Liam O’Faolain

Chemical sensing in slotted photonic crystal heterostructure cavities

We fabricated slotted photonic crystal waveguides and cavities supporting resonant modes in air. Their peculiar geometry enables the detection of refractive index changes in a given analyte with high sensitivity because of the large overlap between the optical mode and the analyte. This yields a high figure of merit for the sensitivity of the device and we are able to report values of S=Δλ/Δn over 1500. By applying a photonic crystal heterostructure to the slotted geometry, we are able to create high quality-factor cavities essential for realizing low detection limits up to Q=50 000.

Light scattering and Fano resonances in high-Q photonic crystal nanocavities

The authors show that light scattering from high-Q planar photonic crystal nanocavities can display Fano-like resonances corresponding to the excitation of localized cavity modes. By changing the scattering conditions, we are able to tune the observed lineshapes from strongly asymmetric and dispersivelike resonances to symmetric Lorentzians. Results are interpreted according to the Fano model of quantum interference between two coupled scattering channels. Combined measurements and line shape analysis on a series of silicon L3 nanocavities as a function of nearby hole displacement demonstrate that Q factors as high as 1.1×105 can be directly measured in these structures. Furthermore, a comparison with theoretically calculated Q factors allows to extract the rms deviation of hole radii due to weak disorder of the photonic lattice.

Dispersion control and slow light in slotted photonic crystal waveguides

Slotted photonic crystal waveguides combine the ability to confine light in air with the dispersion control available from photonic crystals. Here, we study the dependence of their dispersion properties on geometry, especially the slot width, and demonstrate slow light operation with group indices in excess of 100.

Direct measurement of the group index of photonic crystal waveguides via Fourier transform spectral interferometry

The authors report a direct, single-shot measurement of the group index profile of photonic crystal waveguides, combining spectral interferometry with Fourier transform analysis. This technique’s versatility allows them to resolve subtle changes in dispersion and to quantify the “slow light” effect at the photonic crystal waveguide mode cutoff. For a waveguide 99μm long, they measure a group index up to 85, whereas for lengths of 397 and 695μm, they measure maximum values of 30 and 25, respectively. These results show the relationship between transmission characteristics and the maximum group delay observed in photonic crystals.

Low-loss photonic crystal defect waveguides in InP

We have fabricated high-quality planar photonic crystal defect waveguides in InP/InGaAsP material. Using Fourier analysis of the Fabry-Perot fringes obtained in transmission, we derive the propagation losses. Values as small as 1.8 dB/mm for waveguides consisting of three rows of missing holes (W3) were measured. We believe that the reduction in losses is due to the high quality of etching carried out using a high beam voltage–current ratio regime of chemically assisted ion-beam etching.

High-aspect-ratio chemically assisted ion-beam etching for photonic crystals using a high beam voltage-current ratio

We investigate etching conditions for photonic crystals (PhCs) in InGaAsP/InP and AlGaAs/GaAs using a new regime of CAIBE operation. We show that the beam voltage-current ratio is critical in obtaining high material/mask selectivity. For one-dimensional PhCs, i.e., air slots, selectivities of 22:1 and 50:1 were achieved in InP and GaAs, respectively, using a very high beam voltage (about 1500 V) and a low beam current (about 10 mA). Etched features were observed to be very smooth, i.e., edge roughness was low. Two-dimensional PhCs were etched in InGaAsP/InP under similar conditions achieving selectivities up to 27:1 and 34:1 for hole diameters of 170 and 270 nm, respectively.

Temperature stabilization of optofluidic photonic crystal cavities

We present a principle for the temperature stabilization of photonic crystal (PhC) cavities based on optofluidics. We introduce an analytic method enabling a specific mode of a cavity to be made wavelength insensitive to changes in ambient temperature. Using this analysis, we experimentally demonstrate a PhC cavity with a quality factor of Q≈15 000 that exhibits a temperature-independent resonance. Temperature-stable cavities constitute a major building block in the development of a large suite of applications from high-sensitivity sensor systems for chemical and biomedical applications to microlasers, optical filters, and switches.

Investigation of transition dynamics in a quantum-dot laser optically pumped by femtosecond pulses

The behavior of a quantum-dot edge-emitting laser, optically pumped by femtosecond pulses, has been investigated. It has been observed that pulses generated by the laser from ground-state transitions have longer durations than those generated from the excited states. Interestingly, the shortest pulses were generated when the transitions from the first excited-state were dominant.

Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals

We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index neff is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G=1.5k0, which is 47% better than an unstructured reference device.

Fabrication of photonic crystals using a spin-coated hydrogen silsesquioxane hard maska)

We present a method for creating a hard mask for the dry etching of microphotonic structures and devices. We demonstrate that spin-on glass [hydrogen silsesquioxane (HSQ)] has sufficient dry etch resistance to allow the creation of high-quality, deeply etched photonic crystals. Furthermore, HSQ is a more favorable hard mask for the creation of active devices than plasma-enhanced chemical-vapor deposition (PECVD) silica, as less damage is incurred. It is also an economic and convenient replacement for PEVCD in photonic crystal fabrication. We examine this method and show that it can create photonic crystals of equivalent quality to those created using PEVCD masking.