Time-resolved non-linear optical diagnostics for multiscale probing of flow dynamics

APL Photonics, vol. 11

Abstract

This letter presents a novel nonlinear optical diagnostic technique to simultaneously investigate flow phenomena over molecular and fluid timescales, using a single femtosecond laser source. We combine hybrid femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (fs/ps CARS) and femtosecond laser-induced grating spectroscopy (fs LIGS), adopting a new planar phase-matching geometry, whereby 1D imaging spectroscopy is realized over a shared probe volume. This dual diagnostic approach uniquely bridges physical scales: fs/ps CARS provides molecular-level snapshots, while fs LIGS probes macroscopic phenomena, such as wave propagation in compressible gases. We demonstrate this technique in an underexpanded supersonic air jet, resolving the temperature and speed of sound fields across the Mach disk. Hybrid fs/ps CARS probes the rotational energy distribution of the Raman-active molecules, providing the local temperature and composition fields with a 20 μm resolution. Through multi-photon ionization, we generate a volumetric plasma Bragg grating, which enables thermal fs LIGS imaging in virtually any gas mixture, from which we extract the acoustic response of the fluid medium and, thus, determine the local speed of sound. Overall, this work establishes a general framework for high-precision, time- and space-resolved characterization of complex flows where microscopic inter-molecular interactions determine the macroscopic fluid dynamics, with significant implications for high-enthalpy aerodynamics and supercritical fluid technologies.

Authors 3

1. Leonardo Castellanos

   Delft University of Technology

   Affiliation as printed

   Faculty of Aerospace Engineering, Delft University of Technology 1 , Kluyverweg 1, Delft 2629 HS,

2. Francesco Mazza Aachen Chair of Optical Diagnostics in Energy

   RWTH Aachen University · Delft University of Technology

   Affiliation as printed

   Chair of Optical Diagnostics in Energy, Process and Chemical Engineering, RWTH Aachen University 2 , Schinkelstraße 8, Aachen 52062,

   Faculty of Aerospace Engineering, Delft University of Technology 1 , Kluyverweg 1, Delft 2629 HS,

3. Francesca De Domenico

   Delft University of Technology

   Affiliation as printed

   Faculty of Aerospace Engineering, Delft University of Technology 1 , Kluyverweg 1, Delft 2629 HS,