Quantum cascade laser: 30 years of discoveries

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.

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.

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. ...

Abstract. The development of fast-response analysers for the measurement of nitrous oxide (N2O) has resulted in exciting opportunities for new experimental techniques beyond commonly used static chambers and gas chromatography (GC) analysis. For example, quantum cascade laser (QCL) absorption spectrometers are now being used with eddy covariance (EC) or automated chambers. However, using a field-based QCL EC system to also quantify N2O concentrations in gas samples taken from static chambers has not yet been explored. Gas samples from static chambers are often analysed by GC, a method that requires labour and time-consuming procedures off-site. Here, we developed a novel field-based injection technique that allowed the use of a single QCL for (1) micrometeorological EC and (2) immediate manual injection of headspace samples taken from static chambers. To test this approach across a range of low to high N2O concentrations and fluxes, we applied ammonium nitrate (AN) at 0, 300, 600 and 900 kg N ha−1 (AN0, AN300, AN600, AN900) to plots on a pasture soil. After analysis, calculated N2O fluxes from QCL (FN2O_QCL) were compared with fluxes determined by a standard method, i.e. laboratory-based GC (FN2O_GC). Subsequently, the comparability of QCL and GC data was tested using orthogonal regression, Bland–Altman and bioequivalence statistics. For AN-treated plots, mean cumulative N2O emissions across the 7 d campaign were 0.97 (AN300), 1.26 (AN600) and 2.00 kg N2O-N ha−1 (AN900) for FN2O_QCL and 0.99 (AN300), 1.31 (AN600) and 2.03 kg N2O-N ha−1 (AN900) for FN2O_GC. These FN2O_QCL and FN2O_GC were highly correlated (r=0.996, n=81) based on orthogonal regression, in agreement following the Bland–Altman approach (i.e. within ±1.96 standard deviation of the mean difference) and shown to be for all intents and purposes the same (i.e. equivalent). The FN2O_QCL and FN2O_GC derived under near-zero flux conditions (AN0) were weakly correlated (r=0.306, n=27) and not found to agree or to be equivalent. This was likely caused by the calculation of small, but apparent positive and negative, FN2O when in fact the actual flux was below the detection limit of static chambers. Our study demonstrated (1) that the capability of using one QCL to measure N2O at different scales, including manual injections, offers great potential to advance field measurements of N2O (and other greenhouse gases) in the future and (2) that suitable statistics have to be adopted when formally assessing the agreement and difference (not only the correlation) between two methods of measurement.

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.

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.

Under the Quantum Cascade Laser (QCL) Calibrations project, Pacific Northwest National Laboratory (PNNL) is collaborating with Utah State University and Space Dynamics Laboratory to design, build, and test a QCL calibration source system suitable for insertion into remote sensing platforms. In FY14, the second year of this project, PNNL completed the radiation testing of QCLs using radiation levels that are typical for a space environment. These tests involved both 64-MeV protons, and 1.17-MeV and 1.33-MeV gamma rays. No changes were observed above the measurement uncertainty indicating that QCLs are viable sources for space-based missions. High-level testing was also carried out with gamma rays to determine if QCLs are sensitive to high doses of gamma radiation. After an accumulated dose of 1.5 Mrad(Si), the QCLs continue to demonstrate high performance although a small increase in threshold current, 0.11 ± 0.14% and 0.51 ± 0.15% for M1244D and M664M, respectively, was observed. At this stage, it is unclear if this small increase is from the radiation exposure, because generally it is within the observed experimental uncertainty, which is most likely because of mechanical changes through repeated mounting cycles. There was also some accidental damage to the laser chips and their front facets from the extensive but necessary handling of the devices during these tests. Regardless, the QCLs are robust and should not be susceptible to degradation for most missions.

Role of linewidth enhancement factor on the frequency response of the synchronized quantum cascade laser

The quantum cascade laser (QCL) is an important laser source in the mid-infrared and terahertz frequency range. The past twenty years have witnessed its tremendous development in power, wall plug efficiency, frequency coverage and tunability, beam quality, as well as various applications based on QCL technology. Nowadays, QCLs can deliver high continuous wave power output up to 5.1 W at room temperature, and cover a wide frequency range from 3 to 300 μm by simply varying the material components. Broadband heterogeneous QCLs with a broad spectral range from 3 to 12 μm, wavelength agile QCLs based on monolithic sampled grating design, and on-chip beam QCL combiner are being developed for the next generation tunable mid-infrared source for spectroscopy and sensing. Terahertz sources based on nonlinear generation in QCLs further extend the accessible wavelength into the terahertz range. Room temperature continuous wave operation, high terahertz power up to 1.9 mW, and wide frequency tunability form 1 to 5 THz makes this type of device suitable for many applications in terahertz spectroscopy, imaging, and communication.

