Micromirror Array with Adjustable Reflection Characteristics Based on Different Microstructures and Its Application.

The conventional reflective optical surface with adjustable reflection characteristics requires a complex external power source. The complicated structure and preparation process of the power system leads to the limited modulation of the reflective properties and difficulty of use in large-scale applications. Inspired by the biological compound eye, different microstructures are utilized to modulate the optical performance. Convex aspheric micromirror arrays (MMAs) can increase the luminance gain while expanding the field of view, with a luminance gain wide angle > 90° and a field-of-view wide angle close to 180°, which has the reflective characteristics of a large gain wide angle and a large field-of-view wide angle. Concave aspheric micromirror arrays can increase the luminance gain by a relatively large amount of up to 2.66, which has the reflective characteristics of high gain. Industrial-level production and practical applications in the projection display segment were carried out. The results confirmed that convex MMAs are able to realize luminance gain over a wide spectrum and a wide range of angles, and concave MMAs are able to substantially enhance luminance gain, which may provide new opportunities in developing advanced reflective optical surfaces.

An Improved 3D OPC Method for the Fabrication of High-Fidelity Micro Fresnel Lenses.

Based on three-dimensional optical proximity correction (3D OPC), recent advancements in 3D lithography have enabled the high-fidelity customization of 3D micro-optical elements. However, the micron-to-millimeter-scale structures represented by the Fresnel lens design bring more stringent requirements for 3D OPC, which poses significant challenges to the accuracy of models and the efficiency of algorithms. Thus, a lithographic model based on optical imaging and photochemical reaction curves is developed in this paper, and a subdomain division method with a statistics principle is proposed to improve the efficiency and accuracy of 3D OPC. Both the simulation and the experimental results show the superiority of the proposed 3D OPC method in the fabrication of Fresnel lenses. The computation memory requirements of the 3D OPC are reduced to below 1%, and the profile error of the fabricated Fresnel lens is reduced 79.98%. Applying the Fresnel lenses to an imaging system, the average peak signal to noise ratio (PSNR) of the image is increased by 18.92%, and the average contrast of the image is enhanced by 36%. We believe that the proposed 3D OPC method can be extended to the fabrication of vision-correcting ophthalmological lenses.

Superhydrophobic Multifocal Microlens Array with Depth-of-Field Detection for a Humid Environment.

Microlens array (MLA) has been widely applied in augmented reality and optical imaging. When used in a humid environment or medical endoscopy, MLA needs to be both superhydrophobic and multifocal. However, it is not easy to achieve both superhydrophobic and multifocal function by integrating superhydrophobic and multifocal structures on the same surface by means of a simple, efficient, and precise method. In this paper, the superhydrophobic multifocal MLA with superhydrophobic properties and multifocal functions is successfully designed for preparation based on a method of 3D lithography and soft lithography. The 3D lithography can further help the preparation of a multifocal MLA with varying apertures and a multistep superhydrophobic structure with a round dome. The superhydrophobic multifocal MLA with periods 50 and 120 µm has perfect superhydrophobic property. The water droplet can slide and bounce off the surface at a roll angle of less than 12.9° with both multifocal and integrated imaging function, as well as up to 397 µm depth-of-field (DOF) detection range; this greatly exceeds the conventional MLA. The perfect superhydrophobic and optical property can be achieved in an extremely humid environment. The superhydrophobic multifocal MLA proposed in this paper has a promising prospect for actual practices.

Fiber-to-Chip Three-Dimensional Silicon-on-Insulator Edge Couplers with High Efficiency and Tolerance.

The edge coupler is an indispensable optical device for connecting an external fiber and on-chip waveguide. The coupling efficiency of the edge coupler affects the effective integration of optical circuits. In this study, three-dimensional (3D) edge couplers with high efficiency and tolerance are proposed. The high coupling efficiency of the 3D edge couplers is verified by theoretical calculations. Three couplers are fabricated on a thick-silicon platform via 3D grayscale lithography. At the 1550 nm band, the fiber-to-chip experimental data show that the maximum coupling efficiencies of the three edge couplers are 0.70 dB and 1.34 dB, 0.80 dB and 1.60 dB, and 1.00 dB and 1.14 dB for the TE and TM modes, respectively. At the 1550 nm band, misalignment tolerances measurement data reveal 0.8 dB/0.9 dB tolerance of ±5 µm in the horizontal direction, and 1.7 dB/1.0 dB tolerance of ±2 µm in the vertical direction for TE/TM mode. This study provides a new idea for the design of 3D edge couplers and demonstrates significant superiority in research and industrial applications.

