Photonic Crystal Light-emitting Diodes Research Articles

Design of photonic crystals for light‐emitting diodes

AbstractPhotonic crystals (PCs) can greatly enhance the optoelectronic performance of light‐emitting diodes (LEDs) due to their distinctive color, photonic band gap, etc. Therefore, many scholars have conducted extensive research based on the high light extraction efficiency, good monochromaticity, and other excellent optoelectronic properties of PC LEDs. This review discusses the main principles of photonic crystals to improve the optoelectronic performance of LEDs and summarizes 12 structural applications of photonic crystal LEDs, such as PC slabs, Bragg grating, backside reflectors, surface PC, embedded PC, dual PC, PC beads, CPC, PC thin films, LIPC, defective PC, and composite architectures with other materials that boost LED optoelectronic qualities. In summary, it is found that photonic crystals can not only greatly improve the light extraction efficiency of LEDs but also improve other optoelectronic properties such as luminescent color and directional radiation angle, and reduce the manufacturing cost of LEDs. Photonic crystal LEDs are expected to be a strong candidate for future lighting technology. Finally, the prospects and challenges of PC LEDs are summarized.

Effect of radiative and nonradiative energy transfer processes of light-emitting diodes combined with quantum dots for visible light communication

Though light-emitting diodes (LEDs) combined with various color conversion techniques have been widely explored for VLC (visible light communication), E-O (electro-optical) frequency responses of devices with quantum dots (QDs) embedded within the nanoholes have rarely been addressed. Here we propose LEDs with embedded photonic crystal (PhC) nanohole patterns and green light QDs for studying small-signal E-O frequency bandwidths and large signal on–off keying E-O responses. We observe that the E-O modulation quality of PhC LEDs with QDs is better than a conventional LED with QDs when the overall blue mixed with green light output signal is considered. However, the optical response of only QD converted green light shows a contradictory result. The slower E-O conversion response is attributed to multi-path green light generation from both radiative and nonradiative energy transfer processes for QDs coated on the PhC LEDs.

Ultra High Luminous Efficiency p-Type Surface Defect Structure GaN LED

In this paper, we propose a double-layer SiO2 photonic crystal LED with a linear defect structure in the [Formula: see text]-GaN layer, the purpose is to solve the problem of low light extraction efficiency caused by diffusion scattering effect. We used FDTD modeling to analyze the light-emitting characteristics of GaN LED, after optimizing the structural parameters of two-dimensional photonic crystals, the light extraction efficiency is improved from 19.4% to 35.2%, the light extraction efficiency is increased by 1.8 times, meanwhile the light output power is also increased by 1.86 times. The introduction of the defect structure does not affect the [Formula: see text]–[Formula: see text] characteristics of the LED, and the output power of the LED is increased by 11% under the current of 300[Formula: see text]mA.

Monolithically integrated green-to-orange color InGaN-based nanocolumn photonic crystal LEDs with directional radiation beam profiles

In this study, the monolithic integration of LEDs with different emission colors (wavelengths of 543, 573, and 597 nm) with the directional radiation profiles was demonstrated. InGaN/GaN nanocolumn arrays ordered in a triangular lattice were prepared side by side, changing the diameter of the n-GaN nanocolumn (D n-GaN). The periodic arrangement of the nanocolumns led to the photonic crystal (PC) effect. The photonic band edge wavelength (λ B) and the InGaN bandgap were controlled by the D n-GaN. By controlling λ B closely at the bandgap wavelength, the PC effect provided directional beam radiation from the LEDs with radiation angles of approximately ±30°.

Photonic crystal structured blue µLED with aperture size dependent data transmission performance in plastic fiber link

A comprehensive comparison on the data transmission performance of photonic crystal structured micro light-emitting diodes (PC-µLEDs) with different aperture sizes is realized for high-speed visible light communication application. The 120×120-µm2 large PC-µLED exhibits the largest optical power of 580 µW and the highest external quantum efficiency of 2.5%; however, it also demonstrates the lowest analog modulation bandwidth of only 72 MHz. By contrast, the smallest PC-µLED with 20×20-µm2 aperture emits the lowest optical power of 37 µW but provides the highest 3-dB bandwidth of 162 MHz. After optimizing the operating parameters for data transmission, the trade-off between output power and encoding bandwidth is observed to improve the transmission performance. The PC-µLED with mesa length of 60–80 µm can transmit on-off keying (OOK) data format at 500 Mbit/s under error-free BER criterion. In particular, the device with a mesa area of 80×80-µm2 successfully carries the 300-MBaud 4-level pulse amplitude modulation (PAM-4) data with corresponding data rate of 600 Mbit/s under KP4 forward error correction (FEC) required BER. Furthermore, the quadrature-amplitude-modulation (QAM) orthogonal frequency division multiplexing (OFDM) data transmission is also performed, and the highest data rate of 2 Gbit/s under FEC criterion is allowable by using the PC-µLED with a mesa aperture of 80×80-µm2.

