Tobias Gresch

High-Performance Bound-to-Continuum Quantum-Cascade Lasers for Broad-Gain Applications

Based on the bound-to-continuum active region design, we shall present a high performance continuous-wave (CW) quantum-cascade laser. In contrast to high performance lasers based on a two-phonon resonance transition and a narrow linewidth (< 165 cm-1), the device presented here exhibits a spontaneous emission full-width at half-maximum as large as 295 cm-1. Thus, such devices are very suitable for broadband tuning. At 30degC, it shows a maximum output power and slope efficiency of 188 mW and 500 mW/A, as well as a threshold current density of only 1.79 kA/cm2. Furthermore, at this temperature, the device demonstrates an internal differential quantum efficiency of 71% and a wall plug efficiency of 2.0%. The maximum CW operation temperature reached is 110degC. A thermal resistance of 4.3 K/W was attained by epi-down mounting on diamond submounts. The waveguide losses of 14 cm-1 are explained by intersubband absorption in addition to free-carrier absorption.

Surface plasmon resonance sensor showing enhanced sensitivity for CO2 detection in the mid-infrared range

We present the first optical sensor based on Surface Plasmon Resonance (SPR) operating in the mid-infrared range. The experimental setup is based on a Kretschmann geometry with Ti/Au layers deposited on a CaF(2) prism where light excitation is provided by a Quantum Cascade Laser (QCL) source. Evidence of SPR is presented and the sensing capability of the system is demonstrated by using CO(2) and N(2) mixtures as test samples. Due to the absorption of CO(2) at this wavelength, it is shown that the sensitivity of this configuration is five times higher than a similar SPR sensor operating in the visible range of the spectrum.

Influence of InAs, AlAs δ layers on the optical, electronic, and thermal characteristics of strain-compensated GaInAs∕AlInAs quantum-cascade lasers

We extracted the electronic temperatures, the thermal resistance (RL=11.5K∕W), the cross-plane thermal conductivity [k⊥=2.0±0.1W∕(Km)], and the thermal boundary resistance [TBR=(5–11)×10−10K∕Wm2] in strain-compensated Ga0.609In0.391As∕AlIn0.546As0.454 quantum-cascade lasers operating at 4.78μm in continuous wave up to 15°C and in pulsed mode up to 40°C. Submonolayer thick InAs and AlAs δ layers are included in the active region to increase the conduction band discontinuity. We found that potential interface broadening caused by the insertion of these δ layers allows for a 60% improvement of the thermal conductivity with respect to conventional lattice-matched GaInAs∕AlInAs heterostructures.

High performance, low dissipation quantum cascade lasers across the mid-IR range

In this work, we present the development of low consumption quantum cascade lasers across the mid-IR range. In particular, short cavity single-mode lasers with optimised facet reflectivities have been fabricated from 4.5 to 9.2 μm. Threshold dissipated powers as low as 0.5 W were obtained in continuous wave operation at room temperature. In addition, the beneficial impact of reducing chip length on laser mounting yield is discussed. High power single-mode lasers from the same processed wafers are also presented.

Sequential resonant tunneling in quantum cascade lasers

A model of sequential resonant tunneling transport among two-dimensional subbands that takes into account explicitly elastic scattering is investigated. It is compared to transport measurements performed on quantum cascade lasers, where resonant tunneling processes are known to be dominating. Excellent agreement is found between experiment and theory over a large range of current, temperature, and device structures.

Quantum cascade lasers with large optical waveguides

Quantum cascade lasers with large optical cavities that exhibit low waveguide losses, high saturation intensities, and low beam divergence, emitting between lambdaap4.3 and 5.2mum were studied. Peak-pulsed powers up to 14 W at 15 K and 5 W at 280 K at lambdaap5.2 mum are demonstrated. A full-width at half-maximum of only 30deg was measured for the transverse far-field pattern of the device operating at lambdaap4.3 mum, a significant reduction compared to conventional waveguides

Large cavity quantum cascade lasers with InP interstacks

Multicore quantum cascade lasers with large cavities show low optical losses, high saturation intensity, and low beam divergence. We present a four active core laser based on InGaAs–AlInAs material system emitting at 10.5 μm. To improve thermal conductance, InP interstacks were used to separate active regions. Selective lateral etching of the active regions was used to reduce optical losses. Peak powers up to 4.6 W, average powers up to 310 mW, and wall-plug efficiencies up to 4.6% were measured at 300 K. A far field with full width at half maximum of 18.7° and 49.8° in the lateral and in the growth direction, respectively, was observed.

Extended tuning of mid-ir quantum cascade lasers using integrated resistive heaters

We present single mode quantum cascade lasers including a microscopic heater for spectral emission tuning. Through the use of a buried heater element, the active region temperature can be modified without changing the submount one. Emission frequency tuning in continuous-wave as large as 9 cm(-1) at 1270 cm(-1) and 14 cm(-1) at 2040 cm(-1) are observed, corresponding to an increase of the active region temperatures of ∼ 90 K. Due to the proximity of the heaters to the active region, emission can be modulated at several kHz range and the absence of moving parts guarantees the mechanical stability of the system. This method can be successfully applied to all buried heterostructure lasers, becoming an attractive solution for molecular spectroscopy in the IR. Using the presented devices, molecular absorptions of N(2)O have been measured between 1270 cm(-1) and 1280 cm(-1) and are in agreement with data from the HITRAN database.

Femtosecond dynamics of resonant tunneling and superlattice relaxation in quantum cascade lasers

Time-resolved mid-infrared pump-probe measurements are performed on a quantum cascade laser below and above the threshold. The gain recovery is determined by the electron transport through the cascade heterostructure. Subpicosecond resonant tunneling injection from the injector ground state into the upper lasing state is found to be incoherent due to the strong dephasing in the active subband. The gain recovery due to transport through superlattice is interpreted in terms of dielectric relaxation within the superlattice miniband.

Gain measurements in strain-compensated quantum cascade laser

Spectrally resolved gain measurements of a strain-compensated quantum cascade laser, emitting at 4.8 μm, are presented. Measurements have been acquired using a multisection cavity technique. The shape of the gain is compared with electroluminescence measurements whereas gain coefficients are compared to values obtained from laser data, studying the dependence of the laser threshold current with cavity length. Gain coefficients of 9.9 cm kA−1 at 80 K and 3.2 cm kA−1 at 322 K are found and we show that the temperature dependence of the gain coefficient is governed by optical phonon scattering.