A General Approach to Combine the Advantages of Collinear and Non-Collinear Spectrometer Designs in Phase-Resolved Second Order Nonlinear Spectroscopy
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. These interferometric techniques involve the nonlinear interaction between three short laser pulses with the sample. In order to obtain accurate phase information, the relative phases between the 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 non-collinear 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. Based on 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 al- lows 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 sup- press nonlinear background signals and extend the capabilities of our spectrometer to measure phase resolved sum frequency spectra of inter- faces in a liquid cell.