**1. Introduction**

20 Photonic Crystals – Innovative Systems, Lasers and Waveguides

[6] Imada, M.; Chutinan, A.; Noda S. & Mochizuki M. Multidirectionally distributed

[7] Yokoyama M. & Noda S. Finite-difference time-domain simulation of two-dimensional

[8] Wang, S. C.; Lu, T. C.; Kao, C. C.; Chu, J. T.; Huang, G. S.; Kuo, H. C.; Chen, S. W.; Kao,

[9] Nojima, S. Theoretical analysis of feedback mechanisms of two-dimensional finite-sized

[10] Lu, T. C.; Chen, S. W.; Lin, L. F.; Kao, T. T.; Kao, C. C.; Yu, P.; Kuo, H. C.; Wang, S. C. &

[11] Chen, S. W.; Lu, T. C.; Hou, Y. J.; Liu, T. C.; Kuo H. C. & Wang, S. C. Lasing

lasers. *Applied Physics Letters*, Vol. 96, (2010), pp. 071108, ISSN 0003-6951. [12] Chen Y. Y. & Ye, Z. Propagation inhibition and wave localization in a two-dimensional

[13] Notomi, M.; Suzuki, H. & Tamamura, T. Directional lasing oscillation of two-

[14] Soller, B. J.; Stuart, H. R. & Hall, D. G. Energy transfer at optical frequencies to silicon-

*Physics Letters*, Vol. 78, (2001), pp. 1325-1327, ISSN 0003-6951.

1098-0121.

0021-8979.

3755.

9592.

2880, ISSN 1094-4087.

pp. 011129, ISSN 0003-6951.

Vol. 46, (2007), pp. 5397-5407, ISSN 0021-4922.

feedback photonic crystal lasers. *Physical Review B*, Vol. 65, (2002), pp. 195306, ISSN

photonic crystal surface-emitting laser. *Optics Express*, Vol. 13, (2005), pp. 2869-

T. T.; Chen, J. R. & Lin, L. F. Optically Pumped GaN-based Vertical Cavity Surface Emitting Lasers: Technology and Characteristics. *Japanese Journal of Applied Physics*,

photonic-crystal lasers. *Journal of Applied Physics*, Vol. 98, (2005), pp. 043102, ISSN

Fan, S. H. GaN-based two-dimensional surface-emitting photonic crystal lasers with AlN/GaN distributed Bragg reflector. *Applied Physics Letters*, Vol. 92, (2008),

characteristics at different band edges in GaN photonic crystal surface emitting

random liquid medium. *Physical Review E*, Vol. 65, (2002), pp. 056612, ISSN 1539-

dimensional organic photonic crystal lasers at several photonic band gaps. *Applied* 

on-insulator structures. *Optics Letters*, Vol. 26, (2001), pp. 1421-1423, ISSN 0146-

Vertical-Cavity Surface-Emitting Laser (VCSEL) is a type of semiconductor laser with laser beam perpendicular to the surface of the semiconductor substrate, as shown in Fig.1(a) [1]. VCSEL has many advantages, such as non-divergence output beam, fabrication and test on wafer, easy two-dimensional integration, and single longitudinal mode work. VCSEL is composed of an active region sandwiched between top and bottom highly reflective DBR mirror [2,3]. Generally high power VCSEL could be realized through large emission window, but suffers multi-mode operation due to the inhomogeneous current distribution across the active region. On the other hand single-mode operation is required in many applications including optical communications. Single-mode can transport longer distance and meet the requirements of high-speed data transmission [4,5]. Several approaches such as confined aperture less than 3m, proton implantation, oxide and proton implantation mixed structure have been reported to achieve single-mode VCSEL. Due to the small aperture of emission window, these VCSELs are lasing at low output power. Besides the requirements of high output power and single mode operation, the wavelength range of VCSEL is broadened by applying InAs quantum dots or InGaAsN quantum well of the wavelength range of 1300nm and nitride quantum well of the blue light range for the applications of fiber communication and display.

