Slow light superstructure gratings for optical storage and enhancing second harmonic generation

Abstract

Slow light is the phenomenon whereby a large reduction in group velocity is observed close to a resonance. Devices which exhibit this behaviour are of great general interest and have numerous applications. The primary focus for such devices is in the creation of optical storage and quantum memories. They also have additional applications in optical switching and enhancing optical nonlinearities. The field of slow light generated a great deal of interest when it was demonstrated that exceptionally low group velocities down to the speed of a cyclist were achieved using electromagnetically induced transparency. Since that time there has been significant activity in developing new slow light devices. Along with electromagnetically induced transparency, devices have been developed using fibres and photonic crystals. However, all such devices are limited by the delay-bandwidth product. The greater the reduction in the group velocity, the more delay the device can generate. Unfortunately, this is also accompanied by a decrease in the available bandwidth which limits the device storage capacity. In this work a slow device based on a dynamic moire grating is explored. A moire grating produces a double resonant structure with a slow light transmission band. The amount that the group velocity is reduced in a moire grating is dependent on the size of the transmission band. A smaller transmission band leads to lower group velocities and so the device is bounded by the delay-bandwidth product. Here I show that by dynamically varying the coupling strength of a moire grating, the limits imposed by the delay-bandwidth product can be potentially broken by many orders of magnitude. This result relies on symmetric switching of the coupling strength. I also show that asymmetric switching can generate bandwidth modulation. A potential realisation of such a device is presented using an electro-optic grating in a periodically poled medium. Applying an external quasi-static field allows for the dynamic control of the grating coupling strength. Another feature of moire gratings is that the generated slow light is accompanied by pulse compression and field enhancement. I explore in this work as means of enhancing second-harmonic generation. I then show that the slow light transmission band creates forward and backward second-harmonic modes with greater efficiency than a standard device. By including a second grating at the front of the device, the backward second-harmonic mode can be suppressed producing unidirectional output. The additional grating creates a resonance within the device which further enhances the second-harmonic generation. I show that by tuning the device parameters it is possible to achieve near complete conversion efficiency in regimes where conversion efficiency would ordinarily be negligible.

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Identifiers

ORCID for Thomas, Edward Maybour: orcid.org/0000-0002-5259-9604

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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