Showing papers on "Total internal reflection published in 2014"

Controllable transmission and total reflection through an impedance-matched acoustic metasurface

Abstract: A general design paradigm for a novel type of acoustic metasurface is proposed by introducing periodically repeated supercells on a rigid thin plate, where each supercell contains multiple cut-through slits that are filled with materials possessing different refractive indices but the same impedance as that of the host medium. When the wavelength of the incident wave is smaller than the periodicity, the direction of the transmitted wave with nearly unity transmittance can be chosen by engineering the phase discontinuities along the transverse direction. When the wavelength is larger than the periodicity, even though the metasurface is impedance matched to the host medium, most of the incident energy is reflected back and the remaining portion is converted into a surface-bound mode. We show that both the transmitted wave control and the high reflection with the surface mode excitation can be interpreted by a unified analytic model based on mode-coupling theory. Our general design principle not only supplies the functionalities of reflection-type acoustic metasurfaces, but also exhibits unprecedented flexibility and efficiency in various domains of wave manipulation for possible applications in fields like refracting, collimating, focusing or absorbing wave energy.

Organic electroluminescent element

Abstract: PROBLEM TO BE SOLVED: To provide an organic electroluminescent element whose boundary surface structure can restrict a decline in the amount of light passing through a glass substrate due to total reflection which is observed when light emitted from an organic luminescent layer enters from an ITO transparent electrode having a high refractive index to the boundary surface of the glass substrate having a lower refractive index than the ITO transparent electrode.SOLUTION: The organic electroluminescent element comprises at least a high refractive index resin layer 10 about 1.6 to 1.8 in refractive index, an ITO transparent electrode 2, an organic luminescent layer 7, and an anode layer 6 which are laminated in that order on a glass substrate 1, or the organic electroluminescent element comprises at least a scattering layer 11, a high refractive index resin layer 10 about 1.6 to 1.8 in refractive index, an ITO transparent electrode 2, an organic luminescent layer 7, and an anode layer 6 which are laminated in that order on a glass substrate 1.

Evanescent-wave and ambient chiral sensing by signal-reversing cavity ringdown polarimetry

Abstract: By passing light through a chiral sample — here vapours and solutions — in a specially designed ring cavity, the resulting chiral signals can be isolated from the achiral backgrounds and enhanced by a factor of more than 1,000, making them detectable in situations where conventional means of measurement fail. Detecting and quantifying chirality is important in fields ranging from analytical and biological chemistry to pharmacology and fundamental physics. It is usually done by measuring circular dichroism or optical rotation, procedures that are simple to do in principle but often limited by low signal strength against a large and fluctuating background. Dimitris Sofikitis et al. now show that chiral signals can be selectively enhanced over their background by passing them through a specially designed ring cavity more than a thousand times. With further optimization, the method should exceed current chiral detection limits by several orders of magnitude, an advance that could transform chiral sensing in many fields. Detecting and quantifying chirality is important in fields ranging from analytical and biological chemistry to pharmacology1 and fundamental physics2: it can aid drug design and synthesis, contribute to protein structure determination, and help detect parity violation of the weak force. Recent developments employ microwaves3, femtosecond pulses4, superchiral light5 or photoionization6 to determine chirality, yet the most widely used methods remain the traditional methods of measuring circular dichroism and optical rotation. However, these signals are typically very weak against larger time-dependent backgrounds7. Cavity-enhanced optical methods can be used to amplify weak signals by passing them repeatedly through an optical cavity, and two-mirror cavities achieving up to 105 cavity passes have enabled absorption and birefringence measurements with record sensitivities8,9,10. But chiral signals cancel when passing back and forth through a cavity, while the ubiquitous spurious linear birefringence background is enhanced. Even when intracavity optics overcome these problems11,12,13,14,15, absolute chirality measurements remain difficult and sometimes impossible. Here we use a pulsed-laser bowtie cavity ringdown polarimeter with counter-propagating beams16,17 to enhance chiral signals by a factor equal to the number of cavity passes (typically >103); to suppress the effects of linear birefringence by means of a large induced intracavity Faraday rotation; and to effect rapid signal reversals by reversing the Faraday rotation and subtracting signals from the counter-propagating beams. These features allow absolute chiral signal measurements in environments where background subtraction is not feasible: we determine optical rotation from α-pinene vapour in open air, and from maltodextrin and fructose solutions in the evanescent wave produced by total internal reflection at a prism surface. The limits of the present polarimeter, when using a continuous-wave laser locked to a stable, high-finesse cavity, should match the sensitivity of linear birefringence measurements8 (3 × 10−13 radians), which is several orders of magnitude more sensitive than current chiral detection limits7,14,15 and is expected to transform chiral sensing in many fields.

