Low-index Layer Research Articles

High-efficiency unidirectional vertical emitter achieved by an aperture-coupling nanoslot antenna array.

Coupling light from in-plane guided light into free space or optical fibers is crucial for many photonic integrated circuits and vice versa. However, traditional grating couplers or waveguide grating antennas suffer from low upward coupling efficiency due to the light radiating in both upward and downward directions simultaneously. In this paper, a compact aperture-coupling nanoslot antenna array is proposed for high-efficiency unidirectional radiation, where a two-dimensional high-contrast grating (HCG) is employed as a mirror to reflect the undesired downward radiation. Upon the HCG separated by a low-index spacing layer, a thin silver layer is deposited. Finally, a series of H-shaped slots are patterned on the silver thin film to arrange the aperture fields and radiate the in-plane guided light into free space. The proposed nanoslot antenna array features a front-to-back ratio (F/B) over 10 dB within the wavelength range of 1500 ∼ 1600 nm. At the same time, a high radiation efficiency of over 75% and a maximum radiation efficiency of 87.6% are achieved within the 100 nm bandwidth. The high-efficiency unidirectional antenna array is promising for the integrated photonic applications including wireless optical communications, light detection and ranging, and fiber input/output couplers.

Detection of oral cancerous cells using highly sensitive one-dimensional distributed Bragg’s Reflector Fabry Perot Microcavity

In the proposed work, a multi-layered Distributed Bragg’s Reflector (DBR) Fabry Perot Microcavity resonator is proposed theoretically to sense bio-analyte. One dimensional Photonic-Crystal (PhC) sensor is designed and analysed to sense the presence of oral cancerous cells in the analyte. The Characteristic Matrix Method (CMM) is used to design, model and analyse the proposed sensor. A multi-layer structure with a central defect having 3 pairs of high and low refractive index layers on either side of the defect is analysed for its sensing performance. The incident light having wavelength in the range of mid-infrared frequency is used at input source, which enhances the sensor sensitivity. Five normal (INOK) cells and oral cancerous (YD-10B) cells are considered for the analysis of sensor performance. The effect of variation in the geometrical length of central defect layer and the number DBR layers on resonant wavelength, sensitivity, and Q factor is performed. A highest sensitivity of 3630 nm/RIU with a Q-factor of 11,323 and a very minimum resolution of 9.5 × 10−5 RIU is obtained. The sensor proposed in this work is suitable for label-free, easy fabrication, cost-effective, and highly sensitive sensor designs for biomedical applications.

Fiber Bragg grating-electromagnetically induced transparent fast optical switch

Here, an all-optical switch is modeled in the form of fiber Bragg grating (FBG) based on electromagnetically induced transparency (EIT) phenomena dealing with a three-level EIT silicon (Si) nanocrystalline medium. The EIT consists of two fields, namely a strong control and weak probe fields that interact with the medium. Fast-optical switching of the probe is achieved by applying a pulsed control field, which relies on both steady-state and transient density matrices. The effects of four significant parameters of interest, i.e., Rabi frequency of the control field, EIT nanocrystal density, FBG length, and spontaneous decay rate, have been extensively investigated to optimize the probe transmission. Subsequently, the time-dependent density matrix is solved to obtain the response time of the FBG-EIT switch. Eventually, FBG is modeled such that low index layers contain EIT material and the high index layers do not. The FBG-EIT switch satisfies the criteria of maximum transmission and minimum rise time to give out the optimal operational condition. Despite the optical switching is in OFF mode at Ωc=0 THz demonstrating a high FBG reflectance, the reflectance suddenly vanishes at a certain Ω c s to satisfy the equivalence of low and high refractive indices (switching ON). Therefore, a fast-optical switch is envisaged operating as large as 100 GHz in theory. The future applications of the FBG- EIT include the fast Q-switching of fiber lasers, ultra-fast modulation, optical quantum communication, and high speed optical processing and computation.