In the past few years photonic crystal materials became of a great interest due to their powerful properties allowing for previously unknown flexibility in shaping the light. On the contrary to conventional edge emitting laser, the cavity length of VCSEL is of the size of optical wavelength. This brings VCSEL actually into microcavity field, where spontaneous emission is believed not to be an intrinsic atomic property anymore. Spontaneous emission can be enhanced or inhibited by tailoring the electromagnetic environment that the atom can radiate into. In a conventional edge emitting laser made of large cavity, most of the spontaneous emission is lost to free space as radiation modes and only a small fraction couples to the resonant mode of the cavity formed by the mirrors. Therefore, significant stimulated emission output can only be obtained when the input power crosses a threshold to overcome the free-space loss. In a wavelength-sized microcavity, the photon-mode density develops singularities, just as in the case of carrier confinement in quantum well. In this case, a single spectrally distinct mode can receive most or all of the spontaneous

980nm Photonic Microcavity Vertical Cavity Surface Emitting Laser 23

randomly follow one of the crystal axes and fluctuates with current. For applications such as 10-Gbit/s-class high-speed modulation1 and free-space interconnect using polarizationdependent optical components, a pinned polarization gives better performance. The competition between the modes with orthogonal polarizations can lead to polarization switching and mode hopping [9,10]. Such behavior is unacceptable for many practical applications such as intra-cavity frequency doubling, where other elements are polarizationdependent. Several approaches for polarization control have been reported based on the introduction of anisotropy to either gain or losses. These approaches include asymmetric shape resonator, metal-semiconductor gratings, or sub-wavelength grating by directly etching the top surface. In order to make use of the PhC structure for polarization control in VCSELs, PhC with elliptic air holes has been reported with polarization mode suppression ratio (PMSR) of over 20 dB in [11]. Triangular lattice PhC has been implemented with air holes elongated either along CK or CM directions. Disadvantages of etching photonic crystal holes include increased resistance and optical losses leading to higher threshold currents

In this paper two-dimensional photonic crystal structure of hexagonal lattice of air holes on the top DBR reflector was introduced in VCSEL to suppress higher order mode operation. Defect structure of photonic crystal was created by filling one air hole (H1 microcavity) or seven air holes (H2 microcavity) to investigate the mode characteristics of VCSEL. With the proper selection of hole depths, diameters, and arrangement, this index confinement can be exploited to create single mode photonic crystal defect VCSELs that have the potential for low threshold currents and high output powers. The specific parameters of hexagonal lattice

The active region of 980nm VCSEL was composed of three 8nm thick In0.2Ga0.8As quantum well layers with 10nm thick GaAs barrier layer. Al0.98Ga0.02As layer is incorporated between the P-type DBR and the active region to form lateral oxidation and provide both current and optical confinements. The reflectors were DBR mirrors with the reflectivity higher than 99%. In this work a periodic arrangement of air holes on the top DBR reflector was designed to form two-dimensional photonic crystal structure. Two kinds of lattice defect were produced to evaluate the Q factor of the microcavity. Schematic diagram of the structure was shown in

Generally there were two types of two-dimensional periodic arrangement of photonic crystals: hexagonal lattice and square lattice. Under the similar lattice parameters of hole depth, diameter and distance, hexagonal lattice was suggested to obtain photonic band gap easily than the square lattice does. Once the photonic band gap was created, the band gap of hexagonal lattice was wider than that of square lattice. Therefore hexagonal lattice was often used in the design of PhC-VCSEL. When one or several holes were removed from the lattice, the periodicity of the lattice structure was destroyed. The simplest way is to remove one air hole from the center of the lattice. This created the H1 cavity, shown in Figure 3(a). The second photonic crystal defect structure, H2 microcavity, was to remove seven air holes from the center, as shown in Figure 3(b). In our simulation the air hole was etched through

were optimized to achieve high Q factor of the microcavity.

**2.1 Photonic crystal micro-cavity VCSEL model** 

and voltage.

Fig. 2.

**2. Model and calculation** 

emission, indicating threshold-free stimulation. The rate of spontaneous emission is enhanced in such a microcavity, due to the change in the mode density. Photons whose energies lie within the band gap of photonic crystal cannot propagate through the structure. A point defect in the photonic crystal structure will generate localized state inside the band gap and form a microcavity. All the photons corresponding to the wavelength of the defect can propagate in the crystal. An example of such microcavity is DBR with high-reflectivity mirrors in the direction of the guided modes.

Fig. 1. VCSEL structure ((a): conventional VCSEL, (b): PhC-VCSEL)

