Showing papers on "Total internal reflection published in 2020"

Twisted Nano-Optics: Manipulating Light at the Nanoscale with Twisted Phonon Polaritonic Slabs.

Abstract: Recent discoveries have shown that, when two layers of van der Waals (vdW) materials are superimposed with a relative twist angle between them, the electronic properties of the coupled system can be dramatically altered. Here, we demonstrate that a similar concept can be extended to the optics realm, particularly to propagating phonon polaritons-hybrid light-matter interactions. To do this, we fabricate stacks composed of two twisted slabs of a vdW crystal (α-MoO3) supporting anisotropic phonon polaritons (PhPs), and image the propagation of the latter when launched by localized sources. Our images reveal that, under a critical angle, the PhPs isofrequency curve undergoes a topological transition, in which the propagation of PhPs is strongly guided (canalization regime) along predetermined directions without geometric spreading. These results demonstrate a new degree of freedom (twist angle) for controlling the propagation of polaritons at the nanoscale with potential for nanoimaging, (bio)-sensing, or heat management.

Efficient asymmetric transmission of elastic waves in thin plates with lossless metasurfaces

Abstract: Requiring neither active components nor complex designs, we propose and experimentally demonstrate a generic framework for undistorted asymmetric elastic-wave transmission in a thin plate just using a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of +1 and -1 orders on opposite sides of the metasurface, respectively. Compared with previous loss-induced strategies, the present metasurface maintains a nearly total transmission for the transportation side, but a total reflection from the opposite side, exhibiting a higher contrast ratio of transmission. Moreover, we illustrate that this strong asymmetric behavior is robust to the frequency, the incident angle and the loss effect. The present work paves new avenues to compact rectification, high resolution ultrasonography, vibration and noise control in elastodynamics and acoustics.

Extraordinary Manifestation of Evanescent Wave in Biomedical Application

Abstract: The paper summarizes the latest obtained results illustrating the rectilinear and rotational motion of micro-objects in a biological environment. Such a motion is stipulated by the action of the optical force, on the one hand, and the special influence of the toque of the evanescent wave, on the other one. The reason of this unusual action is the presence of a transverse spin component of the evanescent wave. This feature appears due to the method of excitation: a linearly polarized wave with the azimuth of polarization of 45° at total internal reflection (TIR) at the interface of a prism and a biological medium is used. The objects of the study were gold micro-particles located in a biological environment and red blood cells (RBC) in blood plasma. We demonstrate the possibility for controlling these objects using an evanescent wave under specially selected experimental conditions. The obtained results are highly relevant in modern pharmacology and biomedicine.

Broadband switchable terahertz half-/quarter-wave plate based on metal-VO2 metamaterials.

Abstract: We propose a metal-vanadium dioxide (VO2) metamaterial with broadband and functionality-switchable polarization conversion in the terahertz regime. Simulation results show that the function of the proposed metamaterial can be switched from a half-wave plate (HWP) to a quarter-wave plate (QWP) over a broad bandwidth of 0.66–1.40 THz, corresponding to a relative bandwidth of 71.8%. The HWP obtained when VO2 is in the insulating state has reflection of 90% and linear polarization conversion ratio exceeding 98% over the bandwidth of 0.58–1.40 THz. By transiting the phase of VO2 into the conducting state, the obtained QWP can convert the incident linearly-polarized wave to circularly-polarized wave with an ellipticity of 0.99 over 0.66–1.60 THz. Additionally, results show that the proposed broadband switchable HWP/QWP has a large angular tolerance. We expect that this broadband and switchable multi-functional wave plate will find applications in polarization-dependent terahertz systems including sensing, imaging, and telecommunications.

Light-induced rotation of dielectric microparticles around an optical nanofiber

Abstract: Evanescent electromagnetic fields near a waveguide can exert a transverse radiation force on scattering objects. To prove this experimentally, we demonstrate light-induced orbiting of isotropic, dielectric microparticles around an optical nanofiber that guides elliptically polarized, fundamental modes. The orbit frequency is proportional to the helicity of the coupled light. Interestingly, the observed motion is opposite to the energy flow circulation around the fiber. This result verifies the theoretically predicted negative optical torque on a sufficiently large particle in the vicinity of a nanofiber.

Multimode silicon photonic waveguide corner-bend.