Design and Manufacture of Polarization-Independent 3D SOI Vertical Optical Coupler.

An optical coupler is a key input/output (I/O) device in a photonic integrated circuit (PIC), which plays the role of light-source import and modulated light output. In this research, a vertical optical coupler consisting of a concave mirror and a half-cone edge taper was designed. We optimized the structure of mirror curvature and taper through finite-difference-time-domain (FDTD) and ZEMAX simulation to achieve mode matching between SMF (single-mode fiber) and the optical coupler. The device was fabricated via laser-direct-writing 3D lithography, dry etching and deposition on a 3.5 µm silicon-on-insulator (SOI) platform. The test results show that the overall loss of the coupler and its connected waveguide at 1550 nm was 1.11 dB in transverse-electric (TE) mode and 2.25 dB in transverse-magnetic (TM) mode.

3D OPC method for controlling the morphology of micro structures in laser direct writing.

A 3D optical proximity correction (OPC) method for controlling the morphology of micro-structures in laser direct writing is proposed, considering both the optical proximity effect and nonlinear response of a thick-film photoresist. This method can improve the manufacturability and optical performance of devices, and can be used for most 3D micro\nano structures. Its application in the fabrication of a quadratic curvature microlens array shows that the shape of the lens is well controlled; that is, when the height of the lens is 5.25 µm, the average height error of the lens shape is less than 5.22%.

Flexible Superhydrophobic Microlens Arrays for Humid Outdoor Environment Applications.

A microlens array (MLA) is an essential optical imaging device in the applications of augmented and virtual realities. The imaging of MLA would become blurry in a humid outdoor atmosphere. While the incorporation of superhydrophobicity to MLA would prevent the adhesion of droplets, the complex structure and the multiple fabrication process reduce the capability of optical imaging of MLA. Herein, a flexible superhydrophobic MLA with good optical imaging capability is successfully fabricated by the combination of 3D direct laser writing (DLW) and soft lithography. 3D DLW allows the fabrication of MLA with a hierarchical pillar array (h-MLA) in one step, which ensures good optical properties of the resulting polydimethylsiloxane (PDMS) h-MLA. The resulting h-MLAs with pitches ranging between 50 and 100 µm are superhydrophobic from which water droplets slide away at a sliding angle smaller than 15.6° and bounce off from the surface. Meanwhile, the hierarchical pillar array has a limited impact on the imaging capability and the field of view of h-MLA. With an optimized pitch of 60 µm, h-MLA has a transparency as good as MLA. Moreover, PDMS h-MLA retains excellent optical and superhydrophobic properties when bent and in an extremely humid environment. We believe that the proposed h-MLA could find applications in outdoor environments.

Source and mask optimizing with a defocus antagonism for process window enhancement.

With the continuous reduction of critical dimension (CD) of integrated circuits, inverse lithography technology (ILT) is widely adopted for the resolution enhancement to ensure the fidelity of photolithography, and for the process window (PW) improvement to enlarge the depth of focus (DOF) and exposure latitude (EL). In the photolithography, DOF is a critical specification which plays a vital role for the robustness of a lithographical process. DOF has been investigated to evaluate the optimization quality of ILT, but there is not a clear scenario to optimize the DOF directly. In this paper, the source and mask optimization (SMO) based on defocus generative and adversarial method (DGASMO) is proposed, which takes the source, mask and defocus as variables, and the inverse imaging framework employs the Adam algorithm to accelerate the optimization. In the optimization process, the penalty term constantly pushes the defocus outward, while the pattern fidelity pushes the defocus term inward, and the optimal source and mask are constantly searched in the confrontation process to realize the control of DOF. Compared to SMO with the Adam method (SMO-Adam), the PW and DOF (EL = 15%) in DGASMO maximally increased 29.12% and 44.09% at 85 nm technology node, and the PW and DOF (EL = 2%) at 55 nm technology node maximally increased 190.2% and 118.42%. Simulation results confirm the superiority of the proposed DGASMO approach in DOF improvement, process robustness, and process window.

Artificial Hyper Compound Eyes Enable Variable-Focus Imaging on both Curved and Flat Surfaces.

