Gate-Tunable Plasmon-Enhanced Photodetection in a Monolayer MoS2 Phototransistor with Ultrahigh Photoresponsivity.

Monolayer transition metal dichalcogenides (TMDs), direct bandgap materials with an atomically thin nature, are promising materials for electronics and photonics, especially at highly scaled lateral dimensions. However, the characteristically low total absorption of photons in the monolayer TMD has become a challenge in the access to and realization of monolayer TMD-based high-performance optoelectronic functionalities and devices. Here, we demonstrate gate-tunable plasmonic phototransistors (photoFETs) that consist of monolayer molybdenum disulfide (MoS2) photoFETs integrated with the two-dimensional plasmonic crystals. The plasmonic photoFET has an ultrahigh photoresponsivity of 2.7 × 104 AW-1, achieving a 7.2-fold enhancement in the photocurrent compared to pristine photoFETs. This benefits predominately from the combination of the enhancement of the photon-absorption-rate via the strongly localized-electromagnetic-field and the gate-tunable plasmon-induced photocarrier-generation-rate in the monolayer MoS2. These results demonstrate a systematic methodology for designing ultrathin plasmon-enhanced photodetectors based on monolayer TMDs for next-generation ultracompact optoelectronic devices in the trans-Moore era.

Visible to near-infrared octave spanning supercontinuum generation in tantalum pentoxide (Ta2O5) air-cladding waveguide.

In this work, for the first time, to the best of our knowledge, an anomalous dispersion CMOS-compatible Ta2O5 waveguide was realized, and broadband on-chip supercontinuum generation (SCG) was accordingly demonstrated. When pumped at a center wavelength of 1056 nm with pulses of 100 fs duration and peak power of 396 W, a supercontinuum ranging from 585 nm to 1697 nm was generated, comprising a bandwidth of more than 1.5 octaves and leading to an efficient SCG source. The excellent performance for Ta2O5 to generate SCG benefits mainly from its high nonlinear refractive index, which enhances the efficiency of the nonlinear conversion process.

Efficient wavelength conversion with low operation power in a Ta2O5-based micro-ring resonator.

The Ta2O5-based micro-ring resonator with an unloaded quality factor of 182,000 has been demonstrated to realize efficient nonlinear wavelength generation. The propagation loss of the resonator is 0.5 cm-1, and the buildup factor of the ring resonator is estimated to be ∼50. With a high buildup factor of the ring structure, the four-wave-mixing (FWM) conversion efficiency of -30 dB is achieved in the resonator with a pump power of 6 mW. Based on power-dependent FWM results, the nonlinear refractive index of Ta2O5 is estimated to be 1.4×10-14 cm2/W at a wavelength of ∼1550 nm. The demonstration of an enhanced FWM process in the Ta2O5-based micro-ring cavity implies the possibility of realizing FWM-based optical parametric oscillation in a Ta2O5-based micro-ring resonator.

Vernier-like super resolution with guided correlated photon pairs.

We describe a dispersion-enabled, ultra-low power realization of super-resolution in an integrated Mach-Zehnder interferometer. Our scheme is based on a Vernier-like effect in the coincident detection of frequency correlated, non-degenerate photon pairs at the sensor output in the presence of group index dispersion. We design and simulate a realistic integrated refractive index sensor in a silicon nitride on silica platform and characterize its performance in the proposed scheme. We present numerical results showing a sensitivity improvement upward of 40 times over a traditional sensing scheme. The device we design is well within the reach of modern semiconductor fabrication technology. We believe this is the first metrology scheme that uses waveguide group index dispersion as a resource to attain super-resolution.

Improvement of light quality by DBR structure in white LED.

This study demonstrates the application of DBR structure into the remote phosphor structure to improve the angular correlated color temperature (CCT) deviation in white light-emitting diodes (WLEDs). In the experiment, the LED device with DBR structure yielded a higher luminous efficiency than a conventional structure. The CCT deviation can be improved from 1758K to 280K in a range of -70 to 70 degree and the luminous flux increases more than 10% due to the enhancement of the light extraction of the blue light. Moreover, the reflectance of the different DBR structures is analyzed with different angles to reveal the reasons of such improvements. As the result, this LED device with DBR structure shows the great potential to use as the next generation lighting source.

Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K.

The hybrid white light-emitting didoes (LED) with polyfluoren (PFO) polymer and quantum dot (QD) was investigated using dispensing method at the different correlated color temperature (CCT) for cool and warm color temperature. This result indicates that the hybrid white LED device has the higher luminous efficiency than the convention one, which could be attributed to the increased utilization rate of the UV light. Furthermore, the CIE 1931 coordinate of high quality white hybrid LED with different CCT range from 3000K to 9000K is demonstrated. Consequently, the angular-dependent CCT and the thermal issue of the hybrid white LED device were also analyzed in this study.

Optimization of high-Q coupled nanobeam cavity for label-free sensing.

We numerically and experimentally investigated the lateral coupling between photonic crystal (PhC) nanobeam (NB) cavities, pursuing high sensitivity and figure of merit (FOM) label-free biosensor. We numerically carried out 3D finite-difference time-domain (3D-FDTD) and the finite element method (FEM) simulations. We showed that when two PhC NB cavities separated by a small gap are evanescently coupled, the variation in the gap width significantly changes the coupling efficiency between the two coupled NB cavities and the resulting resonant frequencies split. Experimentally, we fabricated laterally-coupled PhC NB cavities using (InGaAsP) layer on the InP substrate. For sensing, we showed that the laterally coupled PhC NB cavities sensor exhibits higher sensitivity than the single PhC NB cavity. The higher sensitivity of laterally coupled PhC NB cavities is due to the strong evanescent coupling between nearby PhC NB cavities, which depends on the gap width and it is attributed to the large confinement of the electromagnetic field in the gap (air or liquid). As a result of the lateral coupling, both even (symmetric) and odd (asymmetric) modes exist. We show that even modes are more sensitive than odd modes. In addition, higher-order modes exhibit higher sensitivity. Hence, we characterized and examined the fabricated PhC NB cavity as a label-free biosensor, and it exhibits high figure of merit due to its high Q-factor. This illustrates a potentially useful method for optical sensing at nanoscale.

Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.

Improvement of emission uniformity by using micro-cone patterned PDMS film.

Micro-patterned PDMS film was fabricated and combined with LED chip on board (COB) package to improve the emission uniformity of LED chip. The micro scale patterned sapphire substrate (PSS) was used as a mold to fabricate micro-cone patterned PDMS (MC-PDMS) film. A strong scattering effect from this MC-PDMS film can be verified by the high haze ratio and the Bi-directional Transmission effect. The angle dependent color temperature measurement system was used to measure the ΔCCT of COB with and without MC-PDMS. The measurement results indicate that the ΔCCT was reduced from 1025K to 428K. This improvement can effectively eliminate the yellow ring effect of LED chip. This technology can be thus considered as a cost-effective way for the next generation of light source packages.

A Colloidal-Quantum-Dot Integrated U-Shape Micro-Light-Emitting-Diode and Its Photonic Characteristics.

A special micro LED whose light emitting area is laid out in a U-like shape is fabricated and integrated with colloidal quantum dots (CQDs). An inkjet-type machine directly dispenses the CQD layer to the central courtyard-like area of this U-shape micro LED. The blue photons emitted by the U-shape mesa with InGaN/GaN quantum wells can excite the CQDs at the central courtyard area and be converted into green or red ones. The U-shape micro LEDs are coated with Al2O3 by an atomic layer deposition system and exhibit moderate external quantum efficiency (6.51% max.) and high surface recombination because of their long peripheries. Low-temperature measurement also confirms the recovery of the external quantum efficiency due to lower non-radiative recombination from the exposed surfaces. The color conversion efficiency brought by the CQD layer can be as high as 33.90%. A further continuous CQD aging test, which was evaluated by the strength of the CQD emission, under current densities of 100 A/cm2 and 200 A/cm2 injected into the micro LED, showed a lifetime extension of the unprotected CQD emission up to 1321 min in the U-shape device compared to a 39 min lifetime in the traditional case, where the same CQD layer was placed on the top surface of a squared LED.

