Investigation of thermal effects in quantum-cascade lasers

The development of a thermal model for quantum cascade lasers (QCLs) is presented. The model is used in conjunction with a self-consistent scattering rate calculation of the electron dynamics of an InGaAs-AlAsSb QCL to calculate the temperature distribution throughout the device which can be a limiting factor for high temperature operation. The model is used to investigate the effects of various driving conditions and device geometries, such as epilayer down bonding and buried heterostructures, on the active region temperature. It is found that buried heterostructures have a factor of eight decrease in thermal time constants compared to standard ridge waveguide structures in pulsed mode and allow a /spl sim/78% increase in heat sink temperature compared to epilayer down mounted devices in continuous-wave mode. The model presented provides a valuable tool for understanding the thermal dynamics inside a quantum cascade laser and will help to improve their operating temperatures

Thermal effects in InGaAs/AlAsSb quantum-cascade lasers

A quantum-cascade laser (QCL) thermal model is presented. On the basis of a finite-difference approach, the model is used in conjunction with a self-consistent carrier transport model to calculate the temperature distribution in a near-infrared InGaAs/AlAsSb QCL. The presented model is used to investigate the effects of driving conditions and device geometries on the active-region temperature, which has a major influence on the device performance. A buried heterostructure combined with epilayer-down mounting is found to offer the best performance compared with alternative structures and has thermal time constants up to eight times smaller. The presented model provides a valuable tool for understanding the thermal dynamics inside a QCL and will help to improve operating temperatures

QUANTUM CASCADE LASER: from 3 to 26 mum

Absorption spectroscopy of gases and liquids is amongst the most widely used methods to measure molecular concentrations. It is used in various fields, amongst them are industrial leak testing, medical analysis and surgery, process control and monitoring. Trace gas analysis of low-mass molecules is preferably performed in the mid-IR wavelength region, where the line strength for many molecules are high. With the QCL, invented in 1994, this spectral range has a laser source that delivers sufficient output power in continuous-wave operation. The semiconductor laser is robust and operates in a wide temperature range. This work is dedicated to explore the capabilities of QCLs and improve their performance in the wavelength region from 3 to 26 mum. Our active region simulations are based on a Density Matrix model. The choice of basis wavefunctions is verified and a method to find the optimal injection barrier is presented. The influence of different interface roughness models is discussed. Our model agrees well with the full quantum Non-Equilibrium Green’s Function model and with experiments. In this thesis we explore the short wavelength boundary of QCLs. Lasing emission around 3.3 mum requires highly strained active region material. We investigate in detail active region designs, growth optimization, the impact of intervalley scattering and interface roughness. We present a device emitting at 3.4 mum with dissipation values of only 250 mW and threshold currents as low as 16 mA in pulsed operation. A boxcar experiment with a 5.6 ms long pulse shows stable spectral behaviour of DFB devices, an important requirement for spectroscopic applications. We perform genetic optimizations of devices in the range from 4 to 26 mum and investigate active region design parameters. The optimizations are performed on ”seed” designs of published devices and designs from our own group. The current record design in wallplug efficiency for 9 mum is optimized. The design was extracted from literature and processed along with the optimized structure. Comparing the measurements, we improve the slope efficiency from 1.9 to 2.5 W/A, the wallplug efficiency from 9 to 12 % and the dynamical range from 1.5 to 2.1. For all optimizations, the seed and optimized structures are compared, resulting in some common strategies for optimization. The active region designs are explored experimentally as single stacks and broadband designs. Heterogeneous stacking is discussed and application examples for DFB, external cavity and comb operation are shown. An attempt for spectral coverage of a full octave is presented. We show laser emission spanning from 1090 to 1960 cm-1 at 80 K

The optical and thermal properties of quantum cascade lasers

The optical and thermal properties of quantum cascade lasers (QCLs) are investigated through the development of comprehensive theoretical models. The optical properties of various multilayer quantum cascade laser waveguides are investigated by solving Maxwell’s equations using a transfer-matrix method. The complex material refractive indices are calculated using a Drude-Lorentz model which takes into account both phonon and plasma contributions to the material properties. A Caughey-Thomas-like mobility model is used to estimate the temperature dependence of the electron mobility which is found to have a significant effect on the optical waveguide properties. The incorporation of this effect leads to better agreement with experimentally measured threshold current densities. In order to investigate the thermal properties of QCLs, a multi-dimensional anisotropic heat diffusion model is developed which includes temperature-dependent material parameters. The model is developed using finite-difference methods in such a way that is can be solved in both the time-domain and in the steady-state. Various heat management techniques were compared in the time-domain in order to extract the heat dissipation time constants. In the steady-state, the model is used to extract the temperature dependence of the cross-plane thermal conductivity of a GaAs-based THz QCL and compare the thermal properties of THz and InP-based mid-infrared QCL optical waveguides. In addition, fully self-consistent scattering rate equation modelling of carrier transport in short-wavelength QCLs is carried out in order to understand the internal carrier dynamics. This knowledge is then used to optimise the device design and the model predicts significant improvements in the performance of the optimised device

