Borislav Hinkov

Four-wave mixing in a quantum cascade laser amplifier

We present the direct observation of four-wave mixing over a detuning range of more than 3 THz in an InGaAs/AlInAs strain-compensated quantum cascade laser (QCL) amplifier emitting at 4.3 μm by simultaneous injection of a single mode QCL and a broadly tunable source. From its intensity, we determine a χ(3) of 0.9 × 10−15 m2 V−2, in good agreement with transport model simulations based on the density matrix approach. This four-wave-mixing mechanism is an important driving factor in mode proliferation occurring in connection with the recent demonstration of comb generation in broadband QCLs.

Fully automatized quantum cascade laser design by genetic optimization

Using a transport model based on the density matrix formalism, a fully automatized technique to design quantum cascade structures in the mid-infrared is presented that implements a genetic algorithm where the wallplug efficiency has been used as merit factor. Starting from a reference design, the model converges after few generations on an optimized design that presents a better carrier injection in the upper lasing state. Both the designs have been fabricated using buried heterostructure process and the optimized design shows a pronounced increase in the laser operation range and higher output powers. In good agreement with the simulations, the laser efficiency increases from 5% to 12%.

Quantum cascade laser in a master oscillator power amplifier configuration with Watt-level optical output power

We present the design and realization of short-wavelength (λ = 4.53 μm) and buried-heterostructure quantum cascade lasers in a master oscillator power amplifier configuration. Watt-level, singlemode peak optical output power is demonstrated for typical non-tapered 4 μm wide and 5.25 mm long devices. Farfield measurements prove a symmetric, single transverse-mode emission in TM(00)-mode with typical divergences of 25° and 27° in and perpendicular to growth direction, respectively. We demonstrate singlemode tuning over a range of 7.9 cm(-1) for temperatures between 263K and 313K and also singlemode emission for different driving currents. The side mode suppression ratio is measured to be higher than 20 dB.

Rf-modulation of mid-infrared distributed feedback quantum cascade lasers

We present the electrical and optical characterization and theoretical modeling of the transient behavior of regular 4.5-μm single-mode emitting distributed feedback (DFB) quantum cascade lasers (QCLs). Low residual capacitance together with a high-frequency optimized three-terminal coplanar waveguide configuration leads to modulation frequencies up to 23.5 GHz (optical) and 26.5 GHz (electrical), respectively. A maximum 3-dB cut-off value of 6.6 GHz in a microwave rectification scheme is obtained, with a significant increase in electrical modulation bandwidth when increasing the DC-current for the entire current range of the devices. Optical measurements by means of FTIR-spectroscopy and a heterodyne beating experiment reveal the presence of a resonance peak, due to coupling of the lasing DFB- with its neighboring below-threshold Fabry-Pérot-(FP-)mode, when modulating around the cavity roundtrip frequency. This resonance is modeled by a 2-mode Maxwell-Bloch formalism. It enhances only one sideband and consequently leads to the first experimental observation of the single-sideband regime in such kind of devices.

Rate equations analysis of external-cavity quantum cascade lasers

We present a comprehensive analysis of an external-cavity (EC) quantum cascade (QC) laser system, which is based on a coupled-mode system with the length of the EC set equal to that of the laser chip and accommodating the large difference in physical cavity length by introducing an effective speed of light for light propagating in the EC. By analyzing the rate equations and discussing the cross sections and losses of the EC QC lasers, numerical results as well as suggestions for a further optimization of the spectral tuning range of the EC system are presented. Besides, the delayed onset of the EC modes with respect to that of the internal Fabry–Perot modes in pulsed operation has been simulated using the rate equations, yielding a good agreement with the experimental data.

Characterization of iron doped indium phosphide as a current blocking layer in buried heterostructure quantum cascade lasers

This work analyzes transport through metal organic chemical vapour deposition grown Iron doped Indium Phosphide (InP:Fe) for use as a current blocking layer in buried heterostructure Quantum Cascade Lasers. The nature of Iron incorporation in InP and electrical transport properties of InP:Fe is investigated via simulation and compared with measurement. Through simulations, we are able to predict the threshold for the onset of current rise in test structures due to avalanche injection of carriers. In addition, the benefit of InAlAs barriers inserted in InP:Fe layers is investigated and found to reduce the leakage current at lower biases while delaying the onset of avalanche. In buried heterostructure configuration, we have determined that non ideal regrowth profiles make the structure more susceptible to high field effects such as avalanche injection and trap filling that induce leakage currents.