Study on Modulation Bandwidth and Light Extraction Efficiency of Flip-Chip Light-Emitting Diode with Photonic Crystals.

In this study, the photonic crystal structure is employed to increase both the light extraction efficiency and the modulation bandwidth of flip-chip GaN-based light-emitting diodes (LEDs). The finite difference time domain method is utilized to investigate the influence of structure of photonic crystals on the Purcell factor and light extraction efficiency of flip-chip GaN-based LEDs. Simulation results show that the modulation bandwidth is estimated to be 202 MHz at current densities of 1000 A/cm2. The experimental result of modulation bandwidth is in accord with the simulation. The optical f-3dB of the device achieves 212 MHz at current densities of 1000 A/cm2 and up to 285 MHz at current densities of 2000 A/cm2. This design of photonic crystal flip-chip LED has the potential for applications in high-frequency visible light communication.

Improvement of light extraction efficiency in GaN-based light-emitting diodes by addition of complex photonic crystal structure

To improve the light extraction efficiency of a GaN-based light-emitting diode (LED), a new complex periodic photonic crystal structure containing two kinds of hemispheres was designed. This complex photonic crystal structure is arrayed on the surface of the planar LED, and a finite-difference time-domain method was used to optimize the hemisphere materials (ZnO, GaN, and SiC) and the structural parameters of the complex photonic crystal (the size of the hemisphere radius and the lattice constant of complex structure). A high improvement of light extraction efficiency is achieved in complex photonic crystal LED, translated into an 8.3-fold enhancement for the case of a planar LED.

Radiation pattern design of photonic crystal LED optimized by using multi-objective grey wolf optimizer

This paper proposes an effective method for shaping the radiation pattern intensity of photonic crystal (PhC) light-emitting diode (LED). In this method, the process of shaping the radiation pattern intensity is first formulated as a multi-objective problem. A multi-objective optimization, called multi-objective grey wolf optimizer, is then utilized to find a set of optimal designs. The proposed shaping method aims to focus the intensity of light in a narrow-angle range and provide uniform radiated light in this range. The proposed method is also described and applied to a case study. The results show that the method proposed is beneficial and could be utilized to design any kind of PhC LEDs. As the lack of analytical method prevents researchers from finding optimal designs, this method is a shortcut to systematically shape the radiated intensity of PhC LED light.

Large-Signal Modulation Performance of Light-Emitting Diodes With Photonic Crystals for Visible Light Communication

Recently, GaN-based light-emitting diodes (LEDs) find their applications in, other than general lighting, visible light communication (VLC) for high-speed wireless data networks. The pursuit for more bandwidth from the LED can be realized by engineering the LED material structure and device design, or by modulation techniques. In this paper, we proposed GaN-based LEDs with the photonic crystal (PhC) structure for VLC applications. Contradictory to general thought that higher small frequency response translates to better large-signal performance, our results reveal that a PhC LED may not perform well at low data rate due to its relatively poorer signal-to-noise ratio (SNR). However, at higher data rates, the performance of PhC LEDs surpasses conventional LED structure. We showed data transmission capability of PhC LEDs with a bit rate up to 400 Mb/s using on–off keying modulation. To demonstrate the chip capability for large-signal transmission, without using preemphasis or postemphasis, VLC with 16-quadrature amplitude modulation orthogonal frequency-division multiplexing transmission capacity up to 2 Gbit/s is achieved with an error vector magnitude of 17.07%, SNR of 15.35 dB, and bit error rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.3 \times {10}^{-3}$ </tex-math></inline-formula> . Our LEDs embedded with PhCs pass the forward error correction criterion. The results reveal the advantages of LEDs with PhC for achieving higher data rate transmission.