The artificial compound eye (ACE) with zoom imaging requires complex power sources. Meanwhile, its curved substrate makes it difficult for the ACE to realize the zoom imaging on flat surfaces. To realize a wide field of view and a zoom function on both curved and flat surfaces simultaneously, a novel ACE is proposed, which is a bionic design inspired by an ancient creature, trilobite. Compared with a dragonfly, photosensitive units of a trilobite's compound eye are composed of ommatidia with different focal lengths. By learning from this concept, an artificial hyper compound eye (AHCE) was fabricated. Its basic components are five microlenses with different curvatures, and they are capable of being treated as five ommatidia with different focal lengths. Five ommatidia form a photosensitive unit to realize a zoom function. AHCE is capable of variable-focus imaging on curved surfaces. With the information share function, we found that the AHCE not only images on curved surfaces but also has a zoom-imaging function on flat surfaces. The results confirm that the AHCE demonstrates an advanced imaging capability, a variable-focus imaging function on both curved and flat surfaces, which may open new opportunities in developing advanced micro-optical devices.

Understanding the plasmon-enhanced photothermal effect of a polarized laser on metal nanowires.

Improving photothermal efficiency can reduce the melting threshold of metal nanowires. The photothermal efficiency of a polarized laser to Cu nanowires was investigated by numerical simulation and experiment. Our simulation results reveal that the photothermal efficiency of a polarized laser depends on the intensity and distribution area of surface plasmons excited by the laser. As the angle between the polarization direction of the incident laser and the long axis of the Cu nanowire increases, the laser-excited surface plasmons shift from both ends to the sidewall of the Cu nanowire. Such a distribution of surface plasmons was confirmed by the melting behavior of Cu nanowires irradiated by a 450 nm polarized laser. Increasing the laser wavelength will enhance the intensity of the surface plasmons but reduce the distribution area of the surface plasmons. As a result, a higher photothermal efficiency was achieved using a laser with a polarization direction perpendicular to the long axis of the Cu nanowire and a wavelength less than 550 nm. Due to the higher photothermal efficiency, the melting threshold of Cu nanowire irradiated by a laser with polarization perpendicular to the long axis of the Cu nanowire is 32 mW, which is around 20% lower that of Cu nanowire irradiated by a laser with polarization parallel to the long axis of the Cu nanowire.

Scattering force and heating effect in laser-induced plasmonic welding of silver nanowire junctions.

Light-nanomaterial interaction is accompanied by a scattering force and a heating effect. When silver nanowires are irradiated by a laser pulse with light intensity above the melting threshold, they are observed to melt into nanospheres and fly away from their original position. Simulation and experimental results show that the localized surface plasmon resonance excited by laser pulse will heat the ends and junction areas of silver nanowires, causing the occurrence of local melting at these locations. Since the local melting cannot alter the position of silver nanowire, a mathematical model was developed to evaluate the scattering force acting on silver nanowire. According to the developed mathematical model, the scattering force acting on silver nanowire mainly depends on specific surface area of silver nanowire and incident light intensity. When the light intensity of the laser pulse is ${3.0} \times {{10}^{12}}\;{\rm W}/{{\rm m}^2}$3.0×1012W/m2, the scattering force acting on the silver nanowire can reach ${{10}^5}$105 times the gravity of silver nanowire. To obtain silver nanowires networks, the light intensity of the laser pulse was manipulated to regulate the scattering force and heating effect acting on the silver nanowire. As a result, silver nanowire networks were obtained with light intensity of ${1.4} \times {{10}^{10}}\;{\rm W}/{{\rm m}^2}$1.4×1010W/m2 at a scanning speed of 1000 mm/s. This laser-induced plasmonic welding paves the way for improved understanding and control of fundamental laser-nanomaterial interactions.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate.

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au-In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry-Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Enhanced Light Extraction of Flip-Chip Mini-LEDs with Prism-Structured Sidewall.

Current solutions for improving the light extraction efficiency of flip-chip light-emitting diodes (LEDs) mainly focus on relieving the total internal reflection at sapphire/air interface, but such methods hardly affect the epilayer mode photons. We demonstrated that the prism-structured sidewall based on tetramethylammonium hydroxide (TMAH) etching is a cost-effective solution for promoting light extraction efficiency of flip-chip mini-LEDs. The anisotropic TMAH etching created hierarchical prism structure on sidewall of mini-LEDs for coupling out photons into air without deteriorating the electrical property. Prism-structured sidewall effectively improved light output power of mini-LEDs by 10.3%, owing to the scattering out of waveguided light trapped in the gallium nitride (GaN) epilayer.

Revealing the Role of Sidewall Orientation in Wet Chemical Etching of GaN-Based Ultraviolet Light-Emitting Diodes.

