AlAs/GaAs quantum cascade lasers based on large direct conduction band discontinuity

Quantum cascade (QC) lasers opens new prospects for powerful sources operating at THz frequencies. Up to now the best THz QC lasers are based on intersubband emission in GaAs/AlGaAs quantum well (QW) heterostructures. The maximum operating temperature is 200 K, which is too low for wide-spread applications. This is due to the rather low LO-phonon energy (36 meV) of GaAs-based materials. Indeed, thermal activation allows non-radiative path through electron-phonon interaction which destroys the population inversion. Wide band gap materials such as ZnO have been predicted to provide much higher operating temperatures because of the high value of their LO-phonon energy. However, despite some observations of intersubband absorption in c-plane ZnO/ZnMgO quantum wells, little is known on the fundamental parameters such as the conduction band offset in such heterostructures. In addition the internal field inherent to c-plane grown heterostuctures is an handicap for the design of QC lasers and detectors. In this talk, we will review a systematic investigation of ZnO/ZnMgO QW heterostructures with various Mg content and QW thicknesses grown by plasma molecular beam epitaxy on low-defect m-plane ZnO substrates. We will show that most samples exhibit TM-polarized intersubband absorption at room temperature linked either to bound-to-quasi bound inter-miniband absorption or to bound-to bound intersubband absorption depending on the Mg content of the barrier material. This systematic study allows for the first time to estimate the conduction band offset of ZnO/ZnMgO heterostructures, opening prospects for the design of QC devices operating at THz frequencies. This was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement #665107.

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 (QC) lasers are a rapidly evolving mid-infrared and THz, semiconductor laser technology based on intersubband transitions in multiple coupled quantum wells. The lasers' strengths are their wavelength tailorability, high performance and fascinating design potential. The authors will first give a brief introduction into QC lasers followed by a discussion of several recent highlights, such as the quest for high performance QC lasers, especially high efficiency and single-mode operation, and the implementation of unconventional laser schemes. The authors will also discuss several applications, predominantly trace gas sensing applications.

We design the InxGa1-xAs/InyAl1-yAs quantum cascade laser (QCL) structures, based on the four-quantum well active region with vertical transition with strain compensation towards the shorter wavelengths, operating at λ ~2.93–6.6μm. Considering the objective function related to the optical gain, i.e., \({z_{43}^2 (1-\tau_3 /\tau_{43})\tau_4}\), and the escape energy related to the carrier leakage to continuum, the design of QCLs is carried out in double infinite feedback loops. Each barrier/well layer in the active region is optimized towards the lasing wavelength as short as possible under a properly designed injector. For efficient short wavelength emission, the larger conduction band discontinuity (ΔEc) is achieved by changing the indium mole fractions of strain-compensated InxGa1-xAs/InyAl1-yAs layers. The experimentally verified injectors are properly modified for the stronger coupling between the wavefunctions of the ground state in injector and the upper state in active region. Thus the use of strain-compensated In0.72Ga0.28As/In0.3Al0.7As structure (i.e., ΔEc = 857 meV) leads to the shortest wavelength up to λ ~2.93μm with τ43 = 2.13 ps, τ4 = 0.87 ps, τ3 = 1.43 ps and z43 = 1.03 nm under an electric field of 96 kV/cm.

Since the invention of the quantum cascade (QC) laser, which was demonstrated by Faist et al. in 1994, there has been much progress and advancement to include a wide range of wavelengths on GaAs and InP based material. Specifically, the operating range includes mid- to far-infrared with peak power levels in the watt range and above room temperature pulsed operation for wavelengths from 4.5 to 16 mum. There have been three distinctive designs of the QCL active region- the 'vertical' and 'diagonal' transitions and the 'superlatttice'- all of which are used with either surface-plasmon or conventional dielectric waveguides . Then, there is also the multiple wavelength superlattice, bi-directional, dual wavelength and the injector less quantum cascade lasers. With all the progress made thus far, there is still much work to be done in producing QC lasers that emit efficiently in the atmospheric window (3-5 mum wavelength range), which is important for free-space communications and gas sensing applications. Two known approaches in achieving QC lasers with shorter wavelength emission are through material selection, i.e. wider bandgap materials and type-II structures. There is an alternative approach that also holds much promise: exploitation of the nonlinear optical processes. Within the QC laser intracavity, higher order harmonic generation can be employed because both the fundamental and second-harmonic waves can propagate freely within the cavity without being reabsorbed by the semiconductor. Conveniently, the energy generated in this approach by the fundamental mode and second-harmonic generation is well below the selected material bandgap energy, which is still in the transparency region of the material. Appropriately, our work involves the study of strained active regions in AlGaAs/GaAs QC lasers with the goal of enhancing the nonlinear susceptibility and producing lasers that emit in the 3-5 mum wavelength range.