Improving the Efficiency of Transverse Magnetic Polarized Emission from AlGaN Based LEDs by Using Nanowire Photonic Crystal

AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) are attractive for a wide range of applications. To date, however, the best reported external quantum efficiency (EQE) for LEDs operating in the wavelength range of ~240 nm is well below 1%. In this paper, we have performed detailed studies of the EQE of AlGaN nanowire photonic crystal LEDs in this wavelength range by finite-difference time-domain simulation. By coupling in-plane emission to vertical emission, light-extraction efficiency (LEE) over 90% can, in principle, be expected from the device top surface for transverse magnetic polarized photons that was not previously possible in conventional planar quantum well devices. Taken into account practical limitations including the absorptive p-Al(Ga)N contact and the presence of metal contact grid, LEE in the range of 30-40% is also expected. Moreover, due to the Purcell effect, the radiative recombination rate and, therefore, internal quantum efficiency (IQE) can be significantly modified in nanowire photonic crystal structures. We have established the design principles and ultimate efficiency limit of AlGaN nanowire photonic crystal LEDs: For AlGaN LEDs with very high IQE, EQE >70% can, in principle, be expected by operating near the Γ point of nanowire photonic crystals, whereas for structures with relatively low IQE it is preferred to operate near the M point to enhance the IQE while maintaining reasonably high LEE.

表面微腔光子晶体LED的光提取特性

表面微腔光子晶体LED的光提取特性

High-Speed Visible Light Communication Using GaN-Based Light-emitting Diodes With Photonic Crystals

Recent research and progress have shown that using light-emitting diodes (LEDs) as a high-speed transmitter is a promising candidate in a high-speed data system. The photonic crystal (PhC) LED (PhCLED) can improve the performance of the devices for visible light communication (VLC) due to its unique spatial and temporal management of photonic modes. In this paper, the dynamic behaviors of PhCLEDs at both small- and large-signal modulation are studied. Room-temperature time-resolved photoluminescence (TRPL) and Raman scattering measurements were carried out to understand the photon lifetime and strain in the PhC structure. PhCLEDs demonstrate much better high-frequency performance than conventional planar LEDs. Our study also shows that the common wisdom of shrinking the mesa size of LEDs to achieve high-frequency operation has its limit since the spontaneous emission rate has to be increased accordingly to maintain high light output efficiency in VLC LEDs.

Two-color light-emitting diodes with polarization-sensitive high extraction efficiency based on graphene

In the present study, graphene photonic crystals are employed to enhance the light extraction efficiency (LEE) of two-color, red and blue, light-emitting diode (LED). The transmission characteristics of one-dimensional (1D) Fibonacci graphene photonic crystal LED (FGPC-LED) are investigated by using the transfer matrix method and the scaling study is presented. We analyzed the influence of period, thickness, and permittivity in the structure to enhance the LEE. The transmission spectrum of 1D FGPC has been optimized in detail. In addition, the effects of the angle of incidence and the state of polarization are investigated. As the main result, we found the optimum values of relevant parameters to enhance the extraction of red and blue light from an LED as well as provide perfect omnidirectional and high peak transmission filters for the TE and TM modes.

High-speed modulation from the fast mode extraction of a photonic crystal light-emitting diode

Using light-emitting diodes (LEDs) for visible light communication has become an alternative choice of radio source due to channel crowding of the radio-frequency (RF) signal. The modulation bandwidth of LEDs is usually limited by the spontaneous carrier lifetime in multiple quantum wells. Here, sub-GHz modulation of GaN-based LED employing photonic crystal (PhC) nanostructure is demonstrated. The guided photonic modes of the LEDs are modulated by the RF signal. Both carrier lifetime of lower- and higher-order modes are studied in time-resolved photoluminescence (TRPL) at room temperature. The f-3dB-J curve of the PhC LED exhibits a higher bandwidth than the typical LED structure. At 11.41 kA/cm2, the optical −3-dB bandwidth (f-3dB) up to 234 MHz of the PhC LED (PhCLED) is achieved. Our studies on TRPL at different wavelengths and frequency response at different injection current densities conclude that the higher operation speed is attributed to faster radiative carrier recombination of extracted guided modes from the PhC nanostructure.

How to design photonic crystal LEDs with artificial intelligence techniques

The use of multi-objective optimisation techniques in the field of artificial intelligence for designing photonic crystal (PhC) light-emitting diodes (LEDs) is proposed. The extraction ratio and Purcell factor are first identified and considered as the main objectives when designing such LEDs. These two objectives are then optimised simultaneously even though both are in conflict. The proposed method is developed within a framework and applied to a case study. A comparative study shows that the optimised PhC LEDs completely outperform current works in the literature. These results are obtained by just tuning the PhC structural parameters, which can be considered as a substantial achievement in this field.

