Figure 6.3 shows the schematic diagram of a triangular-lattice photonic crystal surface emitting laser. The nitride sample used in this experiment was similar to the as-grown sample shown in figure 3.8 (in chapter 3). The center wavelength of bottom DBR was about 430 nm.
The DBR here could be a reflector to reflect downward light back to the photonic crystal region. The spontaneous emission of the sample was excited by a focused He-Cd laser (325 nm). Figure 6.4 shows the micro-photoluminescence (μ-PL) emission spectrum of the as-grown structure. The peak wavelength is centered at 425 nm with a FWHM of about 20 nm.
The PC laser was fabricated by a few process steps. In the beginning, the SiN 200 nm was deposited as a hard mask using PECVD. The PMMA layer (150 nm) was spun by spinner and patterned using an e-beam lithography system (Raith, ELPHY QUANTUM) to form a soft mask. The lattice constants of PCs were patterned to be the range between 190 nm and 300 nm. The diameter of each PC device was 50 μm. The PC pattern on soft mask was transferred to SiN film by using ICP-RIE (Oxford Plasmalab system 100), and the PMMA layer was removed by dipping ACE tone. Then, the sample was performed a dry etching in an ICP-RIE system (SAMCO RIE-101PH) to etch GaN as deep as 400 nm. Finally, the sample was dipped in BOE to remove the hard mask to complete PC lasers. Figure 6.5(A) and (B) shows the SEM photos of fabricated photonic crystal in top view and cross-section view, respectively.
6-4 Characteristics of Nitride-Based 2-D Photonic Crystal Surface-Emitting
Lasers
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Threshold characteristics
The threshold characteristics of the nitride-based 2-D PCSEL were measured using the optical pumping with a pumping spot size of around 50 μm. The lasing action could be observed in several different devices (different lattice constant a=190-300 nm) with different lasing wavelength. We plot normalized frequencies of those lasers versus lattice constants as shown in figure 6.6. The figure shows the PC device with larger lattice constant has the larger normalized frequency. Taking one of them (a=290 nm) as example, the laser emission intensity from the 2-D PCSEL as a function of the exciting energy density is shown in Fig.
6.7. The threshold energy density (Eth) was observed to be around 3.5 mJ/cm2. The light intensity increased rapidly and linearly as the excitation energy density was above the threshold. Figure 6.8 shows the lasing spectra at different pumping energy, and the inset shows the emission spectrum at the energy density of 0.66 Eth. The spontaneous emission of the PC device below the threshold was centered at around 405 nm. A sharp and narrow laser emission was then clearly observed as the pumping energy increased above the threshold energy. The lasing wavelength located at around 424.3 nm, and the FWHM of the laser is around 0.11 nm. It evidently expresses the laser action actually happened at the PC device.
Other PC devices also could be excited to lase at the similar threshold energy and emitted different lasing wavelengths. The most interesting is the variation of lasing wavelengths of devices with different lattice constants. Figure 6.9 shows the lasing spectra of PC devices with three different lattice constants, 201 nm, 244 nm, and 290 nm (points I, II, and III marked in figure 6.6). The lasing wavelengths of these three devices were 402.2 nm, 412.4 nm, and 424.3 nm for PC devices with 201 nm, 244 nm, and 290 nm of lattice constant, respectively.
The lasing wavelengths show an obvious red shift with the increase of lattice constant. The lasing wavelength differences among them are around 10 nm and 12 nm. We further put the simulated band diagram (figure 6.2) and experiment data (figure 6.6) together for comparison and show it in figure 6.10. The comparison between these two figures helps us to be able to
classify normalized frequencies of our lasers in figure 6.6 into a few groups. Each group of them could correspond to one of simulated band edges at Brillouin-zone boundaries, Γ、M and K. This means the lasing action could only occur at the specific normalized frequency satisfied the Bragg condition. Furthermore, the device with a larger lattice constant of PC would lase at the PC band edge with a larger normalized frequency.
Figure 6.11(A) and (B) show the spontaneous emission image at 0.92Eth (pumping spot size was about 50μm) and the laser emission image at 1.47Eth, respectively. As the pumping energy was below the threshold, the light emission intensity was uniform across the whole PC region (the diameter is 50μm). With the pumping energy increasing above the threshold, a great portion of the PC region started to brighten substantially. Theoretically, the lasing emission in such device should exist in entire PC region; however, figure 6.11(B) shows the lasing area is not full of the PC region. It could be attributed to the disorder of PC, un-uniformity of pumping laser beam, or inhomogeneiety of InGaN-based gain material.
Nevertheless, the nitride-based 2-D PCSEL could actually have an obviously larger lasing area than that of nitride-based VCSEL, which is just several micro-meters.
Polarization
The polarization of the laser was measured by inserting and rotating a polarizer in front of a fiber which collects light into a spectrometer. Figure 6.12 shows the laser emission intensity as a function of the angle of the polarizer. Here we defined the degree of polarization (DOP) as DOP = (Imax-Imin)/( Imax+Imin), where Imax is the maximum intensity and Imin is the minimum intensity. From the figure, we could estimate the degree of polarization of the laser to be about 53%. In fact, the calculation of electric-field vectors in triangular lattice PCs was reported [2], and results suggested that this kind of the laser has weightless polarization property of the laser emission. This could explain the low DOP of our lasers.
We also varied the detecting angle of a fiber which collects light into a spectrometer to measure the laser emission. The fiber is mounted on a stage which could be rotated from -90°
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to 90°. We measured the emission intensity every 10 degree. Here the 90° is the direction parallel to the sample surface. Figure 6.13 shows the laser emission intensity as a function of the detecting angle of the fiber. The result suggests the laser is vertically emitted. This characteristic is a strong enough evidence to reveal the PCSEL is a kind of considerably excellent single-mode surface emitting laser.
References
1. M. Imada, A. Chutinan, S. Node, and M. Mochizuki, Phy. Rev. B, 65, 195306 (2002) 2. S. L. Chuang, Physics of Optoelectronic Devices, John Wiley & Sons, Inc (1995)
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