Showing papers on "Total internal reflection published in 2008"

Membrane-Type Acoustic Metamaterial with Negative Dynamic Mass

Abstract: We present the experimental realization and theoretical understanding of a membrane-type acoustic metamaterial with very simple construct, capable of breaking the mass density law of sound attenuation in the 100--1000 Hz regime by a significant margin ($\ensuremath{\sim}200$ times). Owing to the membrane's weak elastic moduli, there can be low-frequency oscillation patterns even in a small elastic film with fixed boundaries defined by a rigid grid. The vibrational eigenfrequencies can be tuned by placing a small mass at the center of the membrane sample. Near-total reflection is achieved at a frequency between two eigenmodes where the in-plane average of normal displacement is zero. By using finite element simulations, negative dynamic mass is explicitly demonstrated at frequencies around the total reflection frequency. Excellent agreement between theory and experiment is obtained.

A nearly ideal phosphor-converted white light-emitting diode

Abstract: A phosphor-converted light-emitting diode was obtained with nearly ideal blue-to-white conversion loss of only 1%. This is achieved using internal reflection to steer phosphor emission away from lossy surfaces, a reflector material with high reflectivity, and a remotely located organic phosphor having (1) unity quantum efficiency (ηq), (2) homogeneous refractive index to minimize scattering, and (3) refractive index-matched to the encapsulation to eliminate total internal reflection. An inorganic composite phosphor is also reported with a nearly homogeneous refractive index to minimize diffuse scattering of emitted light, thereby maximizing the effective phosphor ηq and light extraction.

Compensation of loss in propagating surface plasmon polariton by gain in adjacent dielectric medium

Abstract: We report the suppression of loss of surface plasmon polariton propagating at the interface between silver film and optically pumped polymer with dye. The large magnitude of the effect enables a variety of applications of ‘active’ nanoplasmonics. The experimental study is accompanied by the analytical description of the phenomenon. In particular, we resolve the controversy regarding the direction of the wavevector of a wave with a strong evanescent component in an active medium.

Control of the Goos-Hänchen shift of a light beam via a coherent driving field

Abstract: We present a proposal to manipulate the Goos-H\"anchen shift of a light beam via a coherent control field, which is injected into a cavity configuration containing the two-level atomic medium. It is found that the lateral shifts of the reflected and transmitted probe beams can be easily controlled by adjusting the intensity and detuning of the control field. Using this scheme, the lateral shift at the fixed incident angle can be enhanced (positive or negative) under the suitable conditions on the control field, without changing the structure of the cavity.

Long-range spiralling surface plasmon modes on metallic nanowires

Abstract: We discuss the characteristics of surface plasmon modes guided on metallic nanowires of circular cross-section embedded in silica glass. Under certain conditions such wires allow low-loss guided modes, full account being taken of ohmic losses in the metal. We find that these modes can be bound to the wire even when the real part of their axial refractive index is less than that of the surrounding dielectric. We assess in detail the accuracy of a simple model in which SPs are viewed as spiralling around the nanowire in a helical path, forming modes at certain angles of pitch. The results are relevant for understanding the behavior of light in two-dimensional arrays of metallic nanowires in fiber form.

Structured illumination in total internal reflection fluorescence microscopy using a spatial light modulator.

Abstract: In wide-field fluorescence microscopy, illuminating the specimen with evanescent standing waves increases lateral resolution more than twofold. We report a versatile setup for standing-wave illumination in total internal reflection fluorescence microscopy. An adjustable diffraction grating written on a phase-only spatial light modulator controls the illumination field. Selecting appropriate diffraction orders and displaying a sheared (tilted) diffraction grating allows one to tune the penetration depth in very fine steps. The setup achieves 91 nm lateral resolution for green emission.

Optical deflection and sorting of microparticles in a near-field optical geometry.

