A. Sa’ar

Neurons culturing and biophotonic sensing using porous silicon

We report on culturing of Aplysia neurons on porous silicon substrates. Good adhesion of the neurons to the porous silicon substrate and a formation of neuron-semiconductor contact have been accomplished. Cultured neurons survived for at least one week on porous silicon showing normal passive membrane properties and generation of action potentials. We have investigated the possibility of using the photoluminescence from porous silicon for transducing neuronal activity into photonic signals. We found that photoluminescence quenching occurs for cathodic current polarization using aqueous salt-based liquid solution contact. The quenching process is due to diffusion of electrons into the porous silicon, giving rise to Auger nonradiative recombination in the silicon nanocrystallites. The decay time of the photoluminescence was found to be relatively slow due to diffusive nature of the process.

Simulation of optically pumped mid‐infrared intersubband semiconductor laser structures

A theoretical self‐consistent investigation of optically pumped mid‐infrared intersubband semiconductor laser with hot electron effects is presented. Electron dynamics under optical pumping are investigated within a rate equation formulation where particle and energy flow equations are derived from Boltzmann’s equation using Fermi statistics. Electron‐polar optical phonon interactions with suitable screening are calculated by using a macroscopic model with slab and interface phonon modes. Our calculations show that despite hot electron effects, population inversion between the first and second excited states can occur at low temperatures under intersubband optical excitation. It is anticipated that lasing in the mid‐infrared can be achieved with asymmetric quantum well structures optimized for electron concentrations exceeding 1011/cm2.

Infrared multispectral detection using Si/SixGe1−x quantum well infrared photodetectors

A modified p-type Si/SiGe quantum well infrared photodetector for multispectral infrared imaging applications is demonstrated. In order to improve the detector’s performances we have used a SiGe emitter and a low-temperature wet passivation process that give rise to a reduced dark current, even at relatively high bias voltages. Multispectral photoresponse at the long, mid and short wavelength infrared atmospheric windows was observed. The response peaks are assigned to the various classes of intervalence band transitions in the quantum wells and in the SiGe emitter layers.

Observation of optically pumped midinfrared intersubband luminescence in a coupled quantum wells structure

We have observed midinfrared emission due to intersubband transitions in a coupled quantum wells structure. The excitation process involves optical pumping of the carriers from the ground subband into the third subband and a generation of a population in the third subband. The luminescence is produced by a radiative transition of the photoexcited carriers from the third to the second subband, followed by a fast relaxation into the ground subband due to a resonance LO‐phonon assisted relaxation process. The emission signal persists from low temperatures up to room temperature with a maximum efficiency at 190 K.

Enhanced photoluminescence and photonic bandgap modification from composite photonic crystals of macroporous silicon and nanocrystalline PbS thin films

We report on the fabrication of composite photonic crystals (PCs) of macroporous silicon and PbS thin films and about their passive and active optical properties. We have measured a redshift in the composite PC photonic stopbands relative to those of the PC substrate. In addition, we have measured a high extraction efficiency of the photoluminescence from the embedded PbS films due to band-edge singularities and slow-light modes of a defect-free two-dimensional composite PC. The peak extraction efficiency has been found to be six times larger than that of planar unpatterned PbS films.

Photoluminescence anisotropy from laterally anodized porous silicon

We report on a technique to fabricate planar structures of porous silicon using standard photolithography and processing steps. Uniform stripes of porous silicon with smooth interfaces with the surrounding silicon have been observed. Using now the freedom to define the crystallographic direction of the anodization process we studied the correlation between the crystallographic orientation of the porous silicon medium and the polarization selection rules of the emitted light. Our main conclusion is that the polarization selection rules are independent of the crystallographic direction of the porous silicon medium and determined solely by the anodization direction.

LOW-POWER PHOTOCURRENT NONLINEARITY IN QUANTUM WELL INFRARED DETECTORS

We report on the intensity dependence of the responsivity in quantum well infrared photodetectors (QWIP). A strong reduction of the responsivity is observed already at small excitation powers for a QWIP with N=4 periods. This nonlinearity is caused by a partial screening of the electric field across the main part of the active region. The intensity dependence is analyzed using a phenomenological approach, which allows us to calculate the nonlinearity from the measured dark current and responsivity. Applying this approach to a background-limited 50 period QWIP, we find that the nonlinearity is already present below 1 mW cm−2, which is less than the thermal background.

Anisotropy, birefringence, and optical phase retardation related to intersubband transitions in multiple quantum well structures

It is well known that the selection rules for intersubband transitions in quantum well structures require that the infrared light be polarized parallel to the growth direction. As a result, the induced intersubband susceptibility tensor becomes highly anisotropic and the crystal becomes birefringent. We have studied the effect of induced birefringence at the midinfrared range of the spectrum using a number of experimental techniques, including Fourier transform infrared absorption spectroscopy and optical phase retardation measurements using a tunable CO2 laser and a cross polarizer setup. We have observed that linearly polarized light becomes almost circularly polarized due to optical phase retardation between the ordinary and the extraordinary directions over a short optical path length of the order of 20 μm near the resonance (but not at the resonance). The real and the imaginary parts of the induced extraordinary refractive index were measured and have been found to be of the same order of magnitude. ...

Thermally induced charged carrier transfer and intersubband transitions in an asymmetrical coupled quantum well structure

We have observed thermally induced charged carrier transfer in an asymmetrical coupled quantum wells structure with energy separation between the first two subbands of the order of the LO phonon energy. In this structure, the population of the second subband is not negligible and intersubband transitions from the second to higher excited subbands can be measured. Due to charged carrier transfer, the built‐in dc space charge electric field is changed, giving rise to a blueshift with increasing temperature of the ground to the first excited state intersubband transition. Also, we have found that the measurements of thermally induced intersubband transitions from the second subband provide a direct way to estimate the energy separation between the second subband and the Fermi energy.

On the question of intersubband electric quadrupole transitions in quantum well structures

Intersubband electric dipole transitions between subbands of the same symmetry are not allowed in symmetric quantum well structures. When a small asymmetry is introduced this selection rule is broken, and the forbidden transitions become allowed. However, we have found that in many cases where the asymmetry of the structure is not too large, the intersubband electric quadrupole transitions dominate the optical transitions. For example, we show that, when the asymmetry is induced by an external dc‐electric field, then, the electric quadrupole transitions dominate up to fields of the order of 10 kV/cm. It is also shown that for structures with a ‘‘built‐in’’ asymmetry, there is a wide range of structures where the electric quadrupole transitions dominate the optical transitions. The selection rules for these transitions differ from those of the electric dipole transitions. It is found that the maximum strength of the electric quadrupole transitions is achieved when the infrared light is linearly polarized i...