Shiva Vangala

Electrodynamic properties of aqueous spray-deposited SnO2:F films for infrared plasmonics

Abstract. Electrodynamic properties of fluorine-doped tin oxide films grown by aqueous-spray-based heterogeneous reaction on heated hydrophilic substrates were investigated with emphasis on applications to infrared plasmonics. These properties were correlated with physical ones such as crystallinity, dopant and electron concentrations, conductivity, and mobility. The degree of crystallinity for the nanocrystalline films increases with F concentration and growth temperature. The F concentration in the films is proportional to that in the starting solution. Electron concentration and Hall mobility rise more slowly with F concentration. At their highest, both F and electron concentrations are ∼2% of the Sn concentration. In more lightly doped films, the electron concentration significantly exceeds the F concentration. The achieved resistivity of the doped films is lower than for undoped SnO2 film by 20 to 750 times. The infrared complex permittivity spectrum shows a shift in plasma wavelength from 15 to 2  μm with more than two orders increase in F concentration.

Platinum germanides for mid- and long-wave infrared plasmonics

Platinum germanides (PtGe) were investigated for infrared plasmonic applications. Layers of Pt and Ge were deposited and annealed. X-ray diffraction identified PtGe(2) and Pt(2)Ge(3) phases, and x-ray photo-electron spectroscopy determined vertical atomic composition profiles for the films. Complex permittivity spectra were measured by ellipsometry over the 2 to 15 μm wavelength range. Surface plasmon polariton (SPP) characteristics such as propagation length and field penetration depth were calculated. Photon-to-SPP couplers in the form of 1D lamellar gratings were fabricated and characterized in the range 9 - 10.5 μm via wavelength-dependent specular reflection spectra for multiple angles of incidence. The observed resonances compare well with calculated spectra for SPP excitation on PtGe(2). Platinum germanides are CMOS compatible and may serve as SPP hosts for on-chip mid-IR plasmonic components with tighter field confinement than noble-metal hosts.

Metal-oxide-semiconductor photocapacitor for sensing surface plasmon polaritons

An electronic detector of surface plasmon polaritons (SPP) is reported. SPPs optically excited on a metal surface using a prism coupler are detected by using a close-coupled metal-oxide-semiconductor capacitor. Semitransparent metal and graphene gates function similarly. We report the dependence of the photoresponse on substrate carrier type, carrier concentration, and back-contact biasing.

Mid-infrared extraordinary transmission through Ga-doped ZnO films with 2D hole arrays

Extraordinary optical transmission (EOT), through highly conductive ZnO films with sub-wavelength hole arrays is investigated in the long-wavelength infrared regime. EOT is facilitated by the excitation of surface plasmon polaritons (SPPs) on Ga-Doped ZnO films and can be tuned utilizing the physical parameters such as film thickness, period, hole size, and hole shape, as well as doping of the film. Analytical and finite-difference time-domain calculations are completed for 1 micron thick films with square, circular, and triangular hole arrays demonstrating SPP coupling and EOT. The fundamental plasmonic modes are observed in each of these hole shapes at wavelengths that correspond to strong EOT peaks. Doping tunability for these structures is also observed. Ga-doped ZnO films are grown via pulsed laser deposition (PLD) on silicon with plasma frequencies in the near-infrared. The sub-wavelength 2D hole arrays are fabricated in the Ga-doped ZnO films via standard lithography and etching processes. This highly conductive ZnO EOT structure may prove useful in novel integrated components such as tunable biosensors or surface plasmon coupling mechanisms.

Electronic detection of surface plasmon polaritons by metal-oxide-silicon capacitor

An electronic detector of surface plasmon polaritons (SPPs) is reported. SPPs optically excited on a metal surface using a prism coupler are detected by using a close-coupled metal-oxide-silicon (MOS) capacitor. Incidence-angle dependence is explained by Fresnel transmittance calculations, which also are used to investigate the dependence of photo-response on structure dimensions. Electrodynamic simulations agree with theory and experiment and additionally provide spatial intensity distributions on and off the SPP excitation resonance. Experimental dependence of the photoresponse on substrate carrier type, carrier concentration, and back-contact biasing is qualitatively explained by simple theory of MOS capacitors.

