Se Young Yang

Injection behavior and modeling of 100 mW broad area diode lasers

The output from a single-frequency, narrow-linewidth GaAlAs diode laser was injected into a 100-mW broad-area diode laser. The injected laser emitted in a single longitudinal mode with up to 80 mW in a single, diffraction-limited, 0.5 degrees FWHM far-field lobe. The angle of the far-field lobe steered with injection frequency and broad-area laser bias current, displaying behavior similar to that observed with injected gain-guided laser arrays. A Fabry-Perot amplifier model which explains the injection behavior in terms of Gaussian beam propagation in a large optical cavity is proposed. The model provides criteria for optimizing locking bandwidth, beam steering, and injection efficiency. >

Injection behavior of high-power broad-area diode lasers

A single-mode diode laser is injected into a high-power broad-area diode laser to produce single-mode operation with 80 mW in a 0.50 degrees-wide far-field lobe. Spectrally resolved near- and far-field measurements suggest a simple Fabry-Perot amplifier model that qualitatively explains the observed injection and beam-steering behavior in these devices as well as gain-guided multiple-stripe arrays. This model provides criteria for the optimization of injection performance.

Transport of intensity phase imaging in a volume holographic microscope.

We demonstrate a method for single-shot quantitative phase imaging based on the transport of intensity equation (TIE) in a volume holographic microscope (VHM). The VHM system uses a multiplexed volume hologram to laterally separate images from different focal planes. This axial intensity information is then used to solve the TIE and recover object phase quantitatively. Further, we show improved phase recovery by using five multiplexed gratings in one hologram.

Phase and amplitude imaging from noisy images by Kalman filtering.

We propose and demonstrate a computational method for complex-field imaging from many noisy intensity images with varying defocus, using an extended complex Kalman filter. The technique offers dynamic smoothing of noisy measurements and is recursive rather than iterative, so is suitable for adaptive measurements. The Kalman filter provides near-optimal results in very low-light situations and may be adapted to propagation through turbulent, scattering, or nonlinear media.

Design and fabrication of dielectric nanostructured Luneburg lens in optical frequencies

A dielectric subwavelength Luneburg lens structure was designed using Hamiltonian ray tracing, and fabricated using electron beam lithography. Analysis from Hamiltonian ray tracing was in agreement with finite difference in time domain (FDTD) method with wavefront error between the two methods below λ/8, while an improvement in speed of approximately 100 times was observed. The fabricated Luneburg lens structure was tested with a fiber laser with a wavelength of λ=1.55µm, and proved its capabilities of focusing.

High-speed electronic beam steering using injection locking of a laser-diode array

High-speed electronic steering of the output beam of a 10-stripe laser-diode array is reported. The array was injection locked to a single-frequency laser diode. High-speed steering of the locked 0.5 degrees -wide far-field lobe is demonstrated either by modulating the injection current of the array or by modulating the frequency of the master laser. Closed-loop tracking bandwidths of 70 kHz and 3 MHz, respectively, were obtained. The beam-steering bandwidths are limited by the FM responses of the modulated devices for both techniques.

Fabrication of a Microscale Device for Detection of Nitroaromatic Compounds

Polymer layers displaying a specific swelling response in the presence of nitroaromatic compounds are integrated into microscale sensors. Blanket layers of the polymer are grown using initiated chemical vapor deposition, and lithographic techniques are used to define microscale polymer lines. A nanoscale metal line is perpendicularly overlaid across each polymer line. Exposure to nitroaromatic analytes causes the polymeric device component to expand, resulting in plastic deformation of the metal and a permanent change in the resistance measured across the device. The response is rapid and selective for nitroaromatic compounds; additionally, the small area, simplicity, and interchangeability of the device design facilitate the fabrication of sensors selective for other analytes and device arrays. Calculated limits of detection for 2,4,6-trinitrotoluene are 3.7 ppb at 20°C or 0.8 pg in a proof-of-concept device; methods for optimization are explored.

Nanostructured origami™ folding of patternable resist for 3D lithography

A new method to fold free standing poly(methyl methacrylate) (PMMA) resist using e-beam exposure is developed and demonstrated. The results prove controllable folding of the patterned PMMA. An explanation of the folding mechanism is proposed based on experimental characterization and theoretical analysis. 3D lithography is achieved by attaching the patterned resist on an adjacent side wall by folding. Patterns are effectively transferred by depositing metal followed by a lift-off process.

Nano Fracture Chemical Sensor for Explosives Detection

Selective detection of explosive compounds is critical for national defense and homeland security. In this paper we describe the fabrication and demonstration of a chemical sensor capable of detecting nitroaromatic explosives in air. The device has the unique features of nano-scale dimensions, simple and inexpensive fabrication, and low power consumption. It consists of a nano-patterned conductive metal line placed on top of a patterned responsive polymer, poly(4-vinylpyridine) (P4VP). Due to polymer-solvent interactions, P4VP swells when it encounters the target analyte, producing a large stress. Detection takes place by monitoring the change in device resistance as the metal nano line deforms or fractures when P4VP swells and transfers mechanical stress. The sensors would be ideal for discreet, wide-scale deployment over large areas. It is also important to note that device sensitivity can be readily enhanced by scaling down the feature size of the metal line or adjusting the material properties of both the metal and polymer. The fabrication process is readily transferrable to a variety of organic and metal materials, improving the versatility of the sensors. The resulting devices may provide new ways to detect security threats and complement existing complex methods to increase the probability of detection and to reduce false alarms. The same approach may also be applicable outside the military/security domain, for example, for pollution monitoring, for factory safety and operational monitoring, or for food quality inspection; all these applications are contingent upon finding the appropriate polymers for the respective analytes.© 2010 ASME