Yi Chi Lee

Polarization and pressure effects in caesium 6S-8S two-photon spectroscopy

This work analyses the effects of polarization and pressure in caesium 6S–8S two-photon spectroscopy. The linewidth was broadened and the frequency was shifted by a change of polarization states. The frequency shift and the linewidth broadening of the caesium 6S–8S two-photon transition were measured as a function of laser power using one single-frequency Ti:sapphire ring cavity laser, two caesium cells and two quarter-wave plates to ensure polarization states of light, and we showed that the linewidth cannot be evaluated just by fitting data to a Lorentzian shape. As determined by fitting the data to a Voigt profile, the natural linewidth is independent of the polarization states of the pump beams, the laser power and the pressure. Caesium 6S–8S two-photon transitions pumped by a circularly polarized beam have narrower linewidths and smaller shifts than those pumped by a linearly polarized beam. The light shift obtained by pumping with the circularly polarized beam is −6.75(57) Hz (mW mm−2)−1, and that obtained by pumping with a linearly polarized beam is −7.25(45) Hz (mW mm−2)−1. These results agree closely with theoretical calculations. The pressure shift is −588(387) Hz mPa−1. This work shows how to evaluate two-photon transitions with a Voigt profile, and then helps us to understand two-photon transitions with different polarization states, and improve the signal quality obtained when they are used as frequency markers.

A narrow window of Rabi frequency for competition between electromagnetically induced transparency and Raman absorption

This investigation clarifies the transition phenomenon between the electromagnetically induced transparency (EIT) and Raman absorption in a ladder-type system of Doppler-broadened cesium vapor. A competition window of this transition was found to be as narrow as 2 MHz defined by the probe Rabi frequency. For a weak probe, the spectrum of EIT associated with quantum interference suggests that the effect of the Doppler velocity on the spectrum is negligible. When the Rabi frequency of the probe becomes comparable with the effective decay rate, an electromagnetically induced absorption (EIA) dip emerges at the center of the power broadened EIT peak. While the Rabi frequency of the probe exceeds the effective decay rate, decoherence that is generated by the intensified probe field occurs and Raman absorption dominates the interaction process, yielding a pure absorption spectrum; the Doppler velocity plays an important role in the interaction. A theory that is based on density matrix simulation, with or without the Doppler effect, can qualitatively fit the experimental data. In this work, the coherence of atom-photon interactions is created or destroyed using the probe Rabi frequency as a decoherence source.

Inhibition and enhancement of cesium two-photon transition under control field

The probability of two-photon transition (TPT) under a control field to inhibit the quantum interference and enhance the nonlinear optical cross section is observed. Essentially, this is a V-type electromagnetically induced transparency (EIT) with TPT instead of one photon transition. Numerical simulation based on solving the steady state density matrix can qualitatively fit the experimental data. A model of double-Lorentzian profile is used to fit the observed spectrum and give the de-convolution information of the inhibition of TPT spectrum due to EIT and enhancement on the wings of TPT. The frequency shift of the inhibit center is linear to the intensity of the control field (one-photon) and quadratic to the intensity of probe field (two-photon). Under the control field, a factor of 10 enhancements on the wings of the TPT is observed.

Optical switching using controlled two-photon transition

New scheme of a real-time all-optical switching (AOS) was demonstrated as the fluorescence signal of cesium two-photon transition (TPT) with the control field. This work explored the optical phenomenon of the TPT with the control field that shares a common excited state, in which the TPT fluorescence is quenched under the control field intensity. The strong stimulated emission due to the control laser suppresses the TPT fluorescence, working as an optical switch. A “NOT gate” is generated for realization of AOS. The power spectrum of the intensity range, which can be used for the AOS, is determined.

All-optical switching using cesium two-photon transition

New scheme of a real-time all-optical switching (AOS) was demonstrated as the fluorescence signal of cesium two-photon transition (TPT) with the control field. This work explored the optical phenomenon of the TPT with the control field that shares a common excited state, in which the TPT fluorescence quenched under the control field intensity. A “NOT gate” is generated for realization of AOS. The power spectrum of the intensity range which can be used for the AOS is determined.

Determining hyperfine transitions with electromagnetically induced transparency and optical pumping

A system is designed to observe the phenomena of electromagnetically induced transparency and optical pumping in cesium D1 and D2 lines at room temperature. When a pump laser is frequency-locked on the top of a hyperfine transition and the frequency of the probe laser scans over another hyperfine transition, a spectrum of V-type electromagnetically induced transparency or an optical pumping can be observed depending on whether the two lasers share a common ground state. Therefore, these results can be used to identify the unknown hyperfine transitions of the D1 line transitions. For educational purposes, this system is helpful for understanding the electromagnetically induced transparency and the optical pumping.

A narrow Rabi frequency window for competition between coherent population trapping and Raman absorption

We demonstrate the competition mechanism between the coherent population trapping and two-photon Raman absorption in a ladder-type system of cesium cell. When the Rabi frequency of the probe becomes comparable with the effective decay rate, an electromagnetically induced absorption (EIA) dip emerges at the center of the power broadened EIT peak. While the Rabi frequency of the probe exceeds the effective decay rate, a decoherence created by the strong probe field takes place and two-photon Raman absorption dominates the interaction process.

Suppression of two-photon transition by quantum interference effect in atomic system

We have observed the suppression which is caused by electromagnetic induced transparency in two-photon transition of cesium ensembles. In this investigation, an experimental result of this phenomenon is reported.

Optical properties of cesium 6S-8S two-photon transitions

The light shift and linewidth of cesium 6S-8S two-photon transitions are measured in the system. The light shift is −8.60 Hz/(mW/mm2) which is in good agreement with theoretical result. The linewidth is about 1.51 MHz.

Doppler-free two-photon transitions of 6S 1/2 -7D 3/2 , 5/2 in cesium

The Doppler-free two-photon transitions of cesium 6S<inf>1/2</inf>-7D<inf>3/2</inf>,<inf>5/2</inf> are observed using a Ti: Sapphire laser in the temperature-controlled cell. These spectra can be used as a secondary frequency standard.