation energy value of ~14.4 meV extracted from the experimental data. The devices operate in a frequency range of 3.8 - 4.05 THz and have a pulsed maximum operating temperature of ~119 K. The total optical output power from an uncoated laser is 1.57 W and the maximum single-facet emission power from an HR coated one is 1.03 W in pulsed mode at 7 K. The maximum output power of ~256 mW is obtained by a 2.5-mm-long device operating in CW mode with a low threshold current density of 188.6 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 15 K.

Performance evaluation of free space optical link driven by gain switched temperature dependent quantum cascade lasers

Temperature dependent terahertz frequency quantum cascade lasers (QCLs) offer huge potential in a large number of new applications. Reduced rate equations are used for better understanding of their behavior and to predict the optical output power under changing current drive and chip temperature. This letter reports the complete end-to-end free space optical (FSO) link driven by a gain switched temperature dependent QCL as transmitter. The device used in the FSO link consists of 90 periods of injector and active regions, emitting light at 116 µm. The device is driven by various electrical inputs such as square, haversine and tangential hyperbolic pulses. The short pulses generated by gain switching travel 1500 m to reach the quantum well infrared photodetector operating in the same wavelength as the source. The performance parameters; namely signal-to-noise ratio (SNR), bit error rate (BER) and capacity are computed for the various input signals. Under the minimum full width half maximum condition, haversine input provides a better performance providing a BER of 7.8 × 10−5, with a peak SNR of 14.56 dB and a capacity of 4.89 bps Hz−1 at a cold finger temperature of 45 K. When peak power is the criterion, tangential hyperbolic input performs well with a minimum BER of 7.66 × 10−9, a peak SNR of 18.06 dB and a capacity of 6.02 bps Hz−1 at a cold finger temperature of 45 K.

Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

AbstractFull phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed.

All-Electronic Phase-Resolved THz Microscopy Using the Self-Mixing Effect in a Semiconductor Laser

We report all-electronic coherent scattering-type scanning near-field microscopy (s-SNOM) using a terahertz-frequency quantum cascade laser. By exploiting the coherent self-mixing effect in these lasers, in conjunction with electronic frequency tuning of the laser, we demonstrate spatial mapping of both the amplitude and the phase of the scattered field with deeply subwavelength resolution. We apply our technique for coherent microscopy of a phonon-resonant crystal. The extracted amplitude and phase parameters reveal clear contrast when compared to both metallic and nonresonant dielectric materials and show excellent agreement with those calculated using a finite-dipole model of the near-field interaction between the s-SNOM tip and the resonant sample in the Reststrahlen band. Our technique paves the way for fast nanoscale-resolved mapping of the dielectric function of solid state systems and optoelectronic nanodevices at terahertz frequencies.

Dual resonance phonon-photon-phonon terahertz quantum-cascade laser: physics of the electron transport and temperature performance optimization.

The state of the art terahertz-frequency quantum cascade lasers have opened a plethora of applications over the past two decades by testing several designs up to the very limit of operating temperature, optical power and lasing frequency performance. The temperature degradation mechanisms have long been under the debate for limiting the operation up to 210 K in pulsed operation in the GaAs/AlGaAs material system. In this work, we review the existing designs and exploit two main temperature degradation mechanisms by presenting a design in which they both prove beneficial to the lasing operation by dual pumping and dual extracting lasing levels. We have applied the density matrix transport model to select potential candidate structures by simulating over two million active region designs. We present several designs which offer better performance than the current record structure.

