Bandstructure and electronic states in terahertz quantum cascade lasers

Terahertz (THz) quantum cascade lasers (QCL) are currently increasing in popularity. It is expected to become the main source of emerging terahertz radiation technology and applications. However to produce the device within the application specification is costly and time consuming. This is because the manufacturing process of the superlattice growth and the device processing and testing are long and expensive processes. Thus a prediction tool is needed to overcome the problems in designing and producing THz QCL within the needed optical expectation. The density matrix method is used to calculate the performance of this device electronically and optically. The result obtained was compared to the experimental result conducted by previous researchers. The calculation result showed that the gain is 20 when the population inversion occurs at threshold current density of 400 A cm-2. Meanwhile a negative gain or loss occurs below 350 A cm-2. As a conclusion, it is demonstrated that this method has a capability to explain the transport phenomena as well as to predict the performance of the THz QCL device design.

The single lobe far-field patterns produced from terahertz quantum cascade lasers (QCLs) are greatly demanded for various applications, such as imaging, data transmission, etc. However, for a ridge waveguide terahertz QCL, the far-field beam divergence is large due to the fact that the waveguide aperture is far smaller than the terahertz wavelength. This is the case typically for double-metal waveguide terahertz QCL which emits terahertz photons in almost every direction in the space. Even for a single plasmon waveguide terahertz QCL, the divergence angle is as large as 30 in both horizontal and vertical direction. Here, in this work we design and fabricate a double metal third-order distributed feedback terahertz QCL emitting around 4.3 THz, and investigate the characteristics of the longitudinal and transverse modes. This work aims to achieve high beam quality for terahertz QCL by exploiting the third-order distributed feedback geometry, and in the meantime to achieve single longitudinal mode operation. The electromagnetic field distribution in the waveguide is modelled by employing a finite element method. The mode selection mechanism is studied by using the eigen frequency analysis, and the far-field beam is simulated by applying the near-field to far-field Fourier transform technique. The QCL active region used in this work is based on the resonant-phonon design, which is grown by a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The wafer bonding and traditional semiconductor device fabrication technology, i.e., optical lithography, electron beam evaporation, lift-off, wet and dry etching, are used to process the MBE-growth wafer into the third-order distributed feedback geometry with double-metal waveguides. By carefully designing the grating structures and optimizing the fabrication process, we achieve third-order distributed feedback terahertz QCL with quasi-single-longitudinal mode operation and single lobe far-field beam pattern with low beam divergence in both vertical and horizontal directions. The effect of grating duty cycle on the far-field beam divergence is systematically studied theoretically and experimentally. By the simulation, we finally achieve the divergence angle of 1213 for a third-order distributed feedback laser with a grating duty cycle of 12% that results in an effective refractive index close to 3. The experimental results show good agreement with the simulation. There is still room to further reduce the beam divergence of third-order distributed feedback terahertz QCL by improve the accuracy of the simulation and the fabrication.

Results are presented for the first imaging system that combines the high power of terahertz (THz) quantum cascade lasers (QCLs) with the three-dimensional image reconstruction capability of computed tomography (CT). The THz QCL used in this project is a GaAs-AlGaAs bound-to-continuum superlattice design, emitting at 2.9 THz (103 /spl mu/m), delivering 70 mW per facet peak power with a threshold current density of 112 A/cm/sup 2/. The reconstruction algorithm is based on the filtered back projection technique which has been widely used in X-ray CT. Images of phantoms made from polystyrene have been successfully reconstructed revealing both their external and internal structures with high contrast levels between different phases. Furthermore, the reconstructed images are shown to be in good quantitative agreement with the structure of the phantoms. We are currently implementing modifications to the experimental protocol and reconstruction algorithm in order to address the problems of reflection and refraction so as to extend our imaging capability to a wider range of materials.

Terahertz (THz) quantum cascade lasers (QCLs) are technologically important laser sources for the THz range but are complex to model. An efficient extended rate equation model is developed here by incorporating the resonant tunneling mechanism from the density matrix formalism, which permits to simulate THz QCLs with thick carrier injection barriers within the semi-classical formalism. A self-consistent solution is obtained by iteratively solving the Schrödinger–Poisson equation with this transport model. Carrier–light coupling is also included to simulate the current behavior arising from stimulated emission. As a quasi-ab initio model, intermediate parameters, such as pure dephasing time and optical linewidth, are dynamically calculated in the convergence process, and the only fitting parameters are the interface roughness correlation length and height. Good agreement has been achieved by comparing the simulation results of various designs with experiments, and other models such as density matrix Monte Carlo and non-equilibrium Green's function method that, unlike here, require important computational resources. The accuracy, compatibility, and computational efficiency of our model enable many application scenarios, such as design optimization and quantitative insights into THz QCLs. Finally, the source code of the model is also provided in the supplementary material of this article for readers to repeat the results presented here, investigate, and optimize new designs.