High Q factor Electrically Injected Green Micro Cavity

Green micro-cavity (MC) with a quality factor (Q) exceeding 6000 was demonstrated under current injection. The MC consists of two dielectric distributed Bragg reflectors (DBRs) and InGaN/GaN quantum wells (QWs) in between. To form a lateral optical confinement (LOC) structure, a low-index thin AlN layer was used to replace part of the p-GaN. The device showed multi-longitudinal mode emission, from 460 to 540 nm, and higher-order lateral confined modes were clearly observed. By optimizing the cavity fabrication processes and lateral optical confinement, the linewidth of resonant mode is as narrow as 0.082 nm, indicating a high Q value of 6039 in green spectral region. This is the highest value in electrically injected GaN-based MC to the best of our knowledge. In addition, three-dimensional (3D) confined optical states were observed and studied via angle resolved measurement for the first time in an electrically injected GaN-based MC. The emission characteristics of devices as a function of cavity length, as well as the loss mechanism inside the cavity were also systematically analyzed.

Construction of a dual-core hollow waveguide for visible and mid-infrared light transmission based on PTFE tubing and UV gel

This work proposes a facile approach to fabricate a dual-core hollow waveguide based on two common and low-cost raw materials, namely polytetrafluoroethylene (PTFE) tube and ultraviolet (UV) gel. A silica glass tube is nested inside a heat-shrinkable PTFE tube. After heat treatment at 350 °C, the PTFE tube shrinks and then transforms into the low-index polymer on the outside of the silica tube. The PTFE-coated silica glass tubing is filled with UV gel followed by inserting a typical Ag/AgI mid-infrared (IR) hollow optical fiber. The UV gel is cured by UV radiation, forming a solid low-index layer between the PTFE-coated silica glass tubing and the Ag/AgI hollow optical fiber. A visible laser beam can be transmitted at a loss of 0.4 dB/m through the silica annulus between the low-index PTFE and UV gel layer. A 10.6-μm-wavelength CO2 laser beam is delivered through the Ag/AgI hollow optical fiber (core size 530 μm) and the transmission loss goes up from 1.63 to 3.22 dB/m as the bending angle increases from 0° to 90°.

Guided Mode Resonance in a Low-Index Waveguide Layer

In this paper, we show that the guided mode resonance can exist in a low-index waveguide layer on top of a high-index substrate. With the help of the interaction of diffraction from a metal grating and total internal reflection effects, we verify that the guided mode can be supported in the low-index SU8 layer on a high-index substrate. Simulation and experiment show the resonant wavelength can be simply manipulated by controlling the geometrical parameters of the metal grating and waveguide layer. This structure extends the possibilities of guided-mode resonance to a broader class of functional materials and may boost its use in applications such as field enhancement, sensing and display.

Color Glass by Layered Nitride Films for Building Integrated Photovoltaic (BIPV) System

We investigated layered titanium nitride (TiN) and aluminum nitride (AlN) for color glasses in building integrated photovoltaic (BIPV) systems. AlN and TiN are among suitable and cost-effective optical materials to be used as thin multilayer films, owing to the significant difference in their refractive index. To fabricate the structure, we used radio frequency magnetron deposition method to achieve the target thickness uniformly. A simple, fast, and cheap fabrication method is achieved by depositing the multilayer films in a single sputtering chamber. It is demonstrated that a multilayer stack that allows light to be transmitted from a low refractive index layer to a high refractive index layer or vice-versa can effectively create various distinct color reflections for different film thicknesses and multilayer structures. It is investigated from simulation based on wave optics that TiN/AlN multilayer offers better color design freedom and a cheaper fabrication process as compared to AlN/TiN multilayer films. Blue, green, and yellow color glasses with optical transmittance of more than 80% was achieved by indium tin oxide (ITO)-coated glass/TiN/AlN multilayer films. This technology exhibits good potential in commercial BIPV system applications.

Toward Ultra‐Efficient OLEDs: Approaches Based on Low Refractive Index Materials

AbstractOrganic light‐emitting diodes (OLEDs) have successfully established themselves as light sources vital in high‐end mobile displays and television due to various advances made for a wide range of technical aspects from materials, devices, and processes to air‐tight packaging and systems. However, a significant portion of the generated excitons in OLEDs are still lost to various channels. Among them, excitation of surface plasmon polaritons (SPPs) is a critical source of exciton loss, the recovery of which often involves structuring in nanoscale. Herein, a nano‐pattern‐free approach is reviewed to reducing SPP loss that makes use of materials with low refractive index. Because of the limited availability of such low refractive index materials, a method to use a low extraordinary refractive index of birefringent organic semiconductors is explored. With the proposed method combined with common lens‐based outcoupling schemes and high refractive index substrates, it is shown that an ultrahigh external quantum efficiency of ≈72% in a green, single‐junction phosphorescent OLED is within the reach. Transient photoluminescence measurement is used to monitor the change in Purcell factor incurred by the use of the birefringent low refractive index layer and to further verify the validity of the proposed concept.

