Interface electronic excitation spectroscopy by time-domain sum-frequency-generation ptychography

Low signal strength is a major limitation in the development of electronic sum-frequency generation (ESFG) spectroscopy. To achieve sufficiently high frequency resolution, it is usually necessary to spectrally narrow one of the excitation beams, which greatly attenuates the strength of ESFG signal. Here, we use high-power chirped short-wave infrared pulses as the up-conversion light to generate the SFG signal, and employ time-domain ptychography to measure the free induction decay induced by the resonant excitation pulses. This approach eliminates the need for spectral narrowing of any excitation light beam, substantially improving the intensity of the ESFG signal while achieving high enough frequency resolution. A comparison with narrowband ESFG experiments conducted on the same sample under identical conditions reveals that the time-domain ptychography method significantly enhances the signal-to-noise ratio of the interfacial excitation spectrum. Theoretical simulations indicate that this approach not only retrieves the intensity spectrum but also provides access to the relative phase of the signal. Once the absolute phase is determined, accurate real and imaginary components of the spectrum can be obtained.