Integration Technology of Micro-LED for Next-Generation Display.

Inorganic micro light-emitting diodes (micro-LEDs) based on III-V compound semiconductors have been widely studied for self-emissive displays. From chips to applications, integration technology plays an indispensable role in micro-LED displays. For example, large-scale display relies on the integration of discrete device dies to achieve extended micro-LED array, and full color display requires integration of red, green, and blue micro-LED units on the same substrate. Moreover, the integration with transistors or complementary metal-oxide-semiconductor circuits are necessary to control and drive the micro-LED display system. In this review article, we summarized the 3 main integration technologies for micro-LED displays, which are called transfer integration, bonding integration, and growth integration. An overview of the characteristics of these 3 integration technologies is presented, while various strategies and challenges of integrated micro-LED display system are discussed.

A Heterostructured Graphene Quantum Dots/ß-Ga2O3 Solar-Blind Photodetector with Enhanced Photoresponsivity.

The superior optical and electronic characteristics of quasi-two-dimensional ß-Ga2O3 make it suitable for solar-blind (200-280 nm) photodetectors (PDs). The metal-semiconductor-metal (MSM) PDs commonly suffer from low photoresponsivity, slow response speed, and a narrow detection wavelength range despite their simple fabrication process. Herein, we report a high-performance MSM PD by integrating exfoliated ß-Ga2O3 flakes with zero-dimensional graphene quantum dots (GQDs), which exhibits the advantages of enhancing the photoresponsivity, shortening the photoresponse time, and stimulating a broad range of photon detection. The hybrid GQDs/ß-Ga2O3 heterostructure PD is sensitive to deep-ultraviolet (DUV) light (250 nm) with an ultrahigh responsivity (R of ∼2.4 × 105 A/W), a large detectivity (D* of ∼4.3 × 1013 Jones), an excellent external quantum efficiency (EQE of ∼1.2 × 108%), and a fast photoresponse (150 ms), which is superior to the bare ß-Ga2O3 PD. These improvements result from effective charge transfer due to the introduction of GQDs, which enhance the light absorption and the generation of electron-hole pairs. In addition, the hybrid GQDs/ß-Ga2O3 PD also exhibits better photoelectric performance than the bare ß-Ga2O3 PD at a 1000 nm wavelength. As a conclusion, the hybrid GQDs/ß-Ga2O3 DUV photodetector shows potential applications in commercial optoelectronic products and provides an alternative solution for the design and preparation of high-performance photodetectors.

Optical Pulling Using Chiral Metalens as a Photonic Probe.

Optical pulling forces, which can pull objects in the source direction, have emerged as an intensively explored field in recent years. Conventionally, optical pulling forces exerted on objects can be achieved by tailoring the properties of an electromagnetic field, the surrounding environment, or the particles themselves. Recently, the idea of applying conventional lenses or prisms as photonic probes has been proposed to realize an optical pulling force. However, their sizes are far beyond the scope of optical manipulation. Here, we design a chiral metalens as the photonic probe to generate a robust optical pulling force. The induced pulling force exerted on the metalens, characterized by a broadband spectrum over 0.6 µm (from 1.517 to 2.117 µm) bandwidth, reached a maximum value of -83.76 pN/W. Moreover, under the illumination of incident light with different circular polarization states, the longitudinal optical force acting on the metalens showed a circular dichroism response. This means that the longitudinal optical force can be flexibly tuned from a pulling force to a pushing force by controlling the polarization of the incident light. This work could pave the way for a new advanced optical manipulation technique, with potential applications ranging from contactless wafer-scale fabrication to cell assembly and even course control for spacecraft.

Study of a Mode Separation Due to Polarization Existing in a Cavity-Enhanced Absorption Spectroscopy.

A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of αmin = 7.6 × 10-9 cm-1 can be obtained in a single laser scan of 10 s.

Improved electro-optical and photoelectric performance of GaN-based micro-LEDs with an atomic layer deposited AlN passivation layer.

The quantum efficiency of GaN-based micro-light-emitting diodes (micro-LEDs) is of great significance for their luminescence and detection applications. Optimized passivation process can alleviate the trapping of carriers by sidewall defects, such as dangling bonds, and is regarded as an effective way to improve the quantum efficiency of micro-LEDs. In this work, an AlN passivation layer was prepared by atomic layer deposition to improve the electro-optical and photoelectric conversion efficiency in GaN-based micro-LEDs. Compared to conventional Al2O3 passivation, the AlN passivation process has a stronger ability to eliminate the sidewall defects of micro-LEDs due to the homogeneous passivation interface. Our experiments show that the AlN-passivated device exhibits two orders of magnitude lower forward leakage and a smaller ideality factor, which leads to significantly enhanced external quantum efficiency (EQE). For 25*25 µm2 micro-LEDs, the EQE of the AlN-passivated device was 18.3% and 57.7% higher than that of the Al2O3-passivated device in luminescence application and detection application, respectively.