Surface plasmon excitation in semitransparent inverted polymer photovoltaic devices and their applications as label-free optical sensors

Abstract: Herein, we report on surface plasmon (SP)-sensitive semitransparent inverted polymer photovoltaic (PV) devices that are based on multilayered material systems consisting of poly(3-hexylthiophene): fullerene-derivative bulk-heterojunction PV layers and thin gold or silver anodes. We demonstrate that these PV devices allow the simultaneous generation of both electrical power and SPs on their anodes for photoexcitation just above the optical absorption edge of the PV layers, resulting not only in attenuated total reflection, but also in attenuated photocurrent generation (APG) under the SP resonance (SPR) condition. Moreover, we also confirm that the biomolecular interaction of biotin–streptavidin on the PV devices can be precisely detected via apparent SPR angle shifts in the APG spectra, even without the need for complex attenuated total reflection configurations. We highlight our view that APG measurements made using these PV devices show great potential for the development of future generations of compact and highly sensitive SPR-based optical sensors. Researchers investigate surface plasmon excitation in photovoltaic devices and explore their application as highly sensitive optical sensors. Byoungchoo Park and co-workers at Kwangwoon University in South Korea have developed semitransparent inverted polymer photovoltaic devices with a planar multilayer structure that are capable of simultaneously generating electrical power and surface plasmons on irradiation. When surface plasmon resonance is induced in these devices, both total internal reflection and photocurrent generation are attenuated. The researchers demonstrated that this property can be exploited to sensitively detect biological compounds. Specifically, they used apparent shifts in the surface plasmon resonance angle in attenuated photocurrent generation to detect the biomolecular interaction of biotin-streptavidin. They conclude that these devices have great potential as next-generation optical sensors that are simple, inexpensive, compact and highly sensitive.

Ultrasensitive Flow Sensing of a Single Cell Using Graphene-Based Optical Sensors

Abstract: On the basis of the polarization-dependent absorption of graphene under total internal reflection, we designed a graphene-based optical refractive index sensor with high resolution of 1.7 × 10–8 and sensitivity of 4.3 × 107 mV/RIU, as well as an extensive dynamic range. This highly sensitive graphene optical sensor enables label-free, live-cell, and highly accurate detection of a small quantity of cancer cells among normal cells at the single-cell level and the simultaneous detection and distinction of two cell lines without separation. It provides an accurate statistical distribution of normal and cancer cells with fewer cells. This facile and highly sensitive sensing refractive index may expand the practical applications of the biosensor.

The circular Bragg phenomenon

Abstract: Exhibited by structurally chiral materials—such as Reusch piles, cholesteric liquid crystals (CLCs), and chiral sculptured thin films (STFs)—due to their helical nonhomogeneity along a fixed axis, the circular Bragg phenomenon is the almost total reflection of the incident light of the co-handed circular-polarization state but very little reflection of the incident light of the cross-handed circular-polarization state. Manifesting itself in spectral regimes that depend on the angle of incidence, the structural period, and the relative permittivity dyadic, the phenomenon amounts to the formation of a light pipe that bleeds energy backward under appropriate conditions. Mild dissipation and dispersion do not significantly affect the circular Bragg regime. Every structurally chiral material of sufficient thickness is essentially a circular-polarization-sensitive band-rejection filter. Cascades of these materials with or without structural defects can be used to satisfy complex filtering requirements, such as multiband, narrowband, and ultra-narrowband filtering. A shift in the circular Bragg regime due to infiltration of a chiral STF by a fluid enables optical sensing. Sources of circularly polarized light can be fabricated by embedding emission sources in CLCs and chiral STFs.

