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Quantum Electronics

Amplification of light pulses in waveguides with a periodically varying refractive index

Vladimir Pavlovich Torchigin

Published May 31, 1995• 11 citations
Publisher

Abstract

An analysis is made of the mechanisms of frequency conversion of the carrier of an optical wave which propagates in a waveguide with a refractive index modulated at a frequency up to 10 6 less than the carrier frequency of light.
DOI
10.1070/qe1995v025n05abeh000393

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The dynamic manipulation of light can be achieved by the interaction of a signal pulse propagating through or reflected from a refractive index front. Both the frequency and the wave vector of the signal are changed in this case, which is generally referred to as an indirect transition. We have developed a theory to describe such transitions in integrated photonic crystal waveguides. Through indirect transitions, the following effects can be envisaged: large frequency shifts and light stopping and order of magnitude pulse compression and broadening without center frequency shift. All effects can be potentially realized with a refractive index modulation as small as 0.001. For the experimental realization, we have used slow light photonic crystal waveguides in silicon. The refractive index front was obtained by free carriers generation with a switching pulse co-propagating with the signal in the same slow light waveguide. The group velocities of the signal and the front could be varied arbitrarily by choosing the right frequencies of the signal and switching pulses. The indirect transition was unambiguously demonstrated by considering two situations: a) the front overtaking the signal and b) the signal overtaking the front. In both cases, a blue shift of the signal frequency was observed. This blue shift can only be explained by the occurrence of the expected indirect transition and not by a direct transition without wave vector variation.
In this study, we report the experimental demonstration of optical pulse compression with a gradual refractive index front comoving in a periodically modulated silicon waveguide. This spatiotemporal effect, also called optical push broom, captures and confines the input signal pulse inside a faster propagating refractive index front, driven by a pump pulse. The signal is accelerated by the approaching front until its velocity matches the front's velocity, effectively stopping the light relative to the front. In contrast with the previous experiment where we used a cw signal, we now utilized a pulsed signal and thus can vary the initial delay between the signal and the pump. We employ the slowly varying envelope approximation to model the effect and successfully reproduce the experimental frequency shift at the output corresponding to the temporal delay at the input.
Control over the time dependence of the refractive index of a material allows one to modify and manipulate the properties of light propagating through it. While metamaterials provide a promising avenue in this context, another route has been extensively explored by the ultrafast community - the so-called molecular modulators. Indeed, impulsively-aligned diatomic molecules provide a unique medium, where periodic rotational revivals induced by a pump pulse persist for tens of picoseconds, offering an excellent opportunity for the controlled modification of the refractive index and, therefore, of femtosecond laser pulses propagating through these media. Here we present an analytical theory which describes this process and stumble across a novel mechanism revealing exponential transformations of the probe pulse - its compression, amplification and frequency up-conversion. In particular, our analytical results predict the generation of amplified ultrashort (about 20 fs) ultraviolet pulses centered around 550 nm, starting with near infrared input pulses centered on 1 μm of about 30 fs duration, under very realistic experimental conditions.