We present a model for longitudinal mode competition in coupled-cavity (CC) terahertz (THz) quantum cascade lasers (QCLs) by using a scattering matrix method and multi-mode reduced rate equations (RREs). The dependence of the mode selection and tuning characteristics on various device parameters is systematically investigated, including the net waveguide loss, the optical length of the passive cavity, and the heat-sink temperature for different relationship between the active and passive cavity lengths. The changes in eigenmode frequencies due to variations of device parameter are calculated before solving the RREs. The mode selection and tuning results obtained from solving the nonlinear RREs could be well explained by the linear scattering matrix analysis. The mode tuning process simulated by the proposed model is compared with experimentally measured data, yielding good agreement. Comprehensive study of the influence of the key device parameters on the performance of CC THz QCLs provides potential design rules for single-mode operation with either wide frequency tunability or high stability.

Terahertz (THz) quantum cascade lasers (QCLs) have been shown to emit peak powers greater than 1 W from a single facet in a single plasmon geometry. However, this is typically achieved by increasing the laser ridge width, resulting in higher-order transverse modes, limiting the achievable power density. Here, we control and fully suppress these modes through thin metallic side-absorbers, showing laser action solely on the fundamental transverse mode operation without sacrificing high THz peak powers. This leads to enhanced power densities and electric fields of up to 1.8 kW/cm2 and 1.17 kV/cm, respectively, opening up the possibility of applying THz QCLs as pump sources for investigations of nonlinear THz physical phenomena.

Terahertz quantum cascade lasers (QCLs) with a broadband gain medium could play an important role for sensing and spectroscopy since then distributed-feedback schemes could be utilized to produce laser arrays on a single semiconductor chip with wide spectral coverage. QCLs can be designed to emit at two different frequencies when biased with opposing electrical polarities. Here, terahertz QCLs with bidirectional operation are developed to achieve broadband lasing from the same semiconductor chip. A three-well design scheme with shallow-well GaAs/Al0.10Ga0.90As superlattices is developed to achieve high-temperature operation for bidirectional QCLs. It is shown that shallow-well heterostructures lead to optimal quantum-transport in the superlattice for bidirectional operation compared to the prevalent GaAs/Al0.15Ga0.85As material system. Broadband lasing in the frequency range of 3.1–3.7 THz is demonstrated for one QCL design, which achieves maximum operating temperatures of 147 K and 128 K respectively in opposing polarities. Dual-color lasing with large frequency separation is demonstrated for a second QCL, that emits at ~3.7 THz and operates up to 121 K in one polarity, and at ~2.7 THz up to 105 K in the opposing polarity. These are the highest operating temperatures achieved for broadband terahertz QCLs at the respective emission frequencies, and could lead to commercial development of broadband terahertz laser arrays.

Quantum cascade laser (QCL) is a semiconductor based laser in a superlattice structure based on intersubband transitions. Although QCLs have been achieved with high performance such as Watt-level emission, it is always highly desired to further improve the device performance with multiple functions , for example, achieving high beam collimation, arbitrary polarization control, and high speed modulation. It is also desired that those performance could be achieved through an integrated approach for miniaturalization, easy alignment and reducing cost. In this presentation, we will use Terahertz (THz) QCL as a demonstration example to obtain high collimated THz QCLs through plasmonic collimation designs with a record beam divergence, electrically tunable THz polarizations by designing integrated THz metasurfaces on a hybrid dielectric-plasmonic waveguides, and broadband graphene-based integrated THz modulators with a fast modulating speed.

We report our development of terahertz (THz) quantum cascade lasers (QCLs), in which the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO) phonon scattering. This depopulation mechanism, similar to that implemented in all the QCLs operating at mid-infrared frequencies, is robust at high temperatures and high injection levels. The unique feature of resonant LO-phonon scattering in our THz QCL structures allows a highly selective depopulation of the lower radiative level with a sub-picosecond lifetime, while maintaining a relatively long upper level lifetime (more than 5 ps) that is due to upper-to-ground-state scattering. The first QCL based on this mechanism achieved lasing at 3.4 THz (lambda approximately 87 microm) up to 87 K for pulsed operations, with peak power levels exceeding 10 mW at ca. 40 K. Using a novel double-sided metal waveguide for mode confinement, which yields a unity mode confinement factor and therefore a low total cavity loss at THz frequencies, we have also achieved lasing at wavelengths longer than 100 microm.