Measurement and control of optical nonlinearities in dispersive dielectric multilayers.

Dispersive dielectric multilayer mirrors, high-dispersion chirped mirrors in particular, are widely used in modern ultrafast optics to manipulate spectral chirps of ultrashort laser pulses. Dispersive mirrors are routinely designed for dispersion compensation in ultrafast lasers and are assumed to be linear optical components. In this work, we report the experimental characterization of an unexpectedly strong nonlinear response in these chirped mirrors. At modest peak intensities <2 TW/cm2-well below the known laser-induced damage threshold of these dielectric structures-we observed a strong reflectivity decrease, local heating, transient spectral modifications, and time-dependent absorption of the incident pulse. Through computational analysis, we found that the incident laser field can be enhanced by an order of magnitude in the dielectric layers of the structure. The field enhancement leads to a wavelength-dependent nonlinear absorption, that shows no signs of cumulative damage before catastrophic failure. The nonlinear absorption is not a simply two-photon process but instead is likely mediated by defects that facilitate two-photon absorption. To mitigate this issue, we designed and fabricated a dispersive multilayer design that strategically suppresses the field enhancement in the high-index layers, shifting the high-field regions to the larger-bandgap, low-index layers. This strategy significantly increases the maximum peak intensity that the mirror can sustain. However, our finding of an onset of nonlinear absorption even at 'modest' fluence and peak intensity has significant implications for numerous past published experimental works employing dispersive mirrors. Additionally, our results will guide future ultrafast experimental work and ultrafast laser design.

Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

High-index transparent electrodes have been one major origin of light trapping and lower light extraction in organic light emitting diodes (OLEDs). In this work, influences of the bottom transparent electrode thickness on emission properties of OLEDs are systematically studied by both simulation and experiments. Simulation shows that with substantially decreasing the thickness of the high-index indium tin oxide (ITO) electrode, waveguided modes, that otherwise would be significantly induced in regular/thicker ITO devices, can be effectively eliminated. Consequently, the overall coupling efficiencies of OLED emission into substrates can be much enhanced. Through further effective light extraction from the substrate, green phosphorescent OLEDs with a high external quantum efficiency (EQE) of up to ≈57.5% were experimentally demonstrated by adopting the very thin (20 nm) ITO electrode and preferentially horizontal dipole emitters (with a horizontal dipole ratio of 76%). The simulation further predicts that very high optical coupling efficiencies into substrates and EQEs approaching 80% are possible with further adopting purely horizontal dipole emitters and/or low-index electron transport layer (ETL) to suppress surface plasmon modes. Overall, this study clearly reveals the potential of using thin transparent electrodes for highly efficient OLEDs.

Deep-UV hexagonal boron nitride (hBN)/BAlN distributed Bragg reflectors fabricated by RF-sputtering

The hexagonal boron nitride (hBN) and BAlN films were prepared by RF-sputtering, which were used as the low and high refractive index layers. A series of hBN/BAlN distributed Bragg reflectors (DBRs) were prepared on sapphire substrate. The reflectivity of 9-pair hBN/BAlN (39 nm/33 nm) DBR reached 90% at 300 nm with a bandwidth of 45 nm, and which of 6-pair hBN/BAlN (35 nm/29 nm) reached 52% at 280 nm. The hBN/BAlN DBRs can be used to achieve higher reflectivity in shorter UV bands with the improvement of BAlN material quality through the growth condition optimization.

