arXiv subjects

Romain Peretti

Publications and source records attributed to Romain Peretti.

18 recordsLinked to original sources

arXiv2026

Fully optoelectronic coherent THz spectroscopy

Coherent terahertz spectroscopy of molecular transients has so far relied on electronic sources whose bandwidth is limited to a fraction of their center frequency. Photoconductive devices offer a complementary approach, combining intrinsically broadband operation with coherent optoelectronic generation and detection. We report free-induction-decay spectroscopy of carbonyl sulfide at 0.29 THz in which, to our knowledge for the first time, both the pulsed emitter and the coherent heterodyne receiver are LT-GaAs photoconductors simultaneously pumped by a single free-running dual-frequency Ti:Sa laser. The receiver operates within 15 dB of the thermal noise limit, and the shared optical reference maintains phase coherence over more than 1000 averaged acquisitions. The measured transients agree with a time-domain Maxwell-Bloch model based on HITRAN parameters. These results demonstrate the feasibility of fully optoelectronic coherent THz spectroscopy and establish photoconductive optoelectronic mixing as a practical route toward frequency-agile, high-resolution coherent THz spectrometers over broad spectral ranges.

physics.app-ph↗

arXiv2024

Signal estimation and uncertainties extraction in TeraHertz Time Domain Spectroscopy

Terahertz Time Domain Spectroscopy (THz-TDS) systems have emerged as mature technologies with significant potential across various research fields and industries. However, the lack of standardized methods for signal and noise estimation and reduction hinders its full potential. This paper introduces a methodology to significantly reduce noise in THz-TDS time traces, providing a reliable and less biased estimation of the signal. The method results in an improved signal-to-noise ratio, enabling the utilization of the full dynamic range of such setups. Additionally, we investigate the estimation of the covariance matrix to quantify the uncertainties associated with the signal estimator. This matrix is essential for extracting accurate material parameters by normalizing the error function in the fitting process. Our approach addresses practical scenarios where the number of repeated measurements is limited compared to the sampling time axis length. We envision this work as the initial step toward standardizing THz-TDS data processing. We believe it will foster collaboration between the THz and signal processing communities, leading to the development of more sophisticated methods to tackle new challenges introduced by novel setups based on optoelectronic devices and dual-comb spectroscopy.

physics.optics↗

arXiv2024

Can the THz-TDS detect trace gases?

The THz-TDS is a versatile technique and can be used to probe gas phase molecules. We are pushing the technique to its limit to try and detect trace gases at the sub-ppm levels. The experiments served as a reference for a quantitative approach that permits to extract the concentration of a probed gas. The single-parameter quantification model can be considered as the basis for a wider multispecies detection scheme.

physics.ins-det↗

arXiv2024

A new metric for the comparison of permittivity models in terahertz time-domain spectroscopy

We present a robust method, as well as a new metric, for the comparison of permittivity models in terahertz timedomain spectroscopy (THz-TDS). In this work, we perform an extensive noise analysis of a THz-TDS system, we remove and model the unwanted deterministic noises and implement them into our fitting process. This is done using our open-source software, Fit@TDS, available at : https://github.com/THzbiophotonics/Fit-TDS. This work is the first step towards the derivation of uncertainties, and therefore the use of error bars. We hope that this will lead to performing analytical analysis with THz-TDS, as results obtained from different setups will be comparable. Finally, we apply this protocol to the study of a $\alpha$-lactose monohydrate pellet in order to give more insight on the molecular dynamics behind the absorption peaks. The comparison with simulation results is made easier thanks to the probabilities derived from the metric.

physics.app-ph↗

arXiv2022

Exceptional Point Singularities in Multi-Section DFB Lasers

A laser exhibits both controllable gain and loss and, under proper design conditions, is an ideal non-Hermitian system allowing the direct observation and engineering of spectral singularities such as exceptional points (EPs). A dual section distributed feedback (DFB) quantum cascade laser (QCL) is a prototype of such a system, allowing the controlled coupling of a ladder of cavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the coupling strength and the gain difference between these two set of modes enables probing the regimes from weak coupling to strong coupling and the robust observation of exceptional point singularities. At these exceptional points, the laser exhibits a sequence of lasing and coherent prefect absorption dynamics1,2 when pumped above transparency. Additionally, the pumping scheme allows the deliberate lifting of the exceptional point degeneracy. These results show that dual section QCL is a perfect platform to study exceptional points because the coupling parameter and system loss can be tuned in a single device.

physics.optics↗

arXiv2022

On the influence of water on THz vibrational spectral features of molecular crystals