Abstract: An ultra-sharp multimode waveguide bend (MWB) based on a multimode waveguide corner-bend (MWCB) is proposed and realized. With the present MWCB, total internal reflection (TIR) happens and the light propagation direction of all the mode-channels can be modified with low excess losses (ELs) and low inter-mode crosstalk (CT) in the optical communication bands from 1260 nm to 1680 nm. For the MWCB designed for the TE0 and TE1 modes, the ELs are less than 0.18 dB and the inter-mode CTs are less than -36 dB in the wavelength range of 1260-1680 nm. The measurement results show the fabricated MWCB works very well as predicted by the theory. It is very flexible to extend the present MWCB for more mode-channels by simply adjusting the core width. For example, the MWCB designed with a 35 µm-wide core has an EL less than 0.54 dB and inter-mode CT less than -24 dB for the ten TE-polarization modes (i.e., TE0∼TE9) in the wavelength-band of 1260-1680 nm. For the present MWCB, the fabrication is also very convenient because no tiny nano-structure and no additional fabrication steps are needed. It also shows that the present MWCB is not sensitive to the sidewall angles even when the angle is up to 8°. The proposed MWCB is promising for multimode silicon photonics because of the simple structure, easy design, easy fabrication as well as excellent performances in an ultra-broad wavelength-band.

On-Chip Diffraction-Free Beam Guiding beyond the Light Cone

Abstract: On-chip channelless diffraction-free beam guiding enables dense integration of optical circuits in a reconfigurable manner, where total internal reflection, which is considered the cornerstone of guided-wave optics, is utilized to confine light in the out-of-plane direction. Here, we theoretically propose a physical mechanism to achieve on-chip channelless diffraction-free beam guiding beyond the light cone, utilizing the physics of bound states in the continuum. A bound state in the continuum with a tailored spatial dispersion plays an important role in cancelling both the in-plane diffraction and the out-of-plane scattering when the condition for total internal reflection is not satisfied. As a proof-of-concept verification, we experimentally demonstrate such an effect based on an all-dielectric platform using microwaves. The on-chip channelless diffraction-free beam guiding beyond the light cone also allows direct free-space coupling to such self-collimation modes. Our results may open up an avenue for exploring the physics and applications of guided-wave optics.

Efficient Cherenkov-type optical-to-terahertz converter with terahertz beam combining

Abstract: A nonlinear structure for efficient Cherenkov-type terahertz emission from ultrashort laser pulses is proposed, modeled, and experimentally demonstrated. The structure comprises a thin (a few tens of micrometers thick) layer of lithium niobate sandwiched between two silicon prisms. A focused-to-a-line laser pulse propagates in the layer and generates a Cherenkov wedge of terahertz radiation in the prisms. The radiation experiences total internal reflection in the prisms and emerges into free space as two adjacent beams collinear to the pump laser beam. The structure can generate a centimeter-wide terahertz beam with high transverse uniformity and a flat frequency spectrum. An optical-to-terahertz conversion efficiency as high as 0.35% is achieved with 10-µJ laser pulses. It can be further enhanced by reducing the thickness of the lithium niobate layer.

Refractive twisted microaxicons.

Abstract: Complex-shaped light fields with specially designed intensity, phase, and polarization distributions are highly demanded for various applications including optical tweezers, laser material processing, and lithography. Here, we propose a novel (to the best of our knowledge) optical element formed by the twisting of a conic surface, a twisted microaxicon, allowing us to controllably generate high-quality spiral-shaped intensity patterns. Performance of the proposed element was analyzed both analytically and numerically using ray approximation and the rigorous finite difference time domain (FDTD) solution of Maxwell's equation. The main geometric parameters, an apex cone angle and a degree of twisting, were considered to control and optimize the generated spiral-shaped intensity patterns. The three-dimensional structure of such a microaxicon cannot be described by an unambiguous height function; therefore, it has no diffraction analogue in the form of a thin optical element. Such an element can be produced via direct laser ablation of transparent targets with structured laser beams or direct laser writing via two-photon photopolymerization and can be used in various micro- and nano-optical applications.

Polarization-dependent asymmetric transmission using a bifacial metasurface

Abstract: One of the most important research topic in optics and photonics is the design of metasurfaces to substitute conventional optical elements that demonstrate unprecedented merits in terms of performance and form factor. In this context, full-space control of metasurfaces that makes it possible to manipulate scattered light in transmission and reflection spaces simultaneously, is proposed as the next-generation scheme in optics, with a potential for applications such as 360° holographic images and novel optical systems. However, previously designed metasurfaces lacked functionality because the desired operation occurs under preconditioned light; therefore, they are difficult to use in real applications. Here, we present a design method that enables polarization-dependent full-space control, in which two independent and arbitrary phase profiles can be addressed to each space. Upon introducing a phase gradient value to realize the critical angle condition, conversion of transmissive into reflective operation is realized. Then, rectangular nanopillars are utilized to facilitate polarization beam splitting with the desired phase. Three samples were fabricated and measured based on the proposed scheme.

