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Applied Physics Letters

All-optical on-chip dynamic frequency conversion

Michel Castellanos Muñoz, Alexander Yu. Petrov, M. Eich

Published Oct 1, 2012• 28 citations
Publisher

Abstract

Dynamic processes allow controlling light through a fast modification of the optical properties of the confining structure while the light is present in it. Here, we demonstrate the on-chip dynamic frequency conversion of a light signal by performing this modification through a short optical switching pulse co-propagating on-chip with the signal. A frequency-dependent propagation velocity allows the switching pulse to overtake a co-propagating signal pulse running ahead while on its way driving the dynamic process. We show a dynamic frequency shift of up to 70 GHz with a conversion efficiency of up to 25%.

DOI
10.1063/1.4757614

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Dynamic processes allow controlling light through a fast modification of the optical properties of the confining structure while the light is present in it. Here, we demonstrate the on-chip dynamic frequency conversion of a light signal by performing this modification through a short optical switching pulse co-propagating on-chip with the signal. A frequency-dependent propagation velocity allows the switching pulse to overtake a co-propagating signal pulse running ahead while on its way driving the dynamic process. We show a dynamic frequency shift of up to 70 GHz with a conversion efficiency of up to 25%.
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.
Nonlinear degenerate four wave mixing and cross phase modulation are established approaches for all optical frequency manipulation in a silicon chip. These approaches require exact group velocity and/or phase velocity matching of pump, signal, and idler. On the other hand, several experimental demonstrations were presented recently, where frequency of light was changed by a free carrier front propagating in a silicon waveguide. This Doppler-like effect is less known, but has important advantages for frequency manipulation on chip. It requires no phase velocity matching and is not dependent on the shape and duration of the pump pulse. It also allows packet switching and can operate in a pump power independent regime. Here, we shortly review the work on front induced indirect transitions in silicon slow light waveguides. We consider three possible interaction regimes: transmission through the front, reflection from the front, and moving with the front called surfing. We derive analytical equations for the front with a linearly rising edge, which provide a unified description of the frequency shift in all three regimes. Finally, we compare the front induced dynamic frequency conversion to the frequency shifting based on nonlinear effects like cross-phase modulation and four wave mixing.
In this thesis we study ultrafast all-optical switching of microcavities. We employ the electronic Kerr effect to switch the resonance frequency of microcavities operating at telecom wavelengths. We observe the fastest possible switching of a microcavity resonance within 300 fs. The switching speed is only determined by the cavity storage time; only limited by the speed of light and not by material related relaxation properties. For the first time, we derive and measure the non-degenerate three-photon absorption coefficient of GaAs. Using the measured nondegenerate two- and three-photon absorption coefficients we estimate that the trigger laser only lose 22 aJ/µm2 at each switch event, which is a record level of minimum energy loss per switch event. We achieve repeated switching of microcavities within 700 fs, corresponding to THz rates. Therefore to best of our knowledge, we have broken the THz clock rate barrier experimentally using semiconductor cavities for the first time. The THz clock rate that we achieve can be a solution for information technology that is exploring ways to meet the growth rate of the computational demand. The color of light is a main visual perception of light for humans. Indeed, a simple measure for the color of light is given by its frequency since frequency of light is invariant from one medium to another. Although the propagation of light is strongly modified using nanostructures such as photonic crystals and microcavities, the frequency of light stays constant in these structures as well. The color of light can be changed in a nonlinear process or by absorption and re-emission. Using microcavities light is conveniently confined in space and then by dynamical control the medium can be changed to act as a nonlinear medium for light. Remarkably, we show that during the switching of cavities we achieve both up and down frequency conversion of light. The frequency conversion that we observe is a result of the instantaneous refractive index change achieved using the electronic Kerr effect. We achieve repeated frequency conversion at THz rates and generate blue- and red-shifted light pulses at this rate.
Using slow-light photonic crystal waveguides in ultrafast pump-probe experiments, we demonstrate the adiabatic frequency conversion and ultrafast switching of telecoms light pulses on a silicon chip. By storing a 1.3 ps pulse in the slow-light modes of a photonic crystal waveguide, we are able to blue-shift it by 0.3 THz with high (80%) efficiency. We show that the shift occurs when the pulse adiabatically follows the modes of the waveguide in which it travels upon these modes being perturbed by the absorption of a pump pulse. We also show rerouting of optical pulses in a bandwidth limited time of just 3ps. The switch is based on coupled W1 photonic crystal waveguides, and the interaction between the waveguide modes is optimised to give an ultra-short (~3λ) switching length of just 5μm.
Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties.This paper describes the way in which they differ from other optical media, and clarifies what they can do.In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion.First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals.Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management.Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging.The last two sections are devoted to applications of these novel properties.First, I report the fact that strong light confinement and huge light-matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics.As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.