Quantum Transmission Line Modeling and Experimental Investigation of the Output Characteristics of a Terahertz Quantum Cascade Laser

We describe a new approach to modeling the optoelectronic properties of a terahertz-frequency quantum cascade laser (THz QCL) based on a quantum transmission line modelling (Q-TLM) method. Parallel quantum cascade transmission line modeling units are employed to describe the dynamic optical processes in a nine-well THz QCL in both the time and frequency domains. The model is used to simulate the current–power characteristics of a QCL device and good agreement is found with experimental measurements, including an accurate prediction of the threshold current and emitted power. It is also confirmed that the Q-TLM model can accurately predict the Stark-induced blue shift of the emission spectrum of the THz QCL with increasing injection current. Furthermore, we establish the new Q-TLM model to describe the properties of a THz QCL device incorporating a photonic lattice patterned on the laser ridge, by linking the transmission line structure to each scattering module. The predicted effects of the lattice structure on the steady-state emission spectra of the THz QCL, including the side-mode suppression, are found to be in good agreement with experimental results. Our Q-TLM modeling approach is a promising tool for the future design of THz QCLs and analysis of their temporal and spectral behaviors.

Wideband Electrically Controlled Vernier Frequency Tunable Terahertz Quantum Cascade Laser

Frequency tuning in terahertz frequency quantum cascade lasers is challenging because of low thermal and current tuning coefficients. Moreover, photonic designs like Vernier selection based sampled...

Terahertz photonic integrated circuit for frequency tuning and power modulation

The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. However, integrating multiple photonic functions into a monolithic platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimized the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser enabling single-mode emission. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned over ∼81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ∼27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ∼30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit.

Tunable and compact dispersion compensation of broadband THz quantum cascade laser frequency combs.

Miniaturized frequency combs (FCs) can be self-generated at terahertz (THz) frequencies through four-wave mixing in the cavity of a quantum cascade laser (QCL). To date, however, stable comb operation is only observed over a small operational current range in which the bias-depended chromatic dispersion is compensated. As most dispersion compensation techniques in the THz range are not tunable, this limits the spectral coverage of the comb and the emitted output power, restricting potential applications in, for example, metrology and ultrashort THz pulse generation. Here, we demonstrate an alternative architecture that provides a tunable, lithographically independent, control of the free-running coherence properties of THz QCL FCs. This is achieved by integrating an on-chip tightly coupled mirror with the QCL cavity, providing an external cavity and hence a tunable Gires Tournois interferometer (GTI). By finely adjusting the gap between the GTI and the back-facet of an ultra-broadband, high dynamic range QCL, we attain wide dispersion compensation regions, where stable and narrow (~3 kHz linewidth) single beatnotes extend over an operation range that is significantly larger than that of dispersion-dominated bare laser cavity counterparts. Significant reduction of the phase noise is registered over the whole QCL spectral bandwidth (1.35 THz). This agile accommodation of a tunable dispersion compensator will help enable uptake of QCL-combs for metrological, spectroscopic and quantum technology-oriented applications.

Detection sensitivity of laser feedback interferometry using a terahertz quantum cascade laser.

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser (QCL). We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with the reinjection of powers as low as ∼-125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ∼1.4 pW/√Hz, although, after accounting for the reinjection losses, we estimate that this corresponds to only ∼1 fW/√Hz being coupled to the QCL active region.

Retrieval of phase relation and emission profile of quantum cascade laser frequency combs

Recently, the field of optical frequency combs experienced a major development of new sources. They are generally much smaller in size (on the scale of millimetres) and can extend frequency comb emission to other spectral regions, in particular towards the mid- and far-infrared regions. Unlike classical pulsed frequency combs, their mode-locking mechanism relies on four-wave-mixing nonlinear processes, yielding a non-trivial phase relation among the modes and an uncommon emission time profile. Here, by combining dual-comb multi-heterodyne detection with Fourier-transform analysis, we show how to simultaneously acquire and monitor over a wide range of timescales the phase pattern of a generic (unknown) frequency comb. The technique is applied to characterize both a mid-infrared and a terahertz quantum cascade laser frequency comb, conclusively proving the high degree of coherence and the remarkable long-term stability of these sources. Moreover, the technique allows also the reconstruction of the electric field, intensity profile and instantaneous frequency of the emission. The combined technique of dual-comb multi-heterodyne detection and Fourier-transform analysis allows simultaneous acquisition and monitoring of the phase pattern of a generic frequency comb demonstrating the high degree of coherence of the emission of two quantum cascade laser frequency combs.