Nobuhito Hayashi

Interference between electromagnetically induced transparency and two-step excitation in three-level ladder systems

We have performed electromagnetically induced transparency (EIT) and two-photon absorption experiments in ladder-type three-level systems in a hot sodium atomic vapor, using the 3S1/2-3P1/2-4D3/2, 3S1/2-3P3/2-4D3/2,5/2, 3S1/2-3P1/2-5S1/2, and 3S1/2-3P3/2-5S1/2 transitions. In particular, in the most pronounced 3S1/2-3P1/2-4D3/2 system, we have observed quite unique spectral line shapes that are superpositions of sharp dips and peaks, in contrast to ordinary EIT spectra. The peaks and dips have apparently different physical origins, and the line shape can be interpreted as the interference between EIT and two-step excitation.

Parametric oscillation in sodium vapor by using an external cavity

We report on parametric oscillation in sodium atomic vapor by using an external cavity. Output powers of approximately 56 mW are obtained for both the signal and idler waves with the input coupling power of 290 mW, resulting in a conversion efficiency of 39%. The signal and idler frequencies are either upshifted or downshifted from the coupling frequency by the amount of the Na hyperfine splitting frequency, and the two waves are found to be strongly temporally correlated.

Ultraslow matched-pulse propagation in sodium vapor

We have observed ultraslow propagation of matched pulses in a parametric amplifying hot sodium vapor. A fractional delay of more than 1 and a gain of 28 have been simultaneously achieved for a probe pulse of width 100 ns, accompanying a conjugate Stokes pulse with similar gain and delay under a strong coupling beam. The results are compared with the preceding work by Boyer et al. [Phys. Rev. Lett. 99, 143601 (2007)].

Interference between electromagnetically induced transparency and N-type resonances in Na vapor

We show theoretically and experimentally that the line shapes observed in ordinary electromagnetically induced transparency (EIT) experiments in sodium atomic vapor can be explained by the superposition of two processes, EIT and N-type resonances (NTR). When the higher frequency side of the Doppler absorption profile is excited, EIT is dominant, whereas, for the lower frequency side, NTR is dominant, and, in between, interference between EIT and NTR takes place. Theoretical simulations, based on a perturbative approach, agreed reasonably well with the experimental observations. The signal behavior concerning ac Stark shifts has also been studied for EIT and NTR; it was found that, although the signal peak always shifts toward the higher frequency in the case of EIT, for NTR, the shift can be nullified by adjusting the power balance of the two incident beams.

Electromagnetically induced transparency in a V-type multilevel system of Na vapor

We have theoretically and experimentally investigated the V-type three-level system in hot sodium atomic vapor interacting with two optical waves: probe, resonant to the D1 line, and coupling, the D2 line. Theoretically, probe transmission spectra are simulated by using a perturbative treatment including eight hyperfine levels (multilevel system) in the 3S1/2, 3P1/2, and 3P3/2 states. The spectra are composed of linear absorption, saturated absorption, and electromagnetically induced transparency (EIT) signals. By comparing this simulation with the experimental results, we concluded that we have indeed observed EIT signals when the coupling beam is resonant to the D2-line closed transition.

Four-wave parametric oscillation: theory and observations

We present theoretically and experimentally a novel type of parametric fluorescence and oscillation in which four optical waves in off-axis, diagonal directions simultaneously build up under resonant counterpropagating pump-laser excitation in atomic sodium vapor. Two of the four are anti-Stokes shifted and the other two are Stokes shifted, and they are strongly coupled owing to electromagnetically induced diffraction. Theoretically, the Liouville-Maxwell equations are solved to derive four-wave propagation equations. Experimentally, we have observed four-wave parametric oscillation when a weak off-axis seed beam is introduced into otherwise axially symmetric parametric fluorescence patterns. Temporal waveforms are also analyzed, and strong correlations between any pairs of the four emissions are identified.

Pulsed ac Stark shifts monitored by Raman Ramsey fringes in sodium vapor

We have employed Raman Ramsey fringes (RRFs) in sodium vapor to measure ac Stark shifts of the hyperfine levels by an off-resonant shift laser pulse. The ac Stark shifts are directly monitored by the RRF peak shifts and are proportional to the power and the pulse width and antiproportional to the detuning frequency of the shift laser. These findings endorse RRFs to be a strong tool for monitoring small pulsed perturbations.

Arbitrary delay-time control of ultraslow matched pulses

We have theoretically and experimentally analyzed the ultraslow matched-pulse propagation in a parametric amplifying medium, especially focusing on the delay-time behaviors of the input probe pulse (signal) and the generated Stokes pulse (idler) under a strong coupling beam. Theoretically, it is predicted that the Stokes pulse always propagates with a definite group velocity with some initial delay time. The probe pulse, on the other hand, propagates at a fixed delay from Stokes after some transient period. This additional delay time is positive (probe is behind Stokes) if the medium is opaque for probe and transparent for Stokes, or it can be negative (probe is ahead of Stokes), if otherwise. It is also shown that the delay times of probe and Stokes, including the order of the pulses, can be arbitrarily and independently controlled by external parameters. The theoretical predictions and numerical simulations have been compared with experimental observations using a hot sodium vapor and they had a very nice agreement.

Slow light in a parametrically amplifying medium

We theoretically study propagation behavior of probe (signal) and Stokes (idler) pulses under a strong coupling beam. The analyses predict that both pulses are parametrically amplified and delayed by about a pulsewidth.

Four-wave parametric oscillation in Na vapor

A novel type of parametric oscillation has been observed in Na atomic vapor, where four Stokes/anti-Stokes-shifted waves, mutually coupled by electromagnetically induced diffraction, simultaneously burst into oscillation with high conversion efficiency under excitation by resonant counter-propagating pump beams with elliptic beam profiles.