Infrared photonic to plasmonic couplers using spray deposited conductive metal oxides

In recent years, infrared plasmonics has turned towards materials that are wavelength and application tailorable, and which are geared towards CMOS processing. The transparent conductive oxides are very favorable towards infrared plasmonic applications for a number of reasons, one of which being the natural visible transparency due to their relatively large bandgap. Fluorine-doped tin oxide (FTO) is one such transparent and doping-tunable material that in addition is low cost due to spray deposition techniques that result in perfectly conformal coatings. In this work, a deposition recipe that gives high free carrier concentration was used to fabricate structures for demonstration of surface plasmon excitation. 1D gratings with a range of structural parameters were etched in silicon. Then the gratings were conformally coated with FTO by aqueous spray deposition. Excitation of surface plasmon polaritons (SPP) at mid- and long- wave infrared wavelengths on these gratings was demonstrated. The observed (SPP) excitation resonances agree will with analytical excitation calculations and numerical simulations. We show that grating heights of ~10-15% of the wavelength are optimum for achieving the strongest sharpest coupling to plasmonic resonances in the mid- and longwave infrared. The presented results are compared with similar etched silicon gratings coated with Ga-doped ZnO (GZO). The dominant difference between our FTO and GZO measurements is the free carrier concentration. The useful wavelength range is predicted for FTO based plasmonics and compared with other plasmonic host materials. The work presented here could play a key role in novel decreased-cost detectors, filters, and on-chip optoelectronics.

Surface characterization studies of orientation patterned ZnSe doped with Cr2

ZnSe doped with Cr2+ was analyzed by EDS, XPS and Micro-Raman spectroscopy techniques. EDS and XPS analysis revealed that chromium concentration is more than 2% and there are additional impurities, Ga, Ti, and Ta. EDS measurements did not reveal any variation in chromium concentration when a line scan was performed over a 200 μm distance. XPS analysis indicated that the sample surface is inhomogeneous. Photoluminescence was acquired by exciting the sample with 325 nm laser beam. Photoluminescence revealed charge transfer bands. Micro-Raman study revealed the LO, TO and 2TA modes at 252, 205 and 140 cm-1. Under 488 or 514.5 nm excitation background luminescence was predominant due to excitation of Cr2+ electrons into the conduction band. However, 632.8 nm laser excitation revealed, strong Raman signals. Raman data were acquired by exciting the sample on the grain boundary and inside the domain. The ratio of LO and TO peak intensities changed randomly when data were acquired from different points on the grain boundary indicating the presence of random strain in the material. When Raman data were acquired from different points on the sample surface for comparison, it revealed that the LO mode was distorted as well as broadened whereas the TO mode intensity increased. This was due to the presence of local modes induced by the sample inhomogeneity and the interaction of the holes with the LO mode.

Fabrication and characterization of an all optically addressed micromirror array

Abstract. We describe the fabrication process for an optically addressed adaptive optics array. The device consists of a micromirror array cascaded directly on wafer fused gallium arsenide (GaAs)-gallium phosphide (GaP) photodiodes. Optically addressing a photodiode generates a photocurrent which in turn causes a voltage drop across the cascaded mirror via an integrated thin film resistor. This architecture allows parallel optical addressing of individual elements without the need for wire bonding each pixel, which can enable higher density segmented type arrays. We first describe a fabrication process for releasing a free-standing array of low stress SixN micromirrors on an indium phosphide (InP) support substrate. We then present a process for transferring GaAs p-i-n photodiodes on a transparent GaP support substrate using a specially designed wafer fusion fixture. The two samples when stacked together and electrically connected via a specially formulated and patterned semiconductive SU-8 resist form the final device. We report mirror displacements of up to 500 nm using this technique while requiring an optical signal as low as 150  μW.