Henry M. van Driel

Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres

Photonic technology, using light instead of electrons as the information carrier, is increasingly replacing electronics in communication and information management systems. Microscopic light manipulation, for this purpose, is achievable through photonic bandgap materials, a special class of photonic crystals in which three-dimensional, periodic dielectric constant variations controllably prohibit electromagnetic propagation throughout a specified frequency band. This can result in the localization of photons, thus providing a mechanism for controlling and inhibiting spontaneous light emission that can be exploited for photonic device fabrication. In fact, carefully engineered line defects could act as waveguides connecting photonic devices in all-optical microchips, and infiltration of the photonic material with suitable liquid crystals might produce photonic bandgap structures (and hence light-flow patterns) fully tunable by an externally applied voltage. However, the realization of this technology requires a strategy for the efficient synthesis of high-quality, large-scale photonic crystals with photonic bandgaps at micrometre and sub-micrometre wavelengths, and with rationally designed line and point defects for optical circuitry. Here we describe single crystals of silicon inverse opal with a complete three-dimensional photonic bandgap centred on 1.46 µm, produced by growing silicon inside the voids of an opal template of close-packed silica spheres that are connected by small ‘necks’ formed during sintering, followed by removal of the silica template. The synthesis method is simple and inexpensive, yielding photonic crystals of pure silicon that are easily integrated with existing silicon-based microelectronics.

Two-photon absorption and Kerr coefficients of silicon for 850–2200nm

The degenerate two-photon absorption coefficient β and Kerr nonlinearity n2 are measured for bulk Si at 300K using 200fs pulses with carrier wavelength of 850<λ<2200nm for which indirect gap transitions occur. With a broad peak near the indirect gap and maximum value of 2±0.5cm∕GW, the dispersion of β compares favorably with theoretical calculations of Garcia and Kalyanaraman [J. Phys. B 39, 2737 (2006)]. Within our wavelength range, n2 varies by a factor of 4 with a peak value of 1.2×10−13cm2∕W at λ=1800nm.

Ultrafast carrier kinetics in exfoliated graphene and thin graphite films

Time-resolved transmissivity and reflectivity of exfoliated graphene and thin graphite films on a 295 K SiO(2)/Si substrate are measured at 1300 nm following excitation by 150 fs, 800 nm pump pulses. From the extracted transient optical conductivity we identify a fast recovery time constant which increases from approximately 200 to 300 fs and a longer one which increases from 2.5 to 5 ps as the number of atomic layers increases from 1 to approximately 260. We attribute the temporal recovery to carrier cooling and recombination with the layer dependence related to substrate coupling. Results are compared with related measurements for epitaxial, multilayer graphene.

Enhanced second-harmonic generation in AlGaAs microring resonators

Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.

Second harmonic generation from graphene and graphitic films

Optical second harmonic generation (SHG) of 800 nm, 150 fs fundamental pulses is observed from exfoliated graphene and multilayer graphitic films mounted on an oxidized silicon (001) substrate. The SHG anisotropy is observed as a sample is rotated about the surface normal. For p-polarized fundamental and SHG light, the isotropic SHG from a graphene layer only slightly interferes with the fourfold symmetric response of the underlying substrate, while other samples show a threefold symmetry characteristic of significant SHG in the multilayer graphitic films. The dominance of the threefold anisotropy is maintained from bilayer graphene to bulk graphite.

Three photon absorption in silicon for 2300–3300nm

We measure the spectral dependence of the degenerate three photon absorption coefficient, γ, for a Si [100] wafer using 200fs pulses in the range 2300–3300nm, i.e., photon energy between half and one-third the indirect band gap. For pulses linearly polarized along the [001] crystal axis γ increases from a value of near 0cm3∕GW2 at 3300nm to a peak value of 0.035cm3∕GW2 at 2700nm before decreasing with shorter wavelength; this is consistent with the dispersion expected from allowed-allowed-allowed transitions. At 2600nm the γ value is ∼30% larger for light polarized along [011] than along [001].

Externally pumped high repetition rate femtosecond infrared optical parametric oscillator

The first externally pumped high repetition rate optical parametric oscillator (OPO) which produces tunable femtosecond pulses in the near‐infrared is demonstrated. The singly resonant oscillator with a KTiOPO4 (KTP) nonlinear crystal is synchronously pumped by 150‐fs pulses at 645 nm from a mode‐locked dye laser. With a pump power of 280 mW, stable 220‐fs pulses at a 76‐MHz repetition rate, tunable from 1.20 to 1.34 μm with a single set of mirrors, are achieved. The internal power conversion efficiency is 13% and the average infrared power is as large as 30 mW, for the signal beam only. Femtosecond pulses tunable over the wavelength range from 0.9–4.5 μm (with KTP) using multiple mirror sets should be possible. This range includes the technologically important regime from 1.0–1.6 μm.

Period doubling of a femtosecond Ti:sapphire laser by total mode locking.

Period doubling of an 84-MHz repetition-rate Kerr-lens mode-locked Ti:sapphire laser operated at 830 nm and producing ~300mW of average power has been observed and explained in terms of total mode locking of TEM(00) and TEM(01) modes in an effective confocal cavity. This configuration leads to a spatial sweeping action of a single-peaked pulse at 42 MHz. Period tripling and quadrupling is observed for certain cavity configurations.

Ultrafast silicon-based active plasmonics at telecom wavelengths.

Using a gold/silicon grating coupler and modulating the silicon dielectric constant with 775 nm, 800 fs pump pulses we demonstrate an ultrafast spectral shift to a surface plasmon polariton coupling resonance for 1300-1700 nm probe pulses. With a modest pump fluence of 2.2 mJ cm(-2) the pump-induced free carriers shift the resonance by more than its width, with recovery occurring in 103 ps due to surface recombination.

Ultrafast control of grating-assisted light coupling to surface plasmons

We demonstrate subpicosecond control over the coupling of free-space radiation to surface-plasmon polaritons using 830 and 500 nm period gold gratings. Thermal changes to the electron distribution following irradiation by 100 fs, 810 nm pulses produce a shift of the 570 nm plasmon resonance by approximately 0.75 nm with reflectivity change up to 6% and decay time of approximately 1 ps.