High optoelectronic performance of a local-back-gate ReS2/ReSe2 heterojunction phototransistor with hafnium oxide dielectric.

A high optoelectronic performance ReS2/ReSe2 van der Waals (vdW) heterojunction phototransistor utilizing thin hafnium oxide (HfO2) as a local-back-gate dielectric layer was prepared and studied. The heterojunction-based phototransistor exhibits a superior electrical performance with a large rectification ratio of ∼103. Furthermore, unlike diode-like heterojunction devices, the innovative introduction of a local-back-gate in this phototransistor provides an outstanding gate-tunable capability with an ultra-low off-state current of 433 fA and a high on/off current ratio of over 106. And under optical excitation of a wide spectrum from 400 to 633 nm, an excellent photodetection responsivity at the 104 A W-1 level and the maximum normalized detectivity of 1.8 × 1015 Jones @ 633 nm have been demonstrated. Such high performances are attributed to the band alignment of the type-II heterojunction and the suppression of dark current by the local-back-gate. This work provides a promising reference for two-dimensional (2D) Re-based heterojunction optoelectronic devices.

Tunable Broadband Terahertz Waveband Absorbers Based on Fractal Technology of Graphene Metamaterial.

In this paper, a metasurface Terahertz absorber based on the fractal technology of a graphene geometry resonator to realize ultra-wideband, ultrathin, adjustable double-layer cross-fractal formation is introduced. This paper proposes a dynamically tuned graphene absorbing material. The structure is composed of one- to four-level-fractal graphene pattern layers, MgF2 layers and metal reflective layers to form a two-sided mirror of an asymmetric Fabry-Perot cavity. To confine the terahertz electromagnetic wave, four different fractals are integrated into a supercell, and the coupling and superposition of adjacent resonant cavities form a broadband high-absorption absorber. Using finite element-based full-wave electromagnetic simulation software to simulate the response frequency of 0.4-2.0 THz, we found that the absorber achieves a broadband 1.26 THz range (absorption > 80%) and a relative bandwidth of 106.8%. By adjusting the Fermi energy, it can realize free switching and expand to wider broadband terahertz absorption, by adjusting the polarization angle (Φ) from 0 to 90° to prove that the structure is not sensitive to polarization, the absorber provides a 60° large angle of incidence, polarization for TE and TM the absorption pattern remains basically the same. Compared with the previous work, our proposed structure uses fractal technology to expand the bandwidth and provide dynamic adjustable characteristics with great degrees of freedom. The appearance of the fractal structure reduces the difficulty of actual processing.

Tunable multilayer-graphene-based broadband metamaterial selective absorber.

We propose a tunable multilayer-graphene-based broadband metamaterial selective absorber using the finite-difference time domain. The simulation results reveal that the absorption spectra of the proposed metamaterial with the nano-cylinder and 30-layer graphene show high absorption (88.3%) in the range of 250-2300 nm, which covers the entire solar spectrum. Moreover, the graphene-based metamaterial has a low thermal emittance of 3.3% in the mid-infrared range (4-13 µm), which can greatly reduce the heat loss. The proposed metamaterial has a tunable cutoff wavelength, which can be tuned by controlling the Fermi level of graphene. In addition, our structure is an angle-insensitive absorber, and the device has the potential to be widely used in solar cell and thermal detectors.

Dual-channel optical switch, refractive index sensor and slow light device based on a graphene metasurface.

In this paper, we propose a graphene-based metasurface that exhibits multifunctions including tunable filter and slow-light which result from surface plasmon polaritons (SPPs) of graphene and plasmon induced transparency (PIT), respectively. The proposed metasurface is composed by two pairs of graphene nano-rings and a graphene nanoribbon. Each group of graphene rings is separately placed on both sides of the graphene nanoribbon. Adjusting the working state of the nanoribbon can realize the functional conversion of the proposed multifunctional metasurface. After that, in the state of two narrow filters, we put forward the application concept of dual-channel optical switch. Using phase modulation of PIT and flexible Fermi level of graphene, we can achieve tunable slow light. In addition, the result shows that the graphene-based metasurface as a refractive index sensor can achieve a sensitivity of 13670 nm/RIU in terahertz range. These results enable the proposed device to be widely applied in tunable optical switches, slow light, and sensors.