Realization of exceeding 80% external quantum efficiency in organic light-emitting diodes using high-index substrates and highly horizontal emitters

Although both high-index substrates and horizontal-dipole emitters have been shown to be facile approaches for enhancing OLED (organic light emitting diode) light extraction, the full benefits and potential of their combination for OLED optical out-coupling have not been thoroughly studied and explored. Simulation studies indicate that very high optical coupling efficiency into substrates ϕsub (and perhaps similarly high OLED external quantum efficiencies) of ~90% can be possibly obtained with both high-index substrates (refractive index >1.8–1.9) and highly horizontal-dipole emitters (horizontal dipole ratio >85%), together with adoption of low-index or index-matching carrier transport layers and optimization of organic layer and transparent electrode thicknesses. With these judicious device design conditions, all waveguided modes and surface plasmon modes in devices can be effectively suppressed for optimal optical out-coupling. Finally, combining the sapphire substrate having high index of n~1.78, the recently developed OLED emitters having high horizontal emitting dipole ratio of up to 87%, and simple external extraction lens, OLED devices having external quantum efficiency of over 80% was successfully realized.

Ge-on-insulator lateral p-i-n waveguide photodetectors for optical communication.

We report high-performance lateral p-i-n Ge waveguide photodetectors (WGPDs) on a Ge-on-insulator (GOI) platform that could be integrated with electronic-photonic integrated circuits (EPICs) for communication applications. The high-quality Ge layer affords a low absolute dark current. A tensile strain of 0.144% in the Ge active layers narrows the direct bandgap to enable efficient photodetection over the entire range of C- and L-bands. The low-index insulator layer enhances optical confinement, resulting in a good optical responsivity. These results demonstrate the feasibility of planar Ge WGPDs for monolithic GOI-based EPICs.

Single polymer-variable index for the design and fabrication of variable stop band distributed Bragg reflectors

Abstract Thin films of variable refractive index from a single polymer were developed by doping titanium dioxide (TiO2) in poly-vinyl alcohol (PVA) by solution processing. Refractive index as high as 1.80 was achieved by increasing the doping levels of TiO2 in PVA. Polymer distributed Bragg reflectors (DBRs) were fabricated by coating titania doped PVA polymer matrix composite (TPMC) and poly methyl methacrylate (PMMA) polymer as alternating high and low index layers respectively. The reflectivity of DBR was increased by increasing number of periods. The experimental results were in agreement with transfer matrix model (TMM) simulations. TMM simulation results showed that, to achieve reflectivity of more than 90%, 20 periods were required for an index contrast of 0.17 and periods reduced to 5 for an index contrast of 0.51. High index contrast of alternative layers will also benefit in having a relatively broad stop band. The present work suggests fabricating flexible polymer DBRs with any desired stop band by simply varying the concentration of TiO2 in PVA without altering any other thin film processing and DBR fabrication parameters.

Recovering cavity effects in corrugated organic light emitting diodes.

Cavity effects play an important role in determining the out-coupling efficiency of an OLED. By fabricating OLEDs on corrugated substrates, the waveguide and SPP modes can be extracted by diffraction. However, corrugation does not always lead to an enhancement in out-coupling efficiency due to the reduction of the electrode reflectance and hence the cavity effects. Based on the results of our rigorous couple-wave analysis (RCWA) simulation, we found that the cavity effects can be partially recovered using a low index Teflon layer inserted between the ITO anode and the substrate due to the enhancement of the reflectance of the corrugated electrodes. To verify the simulation results, we fabricated corrugated OLEDs having a low-index Teflon interlayer with an EQE of 36%, which is 29% higher than an optimized planar OLED. By experimentally measuring the OLED air mode dispersion, we confirm the cavity emission of a corrugated OLED is enhanced by the low index layer.

Simulation Study on the Improvement of the Luminance and the Color Uniformities of Integrated Quantum-Dot Backlights for LCD Applications

A new optical structure of the quantum dot (QD)-based backlight in which the QD layer was integrated on the light guide plate (LGP) was proposed. The optical performances were studied by using optical simulation, especially from the viewpoint of brightness and color uniformities. A low-index layer was inserted between the LGP and the QD layer to maintain efficient light guiding in the LGP, and an anamorphic lens was put over the blue LED to control the light distribution in the LGP. The simulation results for the optimized structure showed that the luminance uniformity was more than 93% and that the spatial and the angular variations in the color coordinates were less than 0.01. The present study demonstrates that satisfactory luminance and color uniformities can be achieved from the integrated QD backlights via optimization of the light distribution in the LGP. The integrated structure may be helpful in reducing the number of optical components and the cost of backlights for liquid crystal display (LCD) applications.

