Ultrabroadband high-power mid-IR nonlinear laser sources for multi-species gas sensing and photothermal imaging
The mid-infrared (mid-IR) spectral region, from 3 to 15 mm, is ideal for the detection of molecular compounds at high sensitivity and chemical selectivity. The sensitivity derives from the strength of the fundamental rovibrational transitions while the selectivity comes from the molecular fingerprints associated to the shape and fine structure of the absorption bands. Therefore, there has been a strong emphasis in recent years to develop sensors and optical instrumentation targeting mid-IR transitions for a variety of applications. Two of them, in particular, provide the main motivation to this project.
Multi-species trace gas sensing via direct frequency comb spectroscopy. It was almost two decades ago that a properly stabilized mode-locked laser was found to act as an optical frequency combs (OFC), quickly recognized as an unprecedented powerful tool for spectroscopy [1-2]. The strength of OFCs relies on a “comb-like” spectrum composed of a number of evenly spaced and phase coherent modes, extending over a spectral range of several hundred wavenumbers. The broad, discrete and evenly spaced spectrum of OFCs enables parallel probing of absorption spectra over as many spectral channels as the number of comb modes, thus substantially reducing the measurement time. Most importantly, the spectral resolution can be very high as it is limited by the width of the comb modes themselves. This approach, commonly referred to as direct comb spectroscopy, has a unique potential for detection of multiple trace gases [2]. Among the many techniques developed so far to measure comb-resolved spectra, dual-comb spectroscopy (DCS) stands out in terms of measurement speed thanks to the absence of any mechanical delay line or of low-speed detection chains. This is particularly useful to study fast transient phenomena in the gas phase, such as combustion processes. Mainly triggered by this motivation, the research activity at IFN-CNR in Lecco covers i) the development of mid-IR frequency comb synthesizers based on nonlinear processes such as difference-frequency generation, ii) the realization of dual-comb spectrometers operating at a fast rate in the fingerprint region for multispecies gas detection, iii) the design and in a future the realization of a new class of mid-IR spectrometers based on compact electro-optic synthesis of frequency combs.
Photo-thermal imaging. Photo-thermal microscopy (PTM) is a rapidly emerging technique for the imaging of biological samples with high selectivity, sensitivity and spatial resolution. Infrared PTM [3] relies on the photo-thermal effect, which occurs when a visible (VIS) light beam – the so-called probe beam – reads out the local temperature change DT induced by a second laser beam in the mid-IR – the so-called pump beam. The physical quantity sensed by the probe beam is the refractive index-change induced by DT through the thermo-optic effect. This approach elegantly combines high spatial resolution, due to the VIS probing with chemical specificity, due to the mid-IR resonant excitation of target vibrations, without any addition of exogenous labels. Another pro derives from the large heat capacity of water that strongly reduces the PT background from non-resonant water absorption and allows imaging of live cells. Lastly PTM is a quantitative technique that measures a signal linearly proportional to the analyte’s concentration. At IFN-Lecco, together with a strong consortium of a Pathfinder EIC-2022 project, we are developing a novel PTM platform based on an ultrabroadband nonlinear laser source, to image molecular biomarkers with unprecedented speed and chemical selectivity for a rapid, precise, and non-biased tumour analysis. The realization of these nonlinear sources is ongoing, exploiting a variety of solutions, based on both optical parametric amplification and oscillation, from both amplified and non-amplified Yb laser sources.
[1] T. Fortier and E. Baumann, “20 years of developments in optical frequency comb technology and applications”, Commun. Physics 2, 153 (2019).
[2] Kevin C. Cossel, E. M. Waxman, I. A. Finneran, G. A. Blake, J. Ye, and N. R. Newbury, “Gas-phase broadband spectroscopy using active sources: progress, status, and applications [Invited],” J. Opt. Soc. Am. B 34, 104-129 (2017)
[3] Yeran Bai, Jiaze Yin and Ji-Xin Cheng, “Bond-selective imaging by optically sensing the mid-IR photothermal effect”, Sci. Adv. 7 , eabg1559 (2021)
