Limits in Enhancement Factor in Near-Brewster Angle Reflection Pump-Probe Two-Dimensional Infrared Spectroscopy

In this work, we simulated 2D infrared spectroscopy (IR) spectroscopy in both transmission geometry and Brewster-angle reflection geometry. Light dispersion and the leakage of s-polarized light are considered in simulating the enhancement factor of the reflection mode. Our simulation shows that the dispersion in reflection will only alter the 2D IR lineshape slightly and can be corrected. Leaking s-polarized light due to imperfectness of IR polarizers in the reflection geometry may limit the enhancement factor, but such limit is above what a typical experiment can reach. In the current experiment, the enhancement factor is mainly limited by the precision of incident angle, for which ordinary rotation stages are probably not adequate enough. Moreover, traditional energy ratio of pump and probe pulses, which is 9:1, may not be ideal and could be changed to 2:1 in the reflection geometry. Considering all the above factors, the enhancement on the order of 1000 is possible in the current experiment. Nevertheless, near-Brewster angle reflection will enhance both the signal and the noise caused by the signal itself, therefore this method only works if the noise is unrelated to the signal, particularly if the noise is caused by the fluctuation in the probe. It cannot improve the signal to noise ratio when the dominate noise is from the signal itself. The theoretical results here agree reasonably well with published experiment results and pave way for realizing even higher enhancement at nearer-Brewster angle.