Power Of Quantum Cascade Lasers Research Articles

Super-resolution infrared microspectroscopy reveals heterogeneous distribution of photosensitive lipids in human hair medulla

Human hair medulla chemical composition appears mostly homogenous when mapped by FTIR microspectroscopy even when using a synchrotron radiation source (SR-μFTIR) but it is expected to be heterogeneous. We performed sub-micron chemical mapping of hair cortex and medullas using Optical Photothermal Infrared microspectroscopy (OPTIR) and a mid-infrared Quantum Cascade Laser (QCL) source covering the fingerprint and the CH stretching region. Photodamages were observed in the hair cortex at mild laser power and occurred in the hair medulla even at the lowest power settings of the IR QCL pulsed at 100 kHz rate (4 μW/μm2 average power density) and visible probe laser (200 μw/μm2 average power density). Photoconversion of calcium carboxylates in other molecules, possibly sodium carboxylates, was observed. Attenuation of the IR QCL power by 40% using ZnSe filter and/or high-speed measurements (1000 cm−1/s) succeeded in almost completely eliminating the photodamages and photoconversion. OPTIR maps and images showed that the medullas were highly heterogeneous at the submicron scale. We found calcium carboxylates, aliphatic lipids and wax esters in small units, hundreds of nanometers in size. The 1470 cm−1 CO sym stretching peak of calcium carboxylates and the CH2asym stretching peak from aliphatic lipids proved to be the most efficient peaks to track the distribution of these molecules. OPTIR had enough sensitivity to map accurately only the strongest peaks from lipids and calcium carboxylates, weaker peaks such as the ester CO and sulfoxide SO bands were not accurately detected by OPTIR even when they were shown to be present by SR-μFTIR. Quantification of the medulla components by OPTIR is difficult due to several factors: discontinuous QCL emission, and noise. The weaker peaks such as CH3, CO, SO are often underestimated or not detected. We demonstrate here that OPTIR can be used to measure, map and image dark, photosensitive samples using very low IR power.

Nanoscale thermal gradients activated by antenna-enhanced molecular absorption in the mid-infrared

We investigate local heat generation by molecules at the apex of polymer-embedded vertical antennas excited at resonant mid-infrared wavelengths, exploiting the surface enhanced infrared absorption effect. The embedding of vertical nanoantennas in a non-absorbing polymer creates thermal isolation between the apical hotspot and the heat sink represented by the substrate. Vibrational mid-infrared absorption by strongly absorbing molecules located at the antenna apex then generates nanoscale temperature gradients at the surface. We imaged the thermal gradients by using a nano-photothermal expansion microscope, and we found values up to 10 K/μm in conditions where the radiation wavelength resonates with both the molecule vibrations and the plasmonic mode of the antennas. Values up to 1000 K/μm can be foreseen at maximum quantum cascade laser power. The presented system provides a promising thermoplasmonic platform for antenna-assisted thermophoresis and resonant mid-infrared photocatalysis.

Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array

We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability.

Distributed loss method to suppress high order modes in broad area quantum cascade lasers

We describe a method where the standard fabrication of broad area quantum cascade lasers is modified to provide a controlled amount of direct contact of device sidewalls with metal. We demonstrate that this provides sufficient levels of distributed losses to suppress the high order transverse modes in favor of the fundamental or near-fundamental transverse mode operation. We observe that the quantum cascade laser power and slope efficiency are degraded by a small amount, resulting in a large increase in brightness to accompany the power scaling.

8-beam local oscillator array at 4.7 THz generated by a phase grating and a quantum cascade laser.

We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.

Measuring molecular frequencies in the 1\u201310\u2009\u03bcm range at 11-digits accuracy

High-resolution spectroscopy in the 1–10 μm region has never been fully tackled for the lack of widely-tunable and practical light sources. Indeed, all solutions proposed thus far suffer from at least one of three issues: they are feasible only in a narrow spectral range; the power available for spectroscopy is limited; the frequency accuracy is poor. Here, we present a setup for high-resolution spectroscopy, whose approach can be applied in the whole 1–10 μm range. It combines the power of quantum cascade lasers (QCLs) and the accuracy achievable by difference frequency generation using an orientation patterned GaP crystal. The frequency is measured against a primary frequency standard using the Italian metrological fibre link network. We demonstrate the performance of the setup by measuring a vibrational transition in a highly-excited metastable state of CO around 6 μm with 11 digits of precision.

Prospects of using quantum-cascade lasers in optoelectronic countermeasure systems: review

This paper discusses existing and prospective laser devices for countermeasure systems that operate in the IR region. Attention is mainly devoted to questions of counteracting imaging seekers that function in the IR region and of suppressing thermal-vision observation, reconnaissance, and aiming equipment. Our critical analysis of the available literature data makes it possible to conclude that quantum-cascade lasers can be used in creating promising compact multispectral IR countermeasure systems. Estimates are given of how much radiation power quantum-cascade lasers need for the functional suppression of remote photodetector devices. The results of the calculations are in satisfactory agreement with the available literature data. It is shown that existing ways of enhancing the radiation power of quantum-cascade lasers make it possible to achieve the values needed to use them in IR countermeasure systems.

Enhanced light output power of quantum cascade lasers from a tilted front facet

We present a technique for enhancing the light output power of quantum cascade lasers (QCLs) by tilting of the front facet, which leads to a change of the modal reflectivity, resulting in an asymmetric light intensity distribution along the laser cavity. This asymmetry provides most of the light being emitted through one facet of the laser. An experimental study of threshold current, slope efficiency and light output power as a function of the front facet angles were performed and compared to conventional QCLs. The lasers with a front facet angle of 8° shows a 20% improved power output from the front facet.

Part-per-trillion level SF_6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF(6) was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10(-10) W·cm(-1)/Hz(1/2).

Scaling the output power of quantum-cascade lasers with a number of cascades

A series of quantum-cascade lasers emitting at λ≈11μm with different numbers of cascades in the active region – between 25 and 200 –grown by gas-source molecular beam epitaxy, is compared in terms of their room-temperature performance in pulsed mode as a function of the number of cascades. Within the given range of cascade number we observe a superlinear increase in both slope efficiency and differential quantum efficiency as functions of cascade number. The differential quantum efficiency scaled per number of cascades, however, shows a sublinear increase. A differential quantum efficiency of 17% per cascade and pulse power exceeding 7W per facet, both at room temperature, was demonstrated for quantum-cascade lasers with 200 cascades. All lasers with stripe width below 30μm are operated in the fundamental TEM00 spatial mode. This combination of high pulse power and TEM00 mode operation for room-temperature mid-infrared laser emission can be attractive for a number of applications.