Iliya Sigal

Difference Frequency Generation by Quasi-Phase Matching in Periodically Intermixed Semiconductor Superlattice Waveguides

Wavelength conversion by difference frequency generation is demonstrated in domain-disordered quasi-phase-matched waveguides. The waveguide structure consisted of a GaAs/AlGaAs superlattice core that was periodically intermixed by ion implantation. For quasi-phase-matching periods of 3.0-3.8 μm, degeneracy pump wavelengths were found by second-harmonic generation experiments for fundamental wavelengths between 1520 and 1620 nm in both type-I and type-II configurations. In the difference frequency generation experiments, output powers up to 8.7 nW were generated for the type-I phase matching interaction and 1.9 nW for the type-II interaction. The conversion bandwidth was measured to be over 100 nm covering the C, L, and U optical communications bands, which agrees with predictions.

Single-beam differential z-scan technique

We report a single-beam, differential z-scan technique with improved sensitivity for the determination of nonlinear absorption and refraction of materials. A sample is scanned in the direction of beam propagation as usual, but, in addition, its longitudinal position is dithered, producing a detector signal proportional to the spatial derivative of only the nonlinear transmission and therefore giving a background-free signal; the nonlinear transmission for any spatial position of the sample can be recovered by simple integration. For both open and closed aperture scans in GaP, we find an improvement in the signal-to-noise ratio of >5 x compared with a balanced z-scan setup, but this can be improved with apparatus optimization. Nonlinear phase distortions

Evaluation of laser speckle contrast imaging as an intrinsic method to monitor blood brain barrier integrity

The integrity of the blood brain barrier (BBB) can contribute to the development of many brain disorders. We evaluate laser speckle contrast imaging (LSCI) as an intrinsic modality for monitoring BBB disruptions through simultaneous fluorescence and LSCI with vertical cavity surface emitting lasers (VCSELs). We demonstrated that drug-induced BBB opening was associated with a relative change of the arterial and venous blood velocities. Cross-sectional flow velocity ratio (veins/arteries) decreased significantly in rats treated with BBB-opening drugs, ≤0.81 of initial values.

Reducing misfocus-related motion artefacts in laser speckle contrast imaging

Laser Speckle Contrast Imaging (LSCI) is a flexible, easy-to-implement technique for measuring blood flow speeds in-vivo. In order to obtain reliable quantitative data from LSCI the object must remain in the focal plane of the imaging system for the duration of the measurement session. However, since LSCI suffers from inherent frame-to-frame noise, it often requires a moving average filter to produce quantitative results. This frame-to-frame noise also makes the implementation of rapid autofocus system challenging. In this work, we demonstrate an autofocus method and system based on a novel measure of misfocus which serves as an accurate and noise-robust feedback mechanism. This measure of misfocus is shown to enable the localization of best focus with sub-depth-of-field sensitivity, yielding more accurate estimates of blood flow speeds and blood vessel diameters.

Imaging brain activity during seizures in freely behaving rats using a miniature multi-modal imaging system

We report on a miniature label-free imaging system for monitoring brain blood flow and blood oxygenation changes in awake, freely behaving rats. The device, weighing 15 grams, enables imaging in a ∼ 2 × 2 mm field of view with 4.4 μm lateral resolution and 1 - 8 Hz temporal sampling rate. The imaging is performed through a chronically-implanted cranial window that remains optically clear between 2 to > 6 weeks after the craniotomy. This imaging method is well suited for longitudinal studies of chronic models of brain diseases and disorders. In this work, it is applied to monitoring neurovascular coupling during drug-induced absence-like seizures 6 weeks following the craniotomy.

Tailoring of spectral response and spatial field distribution with corrugated photonic crystal slab.