Reflection behavior of elastic waves in the functionally graded piezoelectric microstructures

Abstract: The reflection behaviors of elastic waves in the functionally graded piezoelectric (FGP) microstructures are studied based on the modified couple stress theory. The extended Legendre orthogonal polynomial method (LOPM) is employed to obtain the analytical solutions of governing equations. The LOPM does not require delamination and calculation of the displacements of each partial wave. It is more suitable for solving FGP microstructure. The solutions of the incident P wave and SH wave with the consideration of open-circuit surface and short-circuit surface electrical boundaries are illustrated, respectively. The convergence of the polynomial series method is analyzed through numerical examples. The influences of the length scale parameters, gradient shapes and different electrical boundaries on the reflection behaviors are discussed. It is found that the couple stress can increase the propagation velocity of SH waves in the microstructures, and then reduce the critical angle of the total reflection of the incident SH wave.

Strong Coupling between Tamm and Surface Plasmons for Advanced Optical Bio-Sensing

Abstract: The total internal reflection ellipsometry method was used to analyse the angular spectra of the hybrid Tamm and surface plasmon modes and to compare their results with those obtained using the conventional single SPR method. As such type of measurement is quite common in commercial SPR devices, more detailed attention was paid to the analysis of the p-polarization reflection intensity dependence. The conducted study showed that the presence of strong coupling in the hybrid plasmonic modes increases the sensitivity of the plasmonic-based sensors due to the reduced losses in the metal layer. The experimental results and analysis of the optical responses of three different plasmonic-based samples indicated that the optimized Tamm plasmons ΔRp(TP) and optimized surface plasmons ΔRp(SP) samples produce a response that is about five and six times greater than the conventional surface plasmon resonance ΔRp(SPR) in angular spectra. The sensitivity of the refractive index unit of the spectroscopic measurements for the optimized Tamm plasmon samples was 1.5 times higher than for conventional SPR, while for wavelength scanning, the SPR overcame the optimized TP by 1.5 times.

High-sensitivity temperature sensor based on anti-resonance in high-index polymer-coated optical fiber interferometers

Abstract: Compared to the multimode interference (MMI) effect, the anti-resonance (AR) effect does not rely on the multimode property of the optical waveguide. This Letter shows that fiber bending can suppress the MMI and can break the superposition of AR spectra of multiple modes in a high-index polymer-coated optical fiber interferometer based on a single-mode fiber—polymer-coated no-core fiber—single-mode fiber hetero-structure. This results in the dominance of the AR spectrum of an individual mode and consequently in periodic sharp transmission dips. As a result of this phenomenon and large thermo-optical and thermal expansion coefficients of the polymer, a compact, high-sensitivity and linear response temperature sensor with the sensitivity as high as −3.784nm/∘C has been demonstrated experimentally.

Directional Control and Enhancement of Light Output of Scintillators by Using Microlens Arrays

Abstract: Scintillators play an important role in the field of nuclear radiation detection, such as nuclear medical imaging, dark matter detection, nuclear physics experiments, and national security. However, the light extraction efficiency of a scintillator with a high refractive index is severely restricted because of the total internal reflection. In this paper, microlens arrays have been applied onto the surface of a cerium-doped lutetium-yttrium oxyorthosilicate scintillator to improve the light extraction efficiency and to control the directivity of the light output. Compared to that of a reference sample, a 3.26-fold enhancement with an emission angle of 45° has been obtained by using microlens arrays with optimal parameters. It was also found that the enhancement ratio can be affected by the refractive index of the microlens, the spacing of individual microlens. The control mechanism of microlens arrays is revealed by a combination of simulations and experiments. X-ray imaging characteristics exhibit an improved gray scale amplitude without any loss of the spatial resolution. The present results suggest that the application of microlens arrays to scintillators is beneficial to the field of nuclear radiation detection.