In this work we present the characteristics of a novel type of quantum-cascade (QC) laser: the deep-well (DW) QC device, which, unlike conventional QC lasers, contains a superlattice of quantum wells and barriers of different composition, respectively. The fabrication of DW-QC devices is made possible by the use of metal-organic chemical vapor deposition (MOCVD), a crystal growth technique which allows one to easily vary the composition of wells and barriers within QC structures, thus providing significantly increased flexibility in optimizing the device design. We have designed such varying-composition QC structures to have deep quantum wells in and tall barriers in and around the active region. DW- QC laser structures have fabricated into 19 μm-wide ridges and 3 mm-long chips. Threshold-current densities as low as 1.5 kA/cm² are obtained at room temperature in the 4.6-4.8 μm wavelength region. In conventional QC lasers emitting in the 4.5-5.5μm range there is substantial thermionic carrier leakage from the upper laser level to the continuum, as evidenced by a significant decrease in the slope efficiency above 250 K, which is understandable given the relatively small (i.e., ~ 200 meV) energy differential, δE, between the upper lasing level and the top of the exit barrier. For the DW design carrier leakage is suppressed due to deep active wells and tall barriers, such that δE reaches values in excess of 400 meV. Preliminary results include a threshold-current characteristic temperature, T₀, value of 218 K over the temperature range: 250- 340 K.

The Quantum Cascade (QC) laser is a semiconductor laser based on intersubband transitions in the conduction band of a multiple quantum-well heterostructure, and is a highperformance light source in the mid-infrared and terahertz region. With the aim of improving the quantum efficiency and power efficiency of long-wavelength QC lasers, based on the principle of conventional single-transition QC lasers, we demonstrate a cascaded-transition, long-wavelength (λ ∼ 14.6 μm) QC laser where an electron makes two consecutive, cascading optical transitions and emits two energy-matched photons in each active region. The laser structure grown consists of a cascaded-transition active core with 25 stages sandwiched between two conventional single-transition active cores with 30 stages each. From the grown samples, laser emission from ∼ 680 cm−1 to ∼ 700 cm−1 (λ ∼ 14.7–14.3 μm) with threshold current density of 4.8 kA/cm2 at 80 K is observed. The laser functions in pulsed mode up to ∼ 240 K, with a characteristic temperature of 190–230 K for ridge widths around 20 μm. From the current-voltage (I–V) measurement, two turn-on's, defined as the electric field at which the subbands line up to create a low-resistive state, are observed, which is consistent with the I–V of a cascaded-transition only mesa, which, in turn, is suggestive of the functionality of the cascaded-transition active core; each turn-on relates to electron transport through the upper laser subbands and the middle subband respectively.

Summary form only given. Since their first demonstration in 1995 quantum cascade (QC) lasers have been made only in GaInAs-AlInAs heterostructures grown lattice matched on InP. This is somewhat in contradiction with the principles of QC lasers which, based on intersubband transitions, should be independent of the specific semiconductor heterostructure used. By fabricating QC lasers in the AlGaAs-GaAs we demonstrate that the fundamental concepts and design criteria can be truly extended in other material systems, still preserving the same basic characteristics in terms of threshold currents and output power.

Conventional quantum cascade (QC) lasers are intrinsically edge-emitting devices with mode confinement achieved via a standard mesa stripe configuration. Surface emission in edge emitting QC lasers has therefore necessitated redirecting the waveguided laser emission using a second order grating. This paper describes the methods used to fabricate a 2D photonic crystal (PC) structure with or without a central defect superimposed on an electrically pumped QC laser structure with the goal of achieving direct surface emission. A successful systematic study of PC hole radius and spacing was performed using e-beam lithography. This PC method offers the promise of a number of interesting applications, including miniaturization and integration of QC lasers.

