M. Y. Shverdin

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.

Generation of a single-cycle optical pulse

By electronically adjusting the phases of seven Raman sidebands which span 1.56 µm to 410 nm we generate a train rain of well-formed single-cycle optical pulses with a pulse width of 1.5 fs.

Coherent control of laser-induced breakdown

We demonstrate coherent control of laser-induced optical breakdown in Ar and Xe with a femtosecond time-scale pulse train. By using a genetic algorithm to set the relative phases of seven optical sidebands that span two octaves of bandwidth, we enhance or suppress the probability of breakdown, vary the onset time of the spark, and to some extent, vary the position of the spark and the timing of the laser-produced shock wave.

Raman self-focusing at maximum coherence

We demonstrate a type of Raman self-focusing and -defocusing that is inherent in operation at maximum coherence. In this regime the two-photon detuning from the Raman resonance controls the refractive index of the medium.

Optical frequency conversion by a rotating molecular wave plate

We demonstrate efficient four-wave mixing in low-pressure molecular deuterium without the need for phase matching. We use two laser fields with opposite circular polarizations to produce a strong excitation of a rovibrational transition at a frequency of 3167 cm(-1) . The coherent molecular motion, in turn, modulates a third laser field (also circularly polarized) and results in highly efficient single-sideband conversion.

Measurement of Fourier-synthesized optical waveforms

Following the experiments of Shverdin and colleagues [Phys. Rev. Lett. 94, 033904 (2005)], we describe a technique for determining the temporal envelope of an optical beam whose spectrum consists of n discrete, equally spaced frequency components. Four-wave mixing is employed to generate n-1 higher-frequency sidebands. The relative intensities of these sidebands, together with the intensities of the incident side-bands, determine the unknown relative phases of the incident beam.

A Quasiperiodic Approach to Ultrashort Pulses

To optically control atomic and molecular processes, it must be possible to shape the optical waveform with a high level of precision. This involves maximizing the number of phase-adjustable frequencies. The authors demonstrate a technique to multiplicatively increase the number of generated frequencies through the use of modulators in series.

Coherent control of molecular modulation

We demonstrate coherent control of a molecular modulation process using an incident set of seven optical sidebands spanning two octaves of bandwidth. We utilize a genetic algorithm to optimize the relative phases of the incident sidebands to generate additional UV sidebands with nearly 1% efficiency, change the ratio of energy between sidebands by more than a factor of 50, and efficiently alter individual sideband energies by millijoules.

Fourier-synthesis of optical waveforms

The talk will describe the generation of an optical pulse where the spectrum is sufficiently broad that the waveform is a single cycle in length, and where the temporal shape of this waveform may be synthesized. Specifically, we report the generation of a train of single-cycle optical pulses with a pulse width of 1.6 fs, a pulse separation of 11 fs, and a peak power of 1 MW. To synthesize a periodic waveform that is not sinusoidal, it is required that the radiation be phase coherent across its spectrum, and that the bandwidth of the radiation be large as compared to its central frequency. To obtain a multi-octave phase coherent spectrum, we use Q-switched laser pulses at 1.064 /spl mu/m and 807 nm to drive the fundamental vibrational transition in deuterium slightly off resonance. An essential ingredient of the work that is described here is the use of non-resonant four-wave frequency mixing in xenon as a diagnostic for the generated pulses.

Single-cycle optical pulse generation

By electronically adjusting the phases of seven Raman sidebands which span 1.56 µm to 410 nm we generate a train of well-formed single-cycle optical pulses with a pulsewidth of 1.6 fs.