Phosphorene pnp junctions as perfect electron waveguides

Abstract: The current flow in phosphorene pnp junctions is studied. At the interfaces of the junction, omni-directional total reflection takes place, named anti-super-Klein tunneling, as this effect is not due to an energetically forbidden region but due to pseudo-spin blocking. The anti-super-Klein tunneling confines electrons within the junction, which thus represents a perfect lossless electron waveguide. Calculating the current flow by applying Green’s function method onto a tight-binding model of phosphorene, it is observed that narrow electron beams propagate in these waveguides like light beams in optical fibers. The perfect guiding is found for all steering angles of the electron beam as the total reflection does not rely on the existence of a critical angle. For low electron energies and narrow junctions, the guided modes of the waveguide are observed. The waveguide operates without any loss only for a specific orientation of the junction. For arbitrary orientations, minor leakage currents are found, which, however, decay for low electron energies and grazing incidence angles. It is shown that a crossroad-shaped pnp junction can be used to split and direct the current flow in phosphorene. The proposed device, a phosphorene pnp junction as a lossless electron waveguide may not only find applications in nanoelectronics but also in quantum information technology.

Design of an ultra-thin, wide-angle, stray-light-free near-eye display with a dual-layer geometrical waveguide

Abstract: The field of view (FOV) of a geometrical waveguide display is limited by the total internal reflection (TIR) condition (related with the index of glass) and the stray light generated inside the waveguide. A novel concept of an ultra-thin, wide-angle, stray-light-free, optical see-through near-eye display (NED) with a dual-layer geometrical waveguide is proposed in this paper. In the dual-layer waveguide, the two waveguides have different structures and are responsible for two different FOVs which are spliced together to form the entire FOV. The stray light of the dual-layer waveguide is analyzed and an optimized structure to suppress the stray light is designed. An optimized coupling-in structure is designed and a progressive optimization method is proposed for optimizing the illuminance uniformity of the entire FOV across the exit pupil. A dual-layer waveguide with a total thickness of 3.0 mm and stray light of less than 1% is designed. The FOV is 62° in the pupil-expanding direction, and the diameter of the exit pupil (EPD) is 10 mm at an eye relief (ER) of 18 mm. A compact projection optic is designed and finally is integrated with the dual-layer waveguide.

Bloch waves at the surface of a single-layer coating D-shaped photonic crystal fiber.

Abstract: Bloch surface wave (BSW) platforms are particularly interesting for light confinement and surface sensitivity, as an alternative to the metal-based surface plasmon polaritons (SPP). However, most of the reported BSW platforms require depositing a large number of alternating dielectric layers to realize the excitation of the surface waves. In this Letter, we demonstrate an experimentally feasible D-shaped photonic crystal fiber (PCF) platform consisting of only a single dielectric layer on its flat surface, which can sustain Bloch waves at the boundary between the dielectric layer and the PCF cladding. The presence of the dielectric layer modifies the local effective refractive index, enabling a direct manipulation of the BSWs. In addition, the D-shaped structure provides direct contact with the external medium for sensing applications with an ultrahigh sensing figure of merit ($2451\;{{\rm RIU}^{ - 1}}$2451RIU−1) and has the potential to be used over a wide range of analyte refractive indices.

Ultracompact Chip-Scale Refractometer Based on an InGaN-Based Monolithic Photonic Chip.

Abstract: Optical refractometer constitutes the core element for many applications, from determining the purity and concentration of pharmaceutical ingredients to measuring the sugar content in food and beverages, and the analysis of petroleum. Here, we demonstrated the monolithic integration of light-emitting diodes (LEDs) and photodetectors (PDs) to fabricate ultracompact refractometers with a chip size of 475 × 320 μm2. The light emission and photodetection properties of the devices containing the same InGaN/GaN multi-quantum wells have been characterized, confirming that the PD can respond to the emission of the LED. The flip-chip assembly of the chip enables the exposed sapphire substrate to be in direct contact with the solution, and the refractive index sensing capability governed by the change of critical angle and Fresnel reflection at the sapphire/solution interface has been investigated. The processing of the optically smooth surface of sapphire and the integration of high-reflectance distributed Bragg reflector beneath the devices facilitate the amount of light received by the PD. The monolithic chip is capable of detecting solutions with a refractive index ranging from 1.3325 to 1.5148 RIU and exhibits a sensitivity of 7.77 μA/RIU and a resolution of 6.4 × 10-6 RIU at the LED current of 10 mA. Rapid real-time responses of 33.9 ms for rise time and 34.7 ms for fall time are obtained in the detected photocurrent, thereby verifying the feasibility of the chip-scale refractometer.

