We utilize laser-driven molecular oscillations to modulate light and produce a broadband spectrum of mutually coherent Ramansidebands, aiming toward synthesis of ultrafast optical waveforms with prescribed temporal and spatial shapes.
We use molecular deuterium and two driving lasers to demonstrate collinear generation of mutually coherent equidistant sidebands, covering $50000{\mathrm{cm}}^{\ensuremath{-}1}$ of spectral bandwidth and ranging from $2.94\ensuremath{\mu}\mathrm{m}$ to 195 nm in wavelength. The essential idea is the adiabatic preparation of a single, highly coherent ( $|{\ensuremath{\rho}}_{\mathrm{ab}}|\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.33$) molecular eigenstate.
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.
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.
Programmable control of individual pulses in a high-repetition-rate (typically MHz) ultrafast pulse train is a long-standing goal for optical arbitrary waveform synthesis. Here, we report a programmable pulse-by-pulse shaper that enables deterministic spectral-temporal control of ultrafast pulses at a repetition rate of ∼ 20~MHz. By synchronizing an FPGA-driven electro-optic modulation on the stretched waveform in a temporal 4f shaping system, the regime writes pulse-index-dependent spectral phase profiles onto individual pulses. We demonstrate three levels of programmable ability: zero-order phase coding that maps pulse-by-pulse phase sequences into double-slit-like spectral-temporal interference; first-order phase programming that produces arbitrary temporal trajectories with deterministic delay; and fractional-order phase engineering that generates programmable temporal breathing. By launching the shaped pulse train into a nonlinear fiber stage, the programmed temporal breathing is converted into one spectral breathing. We further construct a phase-level-dependent regime map of nonlinear spectral breathing, revealing transitions from weak single-envelope breathing to multi-peak spectral splitting and strongly breathing merged-spectrum dynamics, in agreement with numerical simulations. This pulse-by-pulse spectral-temporal synthesis platform establishes pulse index as a programmable degree of freedom for ultrafast pulse shaping and provides a route toward real-time and pulse resolved optical arbitary waveform synthesis.