Influence of Anodization Temperature on Geometrical and Optical Properties of Porous Anodic Alumina(PAA)-Based Photonic Structures

In this work, the influence of a wide range anodizing temperature (5–30 °C) on the growth and optical properties of PAA-based distributed Bragg reflector (DBR) was studied. It was demonstrated that above 10 °C both structural and photonic properties of the DBRs strongly deteriorates: the photonic stop bands (PSBs) decay, broaden, and split, which is accompanied by the red shift of the PSBs. However, at 30 °C, new bands in transmission spectra appear including one strong and symmetric peak in the mid-infrared (MIR) spectral region. The PSB in the MIR region is further improved by a small modification of the pulse sequence which smoothen and sharpen the interfaces between consecutive low and high refractive index layers. This is a first report on PAA-based DBR with a good quality PSB in MIR. Moreover, it was shown that in designing good quality DBRs a steady current recovery after subsequent application of high potential (UH) pulses is more important than large contrast between low and high potential pulses (UH-UL contrast). Smaller UH-UL contrast helps to better control the current evolution during pulse anodization. Furthermore, the lower PSB intensity owing to the smaller UH-UL contrast can be partially compensated by the higher anodizing temperature.

Efficient guiding mid-infrared waves with graphene-coated nanowire based plasmon waveguides

A graphene-coated nanowire (GCNW) based plasmon waveguide is proposed to achieve outstanding guiding performance in the mid-infrared band. The modal properties of the fundamental graphene plasmon mode and modal dependence on the elliptical nanowire size, thickness and width of the low-index dielectric layer, chemical potential of graphene and the operating frequency are revealed in detail by using finite element method. Results show that the low-index layer has a significant influence on modal properties, and a propagation length over 13 μm along with an extremely small normalized mode size (~10−6) could be obtained simultaneously, which is more than two orders of magnitude smaller compared with free-standing GCNW and GCNW dimers. Besides, study on crosstalk characteristics between the adjacent structures indicates the proposed waveguide exhibits very low crosstalk. Due to these excellent transmission characteristics, the proposed configuration could be used to fabricate various functional photonic devices for the future nanoscale photonic integrated circuits.

Narrow resonance and ultrahigh sensitivity plasmonic waveguide refractive index sensor

A surface plasmon resonance optical refractive index sensor (for aqueous medium) with high angular and spectral sensitivities S, very small full width at half maxima (FWHM) w (narrow resonance), and very high quality factor, χ = S/w, is proposed, which operates in both angular and spectral interrogation modes. The structure consists of a prism and a composite plasmonic waveguide consisting of a high-index dielectic layer and a thin metal (Ag) film, separated by a low-index layer. Above the metal film, the medium is analyte. It is shown that even with a prism of low index, the proposed sensor offers very small FWHM of reflection spectra, which improves the sensor performance. Numerically, an angular sensitivity of 168°RIU−1 and a quality factor χ = 1680 can be achieved for the proposed configuration. Spectral sensitivity comes out to be 6000 nm RIU−1.

Organic Light-Emitting Diodes: Pushing Toward the Limits and Beyond.

Organic light-emitting diodes (OLEDs) are established as a mainstream light source for display applications and can now be found in a plethora of consumer electronic devices used daily. This success can be attributed to the rich luminescent properties of organic materials, but efficiency enhancement made over the last few decades has also played a significant role in making OLEDs a practically viable technology. This report summarizes the efforts made so far to improve the external quantum efficiency (EQE) of OLEDs and discusses what should further be done to push toward the ultimate efficiency that can be offered by OLEDs. The study indicates that EQE close to 58% and 80% can be within reach without and with additional light extraction structures, respectively, with an optimal combination of cavity engineering, low-index transport layers, and horizontal dipole orientation. In addition, recent endeavors to identify possible applications of OLEDs beyond displays are presented with emphasis on their potential in wearable healthcare, such as OLED-based pulse oximetry as well as phototherapeutic applications based on body-attachable flexible OLED patches. OLEDs with fabric-like form factors and washable encapsulation strategies are also introduced as technologies essential to the success of OLED-based wearable electronics.