We demonstrated that the tetramethylammonium hydroxide (TMAH) solution possesses different etching abilities to the chip sidewalls with different orientations because the orientation of chip sidewall determines the exposed crystallographic plane of gallium nitride (GaN) and these crystallographic planes are with different chemical stability to the TMAH solution. After TMAH etching treatment, trigonal prisms were observed on sidewalls where m-plane GaN was exposed. For the investigated two types of light-emitting diodes (LEDs) with orthogonal arrangements, the LEDs with their larger sidewalls orientated along the [11⁻20] direction exhibited an additional 10% improvement in light output power after TMAH etching treatment compared to the LEDs with larger sidewalls orientated along the [1⁻100] direction.

The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN-based green light-emitting diodes.

The development of efficient green light-emitting diodes (LEDs) is of paramount importance for the realization of colour-mixing white LEDs with a high luminous efficiency. While the insertion of an InGaN/GaN superlattice (SL) with a lower In content before the growth of InGaN/GaN multiple quantum wells (MQWs) is known to increase the efficiency of LEDs, the actual mechanism is still debated. We therefore conduct a systematic study and investigate the different mechanisms for this system. Through cathodoluminescence and Raman measurements, we clearly demonstrate that the potential barrier formed by the V-pit during the low-temperature growth of an InGaN/GaN SL dramatically increases the internal quantum efficiency (IQE) of InGaN quantum wells (QWs) by suppressing non-radiative recombination at threading dislocations (TDs). We find that the V-pit potential barrier height depends on the V-pit diameter, which plays an important role in determining the quantum efficiency, forward voltage and efficiency droop of green LEDs. Furthermore, our study reveals that the low-temperature GaN can act as an alternative to an InGaN/GaN SL structure for promoting the formation of V-pits. Our findings suggest the potential of implementing optimized V-pits embedded in an InGaN/GaN SL or low-temperature GaN structure as a beneficial underlying layer for the realization of highly efficient green LEDs.

Numerical simulation and experimental investigation of GaN-based flip-chip LEDs and top-emitting LEDs.

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta2O5/SiO2 double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO2 current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150 mA, and the light output saturation current was shifted from 130.9 A/cm2 for the TE-LED to 273.8 A/cm2 for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150 mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

Numerical and experimental investigation of GaN-based flip-chip light-emitting diodes with highly reflective Ag/TiW and ITO/DBR Ohmic contacts.

We demonstrate two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with highly reflective Ag/TiW and indium-tin oxide (ITO)/distributed Bragg reflector (DBR) p-type Ohmic contacts. We show that a direct Ohmic contact to p-GaN layer using pure Ag is obtained when annealed at 600°C in N2 ambient. A TiW diffusion barrier layer covered onto Ag is used to suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature annealing process. We develop a strip-shaped SiO2 current blocking layer beneath the ITO/DBR to alleviate current crowding occurring in FCLED with ITO/DBR. Owing to negligibly small spreading resistance of Ag, however, our combined numerical and experimental results show that the FCLED with Ag/TiW has a more favorable current spreading uniformity in comparison to the FCLED with ITO/DBR. As a result, the light output power of FCLED with Ag/TiW is 7.5% higher than that of FCLED with ITO/DBR at 350 mA. The maximum output power of the FCLED with Ag/TiW obtained at 305.6 A/cm2 is 29.3% larger than that of the FCLED with ITO/DBR obtained at 278.9 A/cm2. The improvement appears to be due to the enhanced current spreading and higher optical reflectance provided by the Ag/TiW.

Reflectance bandwidth and efficiency improvement of light-emitting diodes with double-distributed Bragg reflector.

Distributed Bragg reflectors (DBR) with metal film on the bottom have been demonstrated to further improve the light output power of GaN-based light-emitting diodes (LEDs). Periods of TiO2/SiO2 stacks, thickness of metal film, and material of metallic reflector were designed and optimized in simulation software. The maximal bandwidth of double-DBR stacks have reached up to 272 nm, which was 102 nm higher than a single-DBR stack. The average reflectance of LEDs with wavelength from 380 nm to 780 nm in double-DBR stacks is 95.09% at normal incident, which was much higher than that of a single-DBR stack whose average reflectance was 91.38%. Meanwhile, maximal average reflectance of LEDs for double-DBR stacks with an incident angle from 0 to 90° was 97.41%, which was 3.2% higher than that of a single-DBR stack with maximal average reflectance of 94.21%. The light output power of an LED with double-DBR stacks is 3% higher than that of an LED with a single-DBR stack, which was attributed to high reflectance of double-DBR stacks.

Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes.

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10-25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.