Two-photon microscopy has become a powerful tool in neuroscience as it can image and manipulate neural circuits in vivo with cellular resolution. But in conventional two-photon microscopes, a single laser beam scans regions of interest on the sample within a two-dimensional plane. This serial scanning constrains the temporal resolution of imaging and photostimulation. One way to overcome this limit is to increase the number of beamlets on the sample. Here, we discuss our recent progress on holographic beam multiplexing in two-photon microscopy using spatial light modulators (SLMs). The SLM generates a 3D holographic excitation pattern, targeting different cells on the sample simultaneously. In transparent samples such as zebrafish larva where wide field detection can be used, groups of neurons in 3D volume can be imaged simultaneously, and their fluorescence signals can be recorded with extended depth of view techniques. In scattering samples such as mice cortex which needs laser scanning for imaging, multiple planes can be imaged simultaneously, with the signals from different planes being separated by novel statistical algorithms. Using this approach, we also recorded neural activity across multiplanes in moving Hydra. Besides imaging, 3D optogenetics can be performed. We demonstrate 3D patterned photoactivation of groups of target neurons on mice cortex in vivo, while simultaneously monitoring activity of the neural network. Furthermore, spatial light modulators can switch patterns in high speed, facilitating time-multiplexing. SLM-based two-photon microscopy is thus an all-optical platform to study neural circuits in 3D.
Optical manipulation of in vivo neural circuits with cellular resolution could be important for understanding cortical function. Despite recent progress, simultaneous optogenetic activation with cellular precision has either been limited to 2D planes, or a very small numbers of neurons over a limited volume. Here we demonstrate a novel paradigm for simultaneous 3D activation using a low repetition rate pulse-amplified fiber laser system and a spatial light modulator (SLM) to project 3D holographic excitation patterns on the cortex of mice in vivo for targeted volumetric 3D photoactivation. This method is compatible with two-photon imaging, and enables the simultaneous activation of multiple cells in 3D, using red-shifted opsins, such as C1V1 or ReaChR, while simultaneously imaging GFP-based sensors such as GCaMP6. This all-optical imaging and 3D manipulation approach achieves simultaneous reading and writing of cortical activity, and should be a powerful tool for the study of neuronal circuits.
Imaging the neuronal activity throughout the brain with high temporal and spatial resolution is an important step in understanding how the brain works. Two-photon laser scanning microscopy with fluorescent calcium indicators has enabled this type of experiments in vivo. Most of these microscopes acquire images serially, with a single laser beam, limiting the overall imaging speed. To overcome this limit, multiple beamlets can be used to image in parallel multiple regions. Here, we demonstrate a novel scheme of a two-photon laser-scanning microscope that can simultaneously record neuronal activity at multiple planes of the sample with a single photomultiplier tube. A spatial light modulator is used to generate the designated multiple beamlets, and a constrained non-negative matrix factorization algorithm is used to demix the signals from multiple scanned planes. We simultaneously record neuronal activity of multiple layers of a mouse cortex at 10 fps in vivo. This novel imaging scheme provides a powerful tool for mapping the brain activity.
We have designed new THz metastructure waveguides on Si wafers, aimed for low propagation loss and integration with
Si-based integrated circuits. The waveguide has a round cross-sectional hollow-core, surrounded by high reflectioncladding-
walls formed by high-contrast metastructure gratings. We developed a new fabrication technique to fabricate
such a 3D metastructure cage waveguide structure. The waveguide is built using the entire wafer thickness which
involves deep Si etching of periodically spaced holes and using isotropic undercut etching to create a connecting a line
of etched spheres in the middle of the wafer to form the waveguide’s hollow core, then deep etch the high-contrast
grating through the entire wafer thickness to form the cladding for the waveguide. We have successfully modeled and
fabricated such a waveguide structure. The next step is to experimentally test and characterize the waveguide in the THz
We present a unique heterogeneous integration approach for VCSELs on silicon using eutectic bonding. An electrically pumped III-V – silicon heterogeneous VCSEL is demonstrated using a high-contrast grating (HCG) reflector on silicon. CW output power >1.5 mW, thermal resistance of 1.46 K/mW, and 5 Gb/s direct modulation is demonstrated. We also explore the possibility of an all-HCG VCSEL structure that would benefit from stronger thermal performance, larger tuning efficiency, and higher direct modulation speeds.
Optical phased arrays (OPAs) with fast response time are of great interest for various applications such as displays, free space optical communications, and lidar. Existing liquid crystal OPAs have millisecond response time and small beam steering angle. Here, we report on a novel 32×32 MEMS OPA with fast response time (<4 microseconds), large field of view (±2°), and narrow beam divergence (0.1°). The OPA is composed of high-contrast grating (HCG) mirrors which function as phase shifters. Relative to beam steering systems based on a single rotating MEMS mirror, which are typically limited to bandwidths below 50 kHz, the MEMS OPA described here has the advantage of greatly reduced mass and therefore achieves a bandwidth over 500 kHz. The OPA is fabricated using deep UV lithography to create submicron mechanical springs and electrical interconnects, enabling a high (85%) fill-factor. Each HCG mirror is composed of only a single layer of polysilicon and achieves >99% reflectivity through the use of a subwavelength grating patterned into the mirror’s polysilicon surface. Conventional metal-coated MEMS mirrors must be thick (1- 50 μm) to prevent warpage arising from thermal and residual stress. The single material construction used here results in a high degree of flatness even in a thin 400 nm HCG mirror. Beam steering is demonstrated using binary phase patterns and is accomplished with the help of a closed-loop phase control system based on a phase-shifting interferometer that provides in-situ measurement of the phase shift of each mirror in the array.
