Enhanced light extraction from GaN-based LEDs with a bottom-up assembled photonic crystal

Photonic crystal (PhC) structure is an efficient tool for light extraction from light-emitting diodes (LEDs). The fabrication of a large area PhC structure on the light output surface of LEDs often involves sophisticated equipments such as nanoimprint lithography machine. In this study a monolayer of polystyrene (PS) microspheres was employed as a template to fabricate a noninvasive photonic crystal of indium tin oxide (ITO) on the surface of GaN-based LED. PS spheres can help to form periodic arrangement of bowl-like holes, a photonic crystal with gradually changed fill factors. Importantly, the electroluminescence intensity of LED with a photonic crystal was significantly enhanced by 1.5 times compared to that of the conventional one under various forward injection currents.     

Full wafer scale nanoimprint lithography for GaN-based light-emitting diodes

A UV-imprinting process for a full wafer was developed to enhance the light extraction of GaN-based green light-emitting diodes (LEDs). A polyvinyl chloride flexible stamp was used in the imprinting process to compensate for the poor flatness of the LED wafer. Two-dimensional photonic crystal patterns with pitches ranging from 600 to 900 nm were formed on the p-GaN top cladding layer of a 2 inch diameter wafer using nanoimprint and reactive ion etching processes. As a result, the optical output power of the patterned LED device was increased by up to 44% at a driving current of 20 mA by suppressing the total internal reflection and enhancing the irregular scattering of photons at the patterned p-GaN surface.     

Fabrication of photonic crystals on several kinds of semiconductor materials by using focused-ion beam method

In this paper, we introduced the fabrication of photonic crystals on several kinds of semiconductor materials by using focused-ion beam machine, it shows that the method of focused-ion beam can fabricate two-dimensional photonic crystal and photonic crystal device efficiently, and the quality of the fabricated photonic crystal is high. Using the focused-ion beam method, we fabricate photonic crystal wavelength division multiplexer, and its characteristics are analyzed.     

Dispersion of micelle-encapsulated fluorophores in a polymer matrix for control of color of light emitted by light-emitting diodes

In the present study, we demonstrated that fluorescent dyes could be nanoscopically dispersed in a polymer matrix that was immiscible with the dyes; the dyes were encapsulated in micelles. Using a model polymer composite, we also showed that the color of light emitted by light-emitting diodes (LEDs) could be controlled by coating fluorescent polymer composites onto the LEDs. For this purpose, fluorophores that were insoluble in toluene were solubilized into a solution of block copolymer micelles in toluene by the selective incorporation of fluorescent dyes into micellar cores. Because the micelles could be dispersed well in the polymer matrix without the formation of aggregates, fluorescent dyes encapsulated in the micelles were also effectively dispersed in the polymer matrix without macroscopic separation. The polymer composite can be evenly coated onto most substrates, regardless of their surface characteristics. Thus, light-emitting devices with well-controlled emission wavelengths and emission intensities can be fabricated by coating the polymer composite onto the surface of the device.     

Enhanced light extraction efficiency for glass scintillator coupled with two-dimensional photonic crystal structure

In order to improve the light extraction efficiency for Tb³⁺-doped glass scintillator, we propose an effective approach by incorporating a two-dimensional photonic crystal structure: a periodic array of hexagonally close-packed polystyrene nanosphere monolayer onto glass surface. The in-plane vector for the light suffering total internal reflection is changed into light cone resulting light emission out of glass. The optimized diameter of polystyrene nanosphere is 500 nm and the maximum enhancement of extraction efficiency reaches 25%. The enhancement shows angular dependence from the far-filed distribution pattern.     

两种典型结构强流自箍缩二极管技术研究

主要介绍了箍缩聚焦二极管和自箍缩离子束二极管的研究进展。重点介绍了近几年发展的阳极杆箍缩聚焦二极管的理论模拟和实验结果,在“闪光二号”加速器和2MV脉冲功率驱动源上进行了阳极杆箍缩二极管实验,二极管输出电压1．8—2．1MV,电流40—60kA,脉宽（FWHM）50～60n8,1m处的脉冲X剂量约20～30mGy,焦斑直径约Imm,X射线最高能量1．8MeV。在“闪光二号”加速器上开展了高功率离子束的产生和应用研究,给出了自箍缩反射离子束二极管的结构和工作原理,实验获得的离子束峰值电流～160kA,离子的峰值能量-500keV,开展了利用高功率质子束轰击19F靶产生6～7MeV准单能脉冲γ射线,模拟x射线热一力学效应等应用基础研究。     