Ultra-high-efficiency metamaterial polarizer

Abstract: Conventional polarizers operate by rejecting undesired polarization, which limits their transmission efficiency to much less than 50% when illuminated by unpolarized light. We designed, fabricated, and characterized a multilevel metamaterial linear polarizer that rotates light with polarization perpendicular to its principal axis by 90 deg. Light with polarization parallel to its principal axis is transmitted undisturbed. Thereby, such a polarizer is able to output linearly polarized light from unpolarized input with a transmission efficiency that is substantially higher than the theoretical upper limit of 50%. A nonlinear optimization algorithm was used to design the polarizer, while multilevel focused-ion-beam lithography was used to fabricate it in silicon for the vacuum wavelength, λ0=1550 nm. We experimentally confirmed that the fabricated device enhances the transmission of the desired linear polarization by 100% compared to an unpatterned film, corresponding to a transmission efficiency of ∼74% at the design wavelength. Since our method allows for the generalized manipulation of the amplitude, phase, and polarization of light with high transmission efficiency using ultrathin elements, it should enable the efficient generation of complex vector distributions of light.

Tuneable multicoloured patterns from photonic cross-communication between cholesteric liquid crystal droplets

Abstract: Monodisperse droplets of planar-aligned cholesteric (N*) liquid crystal exhibit an intriguing capacity for photonic cross-communication, giving rise to colourful patterns that depend sensitively on the N* pitch, droplet positions and illuminated area. The phenomenon results from a combination of omnidirectional selective reflection of N* droplets—which thus act as spherically symmetric self-assembled photonic crystals—and total internal reflection at the continuous phase surface. We outline how the unique optical properties can be employed in numerous applications.

High-Q MgF₂ whispering gallery mode resonators for refractometric sensing in aqueous environment.

Abstract: We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF₂) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF₂ which is very close to that of water. Compared to fused silica this leads to more than 50% longer evanescent fields and a 4.25 times larger sensitivity. Moreover the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent field decay and the sensitivity of mm-sized MgF₂ whispering gallery mode resonators immersed in water. These data show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above 1 × 10⁸.

Bonding pathways of gold nanocrystals in solution.

Abstract: Nanocrystal bonding is an important phenomenon in crystal growth and nanoscale welding. Here, we show that for gold nanocrystals bonding in solution can follow two distinct pathways: (1) coherent, defect-free bonding occurs when two nanocrystals attach with their lattices aligned to within a critical angle; and (2) beyond this critical angle, defects form at the interfaces where the nanocrystals merge. The critical misalignment angle for ∼10 nm crystals is ∼15° in both in situ experiments and full-atom molecular dynamics simulations. Understanding the origin of this critical angle during bonding may help us predict and manage strain profiles in nanoscale assemblies and inspire techniques toward reproducible and extensible architectures using only basic crystalline blocks.

Experimental observation of a giant Goos-Hänchen shift in graphene using a beam splitter scanning method.

Abstract: A giant Goos–Hanchen (G-H) shift in graphene has been theoretically predicted by previous research. In this Letter, we present experimental measurements of the G-H shift in graphene, in a total internal reflection condition, using a new method we have named “the beam splitter scanning method.” Our results show that a focused light source undergoes significant lateral shift when the polarization of incident light changes from transverse magnetic (TM) to transverse electric (TE) mode, indicating a large G-H shift in graphene that is polarization-dependent. We also observed that the difference in the G-H shift for TM versus TE modes (STM-STE) increases with increasing thickness of graphene material. A maximum difference (STM-STE) of 31.16 μm was observed, which is a significant result. Based on this research, the ability to engineer giant G-H shifts in graphene material has now been experimentally confirmed for the first time to the best of our knowledge. We expect that this result will lead to significant new and interesting applications of graphene in various types of optical sensors, and more.

