Miaochan Zhi

Broadband coherent light generation in a raman-active crystal driven by two-color femtosecond laser pulses

We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO(4)). As many as 20 anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from the infrared, through the visible region, to the ultraviolet, and it consists of discrete spatially separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields and open possibilities for synthesis of subfemtosecond light waveforms.

Broadband coherent light generation in diamond driven by femtosecond pulses

We demonstrate broadband light generation in diamond pumped by two-color femtosecond laser pulses. We find that phase matching plays a critical role in the output angle and frequency of the generated sidebands. When a third femtosecond probe pulse is applied to the crystal in the boxed Coherent anti-Stokes Raman Scattering geometry, a two-dimensional array of multi-color beams is generated through the Raman, four-wave mixing, and six-wave-mixing processes. We test the mutual coherence between the generated sidebands. Such coherence, maintained over the broad spectrum, opens possibilities for synthesis of subfemtosecond light waveforms.

Broadband generation in a Raman crystal driven by a pair of time-delayed linearly chirped pulses

A pair of time-delayed linearly chirped pulses with sub-picosecond duration is used to selectively excite Raman transitions in a lead tungstate crystal. Significant molecular coherence leads to generation of up to 40 anti-Stokes and 5 Stokes sidebands. High conversion efficiency (from the two pump beams to the sidebands) is measured. The broadband generation with chirped pulses whose duration is comparable to the Raman coherence lifetime is considerably more efficient, when compared to the case of excitation by two-color femtosecond pulses. In the future, mutual coherence among the generated sidebands may allow ultrashort pulse synthesis.

Coherent transfer of optical orbital angular momentum in multi-order Raman sideband generation

Experimental results from the generation of Raman sidebands using optical vortices are presented. By generating two sets of sidebands originating from different locations in a Raman-active crystal, one set containing optical orbital angular momentum and the other serving as a reference, Youngs double slit experiment was simultaneously realized for each sideband. The interference between the two sets of sidebands was used to determine the helicity and topological charge in each order. Topological charges in all orders were found to be discrete and follow selection rules predicted by a cascaded Raman process.

Toward Single-Cycle Pulse Generation in Raman-Active Crystals

We study broadband sideband generation in Raman-active crystals. The sidebands come out at different angles and cover infrared, visible, and ultraviolet spectral regions. We combine the sidebands into a collinear beam by using a prism. We use a pulse shaper to control the relative phases of the sidebands aiming to produce single-cycle pulses.

Pulse-shaper-assisted phase control of a coherent broadband spectrum of Raman sidebands

We report pulse-shaper-assisted phase control of a broad spectrum, aiming to synthesize nonsinusoidal waveforms. We first generate multiple-order coherent Raman sidebands by focusing two laser beams into a piece of diamond. Then we combine five sidebands into a single beam by using a prism. Our setup allows for both coarse, manual phase adjustments and fine tuning of the spectral phases. A flat spectral phase across these five frequency-separated sub-bands is achieved, which implies generation of two to three optical-cycle pulses.

Discrimination of dipicolinic acid and its interferents by femtosecond coherent Raman spectroscopy

Measurements of the beat frequencies between vibrational modes of dipicolinic acid (DPA) and a series of other molecules (interferents) are presented. The results were obtained from femtosecond time-resolved coherent Raman scattering, and the vibrational level spacings were determined from a Fourier transform of the signal versus probe pulse delay. The entire spectrum of the generated signal is recorded in order to demonstrate multimode excitation and to explain the variety of qualitatively different traces that can be obtained for the same molecule. Since the spectral signature of DPA is unique enough to be used for identification purposes, this technique has the potential to detect hazardous bacterial species, such as anthrax spores.

Concentration dependence of femtosecond coherent anti-Stokes Raman scattering in the presence of strong absorption

We study near-resonant femtosecond coherent anti-Stokes Raman scattering (CARS) of dipicolinic acid (DPA) by exciting the molecular system with a pair of visible pump and Stokes pulses and probing the resultant molecular coherence with a time-delayed UV probe pulse. We record the generated Stokes and anti-Stokes pulse energies as functions of DPA concentration. We observe that the CARS signal has a maximum and the power-law dependence is steeper than the well-known quadratic one. We present a model that describes the propagation of the generated signal through the medium. From this model, we derive an analytical expression that closely agrees with our experimental data. Since DPA serves as a marker molecule for bacterial spores, our results help to establish the detectability limits for a lethal spore dosage when the present technique is applied.

Energy transfer between laser filaments in liquid methanol.

We demonstrate energy exchange between two filament-forming femtosecond laser beams in liquid methanol. Our results are consistent with those of previous works documenting coupling between filaments in air; in addition, we identify an unreported phenomenon in which the direction of energy exchange oscillates at increments in the relative pulse delay equal to an optical period (2.6 fs). Energy transfer from one filament to another may be used in remote sensing and spectroscopic applications utilizing femtosecond laser filaments in water and air.

Broadband light generation using a relatively weak Raman mode in lead tungstate crystal

We obtain broadband generation in a Raman-active crystal lead tungstate (PbWO4) by selectively exciting a relatively weak Raman mode using two femtosecond laser pulses. A third probe beam is used to identify the Raman mode being excited and to measure the coherence decay time of the two simultaneously excited strong Raman modes in the crystal.