Lasing Characteristics of Metal-coated GaN Nanoring

To investigate the performance of our metal-coated GaN nanoring device, we use the micro-Photoluminescence system mentioned in chapter 2 to measure the lasing characteristics of our devices. We directly pump our device at room temperature, trying to avoid absorption loss from the undoped GaN layer which is beneath our nanoring structure.

In our experiment, there are three different sizes of nanoring structure: 7μm, 5μm, and 3μm in diameter. Fig. 4.2.1 to Fig. 4.2.3 shows the lasing characteristics of metal-coated nanorings with different diameter. The smallest nano structure we observe lasing action is the nanoring structure with 3μm in diameter, 600nm in height, and 310nm in width. The width of this structure is below the nature wavelength of the lasing mode. Due to better control of dry etching processes, there is no SiO2 layer between metal and gain medium for nanoring structure, trying to utilize the advantages of surface plasmon effect as much as possible.

Following paragraphs are the lasing characteristics of our nanoring devices.

For metal-coated GaN nanoring with 7μm in diameter, we observe a lasing action with lasing wavelength around 365nm, which is shown in Fig. 4.2.1 (a). The spectrum above (red) and below (black) the threshold for our nano structure clearly indicate that we observe a lasing action. From Fig, 4.2.1 (b), the threshold pump power density is 0.019 kW/cm² (19mJ/cm²) obtained from the experimental result. Moreover, the quality factor estimated by the ratio of wavelength to linewidth around the transparency is about 910. We believe this high quality factor could attribute to the combination of whispering-gallery mode and surface plasmon mode. This is the reason why we could still observe lasing action even though we shrink the size of our device to a smaller scale.

We further shrink the diameter of the nanoring to 5μm, and the width of the nanoring to 340nm. The width of the nanoring is smaller than the lasing wavelength. The lasing

characteristics are shown in Fig. 4.2.2. The lasing peak is around 363nm, as we could see that the spectrum above and below the threshold for this structure is quite different. For this case, the threshold pump power density is about 0.042kW/cm² (42mJ/cm²), and the quality factor is about 910, which is quite the same as the result of metal-coated GaN nanoring in the previous paragraph. Clear turn-on behavior of L-L curve and the narrowing linewidth of the lasing peak after turn-on both indicate the lasing action at room temperature.

At last, the smallest nanoring structure which we could observe lasing action at room temperature is 3μm in diameter, 600nm in height, and 310nm in width. It is the best result we could obtain under room temperature, pulsed condition. For this case, the lasing wavelength is 364nm and the quality factor is about 800. The difference in quality factor of our device might be due to the smaller device, the stronger interaction between metal and gain medium is under such a small scale. The threshold pump power density is around 0.045kW/cm² (45mJ/cm²) estimated from the L-L curve shown in Fig. 4.2.3 (b). The increase in threshold power density might be due to the beam spot size of our micro-PL system is about 50μm, which is much larger than the nanoring device. Poor injection efficiency might be the reason for increasing threshold power when we have smaller device. Table 4.2 shows the lasing characteristics and the geometric parameter of our device.

4.3 Result and Discussion

Compare to the nanostripe structure, nanoring structure has a unique advantage that

utilize whispering-gallery mode which do not require other feedback structure to form the cavity [1]. Therefore, metal layer coated on the nanoring structure could be specifically used to reduce optical loss from the side of the nanoring structure and makes lasing action possible at room temperature. Higher quality factor is expected for nanoring structure compare to waveguide structure. Higher quality factor indicates that photon in this structure would stay longer and have higher chance to get enough gain from the gain medium to overcome the loss.

The experiment result of metal-coated GaN nanoring laser confirms our hypothesis. Even though both nano structures are benefit from better optical confinement and better thermal conductivity which make lasing action possible at room temperature, the advantages of whispering-gallery-mode make the device performance of nanoring better than nanostripe structure. Moreover, these advantages let us could shrink our device to smaller scale compare to the nanostripe structure. We could still observe lasing action at room temperature with the width of the nanoring down to subwavelength scale, which is not possible for nanostripe case.

Table 4.3 lists a comparison between nanostripe and nanoring structure.

As shown in the Table 4.3, the size of the nano structure has reduced significantly.