Self-action of continuous laser radiation and Pearcey diffraction in a water suspension with light-absorbing particles

Abstract: Water suspension of light-absorbing nano-sized particles is an example of a medium in which non-linear effects are present at moderate light intensities favorable for optical treatment of organic and biological objects. We study experimentally the phenomena emerging in a thin layer of such a medium under the action of inhomogeneous light field formed due to the Pearcey diffraction pattern near a microlens focus. In this high-gradient field, the light energy absorbed by the particles induces inhomogeneous distribution of the medium refraction index, which results in observable self-diffraction of the incident light, here being strongly sensitive to the medium position with respect to the focus. This technique, based on the complex spatial structure of both the incident and the diffracted fields, can be employed for the detection and measurement of weak non-linearities.

Artificial gauge potentials for neutral atoms: an application in evanescent light fields

Abstract: We show that atoms interacting with evanescent light fields, generated at the interface of a dielectric with vacuum, experience artificial gauge potentials. Both the magnitude and the spatial distribution of these potentials depend crucially on the physical parameters that characterize the evanescent fields most notably the refractive index of the dielectric material and the angle of incidence of the laser beam totally internally reflected at the interface. Gauge fields are derived for various evanescent light fields and for both two-level and three-level systems. The use of such artificial gauge potentials for the manipulation of atoms trapped at the interfaces is pointed out and discussed.

Refractive index sensor based on plastic optical fiber with tapered structure

Abstract: This work reports a refractive index sensor made of plastic optical fiber (POF) with tapered structure. Transmission loss is measured when the external environment's refractive index changes from 1.33 to 1.41. Three wavelengths (532, 633, and 780 nm) are used to evaluate the sensitivity of the sensor, and results indicate that 633 nm is the best sensing wavelength due to the increased levels of sensitivity achieved at this wavelength. A biconical sensing structure is designed to enhance the sensitivity of the sensor. A sensitivity of 950 μW/RIU at 633 nm is obtained for a biconical sensing structure when launched power is 1 mW. Due to its sensitivity to the refractive index and simple construction, POF with tapered structure has potential applications in the biosensing field.

Subwavelength multilayer dielectrics: ultrasensitive transmission and breakdown of effective-medium theory.

Abstract: We show that a purely dielectric structure made of alternating layers of deep subwavelength thicknesses exhibits novel transmission effects which completely contradict conventional effective medium theories exactly in the regime in which those theories are commonly used. We study waves incident at the vicinity of the effective medium's critical angle for total internal reflection and show that the transmission through the multilayer structure depends strongly on nanoscale variations even at layer thicknesses smaller than $\ensuremath{\lambda}/50$. In such deep subwavelength structures, we demonstrate dramatic changes in the transmission for variations in properties such as periodicity, order of the layers, and their parity. In addition to its conceptual importance, such sensitivity has important potential applications in sensing and switching.

Condition for perfect antireflection by optical resonance at material interface

Abstract: Reflection occurs at an air–material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air–material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential for providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.

Optimization design of hybrid Fresnel-based concentrator for generating uniformity irradiance with the broad solar spectrum

