Microcavity Research Articles

Phosphor-up-converted white LED materials assisted by 1-D photonic crystals

A series of lanthanide-doped aluminosilicate glasses have been prepared by sol-gel (SG) processing. The present work started by optimizing bulk aluminosilicate glasses doped with Er3+, Tm3+ and Yb3+ for white light generation (WLG) and possible application as white LED materials, using SG as a low-cost high purity processing alternative to conventional phosphor fabrication. Undoped alternating aluminosilicate glass and titania multilayers were also synthesized in the form of Bragg mirrors (BMs) and Fabry-Perot Microcavity (MC) 1-D photonic crystals to assist light emission, e.g. by enhancing it at selected wavelengths within the WLG spectrum. This work utilizes (anti-Stokes) up-conversion (UC) excited at 980 nm to generate white light, instead of conventional (Stokes) down-conversion. The materials prepared were characterized by X-ray diffraction (XRD), Spectroscopic Ellipsometry, FTIR, variable angle specular reflection and UC photoluminescence (UCPL) spectroscopies. The UC light emission was analyzed with the help of CIE chromaticity diagrams. Laser power dependence studies were also performed in order to assess the average number of infrared photons (980 nm) involved in the UC mechanism.

Microneedles with Recessed Microcavities for Electrochemical Sensing in Dermal Interstitial Fluid

Transdermal electrochemical sensors based on microneedle (MN) or MN array technology are the next frontier of personalized wearable devices in health monitoring and diagnostics. The performance of transdermal sensors is impacted mainly by damage or deformation of the sensing layer located on the MN surface, which usually occurs due to friction caused between the MN surface and skin tissue. In this work, we report the development of a 4 × 4 polymeric MN array (PMNA) with an individual MN height of ∼720 μm and integrated conductive recessed microcavities (MC) on the MNs surface to protect the sensing layer during insertion and removal from the skin. Characterization and proof-of-concept experiments on porcine skin reveal that PMNA/MC-based platforms retain their functionality and sensitivity of 2.36 ± 0.06 nA mM–1 even after multiple skin applications. Three-dimensional nanostructuring of MNA hence provides alternative solutions to the critical issues of maintaining performance upon insertion into skin. This approach holds great potential in advancing transdermal sensing technology and affording reliable, cost-effective, and clinically adequate sensing devices.

Constructing Microcavity for Perovskite Laser Power Converter: A Theoretical Study

Laser power converters (LPCs) used in wireless energy transmission can realize noncontact power supply under extreme conditions and provide continuous energy support to unmanned probes, which is believed to play an essential role in the exploration of deep ocean. However, commercially available LPCs usually employ III–V semiconductors as light absorbers due to their industrial maturity. In order to match the underwater 450∼540 nm laser source, it requires an epitaxial growth method to tune the bandgap of those materials, which is highly costly that severely restricts the industrial preparation of LPCs on a large scale. Herein, the potential of perovskite in the application of underwater LPCs is presented theoretically by constructing MAPbBr3 LPC and its theoretical power conversion efficiency (PCE) of 48.55% is demonstrated. Moreover, a microcavity (MC) constructing strategy to further improve the device performance is proposed, in which the transfer matrix method and an admittance‐based antireflection condition approach are combined for the optical simulation. By modulating the MC condition, the significant optical resonance inside the device while avoiding the obvious light reflection is realized. As a result, MC LPCs can obtain a simulated PCE of 64.27%, which is highly competitive with the existing UV–vis LPCs.

