A General Approach To Combine the Advantages of Collinear and Noncollinear Spectrometer Designs in Phase-Resolved Second-Order Nonlinear Spectroscopy.

Tobias Garling,Martin Thämer,R Kramer Campen,Martin Wolf

doi:10.1021/acs.jpca.9b09927

Abstract

Recent years have seen a huge progress in the development of phase-sensitive second-order laser spectroscopy which has proven to be a very powerful tool for the investigation of interfaces. In these techniques, the nonlinear interaction between two short laser pulses and the sample yields a signal pulse which subsequently interferes with a third pulse, the so-called local oscillator. To obtain accurate phase information, the relative phases between the signal and local oscillator pulses must be stabilized and their timings precisely controlled. Despite much progress made, fulfilling both requirements remains a formidable experimental challenge. The two common approaches employ different beam geometries which each yields its particular advantages and deficiencies. While noncollinear spectrometers allow for a relatively simple timing control they typically yield poor phase stability and require a challenging alignment. Collinear approaches in contrast come with a simplified alignment and improved phase stability but typically suffer from a highly limited timing control. In this contribution we present a general experimental solution which allows for combining the advantages of both approaches while being compatible with most of the common spectrometer types. On the basis of a collinear geometry, we exploit different selected polarization states of the light pulses in well-defined places in the spectrometer to achieve a precise timing control. The combination of this technique with a balanced detection scheme allows for the acquisition of highly accurate phase-resolved nonlinear spectra without any loss in experimental flexibility. In fact, we show that the implementation of this technique allows us to employ advanced pulse timing schemes inside the spectrometer, which can be used to suppress nonlinear background signals and extend the capabilities of our spectrometer to measure phase-resolved sum frequency spectra of interfaces in a liquid cell.

Highlights

Second-order nonlinear spectroscopies like sum frequency, difference frequency, or second harmonic generation spectroscopy (SFG, DFG, or SHG, respectively) have recently become an indispensable tool for the characterization of interfaces
After the introduction of the concept for adjusting the delay between the local oscillator (LO) and the nonlinear sample response in collinear spectrometers, we demonstrate its experimental application
In this time domain approach the sum frequency signal and the LO pulse are generated by nonlinear mixing of two ultrashort pump pulses one in the infrared spectral range and the other centered at 800 nm

Summary

■ INTRODUCTION

Second-order nonlinear spectroscopies like sum frequency, difference frequency, or second harmonic generation spectroscopy (SFG, DFG, or SHG, respectively) have recently become an indispensable tool for the characterization of interfaces. The technical solution to include all the advantages of a noncollinear beam geometry in a collinear spectrometer design contains three parts: (1) the control of the relative time delay between the LO and the signal using a birefringent crystal, (2) the balanced detection scheme to extract the interference term, and (3) LO generation with polarization control in a z-cut α-quartz crystal. There have recently been quite similar approaches published where pulse delay control is achieved by exploiting the properties of a birefringent crystal inside the spectrometer (mainly for SHG spectrometers).[29] the way this timing control is implemented requires a certain polarization combination between pump and signal pulses which limits the applicability of the technique to particular experimental settings. It can be implemented in already existing homodyned second-order spectrometers quite to upgrade them into more powerful, collinear, and phase-sensitive heterodyned versions

■ RESULTS AND DISCUSSION

■ SUMMARY

■ REFERENCES