Optical frequency combs, consisting of well‐controlled equidistant frequency lines, have been widely used in precision spectroscopy and metrology. Terahertz combs have been realized in quantum cascade lasers (QCLs) by employing either an active mode‐locking or phase seeding technique, or a dispersion compensator mirror. However, it remains a challenge to achieve the passive comb formation in terahertz semiconductor lasers due to the insufficient nonlinearities of conventional saturable absorbers. Here, a passive terahertz frequency comb is demonstrated by coupling a multilayer graphene sample into a QCL compound cavity. The terahertz modes are self‐stabilized with intermode beat note linewidths down to a record of 700 Hz and the comb operation of graphene‐coupled QCLs is validated by on‐chip dual‐comb measurements. Furthermore, the optical pulse emitted from the graphene‐coupled QCL is directly measured employing a terahertz pump–probe technique. The enhanced passive frequency comb operation is attributed to the saturable absorption behavior of the graphene‐integrated saturable absorber mirror, as well as the dispersion compensation introduced by the graphene sample. The results provide a conceptually different graphene‐based approach for passive comb formation in terahertz QCLs, opening up intriguing opportunities for fast and high‐precision terahertz spectroscopy and nonlinear photonics.

The terahertz (THz) frequency quantum cascade laser (QCL) is a semiconductor heterostructure laser that has attracted much research interest over the past decade. We report on the high performance of THz QCLs based on a three-well resonant-phonon (RP) depopulation active region (AR) and operating in the frequency range 2.7 THz to 4.0 THz. Devices, processed into surface-plasmon waveguides, lased up to 116 K in pulsed mode with threshold current densities as low as 840 Acm−2. The effects of the design frequency and laser cavity length on performance are discussed. We also report on the operation of QCLs with reduced AR thicknesses, and show, for the first time, that the AR thickness of RP QCLs processed in a surface plasmon waveguide can be reduced to as little as 5 µm. Finally, we demonstrate the use of an electrically tuneable THz QCL, based on a heterogeneous AR, for spectroscopic imaging of the high-explosive pentaerythritol tetranitrate.

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.

Dual‐comb sources with equally spaced and low phase noise frequency lines are of great importance for high resolution spectroscopy and metrology. In the terahertz frequency range, electrically pumped semiconductor quantum cascade lasers (QCLs) are suitable candidates for frequency comb and dual‐comb operation. Although free running terahertz QCLs can be operated as frequency combs and dual‐comb sources, the phase noise originated from the carrier frequency and repetition rate instabilities are relatively high, which hinders the high precision applications. The locking techniques that have been used for a single laser comb can be, in principle, applied to a dual‐comb laser source. However, the complete locking of dual‐comb lines considerably complicates the implementation of such a system. Here, a method is proposed to stabilize a terahertz QCL dual‐comb source by phase locking one dual‐comb line. Although only one dual‐comb line is locked, it is shown that the phase noise of other dual‐comb lines close to the phase locked line is significantly reduced. Finally, it is demonstrated that the terahertz QCL comb without a control of the repetition rate can produce pulsed‐type waveforms. The demonstrated approach provides a convenient method to actively stabilize terahertz dual‐comb laser sources, which can be further utilized for fast gas sensing and spectroscopy.

We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4 terahertz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional terahertz instrumentation (detectors, mixers, or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0-800mm round-trip delay line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path and show that a limiting absorption coefficient of ∼1×10-4 cm-1 is resolvable.

Microscopic density matrix analysis on the linewidth enhancement factor (LEF) of both mid-infrared (mid-IR) and Terahertz (THz) quantum cascade lasers (QCLs) is reported, taking into account of the many body Coulomb interactions, coherence of resonant-tunneling transport and non-parabolicity. A non-zero LEF at the gain peak is obtained due to these combined microscopic effects. The results show that, for mid-IR QCLs, the many body Coulomb interaction and non-parabolicity contribute greatly to the non-zero LEF. In contrast, for THz QCLs, the many body Coulomb interactions and the resonant-tunneling effects greatly influence the LEF resulting in a non-zero value at the gain peak. This microscopic model not only partially explains the non-zero LEF of QCLs at the gain peak, which observed in the experiments for a while but cannot be explicitly explained, but also can be employed to improve the active region designs so as to reduce the LEF by optimizing the corresponding parameters.