John M. Choi

Deep and fast live imaging with two-photon scanned light-sheet microscopy

We implemented two-photon scanned light-sheet microscopy, combining nonlinear excitation with orthogonal illumination of light-sheet microscopy, and showed its excellent performance for in vivo, cellular-resolution, three-dimensional imaging of large biological samples. Live imaging of fruit fly and zebrafish embryos confirmed that the technique can be used to image up to twice deeper than with one-photon light-sheet microscopy and more than ten times faster than with point-scanning two-photon microscopy without compromising normal biology.

Control of critical coupling in a ring resonator-fiber configuration: application to wavelength-selective switching, modulation, amplification, and oscillation.

By controlling the internal loss of a ring resonator near critical coupling, we demonstrate control of the transmitted power in a fiber that is coupled to the resonator. We also demonstrate wavelength-selective optical amplification and oscillation.

Hyperspectral phasor analysis enables multiplexed 5D in vivo imaging

Time-lapse imaging of multiple labels is challenging for biological imaging as noise, photobleaching and phototoxicity compromise signal quality, while throughput can be limited by processing time. Here, we report software called Hyper-Spectral Phasors (HySP) for denoising and unmixing multiple spectrally overlapping fluorophores in a low signal-to-noise regime with fast analysis. We show that HySP enables unmixing of seven signals in time-lapse imaging of living zebrafish embryos.

Ring fiber resonators based on fused-fiber grating add-drop filters: application to resonator coupling

We introduce a fiber ring optical resonator based on adiabatic fused-fiber grating couplers. The coupling of a through fiber to the resonator is controlled by the strength of the fiber Bragg gratings. By using two of these couplers and incorporating erbium-doped (ED) fiber in the ring, we control the internal loss of the ring by pumping the ED fiber. The transmission spectra of the through port and the drop port of a four-port configuration, a ring coupled to two waveguides, are measured. We show that the loss/coupling ratio of the ring-fiber system can be changed and thus that the transmission properties of the fiber can be controlled.

Slowing light with Fabry-Perot resonator arrays

We analyze the transmission of light through coupled-resonator optical waveguides in the form of evanescently coupled Fabry-Perot resonator arrays. We develop a transfer matrix method to calculate the amplitude and phase responses of the arrays. We also discuss the inclusion of optical gain in the system to compensate for losses in these structures. Owing to the compact length along the propagation direction in evanescently coupled arrays, large slowing factors of the order of 10^2-10^3 can be achieved even with a weak index contrast of ∼0.1%. The large slowing factor, coupled with weak index contrast, makes this structure a promising candidate for artificial slow light system.

Engineering transverse Bragg resonance waveguides for large modal volume lasers

We recently analyzed a new class of laser amplifier based on transverse Bragg reflection. We show that the unique properties of Bragg confinement make it possible through modal loss discrimination to achieve single-transverse-mode operation with transverse modal size that is an order of magnitude larger than in lasers that depend on total internal reflection for transverse confinement.

Electrically Pumped Two-Dimensional Bragg Grating Lasers

We demonstrate electrically pumped InGaAsP two-dimensional Bragg grating (2DBG) lasers with two line defects. The 2DBG structure uses a weak 2D index perturbation surface grating to control the optical modes in the plane of the wafer. Measurements of the 2DBG lasers show that modal control in both the longitudinal and transverse directions is due to the gratings and defects. The 2DBG lasers are promising candidates for single-mode, high power, and high efficiency large-area lasers.

Two-Dimensional Bragg Grating Lasers Defined by Electron-Beam Lithography

Two-dimensional Bragg grating (2DBG) lasers with two quarter-wave slip line defects have been designed and fabricated by electron-beam lithography and reactive ion etching. Unlike conventional two-dimensional photonic crystal defect lasers, which use a large refractive index perturbation to confine light in a plane, the 2DBG structures described here selectively control the longitudinal and transverse wave vector components using a weak index perturbation. Two line defects perpendicular to each other are introduced in the 2DBG to define the optical resonance condition in the longitudinal and transverse directions. In this article, we describe the lithography process used to pattern these devices. The 2DBG lasers were defined using polymethylmethacrylate resist exposed in a Leica Microsystems EBPG 5000+ electron-beam writer at 100 kV. A proximity correction code was used to obtain a uniform pattern distribution over a large area, and a dosage matrix was used to optimize the laser design parameters. Measurements of electrically pumped 2DBG lasers showed modal selection in both the longitudinal and transverse directions due to proper design of the grating and defects, making them promising candidates for single-mode, high power, high efficiency, large-area lasers.

Active Coupled-Resonator Optical Waveguides. II. Current Injection InP-InGaAsP Fabry-Perot Resonator Arrays

We investigate active, electrically pumped coupled-resonator optical waveguides (CROWs) in the form of InP-InGaAsP Fabry-Perot resonator arrays. We discuss the fabrication of these devices and present measurements of the transmission spectra. The signal-to-noise ratio is found to be a strong function of wavelength and degraded rapidly along the resonator chain away from the input. Our results highlight a number of ingredients toward practical implementations loss-compensated and amplifying CROWs.

Loss optimization of transverse Bragg resonance waveguides

Coupled-mode theory was used to analyze guiding in a transverse Bragg resonance (TBR) waveguide structure composed of a GaAs substrate with air holes. This analysis predicts that propagation loss will be minimized for discrete widths of the waveguide core. Although the coupled-mode theory is normally applied to structures with small index perturbations, two-dimensional finite-difference time-domain simulations of the TBR waveguide show good quantitative agreement with the coupled-mode predictions, and these results corroborate the previously predicted existence of discrete core widths for low-loss propagation.