Enhanced light extraction of light-emitting diodes with micro patterns by femtosecond laser micromachining for visible light communication

Abstract: A significant enhancement of light extraction of light-emitting diodes (LEDs) with micro patterns has been experimentally investigated. The micro patterns on the surface of a polymer layer are fabricated by a femtosecond laser Bessel beam for obtaining microhole arrays with large depth, resulting in the reduction of photon loss by total internal reflection (TIR) at the surface of the LED. The light output power of the LED is apparently increased by introducing the array patterns without influencing its current-voltage (I-V) characteristics. Moreover, the electroluminescence spectra of a multi-color LED and its angular radiation profiles with orthogonal and hexagonal patterns also have been explored. In addition, the optical field distributions of the micro patterns simulated by the finite difference time domain method have expressed the modulation effect of the array depth. Finally, the patterned LED as a transmitter is embedded in the visible light communication system for evaluating the transmission signal quality.

Nanofocusing Optics for an X-Ray Free-Electron Laser Generating an Extreme Intensity of 100 EW/cm2 Using Total Reflection Mirrors

Abstract: A nanofocusing optical system—referred to as 100 exa—for an X-ray free-electron laser (XFEL) was developed to generate an extremely high intensity of 100 EW/cm2 (1020 W/cm2) using total reflection mirrors. The system is based on Kirkpatrick-Baez geometry, with 250-mm-long elliptically figured mirrors optimized for the SPring-8 Angstrom Compact Free-Electron Laser (SACLA) XFEL facility. The nano-precision surface employed is coated with rhodium and offers a high reflectivity of 80%, with a photon energy of up to 12 keV, under total reflection conditions. Incident X-rays on the optics are reflected with a large spatial acceptance of over 900 μm. The focused beam is 210 nm × 120 nm (full width at half maximum) and was evaluated at a photon energy of 10 keV. The optics developed for 100 exa efficiently achieved an intensity of 1 × 1020 W/cm2 with a pulse duration of 7 fs and a pulse energy of 150 μJ (25% of the pulse energy generated at the light source). The experimental chamber, which can provide different stage arrangements and sample conditions, including vacuum environments and atmospheric-pressure helium, was set up with the focusing optics to meet the experimental requirements.

Large Angle Forward Diffraction by Chiral Liquid Crystal Gratings with Inclined Helical Axis

Abstract: A layer of chiral liquid crystal (CLC) with a photonic bandgap in the visible range has excellent reflective properties. Recently, two director configurations have been proposed in the literature for CLC between two substrates with periodic photo-alignment: one with the director parallel to the substrates and one with the director in the bulk parallel to the tilted plane. The transmission experiments under large angles of incidence (AOI) presented in this work prove that, in the bulk, the director does not remain parallel with the substrates. Because of the inclined helical axis, the full reflection band can be observed at a smaller AOI than in planar CLC. For sufficiently large AOI, the reflection of diffracted light is prohibited by total internal reflection and efficient diffraction occurs in the forward direction.

Nucleation and critical conditions for stripe domains in monodomain nematic elastomer sheets under uniaxial loading

Abstract: Liquid crystal elastomers with mobile liquid crystal moieties display unique mechanical instabilities and domain patterns in the strain-induced director reorientation. Based on a continuum mechanical model, we present a complete Lagrangian description of finite element formulation of LCEs by solving the coupled displacement and director fields within the Lagrangian scheme. The nucleation of the stripe domain and critical conditions for its formation are numerically studied for thin monodomain centimeter-sized sheets. Under orthogonal uniaxial loading tests, micrometer-sized stripe domains nucleate after loading to a critical strain at which a stress peak is observed along with a reduction in the neo-classical elastic energy. Further, we observed increase in the semisoft and the Frank elastic energy as indications of strong and heterogeneous director rotations. When the loading axis was not exactly orthogonal to the initial director, we found that the sample morphology was strongly dependent on the material and geometrical parameters. Strain-induced stripe domains could still be observed in the central region of the sample when the loading axis was nearly at right angle to the initial director. If the angle between the loading axis and the initial director was slightly deviated from the right angle, nearly uniform rotations were observed. The critical angle, beyond which strain-induced stripe domains were observed, could be very close to the right angle for samples with rather small shape anisotropy and relatively large length to width ratio. In such cases, the observation of strain-induced stripe domains was highly sensitive to any possible experimental error in orienting the loading axis at right angle, and minor deviations from the orthogonal loading condition can cause uniform rotations.