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Nanoscale Photoluminescence Manipulation in Monolithic Porous Silicon Oxide Microcavity Coated with Fluorescent Polyelectrolytes Via Electrostatic Nanoassembling

Porous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters [1,2], still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO2) microcavity (MC) can be conformally coated with a uniform nm-thick layer of a cationic light-emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH-RhoB), leveraging the self-tuned electrostatic interaction of the positively-charged PAH-RhoB polymer and negatively-charged PSiO2 surface. It is found that the emission of PAH-RhoB in the PSiO2 MC is enhanced (≈2.5×) and narrowed (≈30×) at the resonant wavelength, compared with that of PAH-RhoB in a non-resonant PSiO2 reference structure [3]. The time-resolved photoluminescence analysis highlights a shortening (≈20%) of the PAH-RhoB emission lifetime in the PSiO2 MC at the resonance versus off-resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor Fp = 2.82 is achieved. This is further confirmed by the enhance- ment of the photoluminescence quantum yield of the PAH-RhoB in the PSiO2 MC with respect to the reference structure.By building on these results, we envisage many emerging photonic applications of the electrostatic nanoassembly coating technology for introduction of foreign emitters into PSi-based photonic nano-/mesostructures, though not lim- ited to, including ultrasensitive fluorescence-enhanced optical nanosensors, nanolasers, exciton-polaritonic devices, spintronic devices, and quantum optical devices.

Nanoscale Photoluminescence Manipulation in Monolithic Porous Silicon Oxide Microcavity Coated with Rhodamine‐Labeled Polyelectrolyte via Electrostatic Nanoassembling

AbstractPorous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters, still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO2) microcavity (MC) can be conformally coated with a uniform nm‐thick layer of a cationic light‐emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH‐RhoB), leveraging the self‐tuned electrostatic interaction of the positively‐charged PAH‐RhoB polymer and negatively‐charged PSiO2 surface. It is found that the emission of PAH‐RhoB in the PSiO2 MC is enhanced (≈2.5×) and narrowed (≈30×) at the resonant wavelength, compared with that of PAH‐RhoB in a non‐resonant PSiO2 reference structure. The time‐resolved photoluminescence analysis highlights a shortening (≈20%) of the PAH‐RhoB emission lifetime in the PSiO2 MC at the resonance versus off‐resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor Fp = 2.82 is achieved. This is further confirmed by the enhancement of the photoluminescence quantum yield of the PAH‐RhoB in the PSiO2 MC with respect to the reference structure. Application of the electrostatic nanoassembling approach to other emitting dyes, nanomaterials, and nanophotonic systems is envisaged.

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.

Defect‐State Lasing in Photonic Lattices of Metal–Organic Microcavities

Photonic band structure engineering has developed into an important technique for controlling the emission and interaction of photons and polaritons in microcavities (MCs). Herein, lasing from defect states (DSs) in photonic Kronig–Penney structures embedded into metal–organic MCs is demonstrated. As compared to the more delocalized lasing states associated with photonic bands, these DSs exhibit improved lasing thresholds that lie even below the lasing threshold of the metal‐free cavity. The characteristics of these DSs are determined via full electrodynamic computations based on the discontinuous Galerkin time‐domain method.

Room-temperature cavity-polaritons in planar ZnO microcavities fabricated by a top-down process

Angle-dependent energy shifts in the near-band edge emission peak were observed at room temperature in planar ZnO microcavities (MCs) fabricated by a top-down process that simultaneously maintains high radiative performance of a ZnO active layer and high reflectivity of wide-bandwidth distributed Bragg reflectors (DBRs). An approximately 2λ-thick ZnO active layer with a thickness gradient less than 10 nm/mm (i.e., an angle of gradient less than 2 arc sec) across the entire 10 × 5 mm2 device area was formed by thinning a bulk single crystal of ZnO grown by the hydrothermal method, whose typical threading dislocation densities were lower than 102 cm−2. The DBRs consisting of 10 and 12 pairs of SiO2/ZrO2 multilayers with a thickness gradient of 0.2 nm/mm/pair were deposited by using the nearly surface-damage-free reactive helicon-wave-excited-plasma sputtering method as the top and bottom mirrors, respectively. The quality factor of a passive cavity consisting of the same DBR stacks was in the range between 670 and 720 for the areal size of 1 mm in diameter. Angle-resolved photoluminescence spectra of the ZnO MCs measured at different positions with a macroscopic spot size of 80 μm in diameter exhibited distinct emission from the lower branch of cavity polaritons with apparent detunings ranging from −40 meV to 40 meV at room temperature.