The nanoscale structure of molecular assemblies plays a major role in many ($\mu$)-biological mechanisms. Molecular crystals are one of the most simple of these assemblies and are widely used in a variety of applications from pharmaceuticals and agrochemicals, to nutraceuticals and cosmetics. The collective vibrations in such molecular crystals can be probed using terahertz spectroscopy, providing unique characteristic spectral fingerprints. However, the association of the spectral features to the crystal conformation, crystal phase and its environment is a difficult task. We present a combined computationalexperimental study on the incorporation of water in lactose molecular crystals, and show how simulations can be used to associate spectral features in the THz region to crystal conformations and phases. Using periodic DFT simulations of lactose molecular crystals, the role of water in the observed lactose THz spectrum is clarified, presenting both direct and indirect contributions. A specific experimental setup is built to allow the controlled heating and corresponding dehydration of the sample, providing the monitoring of the crystal phase transformation dynamics. Besides the observation that lactose phases and phase transformation appear to be more complex than previously thoughtincluding several crystal forms in a single phase and a non-negligible water content in the so-called anhydrous phasewe draw two main conclusions from this study. Firstly, THz modes are spread over more than one molecule and require periodic computation rather than a gas-phase one. Secondly, hydration water does not only play a perturbative role but also participates in the facilitation of the THz vibrations.

physics.chem-ph↗

arXiv2021

Broadband super-resolution Terahertz Time domain spectroscopy applied to Gas analysis

Terahertz (THz) Time domain spectroscopy (THz-TDS) is a broadband spectroscopic technique spreading its uses in multiple fields: in science from material science to biology, in industry where it measures the thickness of a paint layer during the painting operation. Using such practical commercial apparatus with broad spectrum for gas spectroscopy could be a major asset for air quality monitoring and tracking of atmospheric composition. However, gas spectroscopy needs high resolution and the usual approach in THz-TDS, where the recorded time trace is Fourier transform, suffers from resolution limitation due to the size of the delay line in the system. In this letter, we introduce the concept of constraint reconstruction for super-resolution spectroscopy based on the modeling of the spectroscopic lines in a sparse spectrum. Light molecule gas typically shows sparse and narrow lines on a broad spectrum and we propose an algorithm reconstructing these lines with a resolution improvement of 10 the ultimate resolution reachable by the apparatus. We envision the proposed technique to lead to broadband, selective, rapid and cheap gas monitoring applications.

physics.app-ph↗

arXiv2019

THz-TDS time-trace analysis for the extraction of material and metamaterial parameters

We report on a method and an associated open source software, Fit@TDS, working on an average personal computer. The method is based on the fitting of a time-trace data of a terahertz time-domain-spectroscopy system enabling the retrieval of the refractive index of a dielectric sample and the resonance parameters of a metasurface (quality factor, absorption losses, etc.). The software includes commonly used methods where the refractive index is extracted from frequency domain data. However, these methods are limited, for instance in case of a high noise level or when an absorption peak saturates the absorption spectrum bringing the signal to the noise level. Our software allows to use a new method where the refractive indices are directly fitted from the time-trace. The idea is to model a material or a metamaterial through parametric physical models (Drude-Lorentz model and time-domain coupled mode theory) and to implement the subsequent refractive index in the propagation model to simulate the time-trace. Then, an optimization algorithm is used to retrieve the parameters of the model corresponding to the studied material/metamaterial. In this paper, we explain the method and test it on fictitious samples to probe the feasibility and reliability of the proposed model. Finally, we used Fit@TDS on real samples of high resistivity silicon, lactose and gold metasurface on quartz to show the capacity of our method

physics.optics↗

arXiv2014

Inhibition of light emission in a 2.5D photonic structure

We analyse inhibition of emission in a 2.5D photonic structures made up a photonic crystal (PhC) and Bragg mirrors using FDTD simulations. A comparison is made between an isolated PhC membrane and the same PhC suspended onto a Bragg mirror or sandwiched between 2 Bragg mirrors. Strong inhibition of the Purcell factor is observed in a broad spectral range, whatever the in-plane orientation and location of the emitting dipole. We analysed these results numerically and theoretically by simulating the experimentally observed lifetime of a collection of randomly distributed emitters, showing that their average emission rate is decreased by more than one decade, both for coupled or isolated emitters.

physics.optics↗

arXiv2014

Optical characterization of active photon cages

Recently, we developed a new family of 3D photonic hollow resonators which theoretically allow tight confinement of light in a fluid (gaz or liquid): the photon cages. These new resonators could be ideal for sensing applications since they not only localize the electromagnetic energy in a small mode volume but also enforce maximal overlap between this localized field and the environment (i.e. a potential volume of nano-particles). In this work, we will present numerical and experimental studies of the interaction of a photon cage optical mode with nano-emitters. For this, PbS quantum dot emitters in a PDMS host matrix have been introduced in photon cages designed to have optimal confinement properties when containing a PDMS-based active medium. Photoluminescence measurements have been performed and the presence of quantum dot emitters in the photon cages has been demonstrated.

physics.optics↗

arXiv2014

Absorption enhancement in photonic crystal thin films by pseudo disordered perturbations

The effects resulting from the introduction of a controlled perturbation in a single pattern membrane on its absorption are first studied and then analyzed on the basis of band folding considerations. The interest of this approach for photovoltaic applications is finally demonstrated by overcoming the integrated absorption of an optimized single pattern membrane through the introduction of a proper pseudo disordered perturbation.

