Justin W. Cleary

Infrared surface plasmons on heavily doped silicon

wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 lm. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 lm wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965). V C 2011 American Institute of Physics. [doi:10.1063/1.3672738]

IR permittivities for silicides and doped silicon

The complex permittivity for Pt, Pd, Ni, and Ti-silicide films as well as heavily doped p- and n-type silicon were determined by ellipsometry over the energy range 0.031 eV to 4.0 eV. Fits to the Drude model gave bulk plasma and relaxation frequencies. Rutherford backscattering spectroscopy, X-ray diffraction, scanning electron microscopy, secondary ion mass spectrometry, and four-point probe measurements complemented the optical characterization. Calculations from measured permittivities of waveguide loss and mode confinement suggest that the considered materials are better suited for long-wavelength surface-plasmon-polariton waveguide applications than metal films.

Mid- to long-wavelength infrared plasmonic-photonics using heavily doped n-Ge/Ge and n-GeSn/GeSn heterostructures

Heavily doped n-type Ge and GeSn are investigated as plasmonic conductors for integration with undoped dielectrics of Si, SiGe, Ge, and GeSn in order to create a foundry-based group IV plasmonics technology. N-type Ge1-xSnx with compositions of 0 ≤ x ≤ 0.115 are investigated utilizing effective-mass theory and Drude considerations. The plasma wavelengths, relaxation times, and complex permittivities are determined as functions of the free carrier concentration over the range of 10(10) to 10(21) cm-3. Basic plasmonic properties such as propagation loss and mode height are calculated and example numerical simulations are shown of a dielectric-conductor-dielectric ribbon waveguide structure are shown. Practical operation in the 2 to 20 μm wavelength range is predicted.

Long-wave infrared surface plasmon grating coupler

We present a simplified analytic formula that may be used to design gratings intended to couple long-wave infrared radiation to surface plasmons. It is based on the theory of Hessel and Oliner (1965). The recipe is semiempirical, in that it requires knowledge of a surface-impedance modulation amplitude, which is found here as a function of the grating groove depth and the wavelength for silver lamellar gratings at CO(2) laser wavelengths. The optimum groove depth for photon-to-surface-plasmon energy conversion was found by experiment and calculation to be approximately 10%-15% of the wavelength. This value is about twice what has been reported previously in the visible spectral range for sinusoidal grating profiles.

Long-wave infrared tunable thin-film perfect absorber utilizing highly doped silicon-on-sapphire

We show that nearly perfect absorption can be achieved in a simple structure with highly doped silicon on a sapphire (SOS) substrate. An SOS structure with the n-Si film being 600 nm thick and having doping concentration of 2e19 cm(-3) has an absorption peak of 96% in the film at a wavelength of 12.1 μm. More generally, 95% absorption in the n-Si can be achieved and tailored to specific wavelengths in the range of 11.6-15.1 μm utilizing dopings of 1-2.4e19 cm(-3) and film thicknesses of 600-1000 nm. Regions of 90% absorption can be achievable down to 11 μm and up to as much as 22 μm with tailoring of doping and film thickness. It is also shown that choice of substrate with large k/n (imaginary over real part of refractive index) is imperative for high absorption in the thin-film and will play a role in tailoring possibilities. Shown here are results for n-Si, but in general these results also apply to p-Si and the methods may be used to investigate structures with alternative films or substrates. This investigated SOS structure has high potential since desired film thickness and doping investigated here for perfect absorption can be purchased commercially and easily tuned by etching the silicon film.

Far-infrared absorber based on standing-wave resonances in metal-dielectric-metal cavity.

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ~53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers.

Infrared surface polaritons on antimony

The semimetal antimony, with a plasma frequency ~80 times less than that of gold, is potentially useful as a host for infrared surface polaritons (SPs). Relevant IR SP properties, including the frequency-dependent propagation length and penetration depths for fields into the media on either side of the interface, were determined from optical constants measured on optically-thick thermally-evaporated Sb films over the wavelength range 1 to 40 μm. Plasma and carrier relaxation frequencies were determined from Drude-model fits to these data. The real part of the permittivity is negative for wavelengths beyond 11 μm. Distinct resonant decreases in specular reflected intensity were observed for Sb lamellar gratings in the wavelength range of 6 to 11 μm, where the real part of the permittivity is positive. Both resonance angles and the angular reflectance spectral line shapes are in agreement with theory for excitation of bound surface electromagnetic waves (SPs). Finite element method (FEM) electrodynamic simulations indicate the existence of SP modes under conditions matching the experiments. FEM results also show that such waves depend on having a significant imaginary part of the permittivity, as has been noted earlier for the case of surface exciton polaritons.

Infrared surface plasmon resonance biosensor

A Surface Plasmon Resonance (SPR) biosensor that operates deep into the infrared (3-11 μm wavelengths) is potentially capable of biomolecule recognition based both on selective binding and on characteristic vibrational modes. A goal is to operate specifically at wavelengths where biological analytes are strongly differentiated by their IR absorption spectra and where the refractive index is increased by dispersion. This will provide enhanced sensitivity and selectivity, when biological analytes bind reversibly to biomolecular recognition elements attached to the sensor surface. This paper describes work on the optical and materials aspects of IR surface plasmon resonances. First, three possible coupling schemes are considered: hemicylindrical prisms, triangular prisms, and gratings. Second, materials with plasma frequencies one order of magnitude smaller than for noble metals are considered, including doped semiconductors and semimetals.

Silicides for Infrared Surface Plasmon Resonance Biosensors

Pt-, Pd-, Ni-, and Ti-silicide films on silicon were evaluated as conducting hosts for surface plasmon polaritons (SPP) in proposed long-wave IR (LWIR) attenuated total reflection biosensors. Original LWIR complex permittivity data was collected, from which SPP properties were determined and compared with those for noble metals. LWIR SPPs on silicide films were found to offer enhanced sensitivity to thinner biological entities than when usual metal films are used.

Propagation of high-frequency surface plasmons on gold

Propagation of surface plasmons on gold in the range 2.8-3.5 eV over 0.1-1.6 μm distances was characterized by cathodoluminescence spectroscopy. Surface plasmons were excited by an electron beam near a grating milled in the gold. The spectra of outcoupled radiation reveal increasingly strong propagation losses as surface plasmon energy increases above 2.8 eV, but little effect in the range 1.6-2.8 eV. These results are in partial agreement with theoretical expectations.