Figure 2.15 The light-in light-out curve (L-L curve) from the microdisk laser.
0.02 0.03 0.04 0.05 0
100 200 300 400 500
Intensi ty (a.u.)
pump power density ( kW/cm 2 )
mode A (377nm) mode B (379nm)
0.02 0.03 0.04 0.05 0
100 200 300 400 500
Intensi ty (a.u.)
pump power density ( kW/cm 2 )
mode A (377nm)
mode B (379nm)
52
To understand more details of the lasing mode of the GaN microdisk, the finite-difference time-domain method (FDTD) with the effective index was used to perform the simulation for this 4.7 μm microdisk cavity.
Figure 2.16 provides the comparison between the simulation and measurement. The blue curve is the measured spectrum, and the red curve is the simulated spectrum form FDTD. The four resonant modes (mode A, B, C and D) are all match well to the high response modes from the simulation. Top two figures of Figure 2.17 are the calculated Hz mode profiles of the lasing modes (mode A and B) at 377 nm and 379 nm wavelength from top view. Two lasing modes, mode A and mode B were verified to be the first-order and third-order whispering-gallery modes from the FDTD calculated mode profiles.
Since the higher-order whispering-gallery mode usually has a lower Q value, the mode B has a higher threshold power density which is observed from the L-L curves in Figure 2.15.
In the GaN-based microdisk cavity, the UV DBR plays an important role to select the lasing wavelength regime. The gain peak of the GaN without the DBR is around 360 nm wavelength, however the lasing and resonant modes are around 377 nm wavelength. This 17 nm difference in wavelength is attributed to the reflection of the UV DBR mirror on the bottom of the microdisk cavity as shown in Figure 2.18. The region of the resonant modes from the cavity also agrees to the bandwidth of the reflectivity spectrum of the DBR in Figure 2.8. This DBR effect in the GaN expitaxial structure had been observed in our previous works [24].
The UV DBR also reduces the number of resonant modes in microdisk cavity because of the limit bandwidth. The reduction of mode number will decrease the energy waste in the non-lasing modes. Therefore a high efficient GaN microdisk laser can be expected.
For future applications in integrated circuits, this cavity integrated with the DBR structure opens a possibility to tune or select wavelength regime for compact lasers.
Figure 2.16 Comparison of FDTD simulation (red curve) and measurement (blue curve).
54
Figure 2.17 (a) The calculated Hz mode profiles for mode A and (b)
mode B from FDTD.
Mode A
(a)
Mode B
(b)
Mode A
(a)
Mode B
(b)
Figure 2.18 17 nm difference in wavelength due to the reflection of the UV DBR.
56
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