An electrically pumped phonon-polariton laser

We report a device that provides coherent emission of phonon polaritons, a mixed state between photons and optical phonons in an ionic crystal. An electrically pumped GaInAs/AlInAs quantum cascade structure provides intersubband gain into the polariton mode at = 26.3 \mu m, allowing self-oscillations close to the longitudinal optical phonon energy of AlAs. Because of the large computed phonon fraction of the polariton of 65%, the emission appears directly on a Raman spectrum measurement exhibiting a Stokes and anti-Stokes component with the expected shift of 48 meV.Comment: Supplementary materials are appended at the end of the main tex

Resonant optical nonlinearities in cascade and coupled quantum well structures

Resonant or near resonant optical nonlinearities in semiconductor coupled quantum-well systems are discussed. Quantum engineered coupled or cascade quantumwell structures can provide giant nonlinear susceptibilities for various optical nonlinear processes. Nonlinearities integrated within quantum cascade lasers (QCL) showed great potential in various applications in the infrared range. Several schemes of nonlinearities are proposed and discussed in this work. Integrating difference frequency generation (DFG) with QCL can yield long wavelength radiation, such as terahertz light. The DFG process does not require population inversion at a transition associated with low photon energy; however, this requirement is necessary to lasers, such as QCL, and is hard to meet, because of the thermal backfilling and inefficient injection or pumping at room temperature. Therefore terahertz radiation due to DFG QCL for room temperature is proposed. On the other hand, the second harmonic generation can double laser frequency, and then push radiation frequency of AlInAs/GaInAs/InP based QCL to short wavelengths such as 3 μm and shorter. Optical nonlinearities can extend working frequencies of light sources, and also can help to improve light detection. For example, a sum frequency generation can upconvert mid/far-IR signal into near-IR signal with strong near-IR pump light, namely high efficient near-IR photon detector could be employed to detect mid/far-IR light. A specific designed quantum well structure of this frequency up-conversion scheme is discussed. A scheme of monolithic in-plane integration of the optical nonlinearities with QCL is also proposed. In this scheme, an optical nonlinear section is made from the same quantum well structure of a QCL, and is under an independent applied bias. Due to the independence of the applied bias, the nonlinearities can be tuned flexibly. In particular, a widely tunable Raman laser based on this scheme could be achieved. A frequency up-conversion based on sum frequency generation process in coupled quantum-well structure is also proposed for mid-infrared detection. By converting mid-IR signal to near-IR, superior near-IR detector such as silicon avalanche photo diode (APD) can be employed. The scheme can provide lower noise equivalent power (NEP) or higher detectivity compared with regular semiconductor photo detectors. A scheme of lasing without inversion (LWI) based on QCL for THz radiation is proposed. A ladder type three-level system for LWI process is integrated into a boundto- continue high power QCL at 10 μm. The proposed LWI generates THz signal at 69 μm. An optical gain about 80 cm-1 is achieved, against a waveguide loss about 30 cm-1 in a semi insulator (SI) surface plasmon waveguide

Monolithic integration for nonlinear optical frequency conversion in semiconductor waveguides

This thesis presents an investigation into the feasibility of tunable, monolithically integrated, nonlinear optical frequency conversion sources which work under the principles of an optical parametric oscillator (OPO). The room-temperature continuous wave (CW) operation of these devices produces narrow line-width, near- and mid-infrared wavelengths, primarily used in chemical sensing applications. The devices detailed here, based on the GaAs–AlGaAs superlattice material system, benefit from post growth, ion implantation induced, quantum well intermixing, to achieve 1st order phase matching. The experiments, which have been performed to optimize the second-order nonlinear processes in our GaAs–AlGaAs superlattice waveguides, have demonstrated improved conversion efficiencies when compared to the performance achieved previously in similar superlattice nonlinear waveguides. We have achieved pulsed type-I phase matched second harmonic generation (SHG) with powers up to 3.65 μW (average pulse power), CW type-I phase matched SHG up to 1.6 μW for the first time, and pulsed type-II phase matched SHG up to 2 μW (average pulse power), again for the first time. Moreover, we have been able to achieve both CW type-I and CW type-II phase matched difference frequency generation, which converts C-band wavelengths into L- and U-band wavelengths, over at least a 20 nm conversion bandwidth. These results have been made possible through the systematic optimization of processes developed to fabricate nonlinear optical waveguides. Fabrication processes have also been developed to facilitate the incorporation of on-chip lasers and optical routing components, required to achieve a fully integrated OPO and nonlinear optical frequency converter. The optical routing in these devices has been demonstrated using a frequency selective multi-mode interference (MMI) coupler. The superlattice laser material has been designed by optimizing the material structure and employing different growth technologies. Room-temperature CW laser action has been achieved in 100 nm thick, superlattice core, half-ring lasers. The laser excitation is measured at 801 nm, and the internal power of the on-chip pump is estimated to be in excess of 200 mW in a full-ring, after accounting for optical routing, linear, bending and nonlinear losses. We have been able to conclude that our designed OPO and frequency converter is just feasible with the performance achieved in different components