Enhanced light output of angled sidewall light-emitting diodes with reflective silver films

A proof-of-concept of applying laser micro-machining to fabricate high performance GaN light-emitting diode (LED) was presented in this study. Laser micro-machining was applied to fabricate GaN LED chip with angled sidewalls (ALED). The inclined sapphire sidewalls were coated with highly reflective silver film which functions as an efficient light out-coupling medium for photons within the LED structure. Thus, more laterally-propagating photons can be redirected to the upward direction of the ALED with silver coating (Ag-ALED). Performances of the Ag-ALED, ALED and conventional planar GaN LED were evaluated. At an injection current of 30 mA, the light output intensity of Ag-ALED was significantly improved by 97% and 195% as compared to ALED and conventional planar LED, respectively. The corresponding wall-plug efficiency of Ag-ALED was remarkably increased by 95% and 193% as compared to ALED and conventional planar LED, respectively. The results of this study demonstrated that the Ag-ALED showed a pronounced increase in light output intensity compared to conventional planar LED, which may have many potential applications in the field of display engineering.     

Slow-light Mach–Zehnder modulators based on Si photonic crystals

Mach–Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p–n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p–n diode. This length is shorter than commercial LiNbO₃ modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index n_g. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.     

Ripple suppression of third-order dispersion by using graded index one-dimensional photonic crystals

A novel one-dimensional photonic crystal design for ripple suppression of third-order dispersion in ultrafast laser systems has been introduced. We investigated and compared the compensation behaviour of a simple photonic crystal and the proposed graded index structure with the same total thicknesses and periodicity at the near infrared region by the transfer matrix method. The results reveal that implementing the proposed graded index structure removes the pulse wings, produces clean pulses without the use of additional optical elements and also results in a dramatic reduction in the fluctuations of the group delay and third-order dispersion curves.     

Light extraction efficiency enhancement of GaN-based light emitting diodes on n-GaN layer using a SiO2 photonic quasi-crystal overgrowth

In this paper, GaN-based LEDs with a SiO2 photonic quasi-crystal (PQC) pattern on an n-GaN layer by nano-imprint lithography (NIL) are fabricated and investigated. At a driving current of 20 mA on Transistor Outline (TO)-can package, the better light output power of LED III (d = 1.2 microm) was enhanced by a factor of 1.20. After 1000 h life test (55 degrees C/50 mA) condition, Normalized output power of LED with a SiO2 PQC pattern (LED III (d = 1.2 microm)) on an n-GaN layer only decreased by 5%. This results offer promising potential to enhance the light output power of commercial light-emitting devices using the technique of nano-imprint lithography.     

Optical manipulation of photonic defect-modes in cholesteric liquid crystals induced by direct laser-lithography

Manipulation of photonic defect-modes in cholesteric liquid crystals (ChLCs), which are one-dimensional pseudo photonic band-gap materials have been demonstrated by an external optical field. A structural defect in which the pitch length of the ChLC in the bulk and the defect are different was introduced by inducing local polymerization in a photo-polymerizable ChLC material by a direct laser-lithography process, and infiltrating a different ChLC material as the defect medium. When an azobenzene dye-doped ChLC was infiltrated in the defect, the trans-cis isomerization of the dye upon ultraviolet (UV) exposure caused the pitch to shorten, changing the contrast in the pitch lengths at the bulk and the defect, leading to a consequent shifting of the defect-mode. The all-optical manipulation was reversible and had high reproducibility.     

Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures

We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface.     

Fabrication of autocloned photonic crystals using electron-beam guns with ion-assisted deposition

The autocloning technique is an attractive deposition method for the making of photonic crystals. With it various photonic crystals can be produced simply by changing the substrate periodicity and the structure of the stacking materials. We report on a method for the fabrication of autocloned photonic crystals. This method has better step-coverage, a higher deposition rate and a larger deposition area than can be achieved by the more traditional sputtering method and the periodic surface corrugation is preserved even after ion-assisted deposition (IAD) of multilayer stacks using E-beam gun evaporation. The shaping process can be freely controlled by controlling the IAD power and the ion source etching time. Ion source etching is a physical etching process which does not require any chemical reaction or dangerous reactive gas. The process parameters are described in this paper. The refractive index can be adjusted by changing the deposition rate and the substrate temperature during the deposition process. The deposition rate is about 0.7-1 nm/s for SiO₂ which is almost ten times faster than that of the sputtering method. This makes this method good for the mass production of photonic crystals.     

