S. W. Leonard

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.

Attenuation of optical transmission within the band gap of thin two-dimensional macroporous silicon photonic crystals

The transmissivity within the photonic band gap of two-dimensional photonic crystals of macroporous silicon is reported as a function of crystal thickness. Measurements were carried out for crystals of nominally 1, 2, 3, and 4 crystal layers using a commercial parametric source, with a wavelength tunable from 3 to 5 μm. For wavelengths well within the 3–5 μm photonic band gap, attenuation of approximately 10 dB/crystal layer is obtained, in agreement with calculations based on plane wave expansion methods. For these materials, one should be able to achieve photonic crystal functionality in many applications with very small crystal volumes.

A model system for two-dimensional and three-dimensional photonic crystals: macroporous silicon

A review of the optical properties of two-dimensional and three-dimensional photonic crystals based on macroporous silicon is given. As macroporous silicon provides structures with aspect ratios exceeding 100, it can be considered to be an ideal two-dimensional photonic crystal. Most of the features of the photonic dispersion relation have been experimentally determined and were compared to theoretical calculations. This includes transmission and reflection of finite and bulk photonic crystals and their variation with the pore radius to determine the gap map. All measurements have been carried out for both polarizations separately since they decouple in two-dimensional photonic crystals. Moreover, by inhibiting the growth of selected pores, point and line defects were realized and the corresponding high-Q microcavity resonances as well as waveguiding properties were studied via transmission. The tunability of the bandgap was demonstrated by changing the refractive index inside the pores caused by an infiltrated liquid crystal undergoing a temperature-induced phase transition. Finally different realizations of three-dimensional photonic crystals using macroporous silicon are discussed. In all cases an excellent agreement between experimental results and theory is observed.

Optical characterisation of 2D macroporous silicon photonic crystals with bandgaps around 3.5 and 1.3 μm

Abstract Transmission measurements were performed on thin 2D silicon photonic crystals (PCs) with 1–4 crystal rows in order to investigate the effect of a finite structure and to obtain an estimate of the crystal thickness necessary to minimize crosstalk between adjacent waveguides. For wavelengths deep within the H-bandgap a strong exponential decay revealing an attenuation constant of 10 dB per crystal row was measured. For opto-electronic applications, the lattice constant of macroporous Si was successfully downscaled from a pitch of 1.5 to 0.5 μm. Reflection measurements performed at these structures show good agreement with corresponding bandstructure calculations exhibiting a complete bandgap around λ=1.3 μm .

Single-mode transmission in two-dimensional macroporous silicon photonic crystal waveguides

We report the infrared operation of a two-dimensional photonic crystal waveguide fabricated in silicon. Measurements of the transmission spectrum reveal a large transmission bandwidth within the 3.1-5.5-microm bulk-crystal photonic bandgap and a rich resonance structure. The calculated transmission spectrum for this structure is in good agreement with the measured spectrum and predicts a 10% single-mode bandwidth for the waveguide.

Subpicosecond spin relaxation in GaAsSb multiple quantum wells

Spin relaxation times in GaAsxSb1−x quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 μm, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with x=0 and 0.188, respectively. These times are >5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga(As)Sb system, consistent with the D’yakonov–Perel theory of spin relaxation [M. I. D’yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 (1974)]. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 μm with response time <250 fs.

Complete three-dimensional band gap in macroporous silicon photonic crystals

The prospect of obtaining a complete three-dimensional band gap in macroporous silicon photonic crystals is investigated theoretically. Band structure calculations indicate that a modified form of the simple cubic lattice of air spheres in silicon exhibits a complete band gap, with a bandwidth of up to 4%. It is further shown that this quasispherical crystal may be fabricated using standard etching procedures. These results provide a practical route to obtaining large-scale, high-quality, silicon photonic crystals with a complete band gap.

All-optical ultrafast tuning of two-dimensional silicon photonic crystals via free-carrier injection

Summary form only given. Photonic crystals are emerging as a potential disruptive technology for the coming decade. The functionality of photonic crystals could be dramatically enhanced by providing a means to tune their spectral properties. In the past, tunable photonic crystals have been demonstrated using liquid crystals to control the refractive index, for which the optimal response time is milliseconds. Recently, it was suggested that thermally injected free-carriers could be used to tune photonic crystals. Here we demonstrate an ultrafast, all-optical-tunable, two-dimensional photonic crystal, where control of band edge wavelength and absorption is achieved through optical free-carrier injection.

Tuning 2D photonic crystals

We demonstrate three ways in which the optical band-gap of 2-D macroporous silicon photonic crystals can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm for H-polarized radiation as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts upt to 30 nm of the high frequency edge of the E-polarized band-gap. Finally, we show that below-band-gap radiation at 2.0 and 1.7 μm can induce changes to the optical properties of silicon via the Kerr effect and tune the band edges of the 2-D macroporous silicon photonic crystal.

Suppression of intervalley scattering in Ga(As)Sb quantum wells

Femtosecond time-resolved reflectivity was measured near the 1.55 μm absorption edge of several GaAsxSb1−x/AlSb quantum well samples. On the basis of differences in the reflectivity recovery kinetics and plateau values, we deduce that Γ–L intervalley scattering can be effectively suppressed for x⩾0.19. This is consistent with calculations incorporating confinement and strain effects which give L–Γ energy separations of 29 (x=0) and 109 meV (x=0.19). Suppression of intervalley scattering can lead to increased internal quantum efficiency and higher carrier mobility in 1.55 μm based devices.