A compact, fast-response, infrared spectrometer using pulsed quantum cascade (QC) lasers has been applied to the analysis of gases in mainstream and sidestream cigarette smoke. QC lasers have many advantages over the traditional lead salt tunable diode lasers, including near-room-temperature operation with thermoelectric cooling and single mode operation with improved long-term stability. The instrument uses a 36-m, 0.3-L multiple pass absorption gas cell to obtain a time response of 0.1 s for the mainstream system and 0.4 s for the sidestream system. A precision of 10-4 absorbance units in 1 s is achieved using pulse normalization to reduce pulse-to-pulse intensity variations. Laser line widths (hwhm) of 0.006 cm-1 at 967 and 0.010 cm-1 at 1900 cm-1 are obtained in pulsed operation. With this instrument, we have measured the concentrations of ammonia and ethylene using a QC laser at 967 cm-1 and nitric oxide using a laser at 1900 cm-1 in both mainstream and sidestream smoke from two types of cigarettes. ...

Laser characteristics have been evaluated for mid-infrared quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the suitability of QC lasers for use in various spectroscopic applications, including Doppler-limited molecular absorption spectroscopy and pressure-limited LIDAR instrumentation. Using rapid-scanning techniques, direct absorbance measurements of nitric oxide, ammonia and nitrous oxide have been performed with QC lasers, operating at either 5.2 or 8.5 micrometers . Measured Doppler-limited absorption profiles show no distortion with increased averaging (up to 103 - 104 samples averaged), thereby minimizing the need for sophisticated data acquisition systems which re- register successive data streams to accommodate for laser frequency jitter and drift. Additionally, the high tuning rates (2.5 cm-1 in 0.6 milliseconds; 5 - 10 kHz sweep repetition rate) achieved with the QC lasers allow for the measure of relatively rapid transient phenomena or a high degree of signal averaging in a short time. Noise- equivalent absorbances of 3 X 10-6 have also been obtained without optimizing the optical arrangement.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.

Quantum cascade lasers (QCL) were first demonstrated in 1994. However, it took nearly eight years before the first continuous wave, room temperature (CW/RT) operation was successful, brought about by an improved design for rapidly depopulating the lower laser level thorough the use of two phonon resonance technique. The progress since 2002 has been remarkable in improving the power output and wallplug efficiency. Furthermore, newer structure design ideas (notably the nonresonant extraction, NRE, principle for rapid depopulation of the lower laser level without the use of two phonon resonance) have emerged, which increase the structure design flexibility for simultaneous improvements in various operating parameters of QCLs. At present, CW/RT power outputs of >;3 W and wall plug efficiency in excess of 15 % have been demonstrated at 4.6 μm in the midwave infrared (MWIR) region. Using the NRE structure design ideas we have demonstrated CW/RT power output in excess of 2 W at 4.0 μm and have extended QCL operation up to 3.6 μm, were average power output of more than 300 mW has been obtained for room temperature operation. The NRE design principle is also applicable to longwave infrared (LWIR) regions to lead to improvements in power output and wallplug efficiency similar to those attained in the MWIR region. Over all CW/RT operation of QCLs now covers a wavelength region from 3.6 μm to longer than 12 μm. Pranalytica, being a vertically integrated (from fundamental design of the structure to final customer usable product, but fabless) manufacturer of QCLs, has been supplying high power CW/RT QCLs as well as cryogenically cooled QCLs producing substantially higher power output (>;7 W, CW) at very high wallplug efficiency (>;30%) to military, homeland security and civilian customers

We propose and study the feasibility of a THz GaN/AlGaN quantum cascade laser (QCL) consisting of only five periods with confinement provided by a spoof surface plasmon (SSP) waveguide for room temperature operation. The QCL design takes advantages of the large optical phonon energy and the ultrafast phonon scattering in GaN that allow for engineering favorable laser state lifetimes, and the SSP waveguide provides the optical confinement for the ultrathin QCL. Our analysis has shown that the waveguide loss is sufficiently low for the QCL to reach its threshold at the injection current density around 6 kA/cm2 at room temperature.

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.