Measurements of angle-resolved reflectivity of PTFE in liquid xenon with IBEX

Abstract: Liquid xenon particle detectors rely on excellent light collection efficiency for their performance. This depends on the high reflectivity of polytetrafluoroethylene (PTFE) at the xenon scintillation wavelength of 178 nm, but the angular dependence of this reflectivity is not well-understood. IBEX is designed to directly measure the angular distribution of xenon scintillation light reflected off PTFE in liquid xenon. These measurements are fully described by a microphysical reflectivity model with few free parameters. Dependence on PTFE type, surface finish, xenon pressure, and wavelength of incident light is explored. Total internal reflection is observed, which results in the dominance of specular over diffuse reflection and a reflectivity near 100% for high angles of incidence.

Analysis of the Imaging Characteristics of Holographic Waveguides Recorded in Photopolymers

Abstract: In this work, we study the imaging characteristics of an optical see-through display based on a holographic waveguide. To fabricate this device, two transmission holograms are recorded on a photopolymer material attached to a glass substrate. The role of the holograms is to couple the incident light between air and the glass substrate, accomplishing total internal reflection. The role of noise reflection gratings and shrinkage on the imaging characteristics of the device will be also explored. The holograms (slanted transmission gratings with a spatial frequency of 1690 lines/mm) were recorded on a polyvinyl alcohol acrylamide holographic polymer dispersed liquid crystal (HPDLC) material. We will show that sufficient refractive index modulation is achieved in the material, in order to obtain high diffraction efficiencies. We will demonstrate that the final device acts as an image formation system.

A novel composite transmission metasurface with dual functions and its application in microstrip antenna

Abstract: Generally, a double corner-cut square structure transmission linear to circular polarization conversion metasurface (DCS-PCM) is difficult to realize circularly polarized radiation when it is applied to a linearly polarized microstrip antenna as electromagnetic surfaces superstrate. A novel composite transmission metasurface (NCTM) with dual functions of linear to circular polarization conversion and polarization selection is proposed and verified. For a y-polarized incident wave, transmission linear to right-hand circular polarization conversion can be realized from 8.43 GHz to 9.50 GHz, and for an x-polarized incident wave, co-polarized total reflection can be realized. Different from the DCS-PCM, the unit cell of the NCTM is made up of a corner-cut square, a substrate, and a polarization gate. Thanks to the novel design of polarization gates, the proposed NCTM is easier to realize circularly polarized radiation when it is applied to a linearly polarized source antenna. The realized gain of the antenna with NCTM can be improved because of the Fabry–Perot resonant cavity, and the maximum amplitude of the gain improvement is 6.8 dBi. The results of simulation and experiment show that the linearly polarized microstrip antenna with NCTM can realize circularly polarized radiation with an axial ratio less than 3 dB and a significant gain improvement simultaneously.

A high quality surface finish grinding process to produce total reflection mirror for x-ray fluorescence analysis

Abstract: Total reflection x-ray fluorescence analysis is applied to trace element detection in liquid for effective environmental monitoring. This analytical approach requires x-ray total reflection mirrors. In order to achieve high sensitivity element detection, the mirrors require high surface quality for high x-ray reflectivity. Surface finishing for x-ray mirrors is typically conducted through a series of abrasive processes, such as grinding and polishing, and is thus time consuming. The purpose of this study is to streamline and enhance the surface finishing process based on unique high quality grinding techniques for the production of x-ray total reflection mirrors.

Enhancing photoluminescence of carbon quantum dots doped PVA films with randomly dispersed silica microspheres

Abstract: As a kind of excellent photoluminescent material, carbon quantum dots have been extensively studied in many fields, including biomedical applications and optoelectronic devices. They have been dispersed in polymer matrices to form luminescent films which can be used in LEDs, displays, sensors, etc. Owing to the total internal reflection at the flat polymer/air interfaces, a significant portion of the emitted light are trapped and dissipated. In this paper, we fabricate free standing flexible PVA films with photoluminescent carbon quantum dots embedded in them. We disperse silica microspheres at the film surfaces to couple out the total internal reflection. The effects of sphere densities and diameters on the enhancement of photoluminescence are experimentally investigated with a homemade microscope. The enhancement of fluorescence intensity is as high as 1.83 when the film is fully covered by spheres of 0.86 [Formula: see text]m diameter. It is worth noting that the light extraction originates from rather the scattering of individual spheres than the diffraction of ordered arrays. The mechanism of scattering is confirmed by numerical simulations. The simulated results show that the evanescent wave at the flat PVA/air interface can be effectively scattered out of the film.