van der Rw Rob Heijden

Cl2∕O2-inductively coupled plasma etching of deep hole-type photonic crystals in InP

We have developed an inductively coupled plasma etching process for fabrication of high-aspect-ratio hole-type photonic crystals in InP, which are of interest for optical devices involving the telecommunication wavelength of 1550nm. The etching was performed at 250°C using Cl2∕O2 chemistry for sidewall passivation. The process yields nearly cylindrical features with an aspect ratio larger than 10 for hole diameters near 0.25μm. This makes them very suitable for high-quality photonic crystal patterns.

Compact Mach-Zehnder interferometer based on self-collimation of light in a silicon photonic crystal

We demonstrate a compact silicon photonic crystal Mach-Zehnder interferometer operating in the self-collimation regime. By tailoring the photonic band structure such as to produce self-collimated beams, it is possible to design beam splitters and mirrors and combine these to a 20 x 20 microm(2) format. With transmission spectroscopy we find a pronounced unidirectional optical output, the output ratio being as high as 25 at the self-collimation wavelength. Furthermore, the self-collimated beams and the unidirectionality are clearly observed in real space using near-field and far-field optical microscopy. Interpretation of the optical data is strongly supported by different types of simulations.

Wavelength tuning of planar photonic crystals by local processing of individual holes

Tuning of the resonant wavelength of a single hole defect cavity in planar photonic crystals was demonstrated using transmission spectroscopy. Local post-production processing of single holes in a planar photonic crystal is carried out after selectively opening a masking layer by focused ion beam milling. The resonance was blue-shifted by enlargement of selected holes using local wet chemical etching and red-shifted by infiltration with liquid crystals. This method can be applied to precisely control the resonant frequency, and can also be used for mode selective tuning.

Observation of the screening signature in the lateral photovoltage of electrons in the quantum Hall regime

The lateral photovoltage generated in the plane of a two-dimensional electron system (2DES) by a focused light spot, exhibits a fine-structure in the quantum oscillations in a magnetic field near the quantum Hall conductivity minima. A double peak structure occurs near the minima of the longitudinal conductivity oscillations. This is the characteristic signature of the interplay between screening and Landau quantization.

Comparative study of Cl2, Cl2∕O2, and Cl2∕N2 inductively coupled plasma processes for etching of high-aspect-ratio photonic-crystal holes in InP

An extensive investigation has been performed on inductively coupled plasma etching of InP. An important motivation for this work is the fabrication of high-aspect-ratio holes for photonic crystals. The essential chemistry is based on Cl2 with the addition of N2 or O2 for sidewall passivation. The influence of different process parameters such as gas flows, temperature, pressure, ion energy, and inductively coupled plasma power on the hole geometry is presented. It is concluded that photonic crystals can be etched with Cl2 only; however, temperature and pressure control is critical. Adding passivation gases largely broadens the window in the parameter space for hole etching. Most importantly, etching of narrow holes can be carried out at higher temperatures where the etching is mass limited and spontaneous etching of InP by Cl2 occurs.

Transmission measurement of the photonic band gap of GaN photonic crystal slabs

A high-contrast-ratio (30 dB) photonic band gap in the near-infrared transmission of hole-type GaN two-dimensional photonic crystals (PhCs) is reported. These crystals are deeply etched in a 650 nm thick GaN layer grown on sapphire. A comparison of the measured spectrum with finite difference time domain simulations gives quantitative agreement for the dielectric band and qualitative agreement for the air band. The particular behavior of the air band arises from the relatively low index contrast between the GaN layer and the sapphire substrate. Our results call for extension of the operation of GaN PhCs to the visible range.

Simple activated transport in ion-implanted Si:As at temperatures below 0.5 K

The temperature dependence of the resistance of ion-implanted Si:As, with impurity concentration about 10 % below the metal-insulator transition, is measured in the temperature range 0.1–4.2 K. The temperature dependence below 0.5 K is much stronger than expected from the variable range hopping laws and is well described by purely activated behaviour. This points to the existence of a rigid gap at the Fermi level. Possible origins are discussed.

Wavelength-sized, tunable nanocavity in deeply etched InP /InGaAsP /InP photonic crystals

Wavelength-sized point defect cavities coupled to access waveguides are reported for deeply etched InP/InGaAsP/InP two-dimensional photonic crystals. The observed quality factor of 60 is comparable to those found for one-row defect Fabry-Perot cavities and for simple point defect cavities in membranes. The quality factor was changed by varying the number of rows of holes. Upon infiltration of the holes with liquid crystal, frequency tuning was demonstrated.

Lateral photoelectric effect studies of a two-dimensional electron gas under quantum Hall conditions

The lateral photovoltage (LP) generated in a two-dimensional electron gas (2DEG) by a focussed laser spot has been shown to yield the potential distribution of a current carrying Hall-bar. The width of this distribution, which exhibits an abrupt change at even integer filling factors, is determined by the equilibration between the bulk and the edge states of the 2DEG. The effects of an anisotropic surface morphology and sample dimensions are reported.

Mobile electrons on a helium film supported by capillary action

A thick film of liquid 4He is suspended by the effect of capillary action on an array of small discs. A two-dimensional electron system (2DES) has been realized on this film. Using standard low-frequency impedance measurements it could be established that the electrons remain mobile when passing from the regime of a bulk liquid to a suspended film. Using these techniques allows the investigation of a new class of low-dimensional electron systems.