High thermal stability of correlated color temperature using current compensation in hybrid warm white high-voltage LEDs.

This study experimentally and numerically examines the correlated color temperature (CCT) stability issue for hybrid warm white high-voltage light-emitting diodes (HV-LEDs) by using a current compensation method. This method could efficiently maintain the CCT stability factor at approximately 1.0 and yield greater color uniformity with Δu'v' values ranging from 0.017 to 0.003 in CIE 1976 chromaticity coordinates. The simulation results show that the red chip intensity drop is the primary cause of CCT instability in the hybrid warm white system when the temperature increases. Therefore, Furthermore, results indicate that the relative lumen drop improves from 21% to 15% by using a current compensation method.

AC-driven multicolor electroluminescence from a hybrid WSe2 monolayer/AlGaInP quantum well light-emitting device.

Light-emitting diodes (LEDs) are used widely, but when operated at a low-voltage direct current (DC), they consume unnecessary power because a converter must be used to convert it to an alternating current (AC). DC flow across devices also causes charge accumulation at a high current density, leading to lowered LED reliability. In contrast, gallium-nitride-based LEDs can be operated without an AC-DC converter being required, potentially leading to greater energy efficiency and reliability. In this study, we developed a multicolor AC-driven light-emitting device by integrating a WSe2 monolayer and AlGaInP-GaInP multiple quantum well (MQW) structures. The CVD-grown WSe2 monolayer was placed on the top of an AlGaInP-based light-emitting diode (LED) wafer to create a two-dimensional/three-dimensional heterostructure. The interfaces of these hybrid devices are characterized and verified through transmission electron microscopy and energy-dispersive X-ray spectroscopy techniques. More than 20% energy conversion from the AlGaInP MQWs to the WSe2 monolayer was observed to boost the WSe2 monolayer emissions. The voltage dependence of the electroluminescence intensity was characterized. Electroluminescence intensity-voltage characteristic curves indicated that thermionic emission was the mechanism underlying carrier injection across the potential barrier at the Ag-WSe2 monolayer interface at low voltage, whereas Fowler-Nordheim emission was the mechanism at voltages higher than approximately 8.0 V. These multi-color hybrid light-emitting devices both expand the wavelength range of 2-D TMDC-based light emitters and support their implementation in applications such as chip-scale optoelectronic integrated systems, broad-band LEDs, and quantum display systems.

Dynamical characteristics of AC-driven hybrid WSe2 monolayer/AlGaInP quantum wells light-emitting device.

The exploration of functional light-emitting devices and numerous optoelectronic applications can be accomplished on an elegant platform provided by rapidly developing transition metal dichalcogenides (TMDCs). However, TMDCs-based light emitting devices encounter certain serious difficulties, such as high resistance losses from ohmic contacts or the need for complex heterostructures, which restricts the device applications. Despite the fact that AC-driven light emitting devices have developed ways to overcome these challenges, there is still a significant demand for multiple wavelength emission from a single device, which is necessary for full color light emitting devices. Here, we developed a dual-color AC-driven light-emitting device by integrating the WSe2 monolayer and AlGaInP-GaInP multiple quantum well (MQW) structures in the form of capacitor structure using AlOx insulating layer between the two emitters. In order to comprehend the characteristics of the hybrid device under various driving circumstances, we investigate the frequency-dependent EL intensity of the hybrid device using an equivalent RC circuit model. The time-resolved electroluminescence (TREL) characteristics of the hybrid device were analyzed in details to elucidate the underlying physical mechanisms governing its performance under varying applied frequencies. This dual-color hybrid light-emitting device enables the use of 2-D TMDC-based light emitters in a wider range of applications, including broad-band LEDs, quantum display systems, and chip-scale optoelectronic integrated systems.

Wide-angle polarization independent infrared broadband absorbers based on metallic multi-sized disk arrays.

