Stephanie Law

Towards nano-scale photonics with micro-scale photons: the opportunities and challenges of mid-infrared plasmonics

Abstract Surface plasmon polaritons and their localized counterparts, surface plasmons, are widely used at visible and near-infrared (near-IR) frequencies to confine, enhance, and manipulate light on the subwavelength scale. At these frequencies, surface plasmons serve as enabling mechanisms for future on-chip communications architectures, high-performance sensors, and high-resolution imaging and lithography systems. Successful implementation of plasmonics-inspired solutions at longer wavelengths, in the mid-infrared (mid-IR) frequency range, would benefit a number of highly important technologies in health- and defense-related fields that include trace-gas detection, heat-signature sensing, mimicking, and cloaking, and source and detector development. However, the body of knowledge of visible/near-IR frequency plasmonics cannot be easily transferred to the mid-IR due to the fundamentally different material response of metals in these two frequency ranges. Therefore, mid-IR plasmonic architectures for subwavelength light manipulation require both new materials and new geometries. In this work we attempt to provide a comprehensive review of recent approaches to realize nano-scale plasmonic devices and structures operating at mid-IR wavelengths. We first discuss the motivation for the development of the field of mid-IR plasmonics and the fundamental differences between plasmonics in the mid-IR and at shorter wavelengths. We then discuss early plasmonics work in the mid-IR using traditional plasmonic metals, illuminating both the impressive results of this work, as well as the challenges arising from the very different behavior of metals in the mid-IR, when compared to shorter wavelengths. Finally, we discuss the potential of new classes of mid-IR plasmonic materials, capable of mimicking the behavior of traditional metals at shorter wavelengths, and allowing for true subwavelength, and ultimately, nano-scale confinement at long wavelengths.

All-Semiconductor Plasmonic Nanoantennas for Infrared Sensing

Infrared absorption spectroscopy of vibro-rotational molecular resonances provides a powerful method for investigation of a wide range of molecules and molecular compounds. However, the wavelength of light required to excite these resonances is often orders of magnitude larger than the absorption cross sections of the molecules under investigation. This mismatch makes infrared detection and identification of nanoscale volumes of material challenging. Here we demonstrate a new type of infrared plasmonic antenna for long-wavelength nanoscale enhanced sensing. The plasmonic materials utilized are epitaxially grown semiconductor engineered metals, which results in high-quality, low-loss infrared plasmonic metals with tunable optical properties. Nanoantennas are fabricated using nanosphere lithography, allowing for cost-effective and large-area fabrication of nanoscale structures. Antenna arrays are optically characterized as a function of both the antenna geometry and the optical properties of the plasmonic semiconductor metals. Thin, weakly absorbing polymer layers are deposited upon the antenna arrays, and we are able to observe very weak molecular absorption signatures when these signatures are in spectral proximity to the antenna resonance. Experimental results are supported with finite element modeling with strong agreement.

Strong absorption and selective emission from engineered metals with dielectric coatings.

We demonstrate strong-to-perfect absorption across a wide range of mid-infrared wavelengths (5-12µm) using a two-layer system consisting of heavily-doped silicon and a thin high-index germanium dielectric layer. We demonstrate spectral control of the absorption resonance by varying the thickness of the dielectric layer. The absorption resonance is shown to be largely polarization-independent and angle-invariant. Upon heating, we observe selective thermal emission from our materials. Experimental data is compared to an analytical model of our structures with strong agreement.

Epitaxial growth of engineered metals for mid-infrared plasmonics

The authors demonstrate the ability of high-quality epitaxial InAs films to be used as wavelength-flexible, low-loss, engineered plasmonic metals across the mid-infrared spectral range. Films are grown by molecular beam epitaxy and characterized by Hall effect measurements, atomic force microscopy, and infrared reflection and transmission spectroscopy. The losses of our plasmonic material are studied as a function of InAs doping density, growth rate, buffer layer type, and substrate type. High growth rates are shown to be integral to obtaining films with low losses and doping densities approaching 1×1020 cm−3.

