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Nonlinear optics of multi-mode planar photonic crystal microcavities McCutcheon, Murray William

Abstract

The nonlinear properties of multi-mode InP and Si planar photonic crystal microcavities are investigated in experiments relevant to integrated schemes for classical and quantum optical information processing. Normally incident, short laser pulses are used to coherently initialize the relative phase and amplitudes of two modes of a single-missing-hole InP microcavity. The two modes are orthogonally polarized, and separated by less than the bandwidth of the ~130 fs excitation pulses. The relative amplitudes of the two modes can be controlled by adjusting the polarization and the centre frequency of the excitation beam. Cross-polarized detection of the resonantly scattered light reveals a well-defined relative phase between the modes that is characteristic of their coherence. When the short-pulse excitation is used to coherently excite two modes in a three-hole line-defect (L3) InP microcavity, second-order harmonic radiation is observed due to the interactions of the resonant fields with the second-order nonlinear susceptibility (χ⁽²⁾) of the host InP slab. Second-harmonic and sum-frequency generated signals are observed due to the intra- and inter-mode nonlinear mixing of the microcavity fields. When a separate non-resonant pulse is focussed onto an InP microcavity, sum-frequency light is generated conditional to the resonant mode population of the microcavity. The conditionally generated signals can be tuned by tuning the frequency of the non-resonant pulse. All of the results can be explained with reference to the bulk χ⁽²⁾ properties of the InP slab. While the transient, multi-mode response of the microcavities is harnessed with the short-pulse technique, a continuous wave excitation laser exploits the local-field enhancement intrinsic to these wavelength-scale microcavities. A single-mode InP L3-microcavity with Q = 3,800 is pumped on resonance with a CW laser, and the 2D pattern of far-field second-harmonic radiation is directly imaged. The second-harmonic light is enhanced by 1000 times compared to non-resonant excitation, demonstrating integrated low-power frequency generation. The spatial pattern of the radiation is consistent with simulations based on the bulk χ⁽²⁾ tensor, and reveals the importance of scattering and material absorption of the harmonic light. Ultrafast, all-optical switching is demonstrated in a Si microcavity with a single Q = 35, 000 resonant mode. The mode is resonantly excited with a weak probe pulse, and a non-resonant 200 pJ pump pulse with a precisely controlled time delay is used to inject free-carriers above the silicon bandgap. The free-carrier dispersion shifts the mode frequency by 9 line-widths, and broadens its width by a factor of 4. When the excited mode is perturbed while it is ringing down, coherent oscillations in the spectra are observed which can be explained in terms of a model of an instantaneously perturbed harmonic oscillator, The implications for frequency conversion and for the generation of squeezed optical states are considered.

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