Topic

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Hypersonics OBJECTIVE: Develop and demonstrate a fully tunable, MHz repetition-rate coherent laser source in the Vacuum-Ultraviolet (VUV) spectral region for spectroscopic diagnostics of hypersonic flows. DESCRIPTION: Several DoD-relevant research applications have a critical need for light in the Vacuum-Ultraviolet (VUV) spectral region, specifically at wavelengths spanning 50-200 nm and in the near-UV region up to 215 nm. This spectral region includes absorption transitions for atomic species (N, O) and molecular species (NO, N₂, O₂) created in hypersonic shock layers, as well as ionization energies. One of the key technical challenges is the detection of low-lying electronic states of atoms, whose spectral lines are in the VUV region [1-3]. These atoms are believed to play a crucial role in plasma generation and dynamics. However, the strong light-matter interaction that makes VUV light useful for spectroscopy also makes it challenging to use. Conventional laser and nonlinear-optical systems are limited to Ultraviolet, Visible, and Infrared wavelengths (λ ≥ 200 nm). Current VUV light sources are either weak (e.g., discharge lamps), extremely limited in spectral coverage and repetition rate, or large-scale synchrotron light sources with limited and insecure access. Recently, new techniques for the upconversion of ultrashort-pulse lasers into the VUV have been demonstrated, addressing many of these limitations. Using a cascaded (stepwise) upconversion technique in a gas-filled structured optical fiber, pulsed coherent VUV light down to λ < 100 nm has been generated using MHz repetition-rate lasers [4]. Previous upconversion techniques, such as high-order harmonic generation (HHG), required much higher peak power achievable only at lower repetition rates. The high repetition rate is essential for applications such as sampling shock-tube or detonation dynamics. To date, HHG has been demonstrated using fixed-wavelength laser drivers that allow only stepwise tuning of the source. This source has been successfully used for photoionization mass spectroscopy (PIMS) applications [5, 6] to discern molecular products for aviation fuels combustion diagnostics, biomarker identification for medical diagnosis, atmospheric chemistry experiments, and excited state molecular spectroscopy [7]. The development of a continuously tunable VUV source would be a fundamental enabler for VUV absorption spectroscopy, allowing tuning to specific absorption resonances to understand shock-layer dynamics in hypersonics. For PIMS, tunability would facilitate the determination of precise ionization energies, enabling unambiguous identification of molecular species. The HHG process is unique in not relying on any resonant interaction in the upconversion process; it can be made fully tunable by tuning the laser driving the process. This project aims to implement full tunability in a MHz repetition-rate coherent VUV light source. The target requirements are: • Continuous tunability ideally over the entire range from <100 nm to ~215 nm. • Photon flux of at least 10¹³ photons/s at 120 nm, corresponding to pulse energy >16 pJ per pulse at 1 MHz. • Continuous repetition rates spanning 100 kHz to 1 MHz for sampling shock-tube or detonation dynamics. • Sub-picosecond pulse duration, but no shorter than 100 femtoseconds. • Maximum spectral linewidth full-width half-maximum (FWHM): ≤ 40 meV (0.5 nm). PHASE I: Establish a feasible pathway for implementing a tunable VUV photon source. Outcomes of Phase I shall include a conceptual design, supported by experiments and/or calculations, demonstrating the suitability of the source for hypersonic and combustion studies, in accordance with the outlined requirements. PHASE II: Phase II shall seek a full implementation and characterization of this light source, along with a demonstration of capabilities in DoD-relevant experiments. Relevant demonstrations shall include probing of microsecond-scale shock-tube chemistry relevant to hypersonics, using VUV absorption spectroscopy. PHASE III DUAL USE APPLICATIONS: Phase III shall seek further improvements to light source performance and reliability, and expand experimental demonstrations for Navy-relevant conditions and geometries. These demonstrations shall be conducted via ground tests in facilities such as shock tubes, shock tunnels, expansion tunnels, and ballistic ranges, once a sufficient Technology Readiness Level (TRL) is achieved. : This new class of light source has significant potential in biomedical and materials R&D, as well as in semiconductor manufacturing. The high repetition rate and focusability make this source potentially transformative for nanometer-resolution mass spectrometry imaging. In materials research, such a source is ideal for ultrafast angle-resolved photoemission spectroscopy (ARPES). Finally, there is a strong need in microelectronics for powerful short-wavelength light sources to identify nanoscale defects using scatterometry. REFERENCES: 1. T. T. Aiken and I. D. Boyd, "Sensitivity Analysis of Ionization in Two-Temperature Models of Hypersonic Air Flows," Journal of Thermophysics and Heat Transfer, pp. 1-13, 2024. 2. Y. Wu, M. Hochlaf, and G. C. Schatz, "Modeling of collision-induced excitation and quenching of atomic nitrogen," The Journal of Chemical Physics, vol. 161, no. 1, 2024. 3. I. Adamovich and J. Rich, "Semiclassical analytic theory of electronic energy transfer in 3D atomic collisions," The Journal of Chemical Physics, vol. 160, no. 19, 2024. 4. D. E. Couch et al., "Ultrafast 1 MHz vacuum-ultraviolet source via highly cascaded harmonic generation in negative-curvature hollow-core fibers," Optica, vol. 7, no. 7, pp. 832-837, 2020. 5. D. E. Couch et al., "Detection of the keto-enol tautomerization in acetaldehyde, acetone, cyclohexanone, and methyl vinyl ketone with a novel VUV light source," Proceedings of the Combustion Institute, vol. 38, no. 1, pp. 1737-1744, 2021. 6. C. O. Rogers, K. S. Lockwood, Q. L. Nguyen, and N. J. Labbe, "Diol isomer revealed as a source of methyl ketene from propionic acid unimolecular decomposition," International Journal of Chemical Kinetics, vol. 53, no. 12, pp. 1272-1284, 2021. 7. G. T. Buckingham et al., "The thermal decomposition of the benzyl radical in a heated micro-reactor. I. Experimental findings," The Journal of chemical physics, vol. 142, no. 4, 2015. KEYWORDS: Hypersonics, Shock Layers, Electronic Excitation, Ionization, Vacuum-Ultraviolet (VUV), Tunable Laser Source, High Repetition Rate, Absorption Spectroscopy, High-Order Harmonic Generation (HHG).