Utsab Khadka

All-optical switching in an N-type four-level atom-cavity system

We report an experimental observation of all-optical switching in an N-type atom-cavity system with a rubidium atomic vapor cell inside an optical ring cavity. Both absorptive and dispersive switching can be realized on dark- or bright-polariton peaks by a weak switching laser beam (with the extinction ratio better than 20:1). The switching mechanism can be explained as the combination of quantum interference and intracavity dispersion properties.

Modulating the multi-wave mixing processes via the polarizable dark states

We have observed multi-wave mixing (MWM) processes in reversed-Y (RY) type system in 87Rb atoms with electromagnetically induced transparency (EIT) windows at different laser polarization configurations. Interesting rules of changing the MWM processes and EIT profiles are obtained. We have found that the degenerate Zeeman sublevels and their dressed-state effects are responsible for these observed phenomena. Polarizable dark states are used to describe the multi-level dressed states. The experimental data are in good agreement with the results from the theoretical calculation that takes into account all the 16 Zeeman sublevels in the RY system.

Controlling four-wave and six-wave mixing processes in multilevel atomic systems

We experimentally demonstrate that four-wave mixing (FWM) and six-wave mixing (SWM) processes can be selectively turned on and off in a close-cycled four-level atomic system. Under certain laser beam configuration, the FWM and SWM processes can be made to coexist with similar signal amplitudes and transmit through the same electromagnetically induced transparency window in such folded four-level atomic system. By introducing an additional pumping laser beam connecting to the fifth energy level, both FWM and SWM signals can be greatly enhanced.

Four-wave-mixing between the upper excited states in a ladder-type atomic configuration.

Four-wave-mixing (FWM) radiation is generated between the hyperfine structures of the 5D and 5P states in a thermally broadened rubidium atomic vapor using resonant atomic coherence. Background-free unidirectional signals having narrow spectral linewidths are isolated and experimentally studied in the frequency domain, and the effects of the driving beam parameters on the properties of the radiation are discussed. The radiation has several new properties compared to traditional FWM radiations generated between the 5P and 5S states. The high-resolution signals obtained in this method could make it favorable in spectroscopic procedures that rely on two-photon fluorescence.

Three-field noise correlation via third-order nonlinear optical processes.

We report experimental observation of three-field noise correlation between two coexisting four-wave mixing (FWM) signals and the probe signal in a coherently-prepared rubidium vapor system. The two FWM signals in a double-Λ type quasi-four-level atomic system are obtained in different directions via their respective phase-matching conditions and can be detected individually. These signals and the probe beam are found to be strongly correlated or anticorrelated with each other.

Spatial Domain Interactions between Ultraweak Optical Beams

We have observed spatial interactions between two ultraweak optical beams that are initially collinear and nonoverlapping. The weak beams are steered towards each other by a spatially varying cross-Kerr refractive index waveguide written by a strong laser beam in a three-level coherently prepared atomic medium. After fusing together, the combined beam shows controllable phase-dependent behaviors. This is the first observation of solitonlike interactions between weak beams and can be useful for all-optically tunable beam combining, switching, and gating for weak photonic signals.

Measurement of ac-Stark shift by a two-photon dressing process via four-wave mixing

By setting laser beams in two-photon Doppler-free configurations for both cascade and Λ-type subsystems in a four-level inverted-Y system, a four-wave mixing (FWM) process is studied and optimized within the Doppler profile. The two-photon ac-Stark effect in the FWM signal is observed, which can be used to determine the energy level shift of the atom due to dressing fields.

Spatial interference between four- and six-wave mixing signals

We experimentally demonstrate spatial interference between simultaneously generated four-wave mixing (FWM) and six-wave mixing (SWM) signals in a multilevel atomic system. By tuning the relative phase between the FWM and SWM processes the relative strengths between these two high-order nonlinear optical processes can be investigated.

Noise correlations in a doubly-resonant atomic optical parametric oscillator

The simultaneous generations of bright Stokes and anti-Stokes fields are realized by using only one pump field in a Doppler-broadened atomic medium confined in an optical ring cavity. A vacuum-induced absorption phenomenon is also observed in such a system. By utilizing an external Fabry-Perot cavity to separate the Stokes and anti-Stokes fields, we investigate the noise correlation and anticorrelation properties between the Stokes, anti-Stokes, and the pump fields.

Interferometric control of parametrically amplified waveforms

Using atomic coherence, four-wave-mixing radiation is generated between the excited states of a ladder-type configuration in rubidium atomic vapor. By using all-optical phase control between the two frequency-swept driving beams connecting the two lower levels, the generated signal is phase modulated across its bandwidth. When homodyned with a local oscillator, such phase control looks promising for applications including waveform shaping and high-resolution metrology. Experimental observations of signal line shape symmetrization, linewidth narrowing, and bandwidth switching are demonstrated.