Daniel Wasserman

Fabrication of 5nm linewidth and 14nm pitch features by nanoimprint lithography

We report advances in nanoimprint lithography, its application in nanogap metal contacts, and related fabrication yield. We have demonstrated 5nm linewidth and 14nm linepitch in resist using nanoimprint lithography at room temperature with a pressure less than 15psi. We fabricated gold contacts (for the application of single macromolecule devices) with 5nm separation by nanoimprint in resist and lift-off of metal. Finally, the uniformity and manufacturability of nanoimprint over a 4in. wafer were demonstrated.

Strong absorption and selective thermal emission from a midinfrared metamaterial

We demonstrate thin-film metamaterials with resonances in the midinfrared (mid-IR) wavelength range. Our structures are numerically modeled and experimentally characterized by reflection and angularly resolved thermal emission spectroscopy. We demonstrate strong and controllable absorption resonances across the mid-IR wavelength range. In addition, the polarized thermal emission from these samples is shown to be highly selective and largely independent of emission angles from normal to 45°. Experimental results are compared to numerical models with excellent agreement. Such structures hold promise for large-area, low-cost metamaterial coatings for control of gray- or black-body thermal signatures, as well as for possible mid-IR sensing applications.

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.

Room-temperature continuous-wave quantum cascade lasers grown by MOCVD without lateral regrowth

We report on room-temperature continuous-wave (CW) operation of lambda~8.2 mum quantum cascade lasers grown by metal-organic chemical vapor deposition without lateral regrowth. The lasers have been processed as double-channel ridge waveguides with thick electroplated gold. CW output power of 5.3 mW is measured at 300 K with a threshold current density of 2.63 kA/cm2. The measured gain at room temperature is close to the theoretical design, which enables the lasers to overcome the relatively high waveguide loss

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.

Review of mid-infrared plasmonic materials

Abstract. The field of plasmonics has the potential to enable unique applications in the mid-infrared (IR) wavelength range. However, as is the case regardless of wavelength, the choice of plasmonic material has significant implications for the ultimate utility of any plasmonic device or structure. In this manuscript, we review the wide range of available plasmonic and phononic materials for mid-IR wavelengths, looking in particular at transition metal nitrides, transparent conducting oxides, silicides, doped semiconductors, and even newer plasmonic materials such as graphene. We also include in our survey materials with strong mid-IR phonon resonances, such as GaN, GaP, SiC, and the perovskite SrTiO3, all of which can support plasmon-like modes over limited wavelength ranges. We will discuss the suitability of each of these plasmonic and phononic materials, as well as the more traditional noble metals for a range of structures and applications and will discuss the potential and limitations of alternative plasmonic materials at these IR wavelengths.

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.

Wafer-scale production of uniform InAs y P 1-y nanowire array on silicon for heterogeneous integration

One-dimensional crystal growth allows the epitaxial integration of compound semiconductors on silicon (Si), as the large lattice-mismatch strain arising from heterointerfaces can be laterally relieved. Here, we report the direct heteroepitaxial growth of a mixed anion ternary InAsyP1-y nanowire array across an entire 2 in. Si wafer with unprecedented spatial, structural, and special uniformity across the entire 2 in. wafer and dramatic improvements in aspect ratio (>100) and area density (>5 × 10(8)/cm(2)). Heterojunction solar cells consisting of n-type InAsyP1-y (y = 0.75) and p-type Si achieve a conversion efficiency of 3.6% under air mass 1.5 illumination. This work demonstrates the potential for large-scale production of these nanowires for heterogeneous integration of optoelectronic devices.

High-Performance Quantum Cascade Lasers: Optimized Design Through Waveguide and Thermal Modeling

We present a comprehensive model to study the thermal effects in quantum cascade (QC) lasers for continuous-wave (CW) operation at and above room temperature. This model self-consistently solves the temperature-dependent threshold current density equation and heat equation to determine the CW threshold current density, maximum heat sink temperature, and core temperature at threshold for a given laser design. The model includes effects from temperature dependence on thermal backfilling, thermal conductivity, phonon lifetimes, gain bandwidth, thermionic emission, and resistive heating in waveguide layers. Studies on these effects yield results not simultaneously considered by previous models. By including these results in laser designs, lasers with lower core temperatures, with higher operating temperatures, and requiring lower electrical power than current high-performance lasers are predicted. Additionally, experimental results are presented, exploring various methods of improving CW laser performance for a lambda ~ 8 mum QC laser and are compared to the model.

Midinfrared doping-tunable extraordinary transmission from sub-wavelength Gratings

The authors demonstrate doping-tunable extraordinary transmission from subwavelength apertures in a periodic two-dimensional metallic grating deposited upon n-doped GaAs. By varying the doping of the underlying GaAs epilayer, they demonstrate wavelength tunability of ∼23cm−1 or approximately 0.15μm. The authors have achieved transmission peaks as narrow as 18cm−1 with such gratings, which suggests that devices based on midinfrared extraordinary transmission gratings could be used in external cavity structures for quantum cascade lasers, or as tunable filters or modulators for midinfrared applications.