Mapping the charge carrier density in semiconductors by THz-QCL based optical feedback interferometry

A THz imaging system based on self-mixing (SM)interferometry in a Quantum Cascade Laser (QCL) is developed to map the distribution of free charges on a semiconductor surface. In the experiment, a free electron plasma is photo-generated in a high resistivity n-type silicon wafer using a near-infrared (NIR)continuous-wave (CW) pump laser. A model based on Drude theory correctly reproduces the experimental results and in prospective promises a quantitative evaluation of the free charges density

Versatile multimodality imaging system based on detectorless and scanless optical feedback interferometry—a retrospective overview for a prospective vision

In this retrospective compendium, we attempt to draw a “fil rouge” along fifteen years of our research in the field of optical feedback interferometry aimed at guiding the readers to the verge of new developments in the field. The general reader will be moved at appreciating the versatility and the still largely uncovered potential of the optical feedback interferometry, for both sensing and imaging applications. By discovering the broad range of available wavelengths (0.4–120 μm), the different types of suitable semiconductor lasers (Fabry–Perot, distributed feedback, vertical-cavity, quantum-cascade), and a number of unconventional tenders in multi-axis displacement, ablation front progression, self-referenced measurements, multispectral, structured light feedback imaging and compressive sensing, the specialist also could find inspirational suggestions to expand his field of research

Optical limiting behavior of bismuth oxide-based glass in the visible range

The authors report experimental results on the optical limiting behavior of a bismuth oxide-based glass by exciting the samples with nanosecond laser pulses at 532 and 598 nm. The results show that two-photon and free-carrier absorption processes contribute for the nonlinear absorption. Values for β, the two-photon absorption coefficient, and σe, the absorption cross section due to free carriers, were determined. The values for β and σe are dependent on the amount of bismuth oxide in the glass composition

Cerebellar Integrity in the Amyotrophic Lateral Sclerosis - Frontotemporal Dementia Continuum

Amyotrophic lateral sclerosis (ALS) and behavioural variant frontotemporal dementia (bvFTD) are multisystem neurodegenerative disorders that manifest overlapping cognitive, neuropsychiatric and motor features. The cerebellum has long been known to be crucial for intact motor function although emerging evidence over the past decade has attributed cognitive and neuropsychiatric processes to this structure. The current study set out i) to establish the integrity of cerebellar subregions in the amyotrophic lateral sclerosis-behavioural variant frontotemporal dementia spectrum (ALS-bvFTD) and ii) determine whether specific cerebellar atrophy regions are associated with cognitive, neuropsychiatric and motor symptoms in the patients. Seventy-eight patients diagnosed with ALS, ALS-bvFTD, behavioural variant frontotemporal dementia (bvFTD), most without C9ORF72 gene abnormalities, and healthy controls were investigated. Participants underwent cognitive, neuropsychiatric and functional evaluation as well as structural imaging using voxel-based morphometry (VBM) to examine the grey matter subregions of the cerebellar lobules, vermis and crus. VBM analyses revealed: i) significant grey matter atrophy in the cerebellum across the whole ALS-bvFTD continuum; ii) atrophy predominantly of the superior cerebellum and crus in bvFTD patients, atrophy of the inferior cerebellum and vermis in ALS patients, while ALS-bvFTD patients had both patterns of atrophy. Post-hoc covariance analyses revealed that cognitive and neuropsychiatric symptoms were particularly associated with atrophy of the crus and superior lobule, while motor symptoms were more associated with atrophy of the inferior lobules. Taken together, these findings indicate an important role of the cerebellum in the ALS-bvFTD disease spectrum, with all three clinical phenotypes demonstrating specific patterns of subregional atrophy that associated with different symptomology

Quantum cascade laser based hybrid dual comb spectrometer

Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a powerful photonic tool, driving a recent revolution in major molecular fingerprint regions, i.e. mid- and far-infrared domains. Their compact and frequency-agile design, together with their high optical power and spectral purity, promise to deliver an all-in-one source for the most challenging spectroscopic applications. Here, we demonstrate a metrological-grade hybrid dual comb spectrometer, combining the advantages of a THz QCL-FC with the accuracy and absolute frequency referencing provided by a free-standing, optically-rectified THz frequency comb. A proof-of-principle application to methanol molecular transitions is presented. The multi-heterodyne molecular spectra retrieved provide state-of-the-art results in line-center determination, achieving the same precision as currently available molecular databases. The devised setup provides a solid platform for a new generation of THz spectrometers, paving the way to more refined and sophisticated systems exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic schemes

Quantum cascade laser-based sensing to investigate fast laser ablation process

We demonstrated a sensing technique for in-line ablation rate detection using a quantum cascade laser (QCL) under external optical feedback. The design of the QCL-based diagnostic system allowed to monitor the voltage modulation at the laser terminals induced by fast dynamics in the ablation process. Real-time detection of the ablation front velocity as well as in-situ investigations of the surface temperature were provided. Experimental results on fast ablation rates per pulse correlate well with the theoretical prediction. The detection range was demonstrated to be limited only by the QCL-probe emission wavelength, which is scalable up to the THz spectral region. © 2013 Copyright SPIE

Tailoring the wettability of surface-textured copper using sub-THz bursts of femtosecond laser pulses

In this work, copper surfaces were textured with sub-Terahertz bursts of femtosecond pulses. The wettability of Cu textured surfaces was investigated by measuring the static water contact angle (WCA) as a function of the number of sub-pulses and the intra-burst frequency. A superhydrophobic, antiadhesive response was observed when using bursts with a high number of sub-pulses (equal to or higher than 16) or a high intra-burst frequency (equal to or higher than 0.09 THz). Such trend was ascribed to the generation, under specific laser irradiation conditions, of a double-scale hierarchical texture on the sample surface, formed by sub-micro patterns with fine periodic ripples (LIPSS, Laser-Induced Periodic Surface Structures) and random nanoparticle decoration. Such texture enhances the hydrophobic behavior given by inherent adsorption of adventitious hydrocarbons on laser-processed and thermally-treated metal targets