Resonant tunneling diodes based on ZnO for quantum cascade structures (Conference Presentation)

The terahertz (THz) spectral range (lambda ~ 30µm – 300µm) is also known as the “THz-gap” because of the lack of compact semiconductor devices. Various real-world applications would strongly benefit from such sources like trace-gas spectroscopy or security-screening. A crucial step is the operation of THz-emitting lasers at room temperature. But this seems out of reach with current devices, of which GaAs-based quantum cascade lasers (QCLs) seem to be the most promising ones. They are limited by the parasitic, non-optical LO-phonon transitions (36meV in GaAs), being on the same order as the thermal energy at room temperature (kT = 26meV). This can be solved by using larger LO-phonon materials like ZnO (E_LO = 72meV). But to master the fabrication of ZnO-based QC structures, a high quality epitaxial growth is crucial followed by a well-controlled fabrication process including ZnO/ZnMgO etching. We use devices grown on m-plane ZnO-substrate by molecular beam epitaxy. They are patterned by reactive ion etching in a CH4-based chemistry (CH4:H2:Ar/30:3:3 sccm) into 50μm to 150μm square mesas. Resonant tunneling diode structures are investigated in this geometry and are presented including different barrier- and well-configurations. We extract contact resistances of 8e-5 Omega cm^2 for un-annealed Ti/Au contacts and an electron mobility of above 130cm^2/Vs, both in good agreement with literature. Proving that resonant electron tunneling can be achieved in ZnO is one of the crucial building blocks of a QCL. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 665107.

High-power single-mode-emission quantum cascade lasers

Several chemical species have characteristic fundamental absorptions in the mid-IR spectral region, i.e., with wavelengths in the 4–12 m range. These species include the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (NO), which are of particular interest in climate research studies.1, 2 Powerful mid-IR detection systems that measure such gases with high levels of precision are therefore required. Quantum cascade (QC) laser systems can emit light in a specific spectral mode (single-mode emission) with high output power. Pulsed-wave and continuous-wave driven trace gas sensors that are based on mid-IR-emitting QC lasers are suitable systems for detecting greenhouse gases, with detection levels in the parts-per-billion range or better.3, 4 Current designs for highpower single-mode-emitting QC lasers achieve high optical output power. These approaches, however, result in poor discrimination and yield of the single-mode emission, an astigmatic light beam, or are limited to low-duty-cycle operations only (a duty cycle is the proportion of a period in which a signal is active).5, 6 We have designed a single-mode QC laser system that combines the advantages of the previously proposed techniques. Our laser devices have a semiconductor-based monolithically integrated asymmetric master-oscillator power amplifer (MOPA) configuration. Our MOPA produces a highly coherent amplified beam and is configured with a straight waveguide in the amplifier section. The waveguide directs the emitted light into a symmetric beam. The laser we use has a relatively short (about 1.25mm in length) single-mode-emitting ‘seeding’ section, which is based on an index-coupled distributed-feedback (DFB) grating and a Fabry-Pérot (FP)-type amplifier that can be 4mm or more in length (see Figure 1).7 Figure 1. Top view of a typical master-oscillator power amplifier (MOPA) device (about 4.5mm in length), mounted on a copper heat sink. The individual distributed feedback (DFB) and Fabry-Pérot (FP) sections are labeled.

Beam steering in mid-infrared emitting quantum-cascade lasers

We report large beam steering effects, observed in the far-field pattern of InP-based mid-infrared quantum-cascade lasers along the slow axis. Changing the temperature by a few degrees around room temperature or varying the drive current strongly affects the lateral direction of the output beam. The position of maximum intensity in the far-field-distribution changes by more than 20°. This beam steering effect is correlated to changes in the lateral mode distribution, as revealed by time-resolved spectroscopy of the lasing spectrum.