Richard E. Muller

Plasmonic nanoparticle arrays with nanometer separation for high-performance SERS substrates.

We demonstrate a method for fabricating arrays of plasmonic nanoparticles with separations on the order of 1 nm using an angle evaporation technique. Samples fabricated on thin SiN membranes are imaged with high-resolution transmission electron microscopy (HRTEM) to resolve the small separations achieved between nanoparticles. When irradiated with laser light, these nearly touching metal nanoparticles produce extremely high electric field intensities, which result in surface-enhanced Raman spectroscopy (SERS) signals. We quantify these enhancements by depositing a p-aminothiophenol dye molecule on the nanoparticle arrays and spatially mapping their Raman intensities using confocal micro-Raman spectroscopy. Our results show significant enhancement when the incident laser is polarized parallel to the axis of the nanoparticle pairs, whereas no enhancement is observed for the perpendicular polarization. These results demonstrate proof-of-principle of this fabrication technique. Finite difference time domain simulations based on HRTEM images predict an electric field intensity enhancement of 82400 at the center of the nanoparticle pair and an electromagnetic SERS enhancement factor of 10(9)-10(10).

High-power high-speed photodetectors-design, analysis, and experimental demonstration

A novel velocity-matched distributed photodetector (VMDP) is proposed to simultaneously achieve high saturation photocurrent and broad bandwidth. Theoretical analysis on the tradeoff between saturation power and bandwidth shows that the VMDP offers fundamental advantages over conventional photodetectors. A comprehensive theoretical model has been developed for the design and simulation of the VMDP. Experimentally, the VMDP with very high saturation (56-mA) photocurrent and instrument-limited 3-dB bandwidth (49 GHz) has been demonstrated. The theoretical analysis and experimental results show that the VMDP is very attractive for high-performance microwave photonic links and high-power optical microwave applications.

Velocity-matched distributed photodetectors with high-saturation power and large bandwidth

A high-power, high-bandwidth photodetector is experimentally demonstrated using a novel velocity-matched distributed photodetector (VMDP). The distributed photodetector structure can achieve large absorption volume and high-saturation power while maintaining the high-speed performance of the fast photodiodes. The VMDP with 56 mA saturation photocurrent and an instrument-limited 3-dB bandwidth of 49 GHz is achieved. The results show that VMDP is ideal for high-performance microwave fiber-optic links and high-power optical-microwave applications.

ErAs:GaAs photomixer with two-decade tunability and 12μW peak output power

This letter reports the fabrication and demonstration of an ErAs:GaAs interdigitated photomixer as a tunable THz source ranging from ∼20GHzto∼2THz, with 12μW maximum power typically around ∼90GHz. Each photomixer is coupled to a composite dipole-spiral planar antenna that emits a Gaussian-type beam into free space. The beam switches from dipole to spiral antenna behavior as the frequency increases. A distributed Bragg reflector is embedded in the device beneath the photomixer to increase its external quantum efficiency. The photomixer has a 900A thick silicon nitride coating which serves as an antireflection and passivation layer, and also improves the reliability and heat tolerance of the device.

9-/spl mu/m cutoff 256/spl times/256 GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetector hand-held camera

A 9-/spl mu/m cutoff 256/spl times/256 hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated. Excellent imagery, with a noise equivalent differential temperature (NE/spl Delta/T) of 26 mK has been achieved. In this paper, we discuss the development of this very sensitive long wavelength infrared (LWIR) camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NE/spl Delta/T, minimum resolvable temperature (MRTD), uniformity, and operability.

Photomixing and photoconductor measurements on ErAs/InGaAs at 1.55 μm

We report here the fabrication and demonstration of the photomixers made from In0.53Ga0.47As epitaxial material lattice-matched to InP. The material consists of layers of ErAs nanoparticles separated by InGaAs and compensated with Be to reduce the photocarrier lifetime to picosecond levels and to increase the resistivity to ∼100 Ω cm. Interdigitated-electrode and planar-antenna structures were fabricated by e-beam lithography and tested for dc electrical characteristics, 1.55-μm optical responsivity, and difference-frequency photomixing. The measured responsivity of 8 mA/W and photomixer output of >0.1 μW beyond 100 GHz are already comparable to GaAs photomixers and suggest that coherent THz generation is now feasible using the abundant 1.55-μm-semiconductor-laser and optical-fiber technologies.

Long-wavelength 640/spl times/486 GaAs-AlGaAs quantum well infrared photodetector snap-shot camera

A 9-/spl mu/m cutoff 640/spl times/486 snap-shot quantum well infrared photodetector (QWIP) camera has been demonstrated. The performance of this QWIP camera is reported including indoor and outdoor imaging. The noise equivalent differential temperature (NE/spl Delta/T) of 36 mK has been achieved at 300 K background with f/2 optics. This is in good agreement with expected focal plane array sensitivity due to the practical limitations on charge handling capacity of the multiplexer, read noise, bias voltage, and operating temperature.

Convex grating types for concentric imaging spectrometers

The properties of convex gratings fabricated by electron-beam lithography are investigated. Three grating types are shown. The first is a single-panel, true blazed grating in which the blaze angle stays constant relative to the local surface normal. This grating provides high peak efficiencies of approximately 88% in the first order and 85% in the second order. The second grating has two concentric panels, with each panel blazed at a different angle. This type permits flexibility in matching the grating response to a desired form. The third type has a groove shape that departs from the sawtooth blazed profile to increase the second-order bandwidth. All these types are difficult or impossible to produce with conventional techniques. The gratings compare favorably with conventional (holographic and ruled) types in terms of efficiency and scatter. Simple scalar models are shown to predict the wavelength response accurately. These gratings allow the optical designer to realize fully the considerable advantages of concentric spectrometer forms.

15-/spl mu/m 128/spl times/128 GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetector focal plane array camera

In this paper, we discuss the development of very sensitive, very long wavelength infrared GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetectors (QWIPs) based on bound-to-quasi-bound intersubband transition, fabrication of random reflectors for efficient light coupling, and the demonstration of a 15-/spl mu/m cutoff 128/spl times/128 focal plane array imaging camera. Excellent imagery, with a noise equivalent differential temperature (NE/spl Delta/T) of 30 mK has been achieved.

15-{micro}m 128 x 128 GaAs/Al{sub x}Ga{sub 1{minus}x}As quantum well infrared photodetector focal plane array camera

In this paper, we discuss the development of very sensitive, very long wavelength infrared GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetectors (QWIPs) based on bound-to-quasi-bound intersubband transition, fabrication of random reflectors for efficient light coupling, and the demonstration of a 15-/spl mu/m cutoff 128/spl times/128 focal plane array imaging camera. Excellent imagery, with a noise equivalent differential temperature (NE/spl Delta/T) of 30 mK has been achieved.