Preparation of one-dimensional photonic crystals by sol-gel process for magneto-optical materials

One-dimensional photonic crystal (PC) of periodically alternating low (SiO₂) and high (TiO₂) refractive index materials was prepared by sol-gel dip coating method that controls the thickness of each layer with nanometer level. The photonic band gap of high reflectivity was verified at wavelengths between 590 nm and 820 nm, which became significant with increasing the number of bilayer. The UV-vis spectra, SEM image and glow discharge optical emission spectroscopy indicate the periodic structure of SiO₂/TiO₂ multilayer. The magnetic layer of CoFe₂O₄ was also prepared by a sol-gel dip coating method. After the annealing at 700 °C, the single phase of CoFe₂O₄ film with spinel structure, without any preferred crystalline orientation, was obtained. In addition, the one-dimensional magnetophotonic crystal (MPC), in which the magnetic defect layer of CoFe₂O₄ is introduced into the periodic structure, was prepared. The light was localized at the magnetic defect due to the interference of the multilayer film, and the localized transmittance was observed around 530 nm. The Faraday rotation of MPC shows a peak at ∼ 570 nm which is close to the localized peak of transmittance. This may correspond to the enhancement of Faraday rotation due to the localization of light.     

Enhancement of light extraction efficiency of ultraviolet light emitting diodes by patterning of SiO2 nanosphere arrays

Here we introduce a simple and robust method to improve the light extraction efficiency of ultraviolet light emitting diodes (UV LEDs). Although many previous efforts have focused on etching the GaN surfaces, we employed a simple solution process to texture the GaN surface. Arrays of SiO₂ nanosphere monolayers were spun cast onto a polymer layer, consisting of benzocyclobutene (BCB) resins; subsequently, the bottom half of the SiO₂ nanospheres sunk into the BCB layer. The resulting array formed in a hexagonal-like pattern of ‘nano-lenses’ and the photoluminescence measurement exhibited that these patterns enhanced the light extracting efficiency of UV LEDs by 23%.     

Fabrication of three-dimensional photonic crystals by interference lithography with low light absorption

We report on the fabrication of polymer templates of photonic crystals by means of holographic (or interference) lithography. The grating is written in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined.     

A study of a phase formalism for calculating the cumulative density of states of one-dimensional photonic crystals

We explore a phase formalism that underpins a method of calculation of the cumulative density of states of one-dimensional photonic crystals based on the node counting theorem. Node counting is achieved by considering the spatial dependence of a phase variable proportional to the logarithmic derivative of the electric field in the structure. The properties of the phase variable are considered for photonic crystals in general, and illustrative algebraic and numerical results are presented for the phase variable and cumulative density of states of a model crystal. It is also shown how a simple extension of the theory can facilitate the calculation of the reflectivity of finite samples. For a disordered model crystal, a differential equation for the distribution function of the phase variable is derived and then used to obtain a closed-form expression for the ensemble-averaged cumulative density of states and numerical results to illustrate band tailing in the photonic bandgap.     

Beam shaping of light sources using circular photonic crystal funnel

A novel two-dimensional circular photonic crystal (CPC) structure with a sectorial opening for shaping the beam of light sources was designed and investigated. When combined with light sources, the structure acts like an antenna emitting a directional beam which could be advantageously used in several nanophotonic applications. Using the two-dimensional finite-difference time-domain (2D FDTD) method, we examined the effects of geometrical parameters of the structure on the directional and transmission properties of emitted radiation. Further, we examined the transmitting and receiving properties of a pair of identical structures as a function of distance between them.     

Genetic optimization of two-dimensional photonic crystals for large absolute band gaps under light line

A binary genetic algorithm with floating crossover and mutation probabilities is used to design two-dimensional photonic crystals for large absolute band gaps under a light line. The unit cell is composed of a small number of round rods and is arranged in a square lattice. The photonic band structure of each chromosome is calculated by the plane-wave expansion method. Starting from randomly generated photonic crystals, the genetic algorithm finally yielded a photonic crystal with an absolute common band gap of 0.0618(2πc/a) at the mid-frequency of 0.4084(2πc/a).     

Modulation instability of light beam propagation near the supercollimation frequency in nonlinear photonic crystals

The modulation instability (MI) of light beam propagation near the supercollimation frequency in nonlinear photonic crystals was investigated. The role of the nonlinear diffraction effect, merged as a result of nonlinear tuning of the equal-frequency contour curvature, in MI is particularly identified. It is found that the nonlinear diffraction effect tends to stimulate or suppress MI depending on the sign of the product of nonlinear diffraction and Kerr effects. Further analysis reveals that the MI and soliton phenomenon may be steered by the tunable supercollimation frequency in nonlinear photonic crystals.