Frequency conversion in lanthanide-doped sol-gel derived materials for energy applications

In order to increase the efficiency of photovoltaic solar cells, photon absorption in an extended range of the solar spectrum is necessary, for example by means of up-conversion (UC) of sub-bandgap photons to minimize transparency losses. While lanthanide (Ln)-doped luminescent layers may be deposited on cell materials to achieve spectral conversion by methods like CVD or PVD, sol-gel (SG) processing is a low-cost technology which may be a good alternative for this purpose. In this work, Ln-doped SG-derived active layers have been deposited by spin-coating onto c-Si substrates for frequency conversion, with particular emphasis on UC, involving host matrices like aluminosilicate glass and titania ceramic doped with the activator/sensitizer pair Er/Yb. White light emitting diodes (WLEDs) based on Ln-doped phosphors still have efficiency limitations, namely because the Stokes shift from UV/blue LED excitation to the emitted lower frequency visible light converts some of the pump energy into heat. However, if white light generation (WLG) is achieved through IR-pumped UC instead, the cost of the LED lamp will be reduced, while the energy loss from Stokes shift can be minimized. SG synthesis of Ln-doped oxide phosphors for UC in WLG has been achieved (Er/Yb and Tm/Er/Yb doping), based on aluminosilicate glass matrices. Photonic crystal-assisted UC emission has also been demonstrated in a Tm/Yb-doped Fabry–Perot microcavity (MC). The different materials have been characterized by XRD, SEM, ellipsometry and photoluminescence (PL) spectroscopies and their light emission has been analyzed with the help of CIE chromaticity diagrams.

Quantum dot polarisation converter in an optomechanical cavity

We propose a scheme of a quantum photon polarisation converter, which is based on controlled electron – photon – phonon transitions in a hybrid semiconductor nanostructure. This structure consists of a GaAs/InAs quantum dot (QD) that has a parallelepiped shape and contains a single electron, and an optomechanical microcavity (MC) based on a photonic crystal (PC) that supports two orthogonally polarised photonic modes and one mechanical (phonon) mode. Within the framework of the microscopic theory, the QD and MC performance characteristics are found. Populations of states of the system as functions of time and its parameters are calculated. The principal possibility of photon polarisation conversion using transitions in a five-level resonance scheme for coherent (single-photon) and steady-state (subphoton) regimes is shown. The MC optical and mechanical spectra are simulated, and the PC structure parameters are selected to ensure the efficient operation of the converter.

Up-conversion enhancement in Er3+/ Yb3+ doped 1-D microcavity based on alternating aluminosilicate glass and titania sol-gel layers

Enhancement of luminescence properties to improve and/or control both the absorption and emission of lanthanide (Ln)-activated glasses, ceramics and glass-ceramics, can be achieved with the development of smart photonic systems. Among different possible strategies, a prospective proof of concept for photoluminescence (PL) enhancement based on a one-dimensional photonic crystal (PC) was designed to tailor the up-conversion (UC) emission of Er3+ from a microcavity (MC) with an aluminosilicate glass film co-doped with Er3+/Yb3+ sandwiched between alternating layers of undoped aluminosilicate and TiO2. For the fabrication, the layers of different materials were deposited by sol-gel (SG), in order to mimic the PC structure obtained by simulation for a pre-defined position of the pass band defect (547 nm) within the stop band. An enhancement of up to 13 times of the Er3+ green UC was observed for the MC in comparison with a system without the PC structure.

The application of porous Si photonic crystals for metal-resonance enhanced fluorescence

This paper has demonstrated the resonance fluorescence enhancement of R6G when gold nanoparticles (Au NPs) deposited in the porous Si photonic crystal device. Both of microcavity (MC) and distributed Bragg reflector (DBR) with different parameters are investigated for making the photon transmission of photonic crystal device play an optimal role in enhancing fluorescence resonance. While minor changes were observed on the DBR substrates, a significant change in the intensity of enhanced fluorescence varies with the defect modes of MC substrates. Particularly, the strongest enhancement has been presented as the MC defect mode wavelength located at the maximum absorption wavelength of Au NPs. In this case, the fluorescence intensity of R6G on MC device is 2.5 times of that of R6G based on DBR device.