Abstract: This paper presents a novel hybrid Fresnel-based concentrator with improved uniformity irradiance distribution on the solar cell without using secondary optical element (SOE) in the concentrator photovoltaic (CPV) system to overcome the Fresnel loss and to increase the solar cell conversion efficiency. The designed hybrid Fresnel-based concentrator is composed of two parts, the inner part and the outer part. The inner part is the conventional Fresnel lens, while the outer part is double total internal reflection (DTIR) lens. According to the simple geometrical relation, the profile of the proposed hybrid Fresnel-based concentrator is calculated as an initial design profile. To obtain good irradiance uniformity on the solar cell, optimal prism displacements are optimized by using a simplex algorithm for collimated incident sunlight based on different prism focus on different position principles. In addition, a Monte-Carlo ray-tracing simulation approach is utilized to verify the optical performance for the hybrid Fresnel-based concentrator. Results indicate that the hybrid Fresnel-based concentrator designed using this method can achieve spatial non-uniformity less than 16.2%, f -number less than 0.59 (focal length to entry aperture diameter ratio), geometrical concentrator ratio 1759.8×, and acceptance angle ±0.23°. Compared to the conventional Fresnel-based lens and the traditional hybrid Fresnel-based lens, the optimized concentrator yields a significant improvement in irradiance uniformity on the solar cell with a wide solar spectrum range. It also has good tolerance to the incident sunlight.

Spatial resolution in prism-based surface plasmon resonance microscopy.

Abstract: Several optical surface sensing techniques, such as Surface Plasmon Resonance (SPR), work by imaging the base of a prism by one of its faces. However, such a fundamental optical concern has not been fully analyzed and understood so far, and spatial resolution remains a critical and controversial issue. In SPR, the propagation length L(x) of the surface plasmon waves has been considered as the limiting factor. Here, we demonstrate that for unoptimized systems geometrical aberrations caused by the prism can be more limiting than the propagation length. By combining line-scan imaging mode with optimized prisms, we access the ultimate lateral resolution which is diffraction-limited by the object light diffusion. We describe several optimized configurations in water and discuss the trade-off between L(x) and sensitivity. The improvement of resolution is confirmed by imaging micro-structured PDMS stamps and individual living eukaryote cells and bacteria on field-of-view from 0.1 to 20 mm(2).

What is the apparent angle of a Kelvin ship wave pattern

Abstract: While the half-angle which encloses a Kelvin ship wave pattern is commonly accepted to be 19.47 degrees, recent observations and calculations for sufficiently fast-moving ships suggest that the apparent wake angle decreases with ship speed. One explanation for this decrease in angle relies on the assumption that a ship cannot generate wavelengths much greater than its hull length. An alternative interpretation is that the wave pattern that is observed in practice is defined by the location of the highest peaks; for wakes created by sufficiently fast-moving objects, these highest peaks no longer lie on the outermost divergent waves, resulting in a smaller apparent angle. In this paper, we focus on the problems of free surface flow past a single submerged point source and past a submerged source doublet. In the linear version of these problems, we measure the apparent wake angle formed by the highest peaks, and observe the following three regimes: a small Froude number pattern, in which the divergent waves are not visible; standard wave patterns for which the maximum peaks occur on the outermost divergent waves; and a third regime in which the highest peaks form a V-shape with an angle much less than the Kelvin angle. For nonlinear flows, we demonstrate that nonlinearity has the effect of increasing the apparent wake angle so that some highly nonlinear solutions have apparent wake angles that are greater than Kelvin's angle. For large Froude numbers, the effect on apparent wake angle can be more dramatic, with the possibility of strong nonlinearity shifting the wave pattern from the third regime to the second. We expect our nonlinear results will translate to other more complicated flow configurations, such as flow due to a steadily moving closed body such as a submarine.

Optimization of angularly resolved Bloch surface wave biosensors

Abstract: Bloch surface wave (BSW) sensors to be used in biochemical analytics are discussed in angularly resolved detection mode and are compared to surface plasmon resonance (SPR) sensors. BSW supported at the surface of a dielectric thin film stack feature many degrees of design freedom that enable tuning of resonance properties. In order to obtain a figure of merit for such optimization, the measurement uncertainty depending on resonance width and depth is deduced from different numerical models. This yields a limit of detection which depends on the sensor’s free measurement range and which is compared to a figure of merit derived previously. Stack design is illustrated for a BSW supporting thin film stack and is compared to the performance of a gold thin film for SPR sensing. Maximum sensitivity is obtained for a variety of stacks with the resonance positioned slightly above the TIR critical angle. Very narrow resonance widths of BSW sensors require sufficient sampling but are also associated with long surface wave propagation lengths as the limiting parameter for the performance of this kind of sensors.

