Georg W. Rieger

Nonlinear propagation of ultrafast 1.5 μm pulses in high-index-contrast silicon-on-insulator waveguides

Propagation through silicon-on-insulator (SOI) waveguide structures of 1.53 μm, 100 fs laser pulses with peak powers up to 400 W is studied experimentally and theoretically. The dominant nonlinear effects are two-photon absorption and self-phase modulation. The two-photon absorption coefficient and the nonlinear refractive index of Si obtained in this work are β2=0.9 cm/GW and n2=0.7×10−13 cm2/W, respectively. At high intensities, free carriers generated by two-photon absorption are demonstrated to have a significant influence on pulse spectra and transmitted power. The figure of merit for all-optical switching obtained in this work (T=1.8) indicates that a switch based on a SOI waveguide structure might be possible at 1.55 μm.

Resonant scattering and second-harmonic spectroscopy of planar photonic crystal microcavities

The resonant modes of two-dimensional planar photonic crystal microcavities patterned in a free-standing InP slab are probed in a novel fashion using a long working distance microscope objective to obtain cross-polarized resonant scattering and second-harmonic spectra. We show that these techniques can be used to do rapid effective assays of large arrays of microcavities that do not necessarily contain resonant light-emitting layers. The techniques are demonstrated using microcavities comprised of single missing-hole defects in hexagonal photonic crystal hosts formed with elliptically shaped holes. These cavities typically support two orthogonally polarized resonant modes, and the resonant scattering and harmonic spectra are well fitted using a coherent sum of Lorentzian functions. The well-defined coherence between the two resonant features is explained in terms of a microscopic harmonic oscillator model. The relative merits of these techniques are quantitatively compared with the more commonly used cavi...

Nonlinear transmission of 1.5 µm pulses through single-mode silicon-on-insulator waveguide structures

An 80 MHz pulse train of ~ 100 fs optical pulses centred at ~ 1.5 microm is propagated through a variety of high-index-contrast silicon-on-insulator waveguide structures less than 1 mm long. All-optical power limiting and negative differential transmission, based only on the intrinsic nonlinear response of the untextured waveguides near 1.5 microm, are demonstrated for average in-guide power levels of ~ 1 mW. Superlinear transmission is observed in a textured silicon waveguide for power levels less than 20 microW.

Emission spectrum of electromagnetic energy stored in a dynamically perturbed optical microcavity

An ultrafast pump-probe experiment is performed on wavelength-scale, silicon-based, optical microcavities that confine light in three dimensions with resonant wavelengths near 1.5 mum, and lifetimes on the order of 20 ps. A below-bandgap probe pulse tuned to overlap the cavity resonant frequency is used to inject electromagnetic energy into the cavity, and an above-bandgap pump pulse is used to generate free carriers in the silicon, thus altering the real and imaginary components of the cavitys refractive index, and hence its resonant frequency and lifetime. When the pump pulse injects a carrier density of ~ 5 x10(17) cm(-3) before the resonant probe pulse strikes the sample, the emitted radiation from the cavity is blue-shifted by 16 times the bare cavity linewidth, and the new linewidth is 3.5 times wider than the original. When the pump pulse injects carriers, and thus suddenly perturbs the cavity properties after the probe pulse has injected energy into the cavity, we show that the emitted radiation is not simply a superposition of Lorentzians centred at the initial and perturbed cavity frequencies. Under these conditions, a simple model and the experimental results show that the power spectrum of radiation emitted by the stored electromagnetic energy when the cavity frequency is perturbed during ring-down consists of a series of coherent oscillations between the original and perturbed cavity frequencies, accompanied by a gradual decrease and broadening of the original cavity line, and the emergence of the new cavity resonance. The modified cavity lifetime is shown to have a significant impact on the evolution of the emission as a function of the pump-probe delay.

All-optical conditional logic with a nonlinear photonic crystal nanocavity

We demonstrate tunable frequency-converted light mediated by a χ(2) nonlinear photonic crystal nanocavity. The InP-based cavity supports two closely spaced localized modes near 1550 nm, which are resonantly excited by a 130 fs laser pulse. The cavity is simultaneously irradiated with a nonresonant probe beam, giving rise to rich second-order scattering spectra showing nonlinear mixing of the different resonant and nonresonant components. We highlight the radiation at the sum frequencies of the probe beam and the respective cavity modes. This would be a useful, minimally invasive monitor of the joint occupancy state of multiple cavities in an integrated optical circuit.

Efficient coupling of photonic crystal microcavity modes to a ridge waveguide

The unidirectional coupling of a microcavity mode to a ridge waveguide in a slab photonic crystal structure was investigated for the first time. Experimental observation of the coupling efficiency for the signal coupled out of the structure is in good agreement with the result of three-dimensional finite-difference time-domain simulations. The coupling efficiency of the cavity mode to the output channel is ∼60%.

Characterization of Integrated Planar Photonic Crystal Circuits Fabricated by a CMOS Foundry

Integrated planar photonic crystal circuits in silicon on insulator were fabricated with a single-etch-step process by a foundry using complementary metal-oxide-semiconductor processing techniques. The devices studied integrate three key elements: i) input/output grating couplers consisting of 2D uniform arrays of holes, ii) single transverse electric (TE)/single transverse magnetic (TM) mode channel waveguides, and iii) a photonic crystal linear three hole defect (L3) microcavity. Experimentally measured s- and p-polarized transmission, both from grating-to-grating through a uniform silicon slab region, and through the channel waveguide/L3 cavity circuit, were quantitatively compared with finite-difference time-domain simulations. Excellent agreement is achieved assuming circular, vertical side-wall holes, but this requires accurate post-fabrication characterization of actual versus nominal device parameters, including especially the silicon device layer thickness. While s-polarized incident radiation excites TE modes that exhibit typical resonant cavity (filter-like) transmission, p-polarized incident radiation excites TM modes that non-resonantly propagate through the circuit with comparable transmission efficiency. The dependence of the grating coupler tuning range on hole diameter, and the addition of a photoresist covering is determined.

Fabrication of spatial modulated second order nonlinear structures and quasi-phase matched second harmonic generation in a poled azo-copolymer planar waveguide

This work demonstrates that arbitrary types of spatially modulated second-order susceptibility (chi((2)) structures such as 1D and 2D, periodic and quasi-periodic structures can be obtained by using the combination of corona poling and direct laser writing (DLW) techniques. The fabrication technique is based on the photodepoling of azo-dye molecules caused by one-photon or two-photon absorption during the DLW process. Polarization and second harmonic generation (SHG) images of the fabricated structures were measured by electrostatic force microscope and SHG mapping techniques, respectively. Furthermore, quasi-phase-matched (QPM) enhanced SHG from a 1D periodically poled azo-copolymer planar waveguide is demonstrated using an optical parametric oscillator laser by scanning wavelength from 1500 to 1600 nm. The resonant wavelength of the QPM enhanced SHG is peaked at 1537 nm with FWHM is congruent to 2.5 nm.

Guided-mode resonance enhanced second- and third-harmonic generation in an azo-polymer resonant waveguide grating

We demonstrate that guided mode resonances enhanced second- and third-harmonic generation (SHG and THG) in an azo-polymer resonant waveguide grating (RWG). The enhancement factor for SHG emission is at least 1000 compared to that of a sample without RWG effect.

Second-order nonlinear mixing of two modes in a planar photonic crystal microcavity

Sum frequency data are reported from the nonlinear interaction of two coherently excited resonant modes of a two-dimensional planar photonic crystal microcavity patterned in a free-standing InP slab.