Michal Balberg

Confocal microscopy with a volume holographic filter

We describe a modified confocal microscope in which depth discrimination results from matched filtering by a volume hologram instead of a pinhole filter. The depth resolution depends on the numerical aperture of the objective lens and the thickness of the hologram, and the dynamic range is determined by the diffraction efficiency. We calculate the depth response of the volume holographic confocal microscope, verify it experimentally, and present the scanned image of a silicon wafer with microfabricated surface structures.

Use of DNA and Peptide Nucleic Acid Molecular Beacons for Detection and Quantification of rRNA in Solution and in Whole Cells

ABSTRACT DNA and peptide nucleic acid (PNA) molecular beacons were successfully used to detect rRNA in solution. In addition, PNA molecular beacon hybridizations were found to be useful for the quantification of rRNA: hybridization signals increased in a linear fashion with the 16S rRNA concentrations used in this experiment (between 0.39 and 25 nM) in the presence of 50 nM PNA MB. DNA and PNA molecular beacons were successfully used to detect whole cells in fluorescence in situ hybridization (FISH) experiments without a wash step. The FISH results with the PNA molecular beacons were superior to those with the DNA molecular beacons: the hybridization kinetics were much faster, the signal-to-noise ratio was much higher, and the specificity was much better for the PNA molecular beacons. Finally, it was demonstrated that the combination of the use of PNA molecular beacons in FISH and flow cytometry makes it possible to rapidly collect quantitative FISH data. Thus, PNA molecular beacons might provide a solution for limitations of traditional FISH methods, such as variable target site accessibility, poor sensitivity for target cells with low rRNA content, background fluorescence, and applications of FISH in microfluidic devices.

Electrical-thermal switching in carbon-black-polymer composites as a local effect.

Following the lack of microscopic information about the intriguing well-known electrical-thermal switching mechanism in carbon-black-polymer composites, we applied atomic force microscopy in order to reveal the local nature of the process and correlated it with the characteristics of the widely used commercial switches. We conclude that the switching events take place in critical interparticle tunneling junctions that carry most of the current. The macroscopic switched state is then a result of a dynamic-stationary state of fast switching and slow reconnection of the corresponding junctions.

Optical properties of surface micromachined mirrors with etch holes

We have investigated the optical properties of surface-micromachined polycrystalline silicon reflectors within the visible spectral range at five different wavelengths. The measurement results of the reflectivity of various microreflectors at four different incident angles (20/spl deg/, 30/spl deg/, 45/spl deg/, and 60/spl deg/) are presented. Optical properties of microreflectors realized using the multiuser MEMS process (MUMPS) have been investigated. Our studies have found that etch holes, widely used in the surface micromachining process to reduce the time for releasing structures by sacrificial undercutting, have a great influence on the optical properties of micromachined mirrors. Diffraction patterns created by two-dimensional etch-hole arrays on micromachined mirrors have been investigated. The diffraction by etch holes obeys the Fraunhofer diffraction theory when a collimated light source (e.g., a laser beam) is incident. We have shown that when the dimension of etch holes increases, an increasing portion of the incident power will be diffracted and transmitted due to etch holes, leading to decreasing reflectivity of surface micromachined mirrors.

Electroholographic neurons implemented on potassium lithium tantalate niobate crystals

We describe a new approach for constructing large-scale artificial neural networks. The novelty of our approach is based on the concept of electroholography (EH), which permits interconnecting of electronic neurons by minute-volume holograms, using the voltage-controlled photorefractive effect in paraelectric crystals. Crystals of potassium lithium tantalate niobate (KLTN) in the paraelectric phase are shown to be suitable for implementing this concept. A small network composed of two KLTN crystals on which holographic connections are recorded is presented to demonstrate the EH approach.

Electric-field multiplexing of volume holograms in paraelectric crystals

Electric-field multiplexing (EFM) results from the tuning of the effective wavelength of the light beam inside a photorefractive crystal. This tuning results from the application of an external electric field to the crystal during holographic recording. We demonstrate the high Bragg selectivity of this multiplexing technique in paraelectric crystals and compare it with the selectivity obtained in the ferroelectric phase. The effects of the two major physical parameters of working in the paraelectric phase, the temperature and the external electric field applied during the writing stage, are investigated. Experimental results of the EFM of three image-bearing holograms recorded in reflection geometry are presented along with a qualitative analysis of the Bragg selectivity in paraelectric crystals.

Non-invasive blood flow measurements using ultrasound modulated diffused light

Adequate capillary blood flow is a critical parameter for tissue vitality. We present a novel non-invasive method for measuring blood flow based on the acousto-optic effect, using ultrasound modulated diffused light. The benefits of the presented method are: deep tissue sampling (> 1cm), continuous real time measurement, simplicity of apparatus and ease of operation. We demonstrate the ability of the method to measure flow of scattering fluid using a calibrated flow phantom model. Fluid flow was generated by a calibrated syringe pump and the phantoms sampled volume contained millimeter size flow channels. Results demonstrate linear dependence of flow as measured by the presented technique (CFI) to actual flow values with R2=0.91 in the range of 0 to 2 ml/min, and a linear correlation to simultaneous readings of a laser Doppler probe from the same phantom. This data demonstrates that CFI readings provide a non-invasive platform form measuring tissue microcirculatory blood flow.

Photorefractive hologram fixing by a 4 K cooldown to the phase transition in K1−xLixTa1−yNbyO3

A fixing process of photorefractive holograms is reported. The process is based on the formation of an ionic space charge which becomes mobile at the phase transition temperature. A fixed grating was formed in paraelectric potassium lithium niobate tantalate by a 4 K cooldown to Tc, and a warmup back to the paraelectric phase. The diffraction efficiency of the fixed grating was 66% of the original photorefractive space charge grating.

Dipolar holographic gratings induced by the photorefractive process in potassium lithium tantalate niobate at the paraelectric phase

We present experimental evidence indicating the formation of dipolar holographic gratings in potassium lithium tantalate niobate at the paraelectric phase slightly above the phase transition. These gratings are formed in the dark, following the writing of space-charge-based photorefractive gratings, under an external electric field. The dipolar gratings create a spatial modulation of the low-frequency dielectric constant, and the latter induces a spatially correlated modulation of the polarization, which is transformed into birefringence gratings through the quadratic electro-optic effect.

Optical detection of molecular beacons in microfluidic devices

Hybridization and detection of E. coli ribosomal RNA with molecular beacons labeled with a fluorescent dye is achieved in a microfluidic device. The device consists of a four ports mixing chamber, where the probes and the target molecules mix by diffusion. The weak fluorescent signal is detected by two large core optical fibers that are placed on both sides of the channel and collect the light emitted in both directions. The fibers are coupled to a spectrophotometer. The detection limit for the system is 0.2 fmol.