physics.optics↗

arXiv2014

Blue light absorption enhancement based on vertically channelling modes in nano-holes arrays

We investigate the specific optical regime occurring at short wavelengths, in the high absorption regime, in silicon thin-films patterned by periodically arranged nano-holes. Near-field scanning optical microscopy indicates that the incoming light is coupled to vertically channelling modes. Optical modelling and simulations show that the light, travelling inside the low-index regions, is absorbed at the direct vicinity of the nano-holes sidewalls. This channelling regime should be taken into account for light management in optoelectronic devices.

physics.optics↗

arXiv2013

Down Converter Device Combining Rare-Earth Doped Thin Layer and Photonic Crystal for c-Si Based Solar Cell

The aim of the study is to develop ultra-compact structures enabling an efficient conversion of single high energy photon (UV) to two lower energy photons (IR). The proposed structure combines rare-earths doped thin layer allowing the down-conversion process with a photonic crystal (PhC), in order to control and enhance the down-conversion using optical resonances. On the top of the rare-earths doped layer, a silicon nitride (SiN) 2D planar PhC is synthesized. For that, SiN is first deposited by PECVD. After holographic lithography and reactive ion etching, a periodic square lattice of holes is generated on the SiN layer. The PhC topographical parameters as well as the layers thickness are optimized using Finite-Difference-Time-Domain simulations. The design and realization of such PhC-assisted down-converter structures is presented. Optical simulations demonstrate that the PhC leads to the establishment of resonant modes located in the underneath doped layer, allowing a drastic enhancement of the absorption of the rare-earth ions without disturbing the transmission in the visible and near-IR parts of the spectrum, hence demonstrating the relevance of such an approach.

physics.optics↗

arXiv2013

Absorption control in pseudodisordered photonic-crystal thin film

The positive effects of various perturbations introduced in a bidimensional photonic-crystal patterned membrane on its integrated absorption are investigated numerically and theoretically. Two phenomena responsible for the enhanced absorption observed are identified: an increase of the spectral density of modes, obtained thanks to folding mechanisms in the reciprocal lattice, and a better coupling of the modes with the incident light. By introducing a proper pseudodisordered pattern, we show that those two effects can be exploited so as to overcome the integrated absorption obtained for an optimized and single pattern unit cell photonic crystal.

physics.optics↗

arXiv2013

Modal approach for tailoring the absorption in a photonic crystal membrane

In this paper, we propose a method for tailoring the absorption in a photonic crystal membrane. For that purpose, we first applied Time Domain Coupled Mode Theory to such a subwavelength membrane and demonstrated that 100% resonant absorption can be reached even for a symmetric membrane, if degenerate modes are involved. Design rules were then derived from this model in order to tune the absorption. Subsequently, Finite Difference Time Domain simulations were used as a proof of concept and carried out on a low absorbing material (extinction coefficient=10-2) with a high refractive index corresponding to the optical indices of amorphous silicon at around 720 nm. In doing so, 85% resonant absorption was obtained, which is significantly higher than the commonly reported 50% maximum value. Those results were finally analyzed and confronted to theory so as to extend our method to other materials, configurations and applications.

physics.optics↗

arXiv2013

Revisiting literature observations on photodarkening in Yb3+doped fiber considering the possible presence of Tm impurities

Ytterbium (Yb) doped fiber lasers are known to be affected by the creation of color centers during lasing (socalled photodarkening (PD)). In a previous work, this defect creation was investigated from a spectroscopic point of view, showing the presence of traces (ppb) of thulium (Tm) in the Yb doped fiber. It was shown that Tm has a strong impact on the defect creation process involved in PD. In this paper, we compare the results from the literature with our Tm hypothesis, without finding any contradiction. Moreover, this hypothesis can be an explanation for the discrepancies in the literature.

physics.optics↗

arXiv2011

Inhomogeneous Gain Saturation in EDF: Experiment and Modeling

Erbium-Doped Fiber Amplifiers can present holes in spectral gain in Wavelength Division Multiplexing operation. The origin of this inhomogeneous saturation behavior is still a subject of controversy. In this paper we present both an experimental methods and a gain's model. Our experimental method allow us to measure the first homogeneous linewidth of the 1.5 $μ$m erbium emission with gain spectral hole burning consistently with the other measurement in the literature and the model explains the differences observed in literature between GSHB and other measurement methods.

physics.optics↗

arXiv2011

Spectroscopic signature of phosphate crystallization in Erbium-doped optical fibre preforms

In rare-earth-doped silica optical fibres, the homogeneous distribution of amplifying ions and part of their spectroscopic properties are usually improved by adding selected elements, such as phosphorus or aluminum, as structural modifier. In erbium ion (Er3+) doped fibres, phosphorus preferentially coordinates to Er3+ ions to form regular cages around it. However, the crystalline structures described in literature never gave particular spectroscopic signature. In this article, we report emission and excitation spectra of Er3+ in a transparent phosphorus-doped silica fibre preform. The observed line features observed at room and low temperature are attributed to ErPO4 crystallites.

physics.optics↗