Abstract: Near-field optical micromanipulation permits new possibilities for controlled motion of trapped objects. In this work, we report an original geometry for optically deflecting and sorting micro-objects employing a total internal reflection microscope system. A small beam of laser light is delivered off-axis through a total internal reflection objective which creates an elongated evanescent illumination of light at a glass/water interface. Asymmetrical gradient and scattering forces from this light field are seen to deflect and sort polystyrene microparticles within a fluid flow. The speed of the deflected objects is dependent upon their intrinsic properties. We present a finite element method to calculate the optical forces for the evanescent waves. The numerical simulations are in good qualitative agreement with the experimental observations and elucidate features of the particle trajectory. In the size range of 1 µm to 5 µm in diameter, polystyrene spheres were found to be guided on average 2.9 ± 0.7 faster than silica spheres. The velocity increased by 3.00.5 µms−1 per µm increase in diameter for polystyrene spheres and 0.7 ± 0.2 µms−1 per µm for silica. We employ this size dependence for performing passive optical sorting within a microfluidic chip and is demonstrated in the accompanying video.

Power Waves and Conjugate Matching

Abstract: The concept of power waves gives more natural relations between incident and reflected power in a microwave network than the typically used traveling waves. The reflection coefficient for power waves directly describes the reflection of power whereas the reflection coefficient of traveling waves describes the reflection of the waves themselves. In this brief, new physical reasoning of power waves is given starting from the principle of conjugate matching. In addition, a new formula for the reference impedances for a two-port system is given such that the system is simultaneously conjugate matched for both ports.

Surface plasmon resonance imaging of cell-substrate contacts with radially polarized beams

Abstract: We demonstrate the proof-of-concept for surface plasmon resonance sensing and imaging via a virtual probe at the cell-substrate interface of a biological cell in aqueous media. The technique is based on the optical excitation by focused radially polarized beams of localized surface plasmons, which forms a virtual probe on the metal substrate. The intensity distribution at the back focal plane of the objective lens enables quantitative measurements to be made of the cell-substrate contact. The acquired data is then visualized in the form of a local refractive index map.

Rugate filter for light-trapping in solar cells

Abstract: We suggest a design for a coating that could be applied on top of any solar cell having at least one diffusing surface. This coating acts as an angle and wavelength selective filter, which increases the average path length and absorptance at long wavelengths without altering the solar cell performance at short wavelengths. The filter design is based on a continuous variation of the refractive index in order to minimize undesired reflection losses. Numerical procedures are used to optimize the filter for a 10 µm thick monocrystalline silicon solar cell, which lifts the efficiency above the Auger limit for unconcentrated illumination. The feasibility to fabricate such filters is also discussed, considering a finite available refractive index range.

All-optical steering of soliton waveguides in dye-doped liquid crystals

Abstract: Soliton induced waveguides can be all-optically readdressed in planar cells by modifying the molecular anchoring with an external light source. Using an elliptically shaped control beam we demonstrate refraction and total internal reflection of spatial solitons in dye-doped nematic liquid crystals.

Elimination of total internal reflection in GaInN light-emitting diodes by graded-refractive-index micropillars

Abstract: A method for enhancing the light-extraction efficiency of GaInN light-emitting diodes (LEDs) by complete elimination of total internal reflection is reported. Analytical calculations show that GaInN LEDs with multilayer graded-refractive-index pillars, in which the thickness and refractive index of each layer are optimized, have no total internal reflection. This results in a remarkable improvement in light-extraction efficiency. GaInN LEDs with five-layer graded-refractive-index pillars, fabricated by cosputtering TiO2 and SiO2, show a light-output power enhanced by 73% and a strong side emission, consistent with analytical calculations and ray-tracing simulations.

Total internal reflection fluorescence (TIRF) microscopy.

Abstract: Total internal reflection fluorescence (TIRF) microscopy represents a method of exciting and visualizing fluorophores present in the near-membrane region of live or fixed cells grown on coverslips. TIRF microscopy is based on the total internal reflection phenomenon that occurs when light passes from a high-refractive medium (e.g., glass) into a low-refractive medium (e.g., cell, water). The evanescent field produced by total internally reflected light excites the fluorescent molecules at the cell-substrate interface and is accompanied by minimal exposure of the remaining cell volume. This technique provides high-contrast fluorescence images, with very low background and virtually no out-of-focus light, ideal for visualization and spectroscopy of single-molecule fluorescence near a surface. This unit presents, in a concise manner, the principle of operation, instrument diversity, and TIRF microscopy applications for the study of biological samples.

