Mariano Troccoli

High-power quantum cascade lasers grown by low-pressure metal organic vapor-phase epitaxy operating in continuous wave above 400K

High-power quantum cascade lasers (QCLs) working in continuous wave (cw) above 400K are presented. The material was grown by low-pressure metal organic vapor-phase epitaxy and processed into narrow buried heterostructure lasers. A cw output power of 204mW was obtained at 300K with an 8.38μm wavelength, 3mm long and 7.5μm wide coated laser. The device operates in cw mode above 400K, which exceeds the previous maximum cw temperature operation of QCLs by approximately 60K. Preliminary reliability data obtained by accelerated aging tests indicate a remarkable robustness of the lasers.

Raman injection laser.

Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material. This effect is the basis for a broad class of tunable sources known as Raman lasers. In general, these sources have only small gain (∼ 10-9 cm W-1) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers: it is based on triply resonant stimulated Raman scattering between quantum-confined states within the active region of a quantum cascade laser that serves as an internal optical pump—the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid-and far-infrared light sources.

Coherent instabilities in a semiconductor laser with fast gain recovery

We report the observation of a coherent multimode instability in quantum cascade lasers QCLs, which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham- Haken RNGH instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.

High-temperature continuous wave operation of strain-balanced quantum cascade lasers grown by metal organic vapor-phase epitaxy

The authors report the fabrication of high-power strained quantum cascade lasers working in continuous mode above 370K. The devices, processed in narrow buried heterostructures, were grown by low-pressure metal organic vapor-phase epitaxy. Continuous wave output power as high as 312mW at 300K was obtained at a wavelength of 5.29μm from a 3.25mm long, 7.5μm wide laser with a high-reflectivity back facet coating. The slope efficiency was in excess of 1.5W∕A and the power conversion efficiency reached almost 5%.

Temperature profile of GaInAs/AlInAs/InP quantum cascade-laser facets measured by microprobe photoluminescence

The local temperature of quantum-cascade lasers operating in continuous wave mode is reported. This information is extracted from the thermal shift of the band-to-band photoluminescence peaks in the AlInAs and InP cladding layers of quantum-cascade laser facets using a high-resolution microprobe setup. Interpolation by means of a two-dimensional heat diffusion model allows to obtain the temperature profile and the thermal conductivity in the waveguide core. Comparison between substrate and epilayer-side mounted lasers shows the superior thermal dissipation capability of the latter, and explains their better performance with respect to threshold current and maximum operating temperature.

Broadly tunable monolithic room-temperature terahertz quantum cascade laser sources

Electrically pumped room-temperature semiconductor sources of tunable terahertz radiation in 1-5 THz spectral range are highly desired to enable compact instrumentation for THz sensing and spectroscopy. Quantum cascade lasers with intra-cavity difference-frequency generation are currently the only room-temperature electrically pumped semiconductor sources that can operate in the entire 1-5 THz spectral range. Here we demonstrate that this technology is suitable to implementing monolithic room-temperature terahertz tuners with broadband electrical control of the emission frequency. Experimentally, we demonstrate ridge waveguide devices electrically tunable between 3.44 and 4.02 THz.

Low-threshold continuous-wave operation of quantum-cascade lasers grown by metalorganic vapor phase epitaxy

We report on the realization of InGaAs∕InAlAs quantum-cascade lasers grown by metalorganic vapor phase epitaxy operating in continuous wave with low-threshold current densities at temperatures as high as 188K. Threshold current densities of 950A∕cm2 and output powers of 125mW are measured at 80K, while 3mW of continuous output power are measured at 180K, with a threshold of 2.5kA∕cm2. In pulsed mode, peak output powers of more than 0.4W were obtained at 80K and of 160mW at 300K with thresholds of 700A∕cm2 and 2.75kA∕cm2, respectively.

Mid-infrared (λ≈7.4 μm) quantum cascade laser amplifier for high power single-mode emission and improved beam quality

A quantum cascade laser amplifier has been developed. It was used to obtain high power single-mode emission at λ≈7.4 μm from a quantum cascade distributed feedback laser, together with enhanced beam quality. Laser and amplifier are directly coupled in a master oscillator power amplifier configuration. Peak optical powers of 0.5 W at 80 K have been obtained. Ninety percent of the total power is thereby emitted within a divergence of 20° in the lateral direction. The device showed single mode operation with a side mode suppression ratio of 30 dB in the temperature range from 10 to 280 K. This allowed tuning of the emission wavelength in the range from 7.36 to 7.46 μm. The estimated peak amplifier gain is 6.4 and 4.9 dB at 80 and 300 K, respectively, and the cavity losses are 12.5 and 22 cm−1 at the corresponding temperatures.

Thermal resistance and temperature characteristics of GaAs/Al0.33Ga0.67As quantum-cascade lasers

We report on the determination of thermal resistance, facet temperature profile, and heat flux of GaAs/Al0.33Ga0.67As quantum-cascade lasers operating in pulsed mode, using a microprobe band-to-band photoluminescence technique. The thermal resistance of epilayer-side mounted lasers is ∼30% smaller than that of substrate-side mounted ones. The dependence of the thermal resistance on the injection conditions and its correlation with the output power is also reported.

Electronic distribution in superlattice quantum cascade lasers

The electron population in the excited miniband of quantum cascade structures with intrinsic superlattice active regions is extracted from the fine structure analysis of spontaneous interminiband electroluminescence spectra. At current densities typical of laser thresholds, the electrons injected into the excited miniband of a (GaInAs)6 nm/(AlInAs)1.8 nm superlattice are described by a nonequilibrium thermal distribution characterized by temperatures Te>200 K, much higher than the lattice temperature TL=15 K.