Microcavity characteristics analysis of micro-shuttered organic light-emitting diodes

We demonstrated the control of the microcavity (MC) effect in micro-shuttered organic light-emitting diodes (OLEDs) with the introduction of an Ag striped pattern (ASP) layer between the organic layer and the indium zinc oxide layer. The ASP electrodes demonstrated high flexibility, conductivity, and transmittance. The optical and electrical properties of the OLED devices with micro-scale patterned metal electrodes with varying thicknesses and uncovered (open) area:entire glass surface area ratios (open ratios, ORs) were analyzed. The ASP OLEDs demonstrated high color purity and narrowed emission spectra because of the MC effect, increased light extraction by modulating the OR of the micro-shutter structure, and adjustability of the viewing angle distortion by controlling the OR. The ASP OLEDs exhibited maximum efficiency at the OR of 0.3 (Ag thickness; tAg = 15 nm) because of their optimum transmittance and conductivity. The viewing angle distortion decreased by approximately 60.3% with an increase in the OR from 0 to 0.7, as revealed by the changes in the color coordinates. The results showed that when deposited on flexible substrates, ASP electrodes can be used in flexible devices.

Demonstration of polarization control GaN-based micro-cavity lasers using a rigid high-contrast grating reflector

We reported on GaN microcavity (MC) lasers combined with one rigid TiO2 high-contrast grating (HCG) structure as the output mirror. The HCG structure was directly fabricated on the GaN structure without an airgap. The entire MC structure comprised a bottom dielectric distributed Bragg reflector; a GaN cavity; and a top HCG reflector, which was designed to yield high reflectance for transverse magnetic (TM)- or transverse electric (TE)-polarized light. The MC device revealed an operation threshold of approximately 0.79 MW/cm2 when pulsed optical pumping was conducted using the HCG structure at room temperature. The laser emission was TM polarized with a degree of polarization of 99.2% and had a small divergence angle of 14° (full width at half maximum). This laser operation demonstration for the GaN-based MC structure employing an HCG exhibited the advantages of HCGs in semiconductor lasers at wavelengths from green to ultraviolet.

Recent Advances of Organic Solar Cells with Optical Microcavities

In recent decades, organic solar cells (OSCs) have drawn increasing interest due to their unique properties such as low cost, solution‐processing, flexibility, semitransparency, and nontoxicity. Due to some shortcomings of limited optical absorption in organic semiconductors as well as low carrier mobility and short exciton diffusion length, light‐trapping technologies such as surface plasmon resonance, photonic crystals, and microcavities (MCs) have been widely developed to improve device performance. Among these methods, the MC effect is liable to form and has unneglectable influences on the device efficiency. However, few reports systematically summarize the development of MC‐based OSCs. Herein, the principle of the MC effect is introduced first, and subsequently, the application and the development of MCs in single and multi‐junction OSCs are described in detail. Furthermore, in addition to the traditional MCs‐enhanced light absorption, other applications based on the MC structure in OSCs and other photo‐electronic conversion devices are also represented. Finally, the problems that need to be solved and the development directions of MC‐based OSCs in the future are outlined. It is believed that this review can provide new thinking for achieving high‐performance OSCs with optical means.

