Chapter 1 Introduction
1.1 Laser
The concept of laser made by the Schawlow and Townes since 1960 [1] has caused considerable interest in the scientific community. The first ruby laser system made by Maiman in 1960 [2]. Then in 1962, gallium arsenide (GaAs) semiconductor laser is also immediately appear [3-5]. Semiconductor laser due to small size, long life and high stability has been widely applied in many different areas, such as optical fiber communication, optical storage and laser printing, molecular spectroscopy and biomedicine, military and blue-ray DVD, entertainment purposes and so on. Laser system is the elementary combination of the pumping source, the gain material, the optical cavity and the output coupler. The principle of operation is the input electricity or light of the pumping source can make the electronic absorption in the gain material and transition to excited state. Until the conduction band electron concentration attain to the population inversion and the electricity holes in the valence band combine into a large number of electron-hole pairs and emit photons, in order to achieve stimulated emission of the state and the optical cavity can limit to photon, choose the operating mode and to repeat the above process to achieve laser gain effect [6].
Laser can be divided into two classes by the direction of emission : edge emitting laser (EEL) and surface emitting laser (SEL). The laser light of edge emitting laser propagates parallel to the wafer surface of the semiconductor chip. Edge emitting laser is reflected or coupled out at a cleaved edge. The light of surface-emitting lasers propagates in the direction perpendicular to the semiconductor wafer surface.
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Edge emitting laser (EEL)
Edge-emitting lasers are the original and still very widely used form of semiconductor lasers. Their resonator length is typically between a few hundred micrometers and a few millimeters. This is sufficient for reaching a high gain, so that an edge emitting laser may lase even if the resonator losses are fairly high. The laser beam within the edge emitting laser structure is guided in a waveguide structure.
Typically, one uses a double heterostructure, which restricts the generated carriers to a narrow region and at the same time serves as a waveguide for the optical field, as shown in Fig 1.1(a). This arrangement leads to a low threshold pump power and a high efficiency.
Surface emitting laser (SEL)
There are several advantages to producing surface emitting lasers, in contrast to the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not function properly, whether due to bad contacts or poor material growth quality, the production time and the processing materials have been wasted. However, surface emitting lasers can be tested at several stages throughout the process to check for material quality and processing issues. Additionally, because surface emitting lasers emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter. Furthermore, even though the surface emitting laser production process is more labor and material intensive, the yield can be controlled to a more predictable outcome. There are three common kind of surface emitting lasers that is vertical cavity surface emitting laser (VCSEL), as shown in Fig 1.1(b), distributed feedback laser (DFB) and photonic crystal surface emitting laser (PCSEL). Here, we mainly discuss the PCSEL.
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Photonic Crystal Surface Emitting Laser (PCSEL)
Fig 1.1(c) shows the schematic diagram of a GaN-based PCSEL devices. The two-dimensional (2D) photonic crystal (PC) surface emitting laser is based on multidirectionally distributed feedback effect near the band edges in a 2D PC structure, which has potential for high-power and single mode surface-emitting lasers.
PCs with photonic band gaps for photons have many advantages in arbitrarily controlling the light emission and propagation and can be utilized to realize various new optical devices. By varying the lattice constant of the PC pattern, different lasing wavelengths corresponding to different band edges are demonstrated. PCSEL utilizing 2D distributed feedback mechanism has been attracted much attention and widely researched during past decades [7-13]. PCSELs have many advantageous characteristics such as single mode operation in a large lasing area, a symmetric beam shape and a low divergence angle. Numerical studies have attempted to explain the distributed feedback mechanism for PCSELs by using different theoretical methods.
Sakoda et al. used group-velocity anomaly to evaluate lasing threshold by the plane wave expansion method (PWEM) [14]. Lee et al. investigated the quality factor near band edges of finite-size photonic crystals (PCs) by the finite-difference time-domain (FDTD) method [15]. Sakai et al. calculated the threshold gain deviated from the Bragg frequency for square PCs by using the coupled wave theory [16-17]. Nojima proposed the multiple scattering method (MSM) to calculate lasing behaviors in PC lattice atoms with optical gains [18]. There are different advantages and limitations while using these theoretical methods to calculate characteristics of PC lasers. For example, the 2D PWEM better applies to the infinite PC structure, which is usually not the case for actual devices. FDTD method consumes numerous computer resources and calculation time to simulate the finite domain structure.
On the contrary, coupled wave theory has many advantages such as less
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calculation time and capability in providing more accurate solutions to modify the designs. Therefore, the purpose of this thesis is to investigate the different parameters of the square lattice and triangular lattice PC including the influence of the coupling constant to the threshold gain and the frequency deviation at different band-edge modes.
(a) (b)
(c)
Fig 1.1 (a)The schematic diagram of an edge emitting laser (b) The schematic diagram of a vertical-cavity surface emitting laser (c)The schematic diagram of a photonic crystal surface emitting laser
AlN/GaN DBR n-type GaN InGaN/GaN MQWs p-type GaN
Sapphire
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