We report an optical phased array (OPA) for two-dimensional free-space beam steering. The array is composed of tunable MEMS all-pass filters (APFs) based on polysilicon high contrast grating (HCG) mirrors. The cavity length of each APF is voltage controlled via an electrostatically-actuated HCG top mirror and a fixed DBR bottom mirror. The HCG mirrors are composed of only a single layer of polysilicon, achieving >99% reflectivity through the use of a subwavelength grating patterned into the polysilicon surface. Conventional metal-coated MEMS mirrors must be thick (1-50 μm) to prevent warpage arising from thermal and residual stress. The single material construction used here results in a high degree of flatness even in a thin 350 nm HCG mirror. Relative to beamsteering systems based on a single rotating MEMS mirror, which are typically limited to bandwidths below 50 kHz, the MEMS OPA described here has the advantage of greatly reduced mass and therefore achieves a bandwidth over 500 kHz. The APF structure affords large (~2π) phase shift at a small displacement (< 50 nm), an order-of-magnitude smaller than the displacement required in a single-mirror phase-shifter design. Precise control of each all-pass-filter is achieved through an interferometric phase measurement system, and beam steering is demonstrated using binary phase patterns.
We present a single crystalline silicon optical phased array using high-contrast-gratings (HCG) for fast two dimensional
beamforming and beamsteering at 0.5 MHz. Since there are various applications for beamforming and beamsteering
such as 3D imaging, optical communications, and light detection and ranging (LIDAR), it is great interest to develop
ultrafast optical phased arrays. However, the beamsteering speed of optical phased arrays using liquid crystal and
electro-wetting are typically limited to tens of milliseconds. Optical phased arrays using micro-electro-mechanical
systems (MEMS) technologies can operate in the submegahertz range, but generally require metal coatings. The metal
coating unfortunately cause bending of mirrors due to thermally induced stress.
The novel MEMS-based optical phased array presented here consists of electrostatically driven 8 × 8 HCG pixels
fabricated on a silicon-on-insulator (SOI) wafer. The HCG mirror is designed to have 99.9% reflectivity at 1550 nm
wavelength without any reflective coating. The size of the HCG mirror is 20 × 20 μm2 and the mass is only 140 pg,
much lighter than traditional MEMS mirrors. Our 8 × 8 optical phased array has a total field of view of ±10° × 10° and a
beam width of 2°. The maximum phase shift regarding the actuation gap defined by a 2 μm buried oxide layer of a SOI
wafer is 1.7π at 20 V.
A novel 8x8 optical phased array based on high-contrast grating (HCG) all-pass filters (APFs) is experimentally demonstrated with high speed beam steering. Highly efficient phase tuning is achieved by micro-electro-mechanical
actuation of the HCG to tune the cavity length of the APFs. Using APF phase-shifters allows a large phase shift with an
actuation range of only tens of nanometers. The ultrathin HCG further ensures a high tuning speed (0.626 MHz). Both one-dimensional and two-dimensional HCGs are demonstrated as the actuation mirrors of the APF arrays with high beam steering performance.
The optical interconnect is the key technology to support the large bandwidth demand of the super computers and data
centers. As the tremendous number of optical links being implemented in the system, wavelength-division multiplexing
(WDM) is the solution to reduce the use of optical fibers. In this work, we propose to use the vertical coupler based on
the high contrast metastructure to enable the multiplexing and input-output coupling on the photonics chip. The coupling
efficiency can reach 90% for 35 nm 1 dB bandwidth by multiplexing four on-chip channels into the optical fiber.
We generalize the theoretical modelling of high contrast gratings (HCGs) to arbitrary incidence angle. We unveil the
HCG band diagram with HCG supermode formulism, and present the intriguing connections between HCG and 1D
photonic crystal (PhC). We show, for the first time, HCG and PhC can be unified in the same theoretical analysis
architecture. They differentiate themselves by operating at different regimes in the band diagram. The HCG band
diagram not only provides a powerful tool to study both HCG and PhC, but also to design various optical components
with different functionalities on the HCG-based optoelectronics platform.
We present a novel form of hollow-core waveguiding that enables chip-scale integration. Light propagates in air along a
zig-zag path between very highly-reflective Si metastructures comprised of a single layer of sub-wavelength high-contrast
gratings (HCGs) without the aid of sidewalls. Top and bottom subwavelength HCGs separated by 9um of air
and with periodicity perpendicular to the propagation of light reflect light at shallow angles with extremely low loss.
The HCGs are patterned on SOI wafers with 340 nm-thick Si device layers engraved in a single etch step, and have been
measured to have a 0.37 dB/cm propagation loss. Our work demonstrates the light-guiding properties of HCG hollow-core
waveguides with a novel form of lateral beam confinement that uses subtle reflection phase changes between core
and cladding HCG regions capable of bending light around 30 mm radius-of-curvature tracks.
We propose a novel hollow-core slow light waveguide using high contrast grating (HCG). Light propagates in air along
a path bounded by two HCG layers. A strong interaction between the light and the HCG leads to a large group index,
and thus the slow light effect. Waveguide loss and group index can be optimized separately by tuning the HCG and
waveguide parameters. High performance slow light is obtained with <0.1 dB/cm loss, >120 group index and >120 GHz