Compare to nanostripe structure, which has a size about 12.5μm³, the size of the smallest nanoring structure we could observe lasing action at room temperature is 1.7μm³, seven times smaller than the nanostripe structure. Moreover, the width of the nanoring has been shrunk down to 310nm, which is smaller than the lasing wavelength we observe. We could not

achieve lasing in such a thin structure in nanostripe case. We believe that the utilization of whispering-gallery mode would be the key reason for this result.

Moreover, compare to the lasing characteristics of these two devices, the nanoring structure has a lower threshold power density and higher quality factor. The quality factor is about 6 times larger for nanoring structures which are 5 or 7μm in diameter, and about 5.5 times for nanoring structure which is 3μm in diameter. The threshold power density is at least 20% lower for the nanoring structure compare to the nanostripe structure.

In conclusion, whispering gallery mode would be a better choice to further improve the performance of metal-coated nanocavity than fabry-perot oscillation from the waveguide structure.

4.3 Summary

Even though some research results about metal-clad microring laser already reported experimentally and theoretically [1,4], they only observed lasing action at 77K and the quality factor was about 160, which was quite similar to other structure reported previously [5-7].

The advantages of whispering-gallery mode weren’t fully utilized by them. Therefore, in our experiment, we demonstrate lasing in metal-coated GaN nanoring at room temperature. It is the first time in the world that demonstrates lasing in metal-coated nanoring cavity at room temperature with GaN-based material system with such a high quality factor.

In our experiment, we fabricate three different size of the device: 7μm, 5μm, and 3μm in

diameter. They all could observe lasing action at room temperature by optical pumping.

Besides, their quality factors are about 800 to 900, much higher than previous reported data.

The lasing wavelength of each device is 365nm, 363nm, and 364nm, and the threshold power density for each one is 0.019kW/cm² (19mJ/cm²), 0.042kW/cm² (42mJ/cm²), and 0.045kW/cm² (45mJ/cm²) respectively.

Compare to the nanostripe structure mentioned in the previous chapter, nanoring structure has higher quality factor and lower threshold power density than it. We could further shrink the volume of the device to one tenth the nanostripe structure, and the width of the nanoring is smaller than the lasing wavelength. We attribute this result to better feedback mechanism brought by nanoring structure. Moreover, from the experimental result, the red-shift of lasing peak wavelength of the reference sample compares to the metal-coated one which shows a steady blue-shift as pumping power increase indicates that metal could provide better thermal dissipation than the air, so the device’s performance won’t be influenced by heat even operate at high power density.

Fig. 4.1.1 Process Flow Chart for Metal-coated GaN Nanoring.

Fig. 4.1.2 SEM Image of Different Size of GaN Nanoring before Shielding Layer Deposition (a) 7μm in diameter. (b) 5μm in diameter. (c) 3μm in diameter.

Fig. 4.1.3 SEM image of metal-coated GaN nanoring, 7μm in diameter.

Fig. 4.1.4 Schematic Diagram of Metal-coated GaN Nanoring.

Fig. 4.2.1 Lasing Characteristics of Metal-coated GaN Nanoring with 7μm in diameter (a) PL Spectrum Above (Red) and Below (black) Threshold. (b) L-L Curve and the Linewidth of the

Lasing Peak.

Fig. 4.2.2 Lasing Characteristics of Metal-coated GaN Nanoring with 5μm in diameter (a) PL Spectrum Above (Red) and Below (black) Threshold. (b) L-L Curve and the Linewidth of the

Lasing Peak.

Fig. 4.2.3 Lasing Characteristics of Metal-coated GaN Nanoring with 3μm in diameter (a) PL Spectrum Above (Red) and Below (black) Threshold. (b) L-L Curve and the Linewidth of the

Lasing Peak.

Table 4.2 Lasing Characteristics of Different Metal-coated GaN Nanorings.

Table 4.3 Comparison between Metal-coated GaN Nanostripe and Nanoring.

References:

[1] M. W. Kim, and P. C. Ku, “Lasing in metal-clad microring resonator,” Appl. Phys. Lett., vol. 98, pp. 131107, 2011.

[2] C. Y. Lu, S. W. Chang, S. H. Yang, and S. L. Chuang, “Quantum-dot laser with a metal-coated waveguide under continuous-wave operation at room temperature,” Appl.

Phys. Lett., vol. 95, pp. 233507, 2009.

[3] C. Y. Lu, S. W. Chang, S. L. Chuang, T. D. Germann, and D. Bimberg, “Metal-cavity surface-emitting microlaser at room temperature,” Appl. Phys. Lett., vol. 96, pp. 251101, 2010.

