Mathias Bonmarin

A picosecond time-resolved vibrational circular dichroism spectrometer

We present an experimental setup to detect transient vibrational circular dichroism signals. A femtosecond laser system is synchronized to a photoelastic modulator to produce alternating left- and right-handed circularly polarized mid-IR pulses at 1 kHz repetition rate. Transient changes in the circular dichroism of the CH-stretch vibrations of a cobalt-sparteine complex were probed in a proof-of-principle experiment and are clearly distinct from conventional transient absorption changes.

Vibrational circular dichroism signal enhancement using self-heterodyning with elliptically polarized laser pulses

Vibrational circular dichroism (VCD) spectra were recorded using elliptically polarized ultrashort laser pulses, produced with the help of a photoelastic modulator. The short polarization axis of the elliptical light acts as a phase-locked local oscillator field, heterodyning the chiral signal generated by the field along the long polarization axis. This leads to VCD signals that increase linearly with the ellipticity of the probe pulses and enhanced signal to noise, which is expected to improve recently reported transient VCD scans. An analogous scheme allows for vibrational optical rotary dispersion measurements. The techniques are compared with similar approaches using both a linear response picture and the Jones matrix calculus.

Lock-in thermal imaging for the early-stage detection of cutaneous melanoma

This paper theoretically evaluates lock-in thermal imaging for the early-stage detection of cutaneous melanoma. Lock-in thermal imaging is based on the periodic thermal excitation of the specimen under test. Resulting surface temperature oscillations are recorded with an infrared camera and allow the detection of variations of the samples thermophysical properties under the surface. In this paper, the steady-state and transient skin surface temperatures are numerically derived for a different stage of development of the melanoma lesion using a two-dimensional axisymmetric multilayer heat-transfer model. The transient skin surface temperature signals are demodulated according to the digital lock-in principle to compute both a phase and an amplitude image of the lesions. The phase image can be advantageously used to accurately detect cutaneous melanoma at an early stage of development while the maximal phase shift can give precious information about the lesion invasion depth. The ability of lock-in thermal imaging to suppress disturbing subcutaneous thermal signals is demonstrated. The method is compared with the previously proposed pulse-based approaches, and the influence of the modulation frequency is further discussed.

Polarization control of ultrashort mid-IR laser pulses for transient vibrational circular dichroism measurements.

Linear dichroism and birefringence artifacts are a major source of concern in transient circular dichroism measurements. They mainly arise from interaction of an imperfectly circular polarized probe beam with a nonisotropic sample. We present in this article a procedure to generate mid-IR pulses of highly symmetric left and right handed circular or elliptical polarization states for transient VCD measurements. An infrared femtosecond laser source is synchronized to the natural frequency of a photo elastic modulator. Residual static birefringence of the modulator and the sample cell can be largely compensated by carefully controlling the arrival time of the ultrashort probe pulses at the modulator.

Electrothermal Finite-Element Modeling for Defect Characterization in Thin-Film Silicon Solar Modules

We present and validate a finite-element model for coupled charge and heat transport in monolithically interconnected thin-film solar modules. Using measured current-voltage ( I-V) and lock-in thermography (LIT) measurements of amorphous silicon minimodules, we experimentally validate our model. The entire module volume is represented by two planes (front and back electrodes) which are coupled in vertical direction using 1-D models, leading to a large reduction of the degrees of freedom in the numerical model and contributing to an efficient solution approach. As compared to 3-D models, the vertical coupling of the charge transport is represented by local temperature-dependent I-V curves. These can be obtained by drift-diffusion calculations, single-cell measurements or, as presented here, by an analytical solar cell diode model. Inhomogeneous heat sources such as Joules heating in the electrodes lead to nonuniform temperature distributions. The explicit temperature dependence in the local I-V curve, therefore, mediates the feedback of the thermal transport on the local electrical cell characteristics. We employ measured I-V curves under partial illumination and analytical solutions for the potential distribution to validate this approach. Further, with LIT measurements of the same modules with and without artificially induced electrical shunts, we verify the computed temperature distributions.

A lock-in thermal imaging setup for dermatological applications.

Lock‐in thermal imaging is a thermographic method that is widely used in the nondestructive testing of materials. The technique allows detecting under the sample surface, small variations of the thermophysical properties in a noninvasive and noncontact manner. Surprisingly, this method has, to our knowledge, never been used in dermatology although it is particularly suited.

OCT imaging of the larynx: a feasibility study

Laryngeal cancer is the most common primary head and neck malignancy and the need for early identification is very important for early treatment. Outpatient fiberoptic examination of the larynx is often unreliable in differentiating between benign, pre-malignant and malignant lesions, and therefore surgeons often have to rely on biopsies for a definite diagnosis. This is an invasive procedure requiring general anaesthesia and may have a detrimental effect on patient’s voice. The aim of our study was to investigate the feasibility of optical coherence tomography in imaging of the larynx, which will lay the foundations for investigating its ability to differentiate between benign and malignant disease. Tissue specimens from normal larynges were imaged with a polarisation sensitive OCT system at 850 nm and a second OCT system at 1300 nm. Both OCT systems were capable of providing both B-scan (longitudinal OCT) images as well as C-scan (en-face OCT or at constant depth) images. Imaged specimens were processed with standard histopathological techniques and sectioned in the plane of the B-scan OCT images. Haematoxylin-Eosin stained specimens were compared to the OCT images. Preliminary results showed good correlation between OCT images and histology sections in normal tissues.

A lock-in-based method to examine the thermal signatures of magnetic nanoparticles in the liquid, solid and aggregated states

We propose a new methodology based on lock-in thermography to study and quantify the heating power of magnetic nanoparticles. Superparamagnetic iron oxide nanoparticles exposed to a modulated alternating magnetic field were used as model materials to demonstrate the potency of the system. Both quantitative and qualitative information on their respective heating power was extracted at high thermal resolutions under increasingly complex conditions, including nanoparticles in the liquid, solid and aggregated states. Compared to conventional techniques, this approach offers a fast, sensitive and non-intrusive alternative to investigate multiple and dilute specimens simultaneously, which is essential for optimizing and accelerating screening procedures and comparative studies.

Thermal Imaging in Dermatology

Abstract Thermal imaging, also commonly referred as thermography, is a visualizing method exhibiting a great potential for dermatological applications. This is because many cutaneous pathologies affect the thermal properties of the different skin layers, as well as the heat transfer processes taking place between them. To ensure the greatest benefit, potential users should be familiar with the basic notions concerning thermal imaging and be aware of the limitations of the technique. This chapter reviews the key concepts of thermal imaging, starting with the skin thermal signals and the different imaging devices used to capture the thermal radiation, to the latest active and passive measurement procedures and demodulation algorithms that can be employed. To illustrate the potential of thermal imaging, pioneer work as well as recent studies devoted to skin cancer diagnostic and burns depth evaluation are presented. Other promising applications are briefly highlighted in the last paragraph.

Time-Resolved Vibrational Circular Dichroism and Optical Rotation with Utrashort Laser Pulses

We present recent progres in enhancing chiral vibrational signals using ellipsometric methods and broad band detection for the improvement of our first transient vibrational circular dichroism measurements.