Optical waveguide including spatially-varying volume hologram

Abstract: An optical waveguide includes a waveguide body and a spatially-varying volume hologram. The volume hologram increases, in a coordinate direction along the volume hologram, an angle of incidence by which light propagating in the waveguide body via total internal reflection is released from the waveguide body. The optical waveguide may form part of an optical system that includes one or more light sources and/or optical sensors.

Tight control of light beams in photonic crystals with spatially-variant lattice orientation.

Abstract: Spatially-variant photonic crystals (SVPCs), in which the orientation of the unit cell changes as a function of position, are shown to be capable of abruptly controlling light beams using just low index materials and can be made to have high polarization selectivity. Multi-photon direct laser writing in the photo-polymer SU-8 was used to fabricate three-dimensional SVPCs that direct the flow of light around a 90 degree bend. The lattice spacing and fill factor were maintained nearly constant throughout the structure. The SVPCs were characterized at a wavelength of 2.94 μm by scanning the faces with optical fibers and the results were compared to electromagnetic simulations. The lattices were shown to direct infrared light of one polarization through sharp bends while the other polarization propagated straight through the SVPC. This work introduces a new scheme for controlling light that should be useful for integrated photonics.

Phase-sensitive Bloch surface wave sensor based on variable angle spectroscopic ellipsometry

Abstract: In this paper, we propose a phase-sensitive Bloch surface wave sensor based on the variable angle spectroscopic ellipsometry and numerically simulate the phase behavior of the sensor. The simulation results show that the dependence of resonant phase is step-like when BSWs are excited. In contrast to the reflectance behavior, even though losses of the dielectric layers are very small, the resonance dip in the reflectivity will be shallow while the step-like change of the reflection phase of the BSW still be remarkable. This means that phase detection is an alternative to reflectivity intensity detection for the sensing applications of the BSWs in this case. Our experimental results indicate that phase detection for the BSW sensors has the potential to achieve the higher sensitivity and the lower limit of detection.

Polymer optical fiber twisted macro-bend coupling system for liquid level detection

Abstract: The liquid level detection principle of cladding mode frustrated total internal reflection (CMFTIR) effect is proposed. The significant enhancement of CMFTIR effect is realized through macro-bend coupling system in which the dark-field coupling phenomenon between two multimode polymer optic fibers is observed through experiment. Especially twisted macro-bend coupling structure (TMBCS) is adopted to achieve stable coupling of two naked POF. The testing result showed that the dark-filed forward coupling efficiency reached 2‰ and the extinction ratio of the liquid level probe reached 4.18dB. Compared with existing optical fiber liquid level sensors, the TMBCS probe is simpler, robuster, and cheaper. In addition, the TMBCS has the potential for displacement or stress sensing.

The mathematical model of reflection and refraction of longitudinal waves in thermo-piezoelectric materials

Abstract: In this paper, the basic equations of motion, of Gauss and of heat conduction, together with constitutive relations for pyro- and piezoelectric media, are presented. Three thermoelastic theories are considered: classical dynamical coupled theory, the Lord–Shulman theory with one relaxation time and Green and Lindsay theory with two relaxation times. For incident elastic longitudinal, potential electric and thermal waves, referred to as qP, φ-mode and T-mode waves, which impinge upon the interface between two different transversal isotropic media, reflection and refraction coefficients are obtained by solving a set of linear algebraic equations. A case study is investigated: a system formed by two semi-infinite, hexagonal symmetric, pyroelectric–piezoelectric media, namely Cadmium Selenide (CdSe) and Barium Titanate (BaTiO3). Numerical results for the reflection and refraction coefficients are obtained, and their behavior versus the incidence angle is analyzed. The interaction with the interface give rises to different kinds of reflected and refracted waves: (i) two reflected elastic waves in the first medium, one longitudinal (qP-wave) and the other transversal (qSV-wave), and a similar situation for the refracted waves in the second medium; (ii) two reflected potential electric waves and a similar situation for the refracted waves; (iii) two reflected thermal waves and a similar situation for the refracted waves. The amplitudes of the reflected and refracted waves are functions of the incident angle, of the thermal relaxation times and of the media elastic, electric, thermal constants. This study is relevant to signal processing, sound systems, wireless communications, surface acoustic wave devices and military defense equipment.