Enhancement of omnidirectional reflection in photonic crystal heterostructures

Abstract: In this paper we have theoretically studied the omnidirectional total reflection frequency range of a multilayered dielectric heterostructures. Three structures of Na3AlF6/Ge multilayer have been studied. The thickness of the two layers of the first and second structure is differing from each other and the third photonic structure is the combination of first and second structures. Using the Transfer Matrix Method (TMM) and the Bloch theorem, the reflectivity of one dimensional periodic structure for TEand TM-modes at different angles of incidence is calculated. From the analysis it is found that the proposed structure has very wide range of omnidirectional total frequency bands for both polarizations.

Even illumination in total internal reflection fluorescence microscopy using laser light.

Abstract: In modern fluorescence microscopy, lasers are a widely used source of light, both for imaging in total internal reflection and epi-illumination modes. In wide-field imaging, scattering of highly coherent laser light due to imperfections in the light path typically leads to nonuniform illumination of the specimen, compromising image analysis. We report the design and construction of an objective-launch total internal reflection fluorescence microscopy system with excellent evenness of specimen illumination achieved by azimuthal rotation of the incoming illuminating laser beam. The system allows quick and precise changes of the incidence angle of the laser beam and thus can also be used in an epifluorescence mode.

Theoretical and experimental analysis of photonic structures for fluorescent concentrators with increased efficiencies

Abstract: In this study we present a theoretical and experimental analy- sis of the application of photonic band stop filters on top of photovoltaic fluorescent concentrators in order to increase the photon collection efficiency. The light guiding effect of the fluorescent concentrator relies on total internal reflection. The escape cone of total internal reflection is their major loss mechanism. Our ray tracing simulation allows to calculate the beneficial effect of photonic band stop reflection filters, which reduce these losses, and to simulate the angular distribu- tion of the light trapped in the concentrator. We present simula- tions of the optical properties of 1D and 3D photonic structures and how 3D structures are realized with colloidal opals. We also show that the application of a 1D photonic structure in- creases the efficiency of a real system by 20% relative.

Optofluidic 1x4 switch.

Abstract: An optofluidic 1×4 switch is designed, fabricated, and tested. The switch is based on a blazed diffraction grating imprinted onto silicone elastomer at the bottom of a microfluidic channel that is filled with liquids with different refractive indices. When the condition of a diffraction maximum is met, the laser beam incident on the grating is deflected by an angle proportional to the refractive index mismatch between the elastomer and the liquid in the channel. The switch was tested using four different aqueous salt solutions generating 0th to 3rd orders of diffraction. The insertion loss was 9.8dB, and the response time was 55 ms. The same basic design can be used to build optofluidic switches with more than 4 outputs.

Sensitive molecular binding assay using a photonic crystal structure in total internal reflection.

Abstract: A novel optical sensor for label-free biomolecular binding assay using a one-dimensional photonic crystal in a total-internal-reflection geometry is proposed and demonstrated. The simple configuration provides a narrow optical resonance to enable sensitive measurements of molecular binding, and at the same time employs an open interface to enable real-time measurements of binding dynamics. Ultrathin aminopropyltriethoxysilane/ glutaraldehyde films adsorbed on the interface were detected by measuring the spectral shift of the photonic crystal resonance and the intensity ratio change in a differential reflectance measurement. A detection limit of 6 x 10(-5) nm for molecular layer thickness was obtained, which corresponds to a detection limit for analyte adsorption of 0.06 pg/mm(2) or a refractive index resolution of 3 x 10(-8) RIU; this represents a significant improvement relative to state-of-the-art surface-plasmon-resonance-based systems.