We report a new physical mechanism for simultaneous tuning of quality factors, spectral responses, and field distributions in photonic crystal slabs through removal of polarization mode degeneracy using a lattice of elliptical nano-holes. The quality factors in these structures can become higher than those obtained with much smaller circular nano-holes. Furthermore, the modes can be superimposed by either rotating or morphing the elliptical nano-holes into a corrugated grating. These findings will enable improved radiation-matter interaction in optical, microwave, and THZ frequencies along with enhanced opto-acoustic coupling.

Label free imaging system for measuring blood flow speeds using a single multi-mode optical fiber (Conference Presentation)

We demonstrate a single multi-mode fiber-based micro-endoscope for measuring blood flow speeds. We use the transmission-matrix wavefront shaping approach to calibrate the multi-mode fiber and raster-scan a focal spot across the distal fiber facet, imaging the cross-polarized back-reflected light at the proximal facet using a camera. This setup allows assessment of the backscattered photon statistics: by computing the mean speckle contrast values across the proximal fiber facet we show that spatially-resolved flow speed maps can be inferred by selecting an appropriate camera integration time. The proposed system is promising for minimally-invasive studies of neurovascular coupling in deep brain structures.

Chronic monitoring of cortical hemodynamics in behaving, freely-moving rats using a miniaturized head-mounted optical microscope

Growing interest within the neurophysiology community in assessing healthy and pathological brain activity in animals that are awake and freely-behaving has triggered the need for optical systems that are suitable for such longitudinal studies. In this work we report label-free multi-modal imaging of cortical hemodynamics in the somatosensory cortex of awake, freely-behaving rats, using a novel head-mounted miniature optical microscope. The microscope employs vertical cavity surface emitting lasers (VCSELs) at three distinct wavelengths (680 nm, 795 nm, and 850 nm) to provide measurements of four hemodynamic markers: blood flow speeds, HbO, HbR, and total Hb concentration, across a > 2 mm field of view. Blood flow speeds are extracted using Laser Speckle Contrast Imaging (LSCI), while oxygenation measurements are performed using Intrinsic Optical Signal Imaging (IOSI). Longitudinal measurements on the same animal are made possible over the course of > 6 weeks using a chronic window that is surgically implanted into the skull. We use the device to examine changes in blood flow and blood oxygenation in superficial cortical blood vessels and tissue in response to drug-induced absence-like seizures, correlating motor behavior with changes in blood flow and blood oxygenation in the brain.

Transition from two dimensional photonic crystal slab to one dimensional corrugated grating

Simultaneous tuning of quality factors, spectral responses, and field distributions is demonstrated by morphing a 2D photonic crystal slab into 1D corrugated grating. Higher quality factor than conventional photonic crystal slabs, along with tunable mode superposition, are observed.

Miniature device for chronic, label-free multi-modal optical imaging of cortical hemodynamics in rats

We report on a novel miniature head-mounted imaging system for simultaneous optical recording of brain blood flow and changes in brain blood oxygenation in a rat. Measurements of blood flow speeds are accomplished using Laser Speckle Contrast Imaging (LSCI) technique, while changes in blood oxygenation are measured via Intrinsic Optical Signal Imaging (IOSI) technique. A single multi-wavelength (wavelength = 680, 795, 850 nm) package of vertical cavity surface emitting lasers (VCSELs) is used as the sole brain illumination source. VCSELs enable rapid toggling between wavelengths, and between high-coherence and low-coherence modes, necessary for LSCI and IOSI, respectively. The combination of a miniature light source and a small 10-bit CCD camera sensor lead to a sub-20 g device mass. The miniature imaging system, including the lens, camera, and illumination lasers, is packaged as a module, and is mounted on a chronic implanted observation window that is surgically placed in the skull, allowing for repeated measurements and removal of the imaging system from the rats head after the imaging session. The imaging system allows for a 2mm-diameter field of view and a resolution of 7.4 µm. It will allow neurophysiologists to correlate standard behavioural assays to neurovascular response in animal models, and thus enrich their understanding of neurovascular coupling dynamics of brain disorders and diseases such as stroke and epilepsy.