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30 MeV silicon ion irradiation of silica glass containing 10 nm silver nanocrystals causes alignment of the nanocrystals in closely spaced linear arrays along the ion tracks. Optical transmission measurements show a 1.5 eV splitting of the surface plasmon resonant absorption bands for polarizations longitudinal and transversal to the arrays. The resulting material is a highly anisotropic glass that absorbs blue light of one polarization, and near-infrared light of the orthogonal polarization. Finite-difference time domain simulations are used to explore the effects of interparticle spacing and total array length on the absorption properties.
This paper presents research funded under the Defense Advanced Research Projects Agency (DARPA) MetaMaterials program for design and development of nanoparticle based, mesoscale electromagnetic and optical materials. Specifically, we present results of formulation and near infrared measurement-model validation for photoassisted, self-assembled multilayer metallic nanoparticle films. The multilayer films may be used as optical filters and absorbers. We demonstrate that nanoparticles can be formed in advanced polymer films that exhibit new electromagnetic constitutive properties. Metal nanoparticle films are produced from a single homogeneous resin containing a soluble precursor. Films cast from doped resins are exposed to UV radiation followed by a controlled thermal cure. The combination of UV exposure and thermal curing creates a multiphase material composed of low volume fractions of dispersed metallic Pd clusters (10–20 nm in size) and high concentrations of Pd nanoparticles which form surface and embedded metallic layers in the films. The layer separation is a function of UV exposure. These materials show significant absorption in the optical and near IR region of the spectrum. Furthermore, these films exhibit mechanical properties similar to bi-metallic layers, specifically, the films display reversible bending with exposure to light and an accompanying rapid temperature increase. This paper presents formulation processes, optical-mechanical measurements and measurement model comparison.
We developed several optical elements with subwavelength-structured surfaces. Antireflection surfaces were fabricated on a diffraction grating. A micro-retarder array realized by the form-birefringent effect has been made for an application to a polarization camera system. And we developed narrow-band reflection wavelength filters called “guided-mode resonant grating filters”. This filter bases on a coupling of guided mode and radiation mode. After describing some examples of the filters, we mention a grating structure for an optical switch with nonlinear optical material.
The enhancement of second- and third-harmonic generation (SHG and THG) in all-silicon coupled microcavities (CMC) formed from mesoporous silicon photonic crystals are observed at the resonance of the fundamental radiation with the CMC eigenmodes. Angular splitting of the peaks in intensity spectra of SHG and THG shows monotonous dependence on magnitude of coupling between two identical microcavity spacers controlled by the reflectivity of the intermediate Bragg reflector.
Using time resolved ultrafast spectroscopy, we have demonstrated that the far infrared (FIR) excitations in ferroelectric crystals may be modified through an arsenal of control techniques from the fields of guided waves, geometrical and Fourier optics, and optical pulse shaping. We show that LiNbO3 and LiTaO3 crystals of 10–250 μm thickness behave as slab waveguides for phonon-polaritons, which are admixtures of electromagnetic waves and lattice vibrations, when the polariton wavelength is on the order of or greater than the crystal thickness. Furthermore, we show that ferroelectric crystals are amenable to processing by ultrafast laser ablation, allowing for milling of user-defined patterns designed for guidance and control of phonon-polariton propagation. We have fabricated several functional structures including THz rectangular waveguides, resonators, splitters/couplers, interferometers, focusing reflectors, and diffractive elements. Electric field enhancement has been obtained with the reflective structures, through spatial shaping, of the optical excitation beam used for phonon-polariton generation, and through temporal pulse shaping to permit repetitive excitation of a phonon-polariton resonant cavity.
