Farnood Khalilzadeh-Rezaie

Robust multimaterial tellurium-based chalcogenide glass fibers for mid-wave and long-wave infrared transmission

We describe an approach for producing robust multimaterial chalcogenide glass fibers for mid-wave and long-wave mid-infrared transmission. By combining the traditional rod-in-tube process with multimaterial coextrusion, we prepare a hybrid glass-polymer preform that is drawn continuously into a robust step-index fiber with a built-in, thermally compatible polymer jacket. Using tellurium-based chalcogenides, the fibers have a transparency window covering the 3-12 μm spectral range, making them particularly attractive for delivering quantum cascade laser light and in space applications.

Infrared surface polaritons on bismuth

Abstract. Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40  μm. The bulk plasma frequency ωp and high-frequency limit to the permittivity ε∞ were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time τ and effective mass m*. This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity.

Fluorine-doped tin oxides for mid-infrared plasmonics

Fluorine-doped tin oxides (FTO) were investigated for infrared plasmonic applications. Nano-crystalline FTO thin films were grown by the SPEED chemical-spray deposition method. Complex permittivity spectra were measured from 1.6 to 12 μm wavelength. These spectra were used to calculate materials parameters, which compared well with values from transport measurements, and to predict characteristics of surface plasmon polaritons (SPP). Reflectivity spectra for lamellar FTO gratings revealed SPP coupling resonances in good agreement with predictions. The FTO film studied here is well suited for plasmonic applications in the important 3-5 μm wavelength range.

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.

Infra-red spectral microscopy of standing-wave resonances in single metal-dielectric-metal thin-film cavity

Resonantly absorbing thin films comprising periodically sub-wavelength structured metal surface, dielectric spacer, and metal ground plane are a topic of current interest with important applications. These structures are frequently described as “metamaterials”, where effective permittivity and permeability with dispersion near electric and magnetic resonances allow impedance matching to free space for maximum absorption. In this paper, we compare synchrotron-based infrared spectral microscopy of a single isolated unit cell and a periodic array, and we show that the resonances have little to do with periodicity. Instead, the observed absorption spectra of usual periodically structured thin films are best described as due to standing-wave resonances within each independent unit cell, rather than as due to effective optical constants of a metamaterial. The effect of having arrays of unit cells is mainly to strengthen the absorption by increasing the fill factor, and such arrays need not be periodic. Initial work toward applying the subject absorbers to room-temperature bolometer arrays is presented.

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.