抄録
Introduction
Vibrational sum-frequency generation (V-SFG) spectroscopy is a powerful technique for investigating molecules at interfaces and surfaces [1]. Multiplex V-SFG measurements have been performed by using a broadband infrared (IR) pulse and a narrowband visible pulse to generate broadband SF lights from the molecules [2]. The IR pulse is usually prepared by difference frequency generation (DFG) with nonlinear crystals such as AgGaS2 and GaSe [3]. However, the IR pulse's bandwidth limits the V-SFG spectral range at most 400 cm-1. In addition, the phonon absorption of these crystals below 500 cm-1 prohibits low-frequency IR light generation, making it challenging to perform SFG measurements at the low-frequency region. Therefore, most current SFG studies focus on intramolecular vibrations at the high-frequency region (> 2000 cm-1). It would be helpful to expand the detection bandwidth of V-SFG spectra and achieve low-frequency V-SFG measurements, including intermolecular vibrations and frustrated translations/rotations. In this study, we have developed ultra-broadband V-SFG spectroscopy with a broadband IR pulse generated by two-color laser-induced air plasma. We demonstrated ultra-broadband V-SFG spectroscopy of polymethylmethacrylate (PMMA) film on a GaAs substrate between 550-3000 cm-1.
Method
Our newly developed V-SFG measurement system uses a broadband IR pulse generated by two-color laser-induced air plasma [4]. The Ti:sapphire laser's output was split into two, and about 90% of the laser output passed through a β-BaB2O4 crystal to generate a second harmonic (SH) light. The SH light (400 nm) and fundamental light (800 nm) with a parallel polarization direction were focused onto the air in an N2-purged box to induce plasma. Another part of the laser output was used to prepare a narrowband visible pulse by a band-pass filter with a half-width half maximum of 1.2 nm. The two pulses were focused onto the sample surface to induce the SFG polarization at the surface. The emitted SFG lights, after being filtered by a short-pass filter, were detected by a polychromator.
To demonstrate ultra-broadband V-SFG spectroscopy, our developed system measured bare GaAs(100) and PMMA film on GaAs(100). The PMMA film 10-µm thick was made by drop casting of 2wt% PMMA in toluene solution on a GaAs(100) substrate. During the V-SFG measurements, the time delay between the two incident pulses was varied, with the visible pulse following the IR pulse.
Results and Discussion
The SFG spectrum of the bare GaAs substrate showed a broadband spectral shape, which reflects the spectrum of the IR pulse generated by air plasma [5]. This result indicates that our SFG system's detection range is roughly 550-4000 cm-1. The V-SFG spectrum of the PMMA film deposited on GaAs(100) at the 0 ps delay time showed a broad spectrum, which is like that of the bare GaAs substrate; this broadband spectrum of the PMMA/GaAs mainly originates from the non-resonant SFG signal from the GaAs substrate. In contrast, the SFG spectrum of the PMMA on the GaAs substrate at > 0.8 ps delay time showed sharp peaks. By comparing these observed peaks with those of Raman and IR absorption spectra of a PMMA film, we conclude that we have successfully detected ultra-broadband V-SFG spectra of the PMMA film between 550-3000 cm-1.
References
[1] Y.-R. Shen, Second Harmonic and Sum-Frequency Spectroscopy, WORLD SCIENTIFIC, 2022.
[2] T.A. Ishibashi and H. Onishi, Appl. Phys. Lett. 81, 1338 (2002).
[3] K. Madeikis, et al., Opt. Express 29, 25344 (2021).
[4] J. Dai, N. Karpowicz, and X.C. Zhang, Phys. Rev. Lett. 103, 023001 (2009).
[5] E. Matsubara, M. Nagai, and M. Ashida, J. Opt. Soc. Am. B 30, 1627 (2013).
