Investigation of Photonic Crystal Light-emitting Diodes and its High-speed Modulation
Date Issued
2016
Date
2016
Author(s)
Yin, Yu-Feng
Abstract
Periodic dielectric nanostructures have shown its outstanding control over the electromagnetic (EM) field and the light-matter interaction in photonics. Much research has investigated the deployment of photonic crystals (PhCs) in light-emitting diodes (LEDs) and lasers in order to manipulate the propagation of light and manage the temporal response of the photonic mode. In this dissertation, we will elaborate the concept of the PhC-embedded LED (PhCLED), its characteristics, and the potential for visible light communication (VLC). In the past, the unique phenomena of negative refraction were demonstrated theoretically and experimentally by pumping optical power into a passive device. Here, we demonstrate an electroluminescent device with negative refraction in the visible wavelength range. Different self-collimation in far-field patterns of polarized light are observed. The experiment was further verified from theoretical calculations of the group velocity on the equifrequency contours (EFCs) based on the plane-wave expansion (PWE) simulation. The angle-dependence in the enhanced spontaneous emission of PhCLEDs was elucidated by. the modal extraction lengths and quality factors obtained from the photonic bands. In our cases, the PhC device with a larger PhC period of 500 nm exhibits the best lower-order mode extraction and self-collimated radiation toward the surface-normal direction. Recent progress has shown that GaN-based LED is one of promising optical transmitters in the physical layer (PHY) of VLC. However, the modulation bandwidth of the front-end LED is usually limited by the spontaneous radiative recombination lifetime of the multiple quantum wells (MQWs). For PhCLEDs, by emitting light more coherently, the guided photonic modes can be extracted and coupled into the high-speed optical signal with a shorter radiative recombination lifetime. The demonstrated devices were investigated based on the temporal modal extraction with the corresponding photoluminescence (PL) measurement and the high-speed electroluminescence (EL) modulation. At 8.56 KA/cm2 of the biasing current density, the electrical-to-optical (E-O) -3-dB bandwidth (f-3dB) up to 234 MHz (a 1.1-fold increment compared to that of a conventional LED) in PhCLED is achieved. A 2.1-fold increment in the bandwidth can be obtained for a larger size of PhCLED. The extracted intrinsic small-signal parameters explain the higher operation speed is attributed to the faster radiative carrier recombination of extracted guided modes from the PhC nanostructure. Finally, we compare various PhC structures with corresponding dynamic behaviors in both small- and large-signal modulation. Faster transient responses and higher efficiency of the out-coupled modes were obtained in the room-temperature time-resolved photoluminescence (TRPL) and Raman scattering measurement. With an optimal design of PhCs, the highest f-3dB of 347 MHz in the PhCLED is achieved, which corresponds a 55% enhancement compared to a convention LED. Moreover, an error-free transmission was tested under a pseudo-random bit sequence (PRBS) at 200 Mbps with a bit-error rate (BER) < 10^-10. Better signal integrity is also shown in digitally modulated eye patterns up to 800 Mbps. Our study reveals that the differences in the high frequency behaviors and strain-related effects of various PhCLEDs result in different improvements in the signal impairment during the data transmission. For the best performance, the PhC with a larger period and a larger air-hole portion characterizes PhCLEDs with the highest f-3dB bandwidth and the best transmitted signal quality.
Subjects
collimation
digital modulation
frequency response
light emitting diodes
photonic crystals
radiative recombination
visible light communication
Type
thesis
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