C-band side-entry Ge quantum-well electroabsorption modulator on SOI operating at 1 V swing

Abstract: Introduction: Development of photonics based on CMOS-compatible materials and processing may lead to inexpensive optoelectronic components and monolithic integration of photonics and electronics for optical interconnects and networks. The lack of physical mechanisms for light emission or optoelectronic modulation in silicon [1] comparable to those in III – V semiconductors has, however, held back silicon-based optics. Recently we discovered a strong quantum-confined Stark effect (QCSE) in Ge quantum wells with SiGe barriers grown on Si substrates, and demonstrated changes in absorption coefficient similar to those used for high-performance III – V modulators [2, 3]. We also recently demonstrated the first modulator device using this effect [4]. The strong QCSE electroabsorption allowed a novel structure without a waveguide and with very relaxed optical alignment tolerance. The 10 V required voltage swing was, however, too high for CMOS electronics, and the device did not operate at the important telecommunications C-band wavelengths. In this Letter, we show a CMOS-compatible 1 V drive, with C-band operation, using a novel frustrated total internal reflection structure. This device retains the alignment tolerance of the previous structure, and demonstrates the compatibility of such modulators with growth on silicon-on-insulator (SOI) substrates, which are often preferred also for other silicon photonics devices. Our previous Ge/SiGe quantum well QCSE modulator [4] used optical entry and exit ports through the polished edges of the substrate, and light was obliquely incident upon a diode mesa containing quantum wells. Oblique incidence led to the formation of an asymmetric FabryPerot resonator owing to total internal reflection (TIR) at the epitaxy/ air interface and 18% reflection at the substrate/epitaxy interface. The lattice mismatch between Si and Ge makes it difficult to grow epitaxial layers of sufficient index contrast and thickness to make distributed Bragg reflectors, and the 18% reflectivity of the Si-SiGe interface cannot be increased much further by increasing the incident angle without making the focal spot projection on the diode mesa prohibitively large. In this Letter we report a side-entry modulator fabricated on an SOI wafer with ten epitaxially grown Ge/SiGe quantum wells. Oblique incidence on the buried oxide layer (BOX) leads to frustrated total internal reflection [5]. Choosing the thickness of a low-index layer contained between two high-index layers allows control of the resulting reflectivity. The 50 nm BOX used has 70% reflectivity at the design angle, increasing the quality factor of the resonator compared to the previous work, reducing the required number of quantum wells and consequently the operating voltage. SOI waveguides typically use a 1 mm BOX. The thin 50 nm BOX used here is attractive for potential integration of modulators with high-performance electronics, the primary application of SOI wafers. Experiment and results: The structure starts with an SOI substrate, with a 730 mm-thick Si substrate, followed by the 50 nm BOX and a 100 nmthick silicon layer. The epitaxial structure grown on this had a 900 nmthick p-type Si0.1Ge0.9 layer B-doped at 3 10 17 cm 23 , a 100 nm-thick

Novel index-guided photonic crystal fiber surface-enhanced Raman scattering probe

Abstract: We demonstrate a novel index-guided (IG) photonic crystal fiber (PCF) surface-enhanced Raman probe. Different from a regular PCF, the IGPCF has four big air holes inserted between the solid silica core and the photonic crystal cladding holes. The gold nanoparticles, serving as the surface enhanced Raman scattering (SERS) substrate, are either coated on the inner surface of the holes or mixed in the analyte solution in two separate experiments, respectively. The analyte solution enters the holes via the capillary effect. The excitation light propagating in the silica core interacts with the gold nanoparticles and the analyte through the evanescent wave which extends significantly into the four big holes when they are filled with liquid leading to a large interaction volume between the excitation light and the nanoparticles/analyte.

Organization of microscale objects using a microfabricated optical fiber.

Abstract: We demonstrate the use of a single fiber-optic axicon device for organization of microscopic objects using longitudinal optical binding. Further, by manipulating the shape of the fiber tip, part of the emanating light was made to undergo total internal reflection in the conical tip region, enabling near-field trapping. Near-field trapping resulted in trapping and self-organization of long chains of particles along azimuthal directions (in contrast to the axial direction, observed in the case of large tip cone angle far-field trapping).