A hexagonal lattice photonic crystal was fabricated inside the metallic microcavity. And a thin film of Alq3 was incorporated inside the textured cavity as an active medium. The microcavity is designed such that the modified photonic modes due to the textured structure can couple to the excited electronic states of Alq3. This leads to changes in the emission characteristics of Alq3. From the angle-resolved transmission (ARTR) results, the photonic bandgap was observed at all angles from normal incident to 60°. The presence of surface plasmon (SP) was observed in both TM and TE modes of the transmission. Compare to the bulk Alq3 photoluminescence spectrum, significant modification of the photoluminescence (PL) spectrum was observed in the angle-resolved photoluminescence (ARPL). The photoluminescence spectra showed clear suppression in luminescence intensity for the range inside the photonic bandgap. We use decouple approximation for the standing wave modes and derive the photonic waveguide characteristics for two-dimensional textured metallic microcavities. The theoretical result is in good agreement to the experimental result.
We present and experimentally validate self-collimation in planar photonic crystals as a new means of achieving structureless confinement of light in optical devices. We demonstrate the ability to arbitrarily guide and route light by exploiting the dispersive characteristics of the photonic crystal. Propagation loss as low as 2.17 dB/mm is measured, and the experimental validation of routing structures are presented.
We demonstrate a micron-size planar silicon photonic device that is able to detect low concentrations of metal nano-particles approaching single particle detection. This sensitivity is achieved by using strong light confining structures that enhance the extinction cross-section of metal nano-particles by orders of magnitude. Structures were fabricated and measurements of the transmission spectra of the devices demonstrate the detection of 10 nm diameter gold particles resting on the device with a density of fewer than 2 particles per 104 nm2 (the area of the sensing region surface). Using such a device, in a fluidic platform, one could detect the presence of a single metal nano-particle specifically bound to various analytes, enabling ultrasensitive detection of analytes including DNA, RNA, proteins, and antigens.
We report a technique for the formation of infiltrated and inverse opal structures that produces high quality, low porosity conformal material structures. ZnS:Mn and TiO2 were deposited within the void space of an opal lattice by atomic layer deposition. The resulting structures were etched with HF to remove the silica opal template. Infiltrated and inverse opals were characterized by SEM, XRD, and transmission/reflection spectroscopy. The reflectance spectra exhibited features corresponding to strong low and high order photonic band gaps in the (111) direction (γ-L). In addition, deliberate partial infiltrations and multi-layered inverse opals have been formed. The effectiveness of a post-deposition heat treatment for converting TiO2 films to rutile was also studied.
Rugate optical reflectance filters with position dependent reflectance peaks in the visible to near infrared spectrum were realized in porous silicon (PS). Filters with strong reflection peaks, near 100%, no detectable higher order harmonics and suppressed sidebands compared to discrete layer filters were obtained by varying the current density continuously and periodically during etching. An in-plane voltage up to 1.5 V was used to obtain refractive index and periodicity change along the filter surface resulting in reflectance peak shifts of up to 100 nm/mm in the direction of the voltage drop. The effect of the lateral change in optical parameters on the filter characteristics is studied by varying the gradient and comparing measurements at different positions with measurements on a non-graded filter. We have observed extra features in the reflectance spectrum of these graded filters compared with reflectance from a non-graded filter which is likely caused by the gradient.
In order to create micrometer-scale functional optical materials or devices, we have investigated on development of a novel electrophoretic deposition (EPD) method using a microelectrode as a counter electrode: This is so-called “μ-EPD method”. The μ-EPD method was applied to fabricate micro colloidal crystals consisting of monodisperse submicron polystyrene latex spheres for micro photonic application. Scanning electron micrographs of the deposit prepared under the optimized μ-EPD parameters showed a formation of microdot consisting of three-dimensionally ordered polystyrene spheres. As a result of the microscopic transmittance spectra, the microdots exhibited a narrow absorption peak and the optical stopband was observed at 460 nm for 204 nm polystyrene spheres, 675 nm for 290 nm polystyrene spheres, and 755 nm for 320 nm polystyrene spheres, respectively. The observed position is due to the Bragg diffraction of light from (111) plane of face-centered cubic opal lattice.
