Besides the phenomenon of multiple laser spots, we also found well-organized mode behaviors in nitride-based VCSEL. Figure 4.5 shows the observed lasing emission images and their intensity distributions in the four different regions. Above the threshold energy, the four different mode patterns similar to TEM00, TEM01, TEM02 and TEM11 were observed in these four regions, respectively. From the light intensity distribution, we could clearly define the mode size of each transverse mode pattern. The size of fundamental mode was about 1.05μm.
Compared to the fundamental mode, the size of other mode patterns was larger. As shown in the figure, the size of mode patterns similar to TEM01, TEM02, and TEM11 was about 4.4, 4.4, and 6.5μm, respectively. It is interesting these modes patterns individually occurred at different regions. The different preference for emitted mode pattern in different regions should be very special compared to other laser diodes. In the following, we mainly discuss the behaviors of fundamental mode (TEM00) (region I) and TEM01 (region II) mode patterns because the behaviors of other mode patterns were similar to those of TEM01.
Figure 4.6 and 4.7 shows the threshold characteristics of pumping region I and II on GaN VCSEL sample, respectively. The intensities of both regions rapidly increase as the pumping energy density was increased. The inserted photos describe the transverse mode patterns from these two regions of GaN VCSEL. As shown in figure 4.6, a lower pumping threshold could be obtained in region I on GaN VCSEL sample compared to that of another. In this region, the distribution of emission intensity in pumping region was uniform below threshold energy. As the pumping energy density was increased above threshold energy density of about 2.8mJ/cm2,
one single emission spot was observed in the center of the pumping region. This single-spot lasing phenomenon has been discussed in the previous chapter and could be characterized as the fundamental mode. However, in another pumping region (region II), we could observe an obviously different emission pattern above pumping energy density of about 6 mJ / cm2 (see figure 4.7). The emission pattern exhibited two separated intense peaks, which could be characterized as TEM01 mode. The mode pattern stably existed and its intensity significantly increased over the range of pumping energy density from 6 to 8 mJ / cm2. The stable transverse mode pattern with pumping energy increasing is a peculiar phenomenon. The results show that as the photons oscillated with specific gain distribution there would be only some transverse mode to appear and keep intensifying at high pumping energy. This stable mode emission could be attributed the short carrier diffusion length that nitride-based matrials have. The carrier diffusion lengths of GaN and InGaN were reported to be around 200 nm [4, 5]. This short carrier diffusion length would cause the carrier not easy to flow away and be shared by other high order modes.
Figure 4.8 and 4.9 shows the emission spectra of the pumping region I and II under different pumping energy, respectively. At pumping region I, a laser emission peak located at 411.77 nm was found above the pumping energy density of 2.8 mJ / cm2. With the increasing of pumping energy, the intensity of the laser emission rapidly increased. At another region, we also observed an emission peak supposed to be fundamental laser emission located at 412.9nm as the pumping energy was around 3 mJ/cm2. However, as energy density was beyond 3 mJ / cm2, another laser emission peak located at 411.73 nm appeared and its spectra intensity was gradually increased to compete with the peak at 412.9 nm within the pumping energy density of 3 and 6 mJ / cm2. As the pumping energy was above 6 mJ / cm2, the emission intensity of TEM01 mode suddenly shows a dramatic increase and the mode pattern became clear. It might imply the gain in this lasing area was shared and competed by two transverse modes between the pumping energy density of 3 and 6 mJ / cm2 and then TEM01
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mode became dominant lasing mode beyond the pumping energy density of 6 mJ / cm2. The gain sharing and competition could further increase the threshold pumping energy. The mode competition also could be seen at other regions on our sample. Figure 4.10 shows the emission spectra of nitride-based VCSEL emitting two kinds of transverse mode patterns, TEM01 and TEM11, at the same region with the same pumping energy. As the image of mode pattern was TEM01, two emission peaks were observed in the emission spectrum. The stronger emission peak was at 413.9 nm and another was at 413.3 nm. As the mode pattern became to be TEM11, the emission peak at 413.3 nm suddenly became dominant. It means the emitting wavelengths of co-existing TEM01 and TEM 11 were about 413.3 nm and 413.9 nm, respectively.
To understand the origin of the mode behavior of nitride-based VCSEL, we further measured the PL emission intensity distribution to realize the gain distribution of our sample.