In-Fiber Mach-Zehnder Interferometer Exploiting a Micro-Cavity For Strain and Temperature Simultaneous Measurement

An in-fiber Mach-Zehnder interferometer (MZI) with a micro-cavity (MC) exploited for strain and temperature simultaneous measurement. The research is carried out experimentally and simulatively. The MC fabricated by splicing a hollow quartz tube between two single-mode fibers serves as a light splitter for the two arms of MZI beams and a Fabry–Perot interferometer (FPI) component at the same time. Thus both functions of the MZI and FPI were implemented in a single sensor. With aid of the different response of the MZI and FPI to the strain and temperature, this device can achieve a simultaneous measurement and the strain/temperature cross-sensitivity issue has been alleviated. Such a composite interference structure, possess the characteristics of easy making, high performance-price ratio provides a promising approach to implement strain and high-temperature simultaneous measurement.

Large Rabi splitting in InGaN quantum wells microcavity at room temperature

Room-temperature luminescence of exciton polaritons was observed from InGaN/GaN quantum wells (QWs) sandwiched in Fabry–Perot (FP) microcavity (MC). Angle resolved photoluminescence measurements revealed unambiguous anti-crossing behavior, indicating strong coupling between excitons and photons. The Rabi splitting value varied from 40 meV–130 meV, depending on the excitation power, which was larger than previous publications. This large Rabi splitting value was ascribed to our high MC quality factor (Q > 3000) and large oscillator strength induced by coupled QWs structure. The sensitive dependence of the Rabi splitting in InGaN QWs on excitation power indicates that exciton-exciton scattering mechanism plays an important role in exciton- photon coupling.

Ultrasensitive Mach-Zehnder Interferometric Temperature Sensor Based on Liquid-Filled D-Shaped Fiber Cavity.

A liquid-filled D-shaped fiber (DF) cavity serving as an in-fiber Mach–Zehnder interferometer (MZI) has been proposed and experimentally demonstrated for temperature sensing with ultrahigh sensitivity. The miniature MZI is constructed by splicing a segment of DF between two single-mode fibers (SMFs) to form a microcavity (MC) for filling and replacement of various refractive index (RI) liquids. By adjusting the effective RI difference between the DF and MC (the two interference arms), experimental and calculated results indicate that the interference spectra show different degrees of temperature dependence. As the effective RI of the liquid-filled MC approaches that of the DF, temperature sensitivity up to −84.72 nm/°C with a linear correlation coefficient of 0.9953 has been experimentally achieved for a device with the MC length of 456 μm, filled with liquid RI of 1.482. Apart from ultrahigh sensitivity, the proposed MCMZI device possesses additional advantages of its miniature size and simple configuration; these features make it promising and competitive in various temperature sensing applications, such as consumer electronics, biological treatments, and medical diagnosis.

Enhancing thermal stability and uniformity in boiling heat transfer using micro-nano hybrid surfaces (MNHS)

In two-phase heat transfer, promising issues include not only improving the boiling performance but also the surface temperature uniformity and stability, which indicate that how long the system maintains thermal stability without a failure on the surface. In this study, the merits of micro-nano hybrid surfaces (MNHS) are discussed for enhancing the thermal uniformity/stability and boiling heat transfer performance. Spatial/temporal heat transfer characteristics are evaluated on MNHS using a local temperature-measuring sensor of resistance temperature detector (RTD). We demonstrate that MNHS can enhance not only boiling performance but also thermal uniformity/stability by delaying bubble coalescence with an appropriate design of the location of the nucleation sites and the nucleated bubble size. The confining effects of nucleated bubbles on nanowire (NW) structures and of uniform bubble nucleation on uniformly distributed micro-cavity (MC) structures lead to the reliable enhancement of thermal uniformity/stability as well as critical heat flux (CHF) in pool boiling environments. These combined effects of the NW and MC structures could delay the bubble coalescence phenomenon by catalyzing bubble nucleation dispersedly and quickly at small bubble sizes. When the normalized pitch of the nucleation sites is 1, namely the pitch of the nucleation sites and the bubble departure size have the same dimension, CHF is significantly enhanced, by more than 170%, on an MNHS versus a plain surface by delaying bubble coalescence and maximizing bubble density. Boiling heat transfer using an MNHS represents a breakthrough reducing the spatial and temporal temperature variation at CHF to less than 1/3 and 1/4, respectively, compared with a plain surface.