Karthik Kumar

Monolithic integration of binary-phase Fresnel zone plate objectives on 2-axis scanning micromirrors for compact microscopes.

We demonstrated a unique monolithic integration of Fresnel elliptical zone plate (EZP) objective on a 2-axis staggered vertical comb-drive micromirror with 500 μm by 800 μm surface area via direct patterning of reflective binary phase modulation elements on a silicon chip. The need for focusing optics is thus obviated, simplifying the micro-endoscope assembly and improving its form factor. The design of binary phase EZP was guided by simulations based on FFT based Rayleigh-Sommerfeld diffraction model. For dual-axis scanning angles up to 9º by 9º at the image plane, the simulated diffracted Airy disks on a spatial map have been demonstrated to vary from 10.5 μm to 28.6 μm. Micromirrors scanning ±9º (optical) about both axes are patterned with elliptical zones designed for 7 mm focal length and 20þ off-axis 635-nm illumination using 635 nm laser. Videos of samples acquired with ~15 μm lateral resolution over 1mm × 0.35 mm field of view (FOV) at 5.0 frames/second using the device in both transmission and reflectance modes bench-top single-fiber laser scanning confocal microscope confirmed the applicability of the device to micro-endoscopy.

Forward-Imaging Swept Source Optical Coherence Tomography using Silicon MEMS Scanner for High-Speed 3-D Volumetric Imaging

We demonstrate a fiber-based forward-imaging swept source OCT (SS-OCT) system using a two-axis silicon micromachined vertical comb-drive microscanner for high-speed, 3-D imaging of biological specimens. Higher signal-to-noise ratio of SS-OCT over traditional time-domain techniques, combined with low beam-steering loss of silver-coated scanning micromirrors with over 90% reflectivity, provide good imaging performance. Fast wavelength scanning of the laser source (scan rate: 20 kHz) over 110 nm spectral bandwidth enabled image acquisition at 8 million voxels/sec (3-D imaging) or 40 fps (2-D imaging, 500 transverse pixels per image). We successfully acquired en face and tomographic in vitro images of rigid structures (microscanner), soft materials (onion and pickle slices), and in vivo images of epidermis. Lateral resolution of 12.5 mum and axial resolution of 10 mum over a 2x1times4 mm3 imaging volume has been demonstrated. The compact forward-imaging OCT probe may be suitable for image-guided minimal-invasive examination of various diseased tissues.

High-Reflectivity Two-Axis Vertical Comb Drive Microscanners for Confocal Imaging Applications

We present 500times700 mum metal-coated scanning micromirrors fabricated from bonded SOI-Si wafers with ~90% reflectivity at 633 nm. Confocal images with 1 mum resolution were generated using single axis actuation of plusmn2.5deg at 1.87 KHz

Nano-Grating Force Sensor for Measurement of Neuron Membrane Characteristics Under Growth and Cellular Differentiation

We fabricated single-layer pitch-variable diffractive nanogratings on silicon nitride probe using e-beam lithography and subsequent pattern transfer techniques. The nanogratings consist of flexure folding beams suspended between two parallel cantilevers of known stiffness. The probe displacement, therefore the force, can be measured through grating transmission spectrum. We measured the mechanical membrane characteristics of PC 12 cells using the force sensors with displacement range of 10 mum and force sensitivity 8 muN/mum. Youngs moduli of 425plusmn30 Pa are measured with membrane deflection of 1% for PC 12 cells cultured on polydimethylsiloxane(PDMS) substrate coated with collagen or laminin in Hams F-12 K medium. We have also observed stimulation of directed neurite contraction up to 6 mum on extended probing for a time period of 30 minutes.

Spatially multiplexed swept source optical coherence tomography

A spatially-multiplexed swept-source optical coherence tomography (SM-SS-OCT) system for rapid acquisition of B-scans of tissue microstructure is described, we believe, for the first time. SM-SS-OCT instrumentation is similar to that of traditional Swept Source OCT (SS-OCT), which uses a widely tunable (~100 nm) laser source to obtain high-resolution images of biological tissue. However, SM-SS- OCT may be considered an improvement over SS-OCT in terms of efficient usage of the wide spectral bandwidth afforded by the frequency-tunable lasers in SS-OCT systems. Commercially available swept-source lasers regularly achieve extremely narrow line widths (~150 KHz), allowing for SS-OCT A-scan depths on the order of meters. Since imaging tissue to such depths is infeasible, the meters-long depth ranging capability of SS-OCT may be utilized for spatially multiplexing many A-scans, each to lesser depth. We achieve this spatial multiplexing by rapidly scanning all lateral positions of the tissue repetitively while simultaneously scanning the laser wavelength continuously, and using appropriate signal processing to reconstruct a B-scan image from acquired data. Our fiber-based design lends itself towards use in endoscopic applications, and our results suggest that SM-SS-OCT can provide rapid acquisition of B-scans, with potential for depth-resolved visualization of transient processes in biological tissue.