Figure 4.11 shows the intensity distribution of photoluminescence at four different regions shown in figure 4.5. The PL intensity distributions at these four different regions were very different but similar to the laser mode patterns they emitted. The light intensity of the bright spot was about two times stronger than that of the neighborhood. As discussed in the section 4-1, these bright regions should embed high gain at corresponding cavity modes. Figure 4.12 shows the PL spectra of three places marked in the photo shown in the inset. Obviously, two bright spots both show similar emission wavelength of around 412nm with linewidth of around 1.6nm (the quality factor is 257.5). On the contrary, the neighbor place (place 3) shows a shorter wavelength of around 409.5nm with a broader linewidth of around 3nm (the quality factor is 136.5). This further confirmed there should be higher gain or lower loss in these bright spots, and they could result from the well alignment of the cavity mode and the gain profile and the higher reflectivity of DBR. In fact, the gain distributions at these four lasing regions are quite similar to their mode patterns. This suggests the lasing modes should have high gain and low threshold at these regions due to the highly overlap between gain
profiles and mode patterns.
Furthermore, for an almost planar resonator, such as VCSEL with DBRs, the mode spaceing is given by[10, 11]
2 0 2 2
0
2 n w
c π eff
= λ ν
Δ (1)
where n is the effective refractive index, and eff w0 is the minimum spot size. Considering the mode spacing between TEM00 and TEM01, we could estimate the spot size of fundamental mode to be 1.15μm with n =2.0 and eff λ =0.413μm. Here the effective index is estimated 0 by considering our resonant wavelength and the effective cavity length included 3λ cavity and DBR penetration depth. Our measured result shows the spot size of fundamental mode is 1.05μm. This value is almost consistent with the estimated value. However, typically, the higher-order mode size should roughly follows the equation: wn = nw0, where n is the mode number. That means the mode size of TEM01 would be about 1.6μm, which is smaller than the measured value. It implies the probability that these mode patterns were just a specific phenomenon resulting from the inhomogeneous gain.
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References
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2. F. Bertram, S. Srinivasan, L. Geng, F. A. Ponce, T. Riemann, and J. Christen, Appl. Phys.
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3. G. Christmann, D. Simeonov, R. Butté, E. Feltin, J.-F. Carlin, and N. Grandjean, Appl.
Phys. Lett., 89, 261101 (2006)
4. S. J. Rosner,a) E. C. Carr, M. J. Ludowise, G. Girolami, and H. I. Erikson, Appl. Phys. Lett., 70, 420 (1997)
5. J.S. Speck and S.J. Rosner, Physica B, 273-274, 24 (1999)
5μm
Figure 4.1 The PL intensity distribution of one region on the as-grown
sample with a scanning area of 20
×20 μm
2.
75
5μm
400 420 440 460 480
0.0 5.0k 10.0k 15.0k 20.0k 25.0k 30.0k 35.0k
0 20 40 60 80 100
~7 nm
~440.2 nm
In te nsi ty ( a rb . uni t)
Wavelength (nm)
Figure 4.2 (A) The wavelength distribution of the region shown in figure
4.1. (B) The PL spectrum of the dotted circle and the
reflectivity spectrum of the region.
I
II
III
@0.8E
th@1.1E
th@1.3E
th@1.4E
th10μm
Figure 4.3 Emission images of multiple laser spots under four different
pumping energy of 0.8 E
th, 1.1E
th, 1.3E
th, and 1.4E
th.
77
430 440 450 460
0.0 20.0k 40.0k 60.0k 80.0k 100.0k
@ 1.4Eth
@ 1.3Eth
@ 1.1Eth
@ 0.8Eth
~447.3 nm
~446.6 nm
~445.2 nm
In te n s ity (a rb . u n it)
Wavelength (nm)
(I)
(II)
(III) (I)
Figure 4.4 Emission spectra of multiple laser spots under four different
pumping energy of 0.8 E
th, 1.1E
th, 1.3E
th, and 1.4E
th.
5μm
1.1μm
5μm
1.1μm
4.4μm
5μm
4.4μm 4.4μm
5μm
4.4μm
5μm
4.4μm 4.4μm 4.4μm
5μm
6.5μm
4μm 6.5μm
4μm 6.5μm
4μm
5μm
Figure 4.5 Four observed lasing emission images and intensity
distributions simialr to TEM
00, TEM
01, TEM
02and TEM
11in
the four different regions.
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