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and fast, miniband-like lower-laser (ll) level depopulation. That has led to an internal-efficiency value of ~ 77 % for ~ 5 μmemitting devices, a record-high value for 4.5-6.0 μm-emitting QCLs. We have recently shown that that value can be basically accounted for when considering both LO-phonon- and interface-roughness (IFR)-triggered carrier leakage from the upper-laser (ul) level and injector states. The same carrier-leakage analysis is applied to MOCVD-grown ~ 8.0 μmemitting, STA-type QCLs, and we find that the internal efficiency reaches a high value of ~ 73.6 %, due to a record-high injection-efficiency value (86.7 %) and to an IFR-enhanced laser-transition efficiency value of ~ 85 %. In contrast, for a conventional MOCVD-grown, ~ 8.0 μm-emitting QCLs the internal efficiency is found to be only ~ 55 %, typical of values extracted from experimental data of mid-infrared-emitting conventional QCLs. The ul-level lifetime is found to be controlled by LO-phonon and alloy-disorder (AD) scattering for typical MOCVD-grown QCLs, just like for 4.5-5.0 μm-emitting QCLs. However, for typical MBE-grown QCLs the ul-level lifetime is controlled by LO-phonon, AD and IFR scattering. The ll-level lifetime is found to controlled by both LO-phonon and IFR scattering. Besides the high internal-efficiency value, the use of excited-state injection and a low voltage defect result in the STA QCL reaching a single-facet wall-plug efficiency value of 10.6 %, a record-high single-facet value for 8-11 μm-emitting QCLs grown by MOCVD and holding potential for CW operation.

Summary form only given. Birefringent filters are routinely used as a wavelength tuning element in bulk optics, broadly tunable lasers such Ti:Sapphire [1] and dye lasers. A detailed description of birefringent filter operation can be found in ref. [2]. In this paper, we report the design, fabrication and characterisation of an integrated electro-optically tunable birefringent filter for wavelength and polarisation tuning of quantum cascade lasers (QCLs) [3].Our generic design for an integrated tunable birefringent filter (ITBF) for QCLs is shown in Fig. 1(a). It consists of 2 quarter waveplates with a differential phase shift (DPS) section between them. The integrated equivalents of waveplates are polarization mode convertors (PMC) [4] that converts the TM (Transverse Magnetic) polarised light from the gain section into circularly polarised light. In a QCL there is only gain for TM modes because of the intersubband transition selection rules. The DPS section adjusts the relative phase of the TM and TE (Transverse Electric) components of the circularly polarized light from the first PMC (PMC1 in figure 1(a)); this is because the birefringence of DPS section can be adjusted by injection current. The relative phase of the TE and TM components at the input into the second PMC (PMC 2 in figure 1(a)) determines the output polarization. In a QCL, only TM light will experience gain and the wavelengths that are rotated back to TM polarization are determined by the current in the DPS section. Thus we have a QCL that has its operating wavelength selected by an electro-optic tunable birefringent filter. For wavelength tuning the gain section was operated pulsed (100 ns pulse width at 60 kHz repetition rate) above threshold, at a current density of 6.8 kA/cm, and wavelength tuning was recorded (shown in Fig. 1(b)) for various values of DC injection current density into the DPS up to 24nm at 0.41kA/cm2. In our polarization control experiments at 0 kA/cm2 a maximum throughput polarisation angle of 37O was recorded and this angle increases up to 82O with an applied DC current of 0.97 kA/cm2 into the DPS and no further change in polarization angle recorded above 0.97 kA/cm2 current density (shown in Fig. 1(c)).In summary we have presented results on a QCL that includes an integrated birefringent filter. The results show that the output wavelength and polarisation can be electro-optically tuned by injecting a DC current into a differential phase shift section.

Terahertz quantum cascade (QC) lasers are well suited for the exploration of active metamaterial concepts in the terahertz frequency range. Terahertz composite right/left handed (CRLH) transmission line metamaterials can be integrated with quantum cascade laser gainmaterial in order to compensate for losses, and enable laser waveguides with new functionality. In particular, we consider the use of metamaterial transmission lines as travelling wave antennas. After presenting the characteristics of a 2.5 THz quantum-cascade laser, calculated radiation characteristics and beam patterns for a leaky-wave antenna based upon a balanced 1D CRLH transmission line waveguide are shown.

The intensity noise of a laser source represents one of the key factors limiting the ultimate sensitivity in laser-based systems for sensing and telecommunication. For advanced applications based on interferometry, the availability of a shot-noise-limited local oscillator is even more important for the effective feasibility of high-precision measurements. This is particularly crucial in quantum optics applications based on homodyne detection schemes to measure non-classical light states, such as squeezed states. This work deeply investigates and analyzes the intensity noise features of the most widely used mid-infrared semiconductor heterostructured lasers: quantum cascade and interband cascade lasers. For this purpose, a comprehensive comparison of three different continuous-wave lasers operating at room temperature around 4.5 µm wavelength is presented. First, a thorough electro-optical characterization is given, highlighting the differences and the shared common characteristics of the tested devices. Then, a detailed intensity noise analysis is reported, identifying their different noise operations with a particular reference to shot-noise-limited operations. Finally, some perspectives towards advanced applications are discussed.