Two-dimensional metallic broadband absorbers on a SiO(2)/Ag/Si substrate were experimentally studied. The absorptivity of such structure can be increased by tailoring the ratio of disk size to the unit cell area. The metallic disk exhibits a localized surface plasmon polariton (LSPP) mode for both TE and TM polarizations. A broadband thermal emitter can be realized because the LSPP mode is independent of the periodicities. By manipulating the ratios and disk sizes, a high-performance, wide-angle, polarization-independent dual band absorber was experimentally achieved. The results demonstrated a substantial flexibility in absorber designs for applications in thermal photovoltaics, sensors, and camouflage.

Improvement in uniformity of emission by ZrO2 nano-particles for white LEDs.

The high luminous efficiency and superior uniformity of angular-dependent correlated color temperature (CCT) white light-emitting diodes have been investigated by ZrO2 nano-particles in a remote phosphor structure. By adding ZrO2 nano-particles with silicone onto the surface of the phosphor layer, the capability of light scattering could be enhanced. In particular, the intensity of blue light at large angles was increased and the CCT deviations could be reduced. Besides, the luminous flux was improved due to the ZrO2 nano-particles with silicone providing a suitable refractive index between air and phosphor layers. This novel structure reduces angular-dependent CCT deviations from 1000 to 420 K in the range of -70° to 70°. Moreover, the enhancement of lumen flux was increased by 2.25% at a driving current 120 mA, compared to a conventional remote phosphor structure without ZrO2 nano-particles. Consequently, the ZrO2 nano-particles in a remote phosphor structure could not only improve the uniformity of lighting but also increase the light output.

Water-Immersion Laser-Scanning Annealing for Improving Polycrystalline Au Films.

A water-immersion laser-scanning annealing (WILSA) method was developed for the heat treatment of a deposited polycrystalline Au film on a glass. The material characterization using X-ray diffraction, field-emission scanning electron microscopy, and electron backscatter diffraction shows improved crystallinity with a more uniform crystallographic orientation of (111) and the grain growth of the annealed Au film. Additionally, the optical constants of the Au film before and after annealing were characterized by spectroscopic ellipsometry in the visible to near-infrared (NIR) regime, and the corresponding optical densities (ODs) were measured by transmittance spectroscopy. Our results show that the extinction coefficient and the OD of the annealed film are significantly reduced, particularly in the NIR regime. This is because the grain growth caused by the annealing reduces the density of grain boundaries, leading to the decrease of the loss of free electrons' scattering at grain boundaries. Hence, the damping effect of the surface plasmon is reduced. Additionally, the integrity of the WILSA-treated thin film is kept intact without pinholes, usually produced by the conventional thermal annealing. Based on the improved optical property of the WILSA-treated Au film, two performances of an insulator-metal-insulator (IMI) layered structure of biosensors are theoretically analyzed. Numerical results show that the propagation length of a long-range surface plasmon polariton along an IMI structure with an annealed Au film is significantly increased, compared to an unannealed film, particular in the NIR region. For the other application of using an IMI sensor to detect the shift of the surface-plasmon-resonance dip in the total internal reflection spectrum for the measurement of a change of the medium's refractive index, the sensitivity is also profoundly improved by the WILSA method. It is worth mentioning that the optimal heating conditions (laser wavelength, fluence, exposure time, and scanning step) depend on the thickness of the Au film. Our study provides a postprocess of WILSA to improve the optical properties of a deposited polycrystalline Au film for raising the sensitivity of the related biosensors.

In situ tunable circular dichroism of flexible chiral metasurfaces composed of plasmonic nanorod trimers.

The circularly polarized light source is one of the keys to chiral photonic circuits and systems. However, it is difficult to integrate conventional light-emitting devices with circular polarization converters directly into compact chip-scale photonic systems partly because of their bulky structures. In this study, in situ optical chirality tunable nanorod trimer metasurfaces consisting of two types of nanorod dimers are demonstrated and integrated with a flexible polydimethylsiloxane (PDMS) substrate. The optical chirality variations originating from the tunable asymmetricity of nanorod trimers under different stretching scenarios are evaluated. Through the processes, the gap distances between nanorods are varied, and the degree of circular polarization of the transmitted wave is controlled through the manipulation of localized surface plasmon resonance (LSPR) coupling. The results reveal the circular dichroism tunability and durability of fabricated chiral metasurfaces which can be important elements for chip-scale flexible optoelectronic integrated circuits for sensing, display and communication applications.