[4] M. W. Kim, and P. C. Ku, “The metal-clad semiconductor nanoring laser and its scaling properties,” Opt. Express, vol. 19, pp. 3218, 2011.

Chapter 5 Conclusion 5.1 Conclusion

Lasing in metal-coated GaN nanocavity at room temperature is demonstrated. With the combination of GaN as gain medium and metal-coated nanocavity, lasing signal has been observed at room temperature in nanostripe and nanoring structure.

First, we observe a lasing peak with its wavelength is about 370nm and the quality factor is 150 for our metal-coated GaN nanostripe. It has a low threshold pump power density which is only 0.055kW/cm² (55mJ/cm²). Moreover, by finite element method and effective index method, we prove that the aluminum layer coated on the nanostripe provides a better optical confinement and better thermal conductivity makes lasing action possible at room temperature and single mode lasing has been confirmed by the band diagram of the nanostripe structure and all these simulation results fit to experimental result pretty with only a small difference between them.

Second, we conduct fabrication process to make metal-coated GaN nanoring laser. The smallest ring we could observe lasing action at room temperature is 3μm in diameter, and the widths of the nanorings is about 310nm, smaller than the lasing wavelength. The quality factor of this device is about 800 and the threshold power density is only 0.045kW/cm² (45mJ/cm²). Better performance and smaller device are obtained by the combination of nanoring structure and metal-coated nanocavity. The quality factor is improved and threshold

power density is smaller compare to metal-coated nanostripe. Also, better thermal stability of metal-coated device is confirmed by analyzing the PL spectrum of the devices. All these results shows promising way to further improve the device performance and shrink the size of device into subwavelength-scale.

5.2 Future Work

In order to achieve lasing in three-dimensional nanocavity or operate under electrically-pump condition, we have to reduce the optical loss in vertical direction. As shown in chapter 3, even though we could see clear standing wave pattern in the nanostripe structure, there is still some energy dissipate into the undoped GaN region beneath the structure. If we wanted to further improve device performance, we have to improve optical confinement vertically by heterostructure or Distributed Bragg reflector (DBR).

Moreover, poor injection efficiency of the measurement system might reduce the device performance. First, the beam spot size of Nd:YVO4 laser is about 50μm, which is much larger than our device, lots of energy is wasted and might be the reason why there is always a signal around 370nm to 380nm in our spectrum. Smaller beam spot size comparable to the size of our device would significantly increase injection efficiency and the threshold power density might be smaller than what we get right now. Second, if we could use bonding technique to remove the sapphire substrate and undoped GaN layer beneath the device, we could collect lasing signal efficiently. It is hard for us to observe lasing signal because we are collect its

signal from top of the wafer, where is coated a high-reflectivity metal layer.

All the issues mentioned in the previous paragraphs are the key points for further development of metal-coated nanocavity. If we could solve these problems, we could achieve lasing in three dimensional or subwavelength nanocavity and photonic integrated circuit in nano-scale in the very near future.

Appendix

1. Thermal Issue

The thermal conductivity of metal is better than the air, therefore, it is believed that the device with metal layer coated on it would show a better performance after reduce the thermal effect. In our cases, the aluminum layer’s thermal conductivity (237W/m-K) coated on the GaN nanocavities is far better than air (0.025W/m-K). We believe that this is also an important feature provided by metal so that we could observe lasing action at room temperature. From the lasing characteristics of GaN nanoring with and without metal, 5μm in diameter, we could observe the differences brought by metal layer.

From the PL spectrum, the lasing peak of the metal-coated nanolaser has a blue shift as the pumping power increase. However, for the GaN nanoring, 5μm in diameter, it has a red shift after a small blue shift as the pumping power increase, this indicates that the heat provided by pumping source influence the device performance. We believe that it is the metal layer coated on the nanoring structure passivates the heat provided by the pumping source, so that the device performance won’t be affected by it. Fig. A. 1 shows the lasing peak wavelength of these two devices after turn on. This evidence shows that the metal layer coated on the nanocavity not only provide better optical confinement, but also better thermal dissipation that the device performance won’t be influenced by heat even operate at high carrier density.

Fig. A.1 The Lasing Peak Wavelength of GaN Nanoring Laser with (Red) or without (Black) Metal.