Terahertz radiation from an ultra-relativistic charge exiting the open end of a waveguide with a dielectric layer.

Abstract: We analyze radiation produced by an ultrarelativistic charge as it exits the open end of a cylindrical waveguide with a dielectric lining. The end of the waveguide can be either orthogonal to the structure axis or skewed. To obtain terahertz radiation from waveguides with centimeter or millimeter radii, we consider high order TM0m modes driven by the beam. We obtain an integral representation which describes the radiation produced by a single waveguide mode in the Fraunhofer zone. We perform a series of numerical calculations for structures which look promising for generation of THz radiation. It is shown that for a mode with large mode number, the aperture of the vacuum channel gives the main contribution to the field if the skew angle of the waveguide aperture is not too small. Simple expressions for the angle of the main pattern lobe maximum are obtained.

Extraction of light trapped due to total internal reflection using porous high refractive index nanoparticle films

Abstract: TiO2 nanoparticle layers composed of columnar TiO2 nanoparticle piles separated with nanoscale pores were fabricated on the bottom surface of the hemispherical glass prism by performing gas phase cluster beam deposition at glancing incidence. The porosity as well as the refractive index of the nanoparticle layer was precisely tuned by the incident angle. Effective extraction of the light trapped in the substrate due to total internal reflection with the TiO2 nanoparticle layers was demonstrated and the extraction efficiency was found to increase with the porosity. An enhanced Rayleigh scattering mechanism, which results from the columnar aggregation of the nanoparticles as well as the strong contrast in the refractive index between pores and TiO2 nanoparticles in the nanoporous structures, was proposed. The porous TiO2 nanoparticle coatings were fabricated on the surface of GaN LEDs to enhance their light output. A nearly 92% PL enhancement as well as a 30% EL enhancement was observed. For LED applications, the enhanced light extraction with the TiO2 nanoparticle porous layers can be a supplement to the microscale texturing process for light extraction enhancement.

Light extraction of trapped optical modes in polymer light-emitting diodes with nanoimprinted double-pattern gratings.

Abstract: Despite the rapid development of polymer light-emitting diodes (PLEDs), the overall device efficiency is still limited because ∼80% of the generated light is trapped in a conventional device architecture by the high refractive index of organic materials and the optical confinement and internal reflection. The implementation of the energy dissipation compensation techniques is urgently required for further enhancement in the efficiency of PLEDs. Here, we demonstrate that incorporating the double-pattern Bragg gratings in the organic layers with soft nanoimprinting lithography can dramatically enhance the light extraction of trapped optical modes in PLEDs. The resulting efficiency is 1.35 times that of a conventional device with a flat architecture used as a comparison. The experimental and theoretical analyses indicate that the enhanced out-coupling efficiency is attributed to the combination of the ordinary Bragg scattering, the guided-mode resonance (GMR), surface plasmon polariton (SPP) modes, and the h...

Achromatic prism-type wave plate for broadband terahertz pulses

Abstract: We demonstrated achromatic half- and quarter-wave plates for broadband terahertz pulses using phase retardation by internal total reflection. Prism-type wave plates realized ultra-broadband retardation stability up to 2.5 THz, which was the limitation of our experimental setup. Novel aspects of our work were use of a 3λ/4 plate as a quarter-wave plate and a multistacked prism-type (MSP) wave plate for a large-aperture THz beam. Real-time polarization imaging of two crossed bunches of hairs was performed to show the efficiency of the MSP wave plate. We clearly observed polarization dependence of the hair direction.