Optical Mode Tuning of Monolayer Tungsten Diselenide (WSe2) by Integrating with One-Dimensional Photonic Crystal through Exciton-Photon Coupling.

Two-dimensional materials, such as transition metal dichalogenides (TMDs), are emerging materials for optoelectronic applications due to their exceptional light-matter interaction characteristics. At room temperature, the coupling of excitons in monolayer TMDs with light opens up promising possibilities for realistic electronics. Controlling light-matter interactions could open up new possibilities for a variety of applications, and it could become a primary focus for mainstream nanophotonics. In this paper, we show how coupling can be achieved between excitons in the tungsten diselenide (WSe2) monolayer with band-edge resonance of one-dimensional (1-D) photonic crystal at room temperature. We achieved a Rabi splitting of 25.0 meV for the coupled system, indicating that the excitons in WSe2 and photons in 1-D photonic crystal were coupled successfully. In addition to this, controlling circularly polarized (CP) states of light is also important for the development of various applications in displays, quantum communications, polarization-tunable photon source, etc. TMDs are excellent chiroptical materials for CP photon emitters because of their intrinsic circular polarized light emissions. In this paper, we also demonstrate that integration between the TMDs and photonic crystal could help to manipulate the circular dichroism and hence the CP light emissions by enhancing the light-mater interaction. The degree of polarization of WSe2 was significantly enhanced through the coupling between excitons in WSe2 and the PhC resonant cavity mode. This coupled system could be used as a platform for manipulating polarized light states, which might be useful in optical information technology, chip-scale biosensing and various opto-valleytronic devices based on 2-D materials.

Enhanced photoluminescence and shortened lifetime of DCJTB by photoinduced metal deposition on a ferroelectric lithography substrate.

The photodeposition of metallic nanostructures onto ferroelectric surfaces could enable new applications based on the assembly of molecules and patterning local surface reactivity by enhancing surface field intensity. DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) is an excellent fluorescent dye and dopant material with a high quantum efficiency used for OLED displays on the market. However, how to raise the photoluminescence (PL) and reduce the lifetime of DCJTB in a substrate remain extraordinary challenges for its application. Here, we demonstrate a tunable ferroelectric lithography plasmon-enhanced substrate to generate photo-reduced silver nanoparticles (AgNPs) and achieve enhanced PL with a shortened lifetime depending on the substrate's annealing time. The enhanced PL with shortened lifetimes can attribute to the localized electromagnetic (EM) wave produced by the nanotextured AgNPs layers' surface and gap plasmon resonances. The simulation is based on the three-dimensional finite element method to explain the mechanism of experimental results. Since the absorption increases, the remarkable enhanced PL of DCJTB can attain in the fabricated periodically proton exchanged (PPE) lithium niobate (LiNbO3) substrate. Furthermore, the proposed fabrication method demonstrates to help tune the surface EM wave distribution in the substrate, which can simultaneously achieve the significantly shortened lifetime and high PL intensity of DCJTB in the substrate. Compared with the un-annealed substrate, the PL intensity of DCJTB in the assembly metallic nanostructures is enhanced 13.70 times, and the PL's lifetime is reduced by 12.50%, respectively. Thus, the fabricated substrate can be a promising candidate, verifying chemically patterned ferroelectrics' satisfaction as a PL-active substrate.

Characterization of the surface plasmon polariton band gap in an Ag/SiO2/Ag T-shaped periodical structure.

In this study, the localized surface plasmon polariton (LSPP) band gap of an Ag/SiO(2)/Ag asymmetric T-shaped periodical structure is demonstrated and characterized. The Ag/SiO(2)/Ag asymmetric T-shaped periodical structure was designed and fabricated to exhibit the LSPP modes in an infrared wavelength regime, and its band gap can be manipulated through the structural geometry. The LSPP band gap was observed experimentally with the absorbance spectra and its angle dependence characterized with different incident angles. Such a T-shaped structure with a LSPP band gap can be widely exploited in various applications, such as emitters and sensors.