Search results for "WAVELENGTH"
showing 10 items of 741 documents
Laser wavelength effects on the refractive index change of waveguides written by femtosecond pulses in silica glasses
2018
We investigate the influence of two fs-laser wavelengths (343 and 800 nm) on the induced refractive index change (Δn) of waveguides written in silica materials. Results show that Δn is higher for waveguides photo-inscribed with UV photons.
Skin chromophore mapping from multi-spectral laser line images
2020
Skin chromophore maps can be used for assessment of various skin malformations and early cancer diagnostics. Commercially available devices are bulky and expensive. We present two portable proof-of-concept device prototypes for multi-spectral laser line imaging with three (448 nm, 532 nm and 659 nm) and four (450 nm, 523 nm, 638 nm and 850 nm) wavelength laser illumination. Laser modules and special optics that ensure uniform light distribution over the region of interest have been exploited. Skin chromophore maps were calculated using Beer-Lambert law, considering light scattering properties in the skin and including photon path length evaluated from the directly measured photon-time-of-fl…
A Monte-Carlo based model of the AX-PET demonstrator and its experimental validation
2013
AX-PET is a novel PET detector based on axially oriented crystals and orthogonal wavelength shifter (WLS) strips, both individually read out by silicon photo-multipliers. Its design decouples sensitivity and spatial resolution, by reducing the parallax error due to the layered arrangement of the crystals. Additionally the granularity of AX-PET enhances the capability to track photons within the detector yielding a large fraction of inter-crystal scatter events. These events, if properly processed, can be included in the reconstruction stage further increasing the sensitivity. Its unique features require dedicated Monte-Carlo simulations, enabling the development of the device, interpreting …
Early decay detection in citrus fruit using laser-light backscattering imaging
2013
Early detection of fungal infections in citrus fruit still remains one of the major problems in postharvest technology. The potential of laser-light backscattering imaging was evaluated for detecting decay in citrus fruit after infection with the pathogen Penicillium digitatum, before the appearance of fruiting structures (green mould). Backscattering images of oranges cv. Navelate with and without decay were obtained using diode lasers emitting at five different wavelengths in the visible and near infrared range for addressing the absorption of fruit carotenoids, chlorophylls and water/carbohydrates. The apparent region of backscattered photons captured by a camera had radial symmetry with…
Plasmonic nanostructures for light trapping in thin-film solar cells
2019
M.J.M. acknowledges funding from FCT through the grant SFRH/BPD/115566/2016. ALTALUZ (Reference PTDC/CTM-ENE/5125/2014). The optical properties of localized surface plasmon resonances (LSPR) sustained by self-assembled silver nanoparticles are of great interest for enhancing light trapping in thin film photovoltaics. First, we report on a systematic investigation of the structural and the optical properties of silver nanostructures fabricated by a solid-state dewetting process on various substrates. Our study allows to identify fabrication conditions in which circular, uniformly spaced nanoparticles are obtainable. The optimized NPs are then integrated into plasmonic back reflector (PBR) st…
Quantitative subsurface defect detection in composite materials using a non-contact ultrasonic system
2002
The results of an experimental study conducted to detect subsurface defects in a thick Gr/PPS composite test sample using a noncontact ultrasonic system are presented. Surface waves are generated by a pulsed laser and detected by an air-coupled capacitance transducer. By controlling the surface wave wavelength through a shadow mask, it is possible to control surface wave penetration depth in the sample. Surface wave peak-to-peak amplitude is related to the near-surface material condition. Results indicate that signal amplitude decreases as the width of the defect increases and an approximately linear relation can be deduced.
Autonomous frequency stabilization of two extended cavity diode lasers at the potassium wavelength on a sounding rocket
2016
We have developed, assembled, and flight-proven a stable, compact, and autonomous extended cavity diode laser (ECDL) system designed for atomic physics experiments in space. To that end, two micro-integrated ECDLs at 766.7 nm were frequency stabilized during a sounding rocket flight by means of frequency modulation spectroscopy (FMS) of 39^K and offset locking techniques based on the beat note of the two ECDLs. The frequency stabilization as well as additional hard- and software to test hot redundancy mechanisms were implemented as part of a state-machine, which controlled the experiment completely autonomously throughout the entire flight mission.
Hyperspectral terahertz microscopy via nonlinear ghost imaging
2020
Ghost imaging, based on single-pixel detection and multiple pattern illumination, is a crucial investigative tool in difficult-to-access wavelength regions. In the terahertz domain, where high-resolution imagers are mostly unavailable, ghost imaging is an optimal approach to embed the temporal dimension, creating a “hyperspectral” imager. In this framework, high resolution is mostly out of reach. Hence, it is particularly critical to developing practical approaches for microscopy. Here we experimentally demonstrate time-resolved nonlinear ghost imaging, a technique based on near-field, optical-to-terahertz nonlinear conversion and detection of illumination patterns. We show how space–time c…
APDs as single-photon detectors for visible and near-infrared wavelengths down to Hz rates
2012
For the SPECTRAP experiment at GSI, Germany, detectors with Single-Photon counting capability in the visible and near-infrared regime are required. For the wavelength region up to 1100 nm we investigate the performance of 2x2 mm^2 avalanche photo diodes (APDs) of type S0223 manufactured by Radiation Monitoring Devices. To minimize thermal noise, the APDs are cooled to approximately -170 deg. C using liquid nitrogen. By operating the diodes close to the breakdown voltage it is possible to achieve relative gains in excess of 2x10^4. Custom-made low noise preamplifiers are used to read out the devices. The measurements presented in this paper have been obtained at a relative gain of 2.2x10^4. …
Reducing the contribution of the photoemission process to the unwanted beam in photoelectron sources at accelerators
2017
Negative electron affinity (NEA) GaAs photocathodes show different pulse responses depending on the wavelength of photoexcitation. The pulse response at 800 nm shows a long and relatively intense tail, whereas at 400 nm, a tail of similar shape but with an intensity lower by around two orders of magnitude is observed. We explain this behavior with the specific properties of NEA photocathodes and compare it with the response of a positive electron affinity photocathode.