G. Y. Yin

Low-light-level nonlinear optics with slow light

Electromagnetically induced transparency in an optically thick, cold medium creates a unique system where pulse-propagation velocities may be orders of magnitude less than c and optical nonlinearities become exceedingly large. As a result, nonlinear processes may be efficient at low-light levels. Using an atomic system with three, independent channels, we demonstrate a quantum interference switch where a laser pulse with an energy per area of {approx}23 photons per {lambda}{sup 2}/(2{pi}) causes a 1/e absorption of a second pulse.

Generation and control of femtosecond pulses by molecular modulation

We have demonstrated that coherent molecular modulation can result in the collinear generation of mutually-coherent spectral sidebands that extend in frequency from the infrared to the far ultraviolet. Our technique is based on adiabatic preparation of a highly coherent molecular superposition-state, which is achieved by using narrow-linewidth lasers slightly detuned from a Raman resonance. The phases of the resultant Stokes and anti-Stokes sidebands are adjusted in order to synthesize desired single-cycle pulse trains at the target. In this article we review recent improvements and developments in this area, including: techniques for increasing the number of generated sidebands; synchronization of the pulse trains with the molecular motion in the given molecular system; laser self-focusing and spatial soliton formation due to the coherent interaction of light with oscillating molecules. In the future, this Raman source may produce sub-cycle optical pulses, and allow synthesis of waveforms where the electric field is a predetermined function of time, not limited to a quasi-sinusoidal oscillation.

Time-gated imaging with an ultrashort-pulse, laser-produced-plasma x-ray source.

We demonstrate a contrast improvement of nearly a factor of 5 in a phantom medical x-ray image, using time-gated detection. This improvement is accomplished by temporal discrimination against scattered x-ray flux and may permit a significant reduction in patient x-ray exposure. A multiterawatt ultrashort-pulse laser-produced-plasma x-ray source (<1 ps) and a time-gated microchannel plate detector (~100 ps) are used.

Efficient gas-phase generation of coherent vacuum ultraviolet radiation.

We report the demonstration of a pulsed atomic lead (Pb) vapor-based vacuum ultraviolet frequency converter from 233 to 186 nm with unity photon-conversion efficiency. This conversion is attained without phase matching.

Observation of electromagnetically induced phase matching

We report the observation of electromagnetically induced phase matching in collisionally broadened Pb vapor. At a critical intensity at which the Rabi frequency of a dressing 1064-nm laser overcomes the Doppler broadening of the vapor, the generated four-frequency-mixing signal at 283 nm increases in a steplike manner by a factor of 59.

Nonlinear optics at maximum coherence

We describe nonlinear optical processes which utilize maximum coherence of a non–allowed transition. The nonlinear susceptibility for such processes is of the same order of magnitude as is the linear susceptibility. This allows frequency converters and optical parametric oscillators with bandwidths which are on order of their centre frequency.

Rotational Raman generation with near-unity conversion efficiency

We demonstrate collinear generation of equidistant rotational sidebands in low-pressure molecular hydrogen with near-unity conversion efficiency. The spectrum consists of 37 coherent sidebands covering over 20, 000 cm(-1) of spectral bandwidth and ranging from 1.37mum to 352 nm in wavelength.

A miniature ultrabright source of temporally long, narrowband biphotons

We demonstrate a miniature source of long biphotons utilizing the cluster effect and double-pass pumping in a monolithic doubly resonant parametric down-converter. We obtain a biphoton correlation time of 17.1 ns with a generation rate of 1.10×105 biphotons/(s mW) and an estimated linewidth of 8.3 MHz.

Pulsed Ti:sapphire laser seeded off the gain peak

An external-cavity diode laser is used to seed a pulsed Ti:sapphire laser from 839 to 860 nm. Because this wavelength range is off the gain peak of Ti:sapphire, a bandpass filter is used in the cavity to permit seeded operation. We describe a tunable, wide-field-of-view birefringent filter especially suited for use in seeded lasers. Measurements of the ratio of unseeded to seeded output as a function of seed power are also presented and demonstrate an approximately reciprocal dependence on the seed power.

Efficient self-seeding of a pulsed Ti3+:Al2O3 laser.

We describe the design considerations and operating characteristics of a compact, widely tunable, narrow-linewidth, megawatt-class pulsed laser system based on Ti(3+):Al(2)O(3) (Ti:sapphire) pumped by the second harmonic of a 1.06-microm Nd:YAG laser. The system delivers 10 mJ in a 5-ns near-transform-limited single-longitudinal-mode (SLM) pulse with a threshold of 20 mJ and a slope efficiency greater than 40%. By the technique of self-seeding, in which a portion of the gain mediums spontaneous fluorescence is coupled back to the laser cavity, SLM operation may be obtained across the entire gain profile of Ti:sapphire with a minimum number of optical components and without external seeding.