Continuously tunable metasurfaces controlled by single electrode uniform bias-voltage based on nonuniform periodic rectangular graphene arrays.

Metasurfaces, the two-dimensional artificial metamaterials, have attracted intensive attention due to their abnormal ability to manipulate the electromagnetic wave. Although there have been considerable efforts to design and fabricate beam steering devices, continuously tunable devices with a uniform bias-voltage have not been achieved. Finding new ways to realize more convenient and simpler wavefront modulation of light still requires research efforts. In this article, a series of novel reflective metasurfaces are proposed to continuously modulate the wavefront of terahertz light by uniformly adjusting the bias-voltage. By introducing the innovation of nonuniform periodic structures, we realize the gradient distribution of the reflected light phase-changing-rate which is the velocity of phase changing with Fermi energy. Based on strict phase distribution design scheme, a beam scanner and a variable-focus reflective metalens are both demonstrated successfully. Furthermore, dynamic and continuous control of either the beam azimuth of beam scanner or the focal length of metalens can be achieved by uniformly tuning the Fermi energy of graphene. Our work provides a potentially efficient method for the development and simplification of the adjustable wavefront controlling devices.

Tunable Broadband THz Waveband Absorbers Based On Graphene for Digital Coding.

A method of coding patterns is proposed to achieve flexible control of absorption response at terahertz frequencies. The designed absorber consists of an Au-graphene pattern layer, a SiO2 layer and a metal reflective layer. Among them, we use concentrical circle structure to achieve broadband absorption, and adjust graphene's Fermi level to achieve tunable absorption. In addition, we propose an encoding method that can achieve flexible control of the absorption response at the terahertz frequency based on the external voltage applied on the graphene membrane, thereby having a programmable function. We also use COMSOL to simulate the electric field distribution diagram to explain the underlying physical mechanism. The programmable broadband adjustable absorber proposed in this paper has potential application prospects in the fields of optical equipment, information transmission, digital coding and artificial intelligence (AI).

Characteristic Test Analysis of Graphene Plus Optical Microfiber Coupler Combined Device and Its Application in Fiber Lasers.

In this study, a graphene and optical microfiber coupler (OMC) integrated device (GOMC) was proposed and fabricated. After its characteristic analysis and testing, it was applied to the development of adjustable multi-wavelength fiber lasers. By integrating the OMC with graphene, the polarization dependence of OMC was enhanced. Meanwhile, the novel GOMC was given the capabilities of filtering, coupling, beam splitting, and polarization correlation. When the GOMC was integrated as a filter and beam splitter into the ring cavity of the fiber laser, the proposed GOMC-based fiber laser could achieve single-wavelength and multi-wavelength regulated output. The laser had a 3 dB linewidth of less than 30 pm, a signal-to-noise ratio of approximately 40 dB, and an output power fluctuation of less than 1 dB. The GOMC could also be used for the development of functional devices, such as adjustable mode lockers and mode coupling selectors, which provide an excellent experimental platform for new fiber lasers and the research of multi-dimensional light-field manipulation.

Author Correction: Near-infrared tunable metalens based on phase change material Ge2Sb2Te5.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

All-optical switch and logic gates based on hybrid silicon-Ge2Sb2Te5 metasurfaces.

We numerically propose a hybrid metasurface (MS) to realize all-optical switch and logic gates in the shortwave infrared (SWIR) band. Such MS consists of one silicon rod and one Ge2Sb2Te5 (GST) rod pair. Utilizing the transition from an amorphous state to a crystalline state of GST, such MS can produce an electromagnetically induced transparency (EIT) analogue with active control. Based on this, we realize all-optical switching at 1770 nm with a modulation depth of 84%. Besides, three different logic gates, NOT, NOR and OR, can also be achieved in this metadevice simultaneously. Thanks to the reversible and fast phase transition process of GST, this device possesses reconfigurable ability as well as fast response time, and has potential applications in future optical networks.

Tunable Duplex Metalens Based on Phase-Change Materials in Communication Range.

Metalenses recently have attracted attention because of their more compact size in comparison with conventional lenses; they can also achieve better optical performance with higher resolution. Duplexer is an interesting function of a metalens that can distinguish different sources and divide them into two parts for specific purposes. In this article, we design tunable duplex metalenses with phase-change material Ge2Sb2Te5 for the first time. Two types of special unit cells are designed to modulate the incident lights, and four metalenses are designed based on the two types of unit cells. Specific phase profiles are calculated for different sections of metalens in which the corresponding unit cells are settled; accordingly, the metalenses can focus the incident lights at any positions according to our design. Moreover, the metalenses become selectable via tuning the state of phase-change material, which means that the output light field can be actively controlled. The proposal of our tunable duplex metalenses will offer new opportunities for active three-dimensional imaging or optical coding.

