Optical phase changes induced by transient perturbations provide a sensitive measure of material properties. One such measure is associated with the change in refractive index with temperature. Another - with thermal expansion. We demonstrate the high sensitivity and speed of such methods using two interferometric techniques: Quantitative Phase Imaging (QPI) in transmission, and phase-resolved Optical Coherence Tomography (OCT) in reflection. Camera frame rate in QPI varied from 10 to 50 kHz, exposure from 1 to 10 µs, and heating pulse – from 0.02 to 1 ms in duration. The phase-stabilized swept-source OCT was operating at 100 kHz repetition rate. Shot-noise limited QPI can resolve energy deposition of about 3.4 mJ/cm^2 in a single pulse, which corresponds to 0.8 ℃ temperature rise in a single cell. OCT can detect deposition of 24 mJ/cm^2 energy between two scattering interfaces producing about 30 dB SNR signals and 4.7 mJ/cm^2 with 45 dB. Finite element modeling of the phase changes in materials heated by laser and by electric current matched the experimental results very well. These techniques can be used for mapping absorption coefficients, electric current density, doping depth in semiconductors, and many other properties. Integration of the phase changes along the penetrating beam path helps increase sensitivity and reveals the size of the hidden objects by looking at the signal relaxation time. These methods may enable multiple applications, ranging from temperature control in retinal laser therapy and in gene expression to characterization of semiconductor devices.
Beating heart cells, cardiomyocytes, are used in drug testing and have the potential for regenerative medicine. Currently their classification into atrial, nodal and ventricular subtypes is performed using destructive and tedious patch clamp measurements. We present a method for analyzing cardiomyocyte contraction cycles using diffraction phase microscopy, a fast quantitative phase imaging method based on off-axis common-path interferometry. The phase variation during the beating cycle can exceed 300 mrad in the most active regions, and is about 40 mrad on average. The phase noise is about 2 mrad per pixel, and it can be reduced by temporal averaging over multiple frames and spatial averaging over the cell. With a maximum acquisition rate exceeding 25,000 fps and with approximately 100 fps required for the characterization of cardiomyocyte motion, 250 frames can be averaged per step, reducing the temporally white noise by a factor of 16. Additional improvements can be obtained by averaging over multiple contraction cycles. Averaging over space does not reduce noise to the same extent due to low-pass spatial filtering during the phase extraction procedure. Low-pass filtering by the pinhole in the reference arm, resulting in high-pass filtering of the image, and low-pass filtering during the phase reconstruction highlight the dynamics of redistribution of dry mass within the cell during a beat cycle. Quantitative phase imaging is a promising approach for rapid, non-invasive, high-throughput characterization of human stem cell-derived cardiomyocytes in culture, with applications to modeling of diseases with patients' specific genes, drug development, and repair of damaged heart tissue.
Patients with retinal degeneration lose sight due to gradual demise of photoreceptors. Electrical stimulation of
the surviving retinal neurons provides an alternative route for delivery of visual information. Subretinal photovoltaic
arrays with 70μm pixels were used to convert pulsed near-IR light (880-915nm) into pulsed current to stimulate the
nearby inner retinal neurons. Network-mediated responses of the retinal ganglion cells (RGCs) could be modulated by
pulse width (1-20ms) and peak irradiance (0.5-10 mW/mm<sup>2</sup>). Similarly to normal vision, retinal response to prosthetic
stimulation exhibited flicker fusion at high frequencies, adaptation to static images, and non-linear spatial summation.
Spatial resolution was assessed in-vitro and in-vivo using alternating gratings with variable stripe width, projected with
rapidly pulsed illumination (20-40Hz). In-vitro, average size of the electrical receptive fields in normal retina was
248±59μm – similar to their visible light RF size: 249±44μm. RGCs responded to grating stripes down to 67μm using
photovoltaic stimulation in degenerate rat retina, and 28μm with visible light in normal retina. In-vivo, visual acuity in
normally-sighted controls was 29±5μm/stripe, vs. 63±4μm/stripe in rats with subretinal photovoltaic arrays,
corresponding to 20/250 acuity in human eye. With the enhanced acuity provided by eye movements and perceptual
learning in human patients, visual acuity might exceed the 20/200 threshold of legal blindness. Ease of implantation and
tiling of these wireless arrays to cover a large visual field, combined with their high resolution opens the door to highly
functional restoration of sight.
We have developed a photovoltaic retinal prosthesis, in which camera-captured images are projected onto the retina using pulsed near-IR light. Each pixel in the subretinal implant directly converts pulsed light into local electric current to stimulate the nearby inner retinal neurons. 30 μm-thick implants with pixel sizes of 280, 140 and 70 μm were successfully implanted in the subretinal space of wild type (WT, Long-Evans) and degenerate (Royal College of Surgeons, RCS) rats. Optical Coherence Tomography and fluorescein angiography demonstrated normal retinal thickness and healthy vasculature above the implants upon 6 months follow-up. Stimulation with NIR pulses over the implant elicited robust visual evoked potentials (VEP) at safe irradiance levels. Thresholds increased with decreasing pulse duration and pixel size: with 10 ms pulses it went from 0.5 mW/mm<sup>2</sup> on 280 μm pixels to 1.1 mW/mm<sup>2</sup> on 140 μm pixels, to 2.1 mW/mm<sup>2</sup> on 70 μm pixels. Latency of the implant-evoked VEP was at least 30 ms shorter than in response evoked by the visible light, due to lack of phototransduction. Like with the visible light stimulation in normal sighted animals, amplitude of the implant-induced VEP increased logarithmically with peak irradiance and pulse duration. It decreased with increasing frequency similar to the visible light response in the range of 2 - 10 Hz, but decreased slower than the visible light response at 20 - 40 Hz. Modular design of the photovoltaic arrays allows scalability to a large number of pixels, and combined with the ease of implantation, offers a promising approach to restoration of sight in patients blinded by retinal degenerative diseases.