The recently expanding field of microstructured optical fibers relies on the controlled fabrication of sub-micron features in a fiber drawn in the viscous fluid state. Microstructured fibers have generated great interest owing to their potential in areas such as photonic bandgap guidance of light in low-index media; high-energy laser transmission; and unique control over waveguide non-linearities, dispersion and modal properties [1–6]. These fibers have been made from a single material with air holes [7, 8] and as multi-material ‘composite’ fibers where air is not a part of the microstructured region [6, 9]. While single-material microstructured fibers generally rely on the established technology base of fused silica, the use of less conventional materials may enable applications not possible using silica . Multi-material fibers may also present certain fabrication advantages due to their incompressible domains and simple cylindrical geometries. However, the use of more than one material raises questions about which types of materials can be combined in the drawing of a microstructured fiber. This problem can be approached by analyzing the relative importance of different materials properties such as viscosity, interfacial energy, and thermal expansion. In this study we focus on the effects of interfacial energy in composite microstructured fibers. We measure the interfacial energies at high temperature of a chalcogenide glass and an organic polymer recently employed in the fabrication of composite photonic bandgap optical fibers. We discuss the effect of interfacial energy during fiber draw, as well as the interplay between surface and viscous forces. Finally, we comment on the implications of this analysis for understanding what classes of materials can be used in composite microstructured fiber fabrication.
Experimental results are reported on various guided optic configurations that combine silicon-based photonic crystals (PC) and Ge/Si quantum island emitters. The feasibility of low-refractive-index-contrast PC waveguides by inductively-coupled-plasma (ICP) etching of buried SiGe/Si waveguides is briefly recalled from a previous work. The main body of the paper is focused on experiments that were carried out on the high-refractive-index-contrast silicon-on-insulator (SOI) system. Self-assembled Ge/Si quantum island layers were deposited on a SOI substrate that was further processed to get two-dimensional PC microcavities and waveguides. The room temperature 1.3–1.55 μm emission from Ge/Si islands is shown to be significantly enhanced in PC microcavities, the strongest enhancement being obtained with the smallest (micropillar-like) cavities surrounded by wide pores. In this latter case, the room-temperature photoluminescence amplitude is more than two-orders of magnitude larger than that of Ge/Si islands grown in unprocessed samples. A superlinear (laser-like) dependence with the optical pumping is observed in the same time. This behavior and other experimental trends would incriminate both a high carrier concentration of the photo-created electron-hole plasma and a good vertical coupling efficiency of the micro-structured silicon. A first attempt to characterize linear PC waveguides is also reported using the wideband luminescence of Ge/Si islands embedded in the guides.
The Jahn-Teller effect in photonic crystals as a prototype of photon-phonon interactions is studied. We are interested in removing the degeneracy of a defect state due to coupling with vibronic mode. Two-dimensional square photonic lattice of the dielectric rods in vacuum doped by the defect rod, giving the doubly degenerate E state in the first TM band gap is studied. We show that coming from the Jahn-Teller theorem, the lattice vibration with the symmetry of B1 and B2 modes should result in splitting the degeneracy of the E photon state, the lattice vibration being frozen. The stable configuration in the presence of the Jahn-Teller effect is determined from the dependence of the energy as a function of the rod displacement. Using the value of the vibronic constants, obtained from the suppercell plane wave calculations and the Finite Difference Time Domain simulations, we find the stable configuration of the lattice. We discuss the conditions to observe the effect.
In this paper, we present a study on quasi-phase matched (QPM) two-dimensional χ(2) lithium niobate (LN) nonlinear photonic crystal (NPC) for frequency doubling at λ = 1064nm. The NPCs were fabricated by electron beam lithography (EBL) through periodic polarization inversion of the ferroelectric domains and characterized with electrostatic force microscopy (EFM), atomic force microscopy and optical microscopy. Domain inversion occurred through the entire wafer thickness of 0.5mm as EFM images on the +c face of the z-cut wafer showed uniform domain structures throughout the corresponding electron beam irradiated regions of the -c face. In addition, the intended periodicity was observed. Moreover, domain inversion was also seen to have taken place in bulk from the optical images of the chemically etched samples. The EBL technique offers great flexibility in superlattice design and relative ease of fabrication as compared to the conventional poling techniques as pattern transfer is direct without the need for a mask and/or a coating of resist. Besides, micro- or sub-micro scale superlattices corresponding to wavelengths in the visible and into the ultraviolet are highly feasible, restricted only by the transparency of the crystals.
