Sameer Walavalkar

Frequency tunable near-infrared metamaterials based on VO_2 phase transition

Engineering metamaterials with tunable resonances from mid-infrared to near-infrared wavelengths could have far-reaching consequences for chip based optical devices, active filters, modulators, and sensors. Utilizing the metal-insulator phase transition in vanadium oxide (VO(2)), we demonstrate frequency-tunable metamaterials in the near-IR range, from 1.5 - 5 microns. Arrays of Ag split ring resonators (SRRs) are patterned with e-beam lithography onto planar VO(2) and etched via reactive ion etching to yield Ag/VO(2) hybrid SRRs. FTIR reflection data and FDTD simulation results show the resonant peak position red shifts upon heating above the phase transition temperature. We also show that, by including coupling elements in the design of these hybrid Ag/VO(2) bi-layer structures, we can achieve resonant peak position tuning of up to 110 nm.

Alumina etch masks for fabrication of high-aspect-ratio silicon micropillars and nanopillars

We introduce using sputtered aluminum oxide (alumina) as a resilient etch mask for fluorinated silicon reactive ion etches. Achieving selectivity of 5000:1 for cryogenic silicon etching and 68:1 for SF(6)/C(4)F(8) silicon etching, we employ this mask for fabrication of high-aspect-ratio silicon micropillars and nanopillars. Nanopillars with diameters ranging from below 50 nm up to several hundred nanometers are etched to heights greater than 2 microm. Micropillars of 5, 10, 20, and 50 microm diameters are etched to heights of over 150 microm with aspect ratios greater than 25. Processing and characterization of the sputtered alumina is also discussed.

Tunable Visible and Near-IR Emission from Sub-10 nm Etched Single-Crystal Si Nanopillars

Visible and near-IR photoluminescence (PL) is reported from sub-10 nm silicon nanopillars. Pillars were plasma etched from single crystal Si wafers and thinned by utilizing strain-induced, self-terminating oxidation of cylindrical structures. PL, lifetime, and transmission electron microscopy were performed to measure the dimensions and emission characteristics of the pillars. The peak PL energy was found to blue shift with narrowing pillar diameter in accordance with a quantum confinement effect. The blue shift was quantified using a tight binding method simulation that incorporated the strain induced by the thermal oxidation process. These pillars show promise as possible complementary metal oxide semiconductor compatible silicon devices in the form of light-emitting diode or laser structures.

Size Tunable Visible and Near-Infrared Photoluminescence from Vertically Etched Silicon Quantum Dots

Corrugated etching techniques were used to fabricate size-tunable silicon quantum dots that luminesce under photoexcitation, tunable over the visible and near infrared. By using the fidelity of lithographic patterning and strain limited, self-terminating oxidation, uniform arrays of pillar containing stacked quantum dots as small as 2 nm were patterned. Furthermore, an array of pillars, with multiple similar sized quantum dots on each pillar, was fabricated and tested. The photoluminescence displayed a multiple, closely peaked emission spectra corresponding to quantum dots with a narrow size distribution. Similar structures can provide quantum confinement effects for future nanophotonic and nanoelectronic devices.

Controllable deformation of silicon nanowires with strain up to 24

Fabricated silicon nanostructures demonstrate mechanical properties unlike their macroscopic counterparts. Here we use a force mediating polymer to controllably and reversibly deform silicon nanowires. This technique is demonstrated on multiple nanowire configurations, which undergo deformation without noticeable macroscopic damage after the polymer is removed. Calculations estimate a maximum of nearly 24% strain induced in 30 nm diameter pillars. The use of an electron activated polymer allows retention of the strained configuration without any external input. As a further illustration of this technique, we demonstrate nanoscale tweezing by capturing 300 nm alumina beads using circular arrays of these silicon nanowires.

Microscaled and Nanoscaled Platinum Sensors

We show small and robust platinum resistive heaters and thermometers that are defined by microlithography on silicon substrates. These devices can be used for a wide range of applications, including thermal sensor arrays, programmable thermal sources, and even incandescent light emitters. To explore the miniaturization of such devices, we have developed microscaled and nanoscaled platinum resistor arrays with wire widths as small as 75 nm, fabricated lithographically to provide highly localized heating and accurate resistance (and hence temperature) measurements. We present some of these potential applications of microfabricated platinum resistors in sensing and spectroscopy.

Scalable Method for the Fabrication and Testing of Glass-Filled, Three-Dimensionally Sculpted Extraordinary Transmission Apertures

This Letter features a new, scalable fabrication method and experimental characterization of glass-filled apertures exhibiting extraordinary transmission. These apertures are fabricated with sizes, aspect ratios, shapes, and side-wall profiles previously impossible to create. The fabrication method presented utilizes top-down lithography to etch silicon nanostructures. These nanostructures are oxidized to provide a transparent template for the deposition of a plasmonic metal. Gold is deposited around these structures, reflowed, and the surface is planarized. Finally, a window is etched through the substrate to provide optical access. Among the structures created and tested are apertures with height to diameter aspect ratios of 8:1, constructed with rectangular, square, cruciform, and coupled cross sections, with tunable polarization sensitivity and displaying unique properties based on their sculpted side-wall shape. Transmission data from these aperture arrays is collected and compared to examine the role of spacing, size, and shape on their overall spectral response. The structures this Letter describes can have a variety of novel applications from the creation of new types of light sources to massively multiplexed biosensors to subdiffraction limit imaging techniques.

Coulomb Blockade in Vertical, Bandgap Engineered Silicon Nanopillars

Vertically oriented, bandgap engineered silicon double tunnel junction nanopillars were fabricated and electrically addressed. The devices were tested at liquid nitrogen and room temperatures. Distinctive staircase steps in current were observed at cryogenic temperatures indicative of the Coulomb blockade effect present in asymmetric double tunnel junction structures. These features disappeared when the device was measured at room temperature.

High-Q Impurity Photon States Bounded by a Photonic Band Pseudogap in an Optically Thick Photonic Crystal Slab

We show that, taking a two-dimensional photonic crystal slab system as an example, surprisingly high quality factors (Q) over 10^5 are achievable, even in the absence of a rigorous photonic band gap. We find that the density of in-plane Bloch modes can be controlled by creating additional photon feedback from a finite-size photonic-crystal boundary that serves as a low-Q resonator. This mechanism enables significant reduction in the coupling strength between the bound state and the extended Bloch modes by more than a factor of 40.

Silicon photonic components for integrated optical systems

We show that the properties of silicon can be changed profoundly when nanofabricated to below 5nm, resulting in larger bandgap emission and more efficient light generation. Moreover, three-dimensional nanostructures can be fabricated at these nanoscale dimensions by dynamic control over the dry etching process.