Description
Strong optical transitions, such as the alkali-metal D lines, provide large oscillator strengths and MHz-level radiative linewidths, making them indispensable for atom manipulation and detection. The electric dipole transition is also crucial for probing unknown interactions and wideband quantum sensing. To probe the transition with conventional frequency-scan spectroscopy is data consuming: One only takes the data after an optical-pumping related relaxation time to ensure steady-state atomic response. This limitation severely limits our ability to probe the transition when the interrogation time is limited, such as due to atomic motion at a nanophotonic interface.
We develop frequency-jump spectroscopy (FJS), a coherent time-domain method in which a well-characterized, rapidly frequency-modulated optical waveform is sent to atoms and the transient transmission is recorded. The abrupt frequency jump makes the atomic forward-scattered field interfere with the incident probe in time, thereby encoding both absorptive and dispersive spectral information into a single transient waveform. This scan-free protocol extracts wideband spectral information with substantially improved data efficiency compared with point-by-point frequency scans. Furthermore, we demonstrate complex spectroscopic with simultaneous retrieval of absorption and phase shift cross-sections.
We demonstrate the technique at an optical nanofiber interface, where atom--surface coupling and atomic motion restrict the available probing window.
The figure below shows the experimental setup diagram of the FJS method and a typical FJS pulse fitting plot. Compared with Steady-state frequency-scan absorption spectroscopy, our method achieve better fitting uncertainty using orders of magnetude lower numbers of interrogating photons.