We present a microscopic theory of electromagnetic energy transport in nanostructured media based on the Lagrangian formulation of semiclassical electrodynamics. We show the importance of the interplay between transverse and longitudinal local fields in determining the light-matter interaction in nanostructured media. We derive rigorously the coupled-dipole equation of the local fields and apply the theory to analyze energy transport in metal nanoparticle chain waveguide.
The photoluminescence (PL) intensity first increases with anodization time (ta) and then decreases at very large ta. The increase in PL intensity with ta may be understood if the PL intensity is taken to be proportional to the effective volume of porous silicon (PS) layer under the probe laser beam. The effective volume of PS layer will be proportional to its thickness and reciprocal to the porosity. For a fixed anodization condition, the thickness and porosity both increase with ta. The increase in thickness increases the effective PS volume, while the increase in porosity causes the effective volume to decrease. Therefore, the intensity variation is governed by these two parameters: thickness and porosity. The observed results suggest that the thickness dominates the PL intensity initially and then the porosity becomes more important for very long ta. The PS layers prepared under ambient light illumination also exhibited the similar behaviour. The intensity variation with ta was explained as the interplay of thickness and porosity variations with ta.
In this paper, we discuss unique light localization in a single line defect, which is effective for constructing photonic crystal light lasers. The localization is based on additional defect doping that breaks the symmetry of the line defect. Even though such a defect is opened to the line defect, the optical field is well confined around the defect at cutoff frequencies of the line defect. This concept expands the design flexibility of microcavities; for example, the composite of point and line defects and waveguide components such as bends and branches can be microcavities. It also allows effective mode controls such as the singlemode operation in relatively large cavities. The lasing operation of these cavities in a GaInAsP photonic crystal slab was experimentally observed by photopumping at room temperature. This paper reports lasing characteristics and the dependence on various structural details.
We are reporting on the analysis of a new design for a thermal source exploiting Si-based suspended micro-bridge structures. A device consists of a metal film perforated by a periodic array of apertures extending into the Si substrate and each of size on order of the wavelength of the light. This perforated film permits resonant coupling of the incident radiation from the underlying silicon photonic crystal with surface plasmons at the metal surface. The coupling provides for unusually high optical emission efficiencies when the structure is thermally excited. The radiation emitted exhibits an enhancement over a narrow wavelength range in the infrared and its spectral response is highly dependent on the direction of observation. The positions of the main resonances, for both reflection and emission from our structures, scale linearly with the periodicity of the metallo-dielectric structure. As one moves off normal incidence, a single main resonance splits into several smaller resonances whose locations scale roughly linearly with observation angle. These structures have been used as emitter/detector sensor chips to selectively detect industrial pollutants like carbon dioxide. Control of the wavelength of resonance, bandwidth and direction of emission play an important role in improving the sensitivity and selectivity of these gas sensors.
The dispersion, which expresses the variation with wavelength of the guided-mode group velocity, is one of the most important properties of optical fibers. Photonic crystal fibers (PCFs) offer much larger flexibility than conventional fibers with respect to tailoring of the dispersion curve. This is partly due to the large refractive-index contrast available in silica/air microstructures, and partly due to the possibility of making complex refractive-index structures over the fiber cross section. We discuss the fundamental physical mechanisms determining the dispersion properties of PCFs guiding by either total internal reflection or photonic bandgap effects, and use these insights to outline design principles and generic behaviours of various types of PCFs. A number of examples from recent modeling and experimental work serve to illustrate our general conclusions.