Digital holography of total internal reflection

Abstract: We introduce a new microscopy technique termed total internal reflection holographic microscopy (TIRHM). Quantitative phase microscopy by digital holography is used to image the phase profile of light in total internal reflection, which is modulated by the materials present on or near the surface of internal reflection. The imaging characteristics are theoretically modeled and imaging capabilities are experimentally demonstrated.

Probing the Concepts of Photonic Crystals on Scintillating Materials

Abstract: The high refractive index of current scintillating materials puts severe restrictions on their effective light yield. In this paper, we describe an approach that uses a photonic crystal pattern machined into the coupling face of the scintillator to partly overcome the problem of total internal reflection. Simulations are performed for 2 mm times 2 mm times 8 mm LuAP and LSO pixels with and without photonic crystal and different types of wrapping. It is shown that by tuning the structure of the photonic crystal and the size of its elements, the extraction efficiency of the surface can be significantly improved compared to a plain exit surface.

Slow light in a dielectric waveguide with negative-refractive-index photonic crystal cladding.

Abstract: A slow light waveguide made of a dielectric slab inserted in a two-dimensional photonic crystal with a negative effective refractive index is proposed and numerically studied. The waveguide may possess modes with zero group velocity, and its frequency varies with the thickness of the waveguide. A linearly tapered left-handed photonic crystal waveguide is also proposed and studied. It is shown that the so-called 'trapped rainbow' proposed by Tsakmakidis, Boardman, and Hess [1] is difficult to realize due to a coupling of forward- and backward-propagating modes near zero group velocity. However, different frequency components of a broadband excitation can still be separated through partial accumulation at waveguide sections of different thicknesses.

Optical confinement methods for continued scaling of CMOS image sensor pixels

Abstract: The pixels that make up CMOS image sensors have steadily decreased in size over the last decade. This scaling has two effects: first, the amount of light incident on each pixel decreases, making optical efficiency, i.e., the collection of each photon, more important. Second, diffraction comes into play when pixel size approaches the wavelength of visible light, resulting in increased spatial optical crosstalk. To address these two effects, we investigate and compare three methods for guiding incident light from the microlens down to the photodiode. Two of these techniques rely on total internal reflection (TIR) at the boundary between dielectric media of different refractive indices, while the third uses reflection at a metal-dielectric interface to confine the light. Simulations are performed using a finite-difference time-domain (FDTD) method on a realistic 1.75-µm pixel model for on-axis as well as angled incidence. We evaluate the optical efficiency and spatial crosstalk performance of these methods compared to a reference pixel and find significant (10%) improvement for the TIR designs with properly chosen parameters and nearly full spatial crosstalk elimination using metal to confine the light. We also show that these improvements are comparable to those achieved by thinning the image sensor stack.

Significant deformations and propagation variations of Laguerre-Gaussian beams reflected and transmitted at a dielectric interface

Abstract: We observe TM and TE Laguerre-Gaussian (LG) light beams reflected and transmitted at a dielectric interface near critical incidence. The intensity distribution of the reflected beam is transversely deformed near the beam waist, and that of the transmitted beam is similar to that of a diagonal Hermite-Gaussian beam. The former rotates around the optical axis by approximately π/2 with propagation, and the latter returns to that of the incident LG beam. These observations agree well with numerical calculations based on an angular spectral analysis and are attributable to the helical wavefront of the LG beams, the sharp incidence-angle dependence of the Fresnel reflection and transmission coefficients, and the Gouy phase.

Double and negative reflection of cold atoms in non-Abelian gauge potentials.

Abstract: Atom reflection is studied in the presence of a non-Abelian vector potential proportional to a spin-1/2 operator. The potential is produced by a relatively simple laser configuration for atoms with a tripod level scheme. We show that the atomic motion is described by two different dispersion branches with positive or negative chirality. As a consequence, atom reflection shows unusual features, since an incident wave may split into two reflected ones at a barrier, an ordinary specular reflection, and an additional nonspecular one. Remarkably, the latter wave can exhibit negative reflection and may become evanescent if the angle of incidence exceeds a critical value. These reflection properties are crucial for future designs in non-Abelian atom optics.