We have investigated the temperature dependence of the fundamental absorption edge of a series ofHg 1 _ x Cd x Te alloys (with composition x ranging from 0.5 to 1). Analyzing our data in the light of the three-dimensional theory of direct-allowed excitons, we find precise .values for the fundamental r -r 6 interband transition energy in a temperature range extendmg from o to 300 K. All experi~ental results, including previous data for HgTe and mercury-rich Hg! _ x Cd x Te alloys, are well accounted for using a simple empirical formula:This expression, which is valid for all compositions O

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In order to determine the angular geometry that satisfies quasi-phase matching conditions for enhanced second-harmonic generation (SHG), the equi-frequency surfaces of the resonant photonic modes (that lie above the light line) of a one-dimensional GaN photonic crystal have been experimentally and theoretically studied as a function of frequency, angle of incidence, and azimuthal direction. Enhancement of the SHG has been observed when the angular configuration satisfies the quasi-phase matching conditions, i.e., when both the fundamental and second-harmonic fields coincide with resonant modes of the photonic crystal. The SHG enhancement achieved to the double resonance was 5000 times with respect to the unpatterned GaN layer. A smaller, but still substantially enhanced SHG level was also observed when the fundamental field is coupled into a resonant mode, while the second-harmonic field is not

CaF2:Er layers have been grown by molecular-beam epitaxy on (100)-oriented CaF2 substrates; the Er concentration ranges from 1% to 50% (mole fraction). The 1.54 μm emission observed under excitation around 800 nm was studied by photoluminescence. Up to 35% Er concentration the integrated emission increases monotonously, quenching appearing for higher doping levels. Photoluminescence results are discussed within the framework of previous studies of Er3+ emission in the near-infrared range (830–860 nm) in order to gain insight into the Er centers involved in the 1.54 μm emission.

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Close spaced vapor transport (CSVT) epitaxial layers have been grown under water vapor partial pressure p ranging from 5×10−2 to 5 mm Hg for source temperatures of 800 and 850 °C and a substrate temperature of 730 °C, using undoped high-purity GaAs as source material and 〈100〉 chromium doped high-resistivity GaAs as substrate. From Hall measurements, all layers were found to be n-type with a majority carrier concentration in the range of (2–3) ×1017 cm−3 and a mobility 3100–3600 cm2 V−1 s−1 as p varies from 5.0 to 5×10−2 mm Hg. Photoluminescence measurements show the following dominant recombination processes: an exitonic peak at 1.514 eV, a free band acceptor at 1.498 eV, a donor acceptor at 1.490 eV, and two peaks involving complexes at 1.47 and 1.42 eV. These peaks depend on the water vapor pressure: for low values of p only the exitonic peaks exist; as p increases the photoluminescence becomes less efficient until it disappears for p=5.0 mm Hg. This study shows that CSVT-GaAs epilayers grown under proper conditions have high quality and could be used for producing some electronic devices.

We established the angular conditions that maintain the quasi-phase matching conditions for enhanced second-harmonic generation. To do that, we investigated the equifrequency surfaces of the resonant Bloch modes of a two-dimensional periodic, hole-array photonic crystal etched into a GaN/sapphire epitaxial structure. The equifrequency surfaces exhibit remarkable shapes, in contrast to the simpler surfaces of a one-dimensional structure. The observed anisotropy agrees well with the surfaces calculated by a scattering matrix method. The equifrequency surfaces at fundamental and second-harmonic frequencies provide the values of polar and azimuthal angles that maintain quasi-phase matching conditions for enhanced second-harmonic generation over an extended tuning range. The predicted values for quasi phase-matching conditions show that frequency tuning for the two-dimensional case covers an about two times larger fractional bandwidth relative to the one-dimensional case.

Porous silicon (PSi) structure is used as support material to detect protein infiltration and to induce fluorescence and second harmonic light enhancement from glucose oxidase (GOX). Functionalization and protein infiltration is monitored by specular reflectometry. Optical response enhancement of PSi microcavity structures compared to PSi single layers or Bragg mirrors is observed, when GOX is impregnated. Penetration of organic molecules along the PSi microcavity structure is demonstrated by energy dispersive X‐ray profile. Enhanced fluorescence emission of GOX when adsorbed on PSi microcavity is evidenced by multi‐photon microscopy (MPM). Second harmonic light generation is observed at some particular pores of PSi and subsequent resonance enhancement of the signal arising from the GOX adsorbed within the pores is detected. Our work evidences an improved device functionality of GOX‐PSi microcavities due to strongly confined and localized light emission within these structures. This opens the way towards the application of PSi microcavity structures as amended biosensors based on their locally enhanced optical response. The second main achievement lies in the novelty of the used techniques. In contrast to the specular reflectometry used to monitor the macroscopic optical response of PSi structures, MPM presents a valuable alternative microscopic technique probing individual pores. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)