High quality factor electromagnetically induced transparency-like effect in coupled guided-mode resonant systems.

We numerically study a dielectric coupled guided-mode resonant (GMR) system, which includes two silicon (Si) grating waveguide layers (GWLs) stacked on CaF2 substrates. It is confirmed that the coupling between the top and bottom GMR modes starts once a Fabry-Perot (F-P) resonator is introduced, and electromagnetically induced transparency (EIT)-like spectral responses occur in the coupled GMR systems. A very narrow transparency window with a high-quality (Q) factor EIT-like effect of up to 288,892 was demonstrated. Furthermore, EIT-like response wavelengths can be flexibly designed in wide wavelength range by modifying either the GMR resonance frequencies or the space between two GWLs. Therefore, this EIT-like response in coupled GMR systems would pave the way towards novel sensors with extremely high sensitivity.

Tunable Graphene-Based Plasmon-Induced Transparency Based on Edge Mode in the Mid-Infrared Region.

A monolayer-graphene-based concentric-double-rings (CDR) structure is reported to achieve broadband plasmon-induced transparency (PIT) on the strength of edge mode in the mid-infrared regime. The theoretical analysis and simulation results reveal that the structure designed here has two plasmonic resonance peaks at 39.1 and 55.4 THz, and a transparency window with high transmission amplitude at the frequency of 44.1 THz. Based on the edge mode coupling between neighbor graphene ribbons, PIT phenomenon is produced through the interference between different (bright and dark) modes. The frequency and bandwidth of the transparency window and slow light time could be effectively adjusted and controlled via changing geometrical parameters of graphene or applying different gate voltages. Additionally, this structure is insensitive to the polarization and incident angle. This work has potential application on the optical switches and slow light modulators.

Near-infrared tunable metalens based on phase change material Ge2Se2Te5.

Metasurfaces draw everyone's attention because they can precisely control the phase, amplitude and polarization of emergent light to achieve light field control in recent years. As one of the most practicable devices among the many applications of metasurface, metalens can extremely reduce the size as well as complexity of optical systems and realizes the higher optical quality compared with conventional lens. So it will be very potential to use metalens in integration systems to reaching higher integration and efficiency. In addition, dynamic control is always desirable in optical systems. In this work, we firstly design a near-infrared tunable metalens treating phase change materials as the meta-atoms which makes the tunable metalens become more compact. At designed wavelength of 1.55 µm, the focusing efficiency of our amorphous metalens is more than 16 times of the efficiency when it works at crystalline state, and its focal length can stay almost unchanged when the GST state is switched. The broadband performance of the metalens is also confirmed. This work may bring some good opportunities for the revolution of the next generation tunable integrated optics.

Active Enhancement of Slow Light Based on Plasmon-Induced Transparency with Gain Materials.

As a plasmonic analogue of electromagnetically induced transparency (EIT), plasmon-induced transparency (PIT) has drawn more attention due to its potential of realizing on-chip sensing, slow light and nonlinear effect enhancement. However, the performance of a plasmonic system is always limited by the metal ohmic loss. Here, we numerically report a PIT system with gain materials based on plasmonic metal-insulator-metal waveguide. The corresponding phenomenon can be theoretically analyzed by coupled mode theory (CMT). After filling gain material into a disk cavity, the system intrinsic loss can be compensated by external pump beam, and the PIT can be greatly fueled to achieve a dramatic enhancement of slow light performance. Finally, a double-channel enhanced slow light is introduced by adding a second gain disk cavity. This work paves way for a potential new high-performance slow light device, which can have significant applications for high-compact plasmonic circuits and optical communication.

Plasmonic Refractive Index Sensor with High Figure of Merit Based on Concentric-Rings Resonator.

A plasmonic refractive index (RI) sensor based on metal-insulator-metal (MIM) waveguide coupled with concentric double rings resonator (CDRR) is proposed and investigated numerically. Utilizing the novel supermodes of the CDRR, the FWHM of the resonant wavelength can be modulated, and a sensitivity of 1060 nm/RIU with high figure of merit (FOM) 203.8 is realized in the near-infrared region. The unordinary modes, as well as the influence of structure parameters on the sensing performance, are also discussed. Such plasmonic sensor with simple framework and high optical resolution could be applied to on-chip sensing systems and integrated optical circuits. Besides, the special cases of bio-sensing and triple rings are also discussed.