The localization of electromagnetic models in single or multiple defects enabled to build photonic-crystal fibers, photonic planar waveguides, filters, splitters etc. Among these novel photonic crystal structures, photonic crystal-based VCSEL (PhC-VCSEL), as shown in Fig.1(b), is becoming an alternative approach and attracting more and more attention. These devices have strong potential due to their unique properties, which make them a perfect choice for many applications. These properties include stable single-mode operation [6], high-speed modulation [7] and polarization control [8]. However, to guarantee the efficient use of photonic crystals one needs careful consideration of the photonic crystal structure, which actually form a microcavity to modulate the spontaneous emission characteristics of VCSEL. Typical PhC-VCSELs consist of a classical VCSEL cavity surrounded by Distributed Bragg Reflectors (DBRs) of high reflectivity. The photonic crystal has a form of cylindrical holes located in various parts of the device. In the simplest case—and therefore the most popular one—the holes are etched in the top DBR. However, there are other possibilities like drilling the whole structure or placing the holes solely in the cavity, which can improve some properties of PhC-VCSEL but although constitutes a technological challenge. Photonic crystal structure with defects at the center was incorporated into the top layer to form microcavity, which provide lateral light confinement and also the modulation to the photon mode. However, large optical loss due to deeply-etched air holes still remains as a problem. The large optical loss is undesirable because it increases not only threshold current but also operating current level. High operating current can limit maximum single-mode output power via heating problem and lead to higher electrical power consumption.

Traditional VCSELs suffer a major drawback of the instability of the polarization, which generally attributed to the symmetric device structure. The polarization of a VCSEL tends to

emission, indicating threshold-free stimulation. The rate of spontaneous emission is enhanced in such a microcavity, due to the change in the mode density. Photons whose energies lie within the band gap of photonic crystal cannot propagate through the structure. A point defect in the photonic crystal structure will generate localized state inside the band gap and form a microcavity. All the photons corresponding to the wavelength of the defect can propagate in the crystal. An example of such microcavity is DBR with high-reflectivity

The localization of electromagnetic models in single or multiple defects enabled to build photonic-crystal fibers, photonic planar waveguides, filters, splitters etc. Among these novel photonic crystal structures, photonic crystal-based VCSEL (PhC-VCSEL), as shown in Fig.1(b), is becoming an alternative approach and attracting more and more attention. These devices have strong potential due to their unique properties, which make them a perfect choice for many applications. These properties include stable single-mode operation [6], high-speed modulation [7] and polarization control [8]. However, to guarantee the efficient use of photonic crystals one needs careful consideration of the photonic crystal structure, which actually form a microcavity to modulate the spontaneous emission characteristics of VCSEL. Typical PhC-VCSELs consist of a classical VCSEL cavity surrounded by Distributed Bragg Reflectors (DBRs) of high reflectivity. The photonic crystal has a form of cylindrical holes located in various parts of the device. In the simplest case—and therefore the most popular one—the holes are etched in the top DBR. However, there are other possibilities like drilling the whole structure or placing the holes solely in the cavity, which can improve some properties of PhC-VCSEL but although constitutes a technological challenge. Photonic crystal structure with defects at the center was incorporated into the top layer to form microcavity, which provide lateral light confinement and also the modulation to the photon mode. However, large optical loss due to deeply-etched air holes still remains as a problem. The large optical loss is undesirable because it increases not only threshold current but also operating current level. High operating current can limit maximum single-mode output

mirrors in the direction of the guided modes.

 (a) (b) Fig. 1. VCSEL structure ((a): conventional VCSEL, (b): PhC-VCSEL)

power via heating problem and lead to higher electrical power consumption.

Traditional VCSELs suffer a major drawback of the instability of the polarization, which generally attributed to the symmetric device structure. The polarization of a VCSEL tends to randomly follow one of the crystal axes and fluctuates with current. For applications such as 10-Gbit/s-class high-speed modulation1 and free-space interconnect using polarizationdependent optical components, a pinned polarization gives better performance. The competition between the modes with orthogonal polarizations can lead to polarization switching and mode hopping [9,10]. Such behavior is unacceptable for many practical applications such as intra-cavity frequency doubling, where other elements are polarizationdependent. Several approaches for polarization control have been reported based on the introduction of anisotropy to either gain or losses. These approaches include asymmetric shape resonator, metal-semiconductor gratings, or sub-wavelength grating by directly etching the top surface. In order to make use of the PhC structure for polarization control in VCSELs, PhC with elliptic air holes has been reported with polarization mode suppression ratio (PMSR) of over 20 dB in [11]. Triangular lattice PhC has been implemented with air holes elongated either along CK or CM directions. Disadvantages of etching photonic crystal holes include increased resistance and optical losses leading to higher threshold currents and voltage.

In this paper two-dimensional photonic crystal structure of hexagonal lattice of air holes on the top DBR reflector was introduced in VCSEL to suppress higher order mode operation. Defect structure of photonic crystal was created by filling one air hole (H1 microcavity) or seven air holes (H2 microcavity) to investigate the mode characteristics of VCSEL. With the proper selection of hole depths, diameters, and arrangement, this index confinement can be exploited to create single mode photonic crystal defect VCSELs that have the potential for low threshold currents and high output powers. The specific parameters of hexagonal lattice were optimized to achieve high Q factor of the microcavity.