Enhanced performance of photonic crystal GaN light-emitting diodes with graphene transparent electrodes.

The two-dimensional (2D) triangle lattice air hole photonic crystal (PC) GaN-based light-emitting diodes (LED) with double-layer graphene transparent electrodes (DGTE) have been produced. The current spreading effect of the double-layer graphene (GR) on the surface of the PC structure of the LED has been researched. Specially, we found that the part of the graphene suspending over the air hole of the PC structure was of much higher conductivity, which reduced the average sheet resistance of the graphene transparent conducting electrode and improved the current spreading of the PC LED. Therefore, the work voltage of the DGTE-PC LED was obviously decreased, and the output power was greatly enhanced. The COMSOL software was used to simulate the current density distribution of the samples. The results show that the etching of PC structure results in the degradation of the current spreading and that the graphene transparent conducting electrode can offer an uniform current spreading in the DGTE-PC LED.PACS: 85.60.Jb; 68.65.Pq; 42.70.Qs

Observation of enhanced visible and infrared emissions in photonic crystal thin-film light-emitting diodes

Photonic crystals, in the form of closed-packed nano-pillar arrays patterned by nanosphere lithography, have been formed on the n-faces of InGaN thin-film vertical light-emitting diodes (LEDs). Through laser lift-off of the sapphire substrate, the thin-film LEDs conduct vertically with reduced dynamic resistances, as well as reduced thermal resistances. The photonic crystal plays a role in enhancing light extraction, not only at visible wavelengths but also at infrared wavelengths boosting heat radiation at high currents, so that heat-induced effects on internal quantum efficiencies are minimized. The observations are consistent with predictions from finite-difference time-domain simulations.

Investigation on bandgap, diffraction, interference, and refraction effects of photonic crystal structure in GaN/InGaN LEDs for light extraction.

In this paper, we have made a clear differentiation among bandgap, diffraction, interference, and refraction effects in photonic crystal structures (PhCs). For observing bandgap, diffraction, and refraction effects, PhCs are considered on the top p-GaN surface of light emitting diodes (LEDs), whereas for interference effect, hole type PhCs are considered to be embedded within n-GaN layer of LED. From analysis, it is observed that at a particular lattice periodicity, for which bandgap lies within the wavelength of interest shows a significant light extraction due to inhibition of guided mode. Beyond a certain periodicity, diffraction effect starts dominating and light extraction improves further. The interference effect is observed in embedded photonic crystal LEDs, where depth of etching supports constructive interference of outward light waves. We have also shed light on refraction effects exhibited by the PhCs and whether negative refraction properties of PhCs may be useful in case of LED light extraction.

Efficiency enhancement of homoepitaxial InGaN/GaN light-emitting diodes on free-standing GaN substrate with double embedded SiO2 photonic crystals.

Homoepitaxially grown InGaN/GaN light emitting diodes (LEDs) with SiO2 nanodisks embedded in n-GaN and p-GaN as photonic crystal (PhC) structures by nanospherical-lens photolithography are presented and investigated. The introduction of SiO2 nanodisks doesn't produce the new dislocations and doesn't also result in the electrical deterioration of PhC LEDs. The light output power of homoepitaxial LEDs with embedded PhC and double PhC at 350 mA current is increased by 29.9% and 47.2%, respectively, compared to that without PhC. The corresponding light radiation patterns in PhC LEDs on GaN substrate show a narrow beam shape due to strong guided light extraction, with a view angle reduction of about 30°. The PhC LEDs are also analyzed in detail by finite-difference time-domain simulation (FDTD) to further reveal the emission characteristics.

Uniform and Duty Cycle Tunable Photonic Crystal LEDs Fabricated by Intermediate Mask Layer: Soft UV Nanoimprint Lithography

As an improvement of the conventional nanoimprint lithography, an intermediate mask layer process is invited for photonic crystal pattern transfer on LEDs. Benefit from the smooth effect of the under-layer resist, uniform photonic crystal structure is successfully transferred to the p-GaN surface. The intermediate SiO2 layer has a high etching selectivity to the under-layer resist, which can be used to fabricate nanostructures with tunable duty cycle via one single initial mold. The photonic crystal LED fabricated by the process of intermediate mask layer nanoimprint lithography shows a 3.31 fold PL enhancement to that of the un-patterned LED.