Doped semiconductors with band-edge plasma frequencies

In this work, the authors demonstrate the potential of epitaxially grown highly doped InSb as an engineered, wavelength-flexible mid-IR plasmonic material. The authors achieve doping concentrations over an order of magnitude larger than previously published results and show that such materials have plasma frequencies corresponding to energies larger than the materials band-gap. These semiconductor-based plasmonic metals open the door to homoepitaxial integration of plasmonic or epsilon-near-zero materials with optoelectronic devices at mid-infrared wavelengths. The materials are characterized by Hall measurements, mid-infrared transmission and reflection spectroscopy, and near-infrared transmission spectroscopy. The opportunities offered and the limitations presented by this material system are discussed and analyzed.

Near-field infrared absorption of plasmonic semiconductor microparticles studied using atomic force microscope infrared spectroscopy

We report measurements of near-field absorption in heavily silicon-doped indium arsenide microparticles using atomic force microscope infrared spectroscopy (AFM-IR). The microparticles exhibit an infrared absorption peak at 5.75u2009μm, which corresponds to a localized surface plasmon resonance within the microparticles. The near-field absorption measurements agree with far-field measurements of transmission and reflection, and with results of numerical solutions of Maxwell equations. AFM-IR measurements of a single microparticle show the temperature increase expected from Ohmic heating within the particle, highlighting the potential for high resolution infrared imaging of plasmonic and metamaterial structures.

Precise measurements of radio-frequency magnetic susceptibility in ferromagnetic and antiferromagnetic materials

Abstract Dynamic magnetic susceptibility, χ , was studied in several intermetallic materials exhibiting ferromagnetic, antiferromagnetic and metamagnetic transitions. Precise measurements by using a 14 MHz tunnel diode oscillator (TDO) allow detailed insight into the field and temperature dependence of χ . In particular, local moment ferromagnets show a sharp peak in χ ( T ) near the Curie temperature, T C . The peak amplitude decreases and shifts to higher temperatures with very small applied dc fields. Anisotropic measurements of CeVSb 3 show that this peak is present provided the magnetic easy axis is aligned with the excitation field. In a striking contrast, small moment, itinerant ferromagnets (i.e., ZrZn 2 ) show a broad maximum in χ ( T ) that responds differently to applied field. We believe that TDO measurements provide a very sensitive way to distinguish between local and itinerant moment magnetic orders. Local moment antiferromagnets do not show a peak at the Neel temperature, T N , but only a sharp decrease of χ below T N due to the loss of spin-disorder scattering changing the penetration depth of the ac excitation field. Furthermore, we show that the TDO is capable of detecting changes in spin order as well as metamagnetic transitions. Finally, critical scaling of χ ( T , H ) in the vicinity of T C is discussed in CeVSb 3 and CeAgSb 2 .

Engineering absorption and blackbody radiation in the far-infrared with surface phonon polaritons on gallium phosphide

We demonstrate excitation of surface phonon polaritons on patterned gallium phosphide surfaces. Control over the light-polariton coupling frequencies is demonstrated by changing the pattern periodicity and used to experimentally determine the gallium phosphide surface phonon polariton dispersion curve. Selective emission via out-coupling of thermally excited surface phonon polaritons is experimentally demonstrated. Samples are characterized experimentally by Fourier transform infrared reflection and emission spectroscopy, and modeled using finite element techniques and rigorous coupled wave analysis. The use of phonon resonances for control of emissivity and excitation of bound surface waves offers a potential tool for the exploration of long-wavelength Reststrahlen band frequencies.

Single-material semiconductor hyperbolic metamaterials.

Layered semiconductor hyperbolic metamaterials for the mid-infrared are grown by molecular beam epitaxy using a single material system, doped and undoped InAs. The onset wavelength for metamaterial behavior can be tuned from 5.8μm to beyond 10μm, while the fill factor ranges from 0.25 to 0.75, resulting in designer optical behavior. The reflection and transmission behavior were studied by Fourier transform spectroscopy and modeled using effective medium theory. We also conducted a geometric optics experiment to demonstrate negative refraction of our materials.

Flat mid-infrared composite plasmonic materials using lateral doping-patterned semiconductors

We demonstrate lateral control of carrier concentration in doped Si for mid-infrared plasmonic applications. Using commercially available spin-dopants, we show that doped silicon can act as a plasmonic material at mid-infrared wavelengths, and that control of the doping pattern allows for the development of flat, single-material plasmonic composites. Our materials are characterized by infrared spectroscopy and microscopy, surface profilometry and infrared emissivity measurements. We demonstrate the ability to fabricate subwavelength doped features and show distinct diffraction from one-dimensional arrays of ‘metal’ lines patterned in our material system.