Mahesh Krishnamurthi

Two-phonon coupling to the antiferromagnetic phase transition in multiferroic BiFeO3

A prominent band centered at ∼1000–1300cm−1 and associated with resonant enhancement of two-phonon Raman scattering is reported in multiferroic BiFeO3 thin films and single crystals. A strong anomaly in this band occurs at the antiferromagnetic Neel temperature, TN∼375°C. This band is composed of three peaks, assigned to 2A4, 2E8, and 2E9 Raman modes. While all three peaks were found to be sensitive to the antiferromagnetic phase transition, the 2E8 mode, in particular, nearly disappears at TN on heating, indicating a strong spin-two-phonon coupling in BiFeO3.

Zinc Selenide Optical Fibers

Semiconductor waveguide fabrication for photonics applications is usually performed in a planar geometry. However, over the past decade a new field of semiconductor-based optical fiber devices has emerged. The drawing of soft chalcogenide semiconductor glasses together with low melting point metals allows for meters-long distributed photoconductive detectors, for example.[1,2] Crystalline unary semiconductors (e.g., Si, Ge) have been chemically deposited at high pressure into silica capillaries,[3,4] allowing the optical and electronic properties of these materials to be exploited for applications such as all-fiber optoelectronics.[5-7] In contrast to planar rib and ridge waveguides with rectilinear cross sections that generally give rise to polarization dependence, the cylindrical fiber waveguides have the advantage of a circular, polarization-independent cross section. Furthermore, the fiber pores, and thus the wires deposited in them, are exceptionally smooth[8] with extremely uniform diameter over their entire length. The high-pressure chemical vapor deposition (HPCVD) technique is simple, low cost, and flexible so that it can be modified to fill a range of capillaries with differing core dimensions, while high production rates can be obtained by parallel fabrication of multiple fibers in a single deposition. It can also be extended to fill the large number of micro- and nanoscale pores in microstructured optical fibers (MOFs), providing additional geometrical design flexibility to enhance the potential application base of the fiber devices.[9] Semiconductor fibers fabricated via HPCVD in silica pores also retain the inherent characteristics of silica fibers, including their robustness and compatibility with existing optical fiber infrastructure, thus presenting considerable advantages over fibers based on multicomponent soft glasses.

Silicon p‐i‐n Junction Fibers

Flexible Si p-i-n junction fibers made by high pressure chemical vapor deposition offer new opportunities in textile photovoltaics and optoelectronics, as exemplified by their photovoltaic properties, gigahertz bandwidth for photodetection, and ability to waveguide light.

Mid-infrared transmission properties of amorphous germanium optical fibers

Germanium optical fibers have been fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The amorphous germanium core material has a small percentage of hydrogen that saturates the dangling bonds to reduce absorption loss. Optical transmission measurements were performed to determine the linear losses over a broad mid-infrared wavelength range with the lowest loss recorded at 10.6 µm. The extended transmission range measured in the germanium fibers demonstrates their potential for use in mid-infrared applications.

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

High‐Pressure Chemical Deposition for Void‐Free Filling of Extreme Aspect Ratio Templates

Extreme aspect ratio semiconductor structures are critical to modern optoelectronic technology because of their ability to waveguide light and transport electrons. Waveguides formed from almost any material by conventional micro/nanofabrication techniques typically have significant surface roughness that scatters light and is a constraining factor in most optoelectronic devices. For example, fabricated planar silica waveguides have optical losses 3 to 5 orders of magnitude higher than silica fibers, in part due to surface roughness. For these reasons silica nanofibers have been proposed as alternatives to fabricated silica or semiconductor channels for waveguiding of light in miniaturized optical devices, as they meet the strict requirements for surface roughness and diameter uniformity required for low loss. An additional advantage of these silica fibers is that they have a circular cross section that can simultaneously guide both transverse electric (TE) and transverse magnetic (TM) polarizations without cutoff. In contrast the rectilinear cross sections of microfabricated planar waveguides can effectively guide only one polarization without cutoff. However, semiconductors in general exhibit a far broader range of useful optoelectronic function than silica glass because of their ability to form hetero and homojunctions, serve as optical gain media over a broad range of wavelengths, and their superior non-linear optical properties.

Two dimensional dynamic focusing of laser light by ferroelectric domain based electro-optic lenses

The authors demonstrate the proof of concept of two dimensional focusing of laser light. This has been achieved by using a combination of two cylindrical electro-optic ferroelectric domain lens stacks in an orthogonal geometry. The devices were fabricated on z-cut lithium tantalate (LiTaO3) wafers and tested with helium-neon laser at 633nm. Continuously tunable optical power ranging from −129m−1to129m−1 is obtained in both directions by varying the applied voltage.

Design and simulation of planar electro-optic switches in ferroelectrics

Conceptual design and numerical simulation of two polarization dependent planar optical switches based on the electro-optic effect in ferroelectrics operating at 1.55 μm wavelength are presented. The first design is a 3×3 optical switch based entirely on electro-optic beam steering (prism) elements and ion-exchanged lenses for collimation. The second design is a 1×N optical switch based on a combination of electro-optic beam steering and electro-optic focusing (lens) elements. The scalability of this device has been improved by compensating the in-plane divergence of the laser. Analytical expressions for the dependence of scalability are presented.

Array of tapered semiconductor waveguides in a fiber for infrared image transfer and magnification

The proof-of-concept of an infrared imaging tip by an array of infrared waveguides tapered as small as 2 μm is demonstrated. The fabrication is based on a high-pressure chemical fluid deposition technique to deposit precisely defined periodic arrays of Ge and Si waveguides within a microstructured optical fiber template made of silica to demonstrate the proposed concept at wavelengths of 10.64 µm and 1.55 µm, respectively. The essential features of the imaging system such as isolation between adjacent pixels, magnification, optical throughput, and image transfer characteristics are investigated. Near-field scanning at 3.39 μm wavelength using a single tapered Ge core is also demonstrated.

A magnifying fiber element with an array of sub-wavelength Ge/ZnSe pixel waveguides for infrared imaging

We demonstrate an array of tapered Ge-core/ZnSe-cladding waveguides in a silica fiber matrix for infrared image transfer and a pixel magnification of 3.5× at 3.39µm and 10.64µm wavelengths. The structure was synthesized by a high-pressure chemical vapor deposition technique to deposit the semiconductor waveguides within the holes of a silica based microstructured optical fiber. The core/cladding structure reduces the optical propagation loss through the waveguides, and good isolation between the pixels is demonstrated. With further material improvements, these structures could be useful for applications such as infrared endoscopic imaging.