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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 21 — Oct. 10, 2011
  • pp: 20389–20394
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Lasing characteristics of an optically pumped single ZnO nanosheet

Kota Okazaki, Daisuke Nakamura, Mitsuhiro Higashihata, Palani Iyamperumal, and Tatsuo Okada  »View Author Affiliations


Optics Express, Vol. 19, Issue 21, pp. 20389-20394 (2011)
http://dx.doi.org/10.1364/OE.19.020389


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Abstract

We report the lasing characteristics of a single ZnO nanosheet optically pumped by ultraviolet laser beam. The ZnO nanosheets were synthesized by a carbothermal chemical vapor deposition method. The ZnO nanosheets dispersed on a silica glass substrate were excited by the third-harmonic of a Q-switched Nd:YAG laser (λ = 355 nm, τ = 5 ns) and photoluminescence from a single ZnO nanosheet was observed. The observed emission spectra showed the obvious lasing characteristics having modal structure and threshold characteristics. The threshold power for lasing was measured to be 50 kW/cm2, which was much lower than 150 kW/cm2, the threshold power of the reference ZnO nanowire. It indicates that the ZnO nanosheet is a superior gain medium for an ultraviolet laser. The oscillation mechanism inside a ZnO nanosheet is attributed to the micro-cavity effect, based on the three-dimensional laser-field simulation.

© 2011 OSA

1. Introduction

It has been shown that the laser action in the UV region takes place using the ZnO nanocrystals due to their excellent crystallinity. These reports show that ZnO nanocrystals can serve as a building block for the UV laser diode (LD) [7

7. E. S. P. Leong, S. F. Yu, and S. P. Lau, “Directional edge-emitting UV random laser diodes,” Appl. Phys. Lett. 89(22), 221109 (2006). [CrossRef]

,8

8. S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett. 93(18), 181106 (2008). [CrossRef]

]. However, in most of these studies, the lasing characteristics have been investigated by exciting a great number of nanocrystals and by observing the light from a large number of nanocrystals. Based on these observation, the lasing mechanism has been attributed to the micro-cavity effect in nanowires [1

1. R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process. 93(4), 843–847 (2008). [CrossRef]

,2

2. R. Q. Guo, M. Matsumoto, T. Matsumoto, M. Higashihata, D. Nakamura, and T. Okada, “Aligned growth of ZnO nanowires by NAPLD and their optical characterizations,” Appl. Surf. Sci. 255(24), 9671–9675 (2009). [CrossRef]

,9

9. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]

13

13. C. Czekalla, T. Nobis, A. Rahm, B. Cao, J. Zúñiga-Pérez, C. Sturm, R. Schmidt-Grund, M. Lorenz, and M. Grundmann, “Whispering gallery modes in zinc oxide micro- and nanowires,” Phys. Status Solidi B 247(6), 1282–1293 (2010). [CrossRef]

], nanosheets [5

5. F. Wang, R. Liu, A. Pan, L. Cao, K. Cheng, B. Xue, G. Wang, Q. Meng, J. Li, Q. Li, Y. Wang, T. Wang, and B. Zou, “The optical properties of ZnO sheets electrodeposited on ITO glass,” Mater. Lett. 61(10), 2000–2003 (2007). [CrossRef]

,6

6. E. S. Jang, X. Chen, J. H. Won, J. H. Chung, D. J. Jang, Y. W. Kim, and J. H. Choy, “Soft-solution route to ZnO nanowall array with low threshold power density,” Appl. Phys. Lett. 97(4), 043109 (2010). [CrossRef]

] and random lasing by nanorod arrays [14

14. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G.-C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004). [CrossRef]

], nanoparticles [15

15. E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater. (Deerfield Beach Fla.) 18(13), 1685–1688 (2006). [CrossRef]

] and thin films [16

16. Y. T. Shih, C. Y. Chiu, C. W. Chang, J. R. Yang, M. Shiojiri, and M. J. Chen, “Stimulated emission in highly (0001)-oriented ZnO films grown by atomic layer deposition on the amorphous glass substrates,” J. Electrochem. Soc. 157(9), H879–H883 (2010). [CrossRef]

]. In order to understand the lasing mechanism as a building block for the laser devises, it is essential to investigate the lasing characteristics of a single nanocrystal.

Compared to the ZnO nanowire, there are only a few reports on lasing from ZnO nanosheets [5

5. F. Wang, R. Liu, A. Pan, L. Cao, K. Cheng, B. Xue, G. Wang, Q. Meng, J. Li, Q. Li, Y. Wang, T. Wang, and B. Zou, “The optical properties of ZnO sheets electrodeposited on ITO glass,” Mater. Lett. 61(10), 2000–2003 (2007). [CrossRef]

,6

6. E. S. Jang, X. Chen, J. H. Won, J. H. Chung, D. J. Jang, Y. W. Kim, and J. H. Choy, “Soft-solution route to ZnO nanowall array with low threshold power density,” Appl. Phys. Lett. 97(4), 043109 (2010). [CrossRef]

]. In these reports, an ensemble of ZnO nanosheets on a substrate was totally examined by photoluminescence (PL) method, and the onset of the stimulated emission was reported, based on the observation of the spectral narrowing in the PL spectra as the excitation intensity increased. It is worth noting that a considerably low threshold (25 kW/cm2) of ZnO nanosheets optically pumped by a Q-switched YAG laser (λ = 355 nm, τ = 6 ns) was reported, and then WGM-like lasing mechanism was also suggested [6

6. E. S. Jang, X. Chen, J. H. Won, J. H. Chung, D. J. Jang, Y. W. Kim, and J. H. Choy, “Soft-solution route to ZnO nanowall array with low threshold power density,” Appl. Phys. Lett. 97(4), 043109 (2010). [CrossRef]

]. However, lasing characteristics on a single ZnO nanosheet have not been investigated so far. Further investigations of lasing characteristics and clarification of lasing mechanisms on a single ZnO nanosheet are required in order to realize the more efficient application to UV LD.

2. Synthesis of ZnO nanocrystals and photoluminescence measurement

ZnO nanocrystals were synthesized by a carbothermal CVD method where the mixed powder of ZnO and graphite was used as a source material. ZnO nanocrystals were deposited on a silicon substrate on which a thin gold film with a thickness of 1.1 nm was deposited as a catalyst promoting the synthesis of ZnO nanocrystals. The mixed powder of ZnO and graphite was placed in an alumina boat, and then the silicon substrate was placed 10 mm above the source. During the deposition, mixed gases of argon (Ar) and oxygen (O2) were flowed at a flow rate of 100 sccm and 3 sccm, respectively.

Typical scanning electron microscope (SEM) (KEYENCE, VE-7800S) images of ZnO nanocrystals are shown in Fig. 1
Fig. 1 SEM images of (a) ZnO nanowires and (b) nanosheets. (c) TEM image and selected area electron diffraction (SAED) pattern of the ZnO nanosheet.
. Depending on the deposition conditions, nanowires or nanosheets were synthesized as shown in Figs. 1 (a) and (b), respectively. The nanowires were obtained at a background gas pressure of 100 Torr and a furnace temperature of 1000 °C, while the nanosheets were obtained at a background gas pressure of 300 Torr and a furnace temperature of 1100 °C. The diameter of the nanowires was about 100~500 nm and the length reached about 10~50 µm. The thicknesses of the nanosheets were in the range from about 100 nm to 500 nm. The ZnO nanosheet was observed by a transmission electron microscopy (TEM) (JEOL, JEM-1300NEF) as shown in Fig. 1 (c). The ZnO nanosheet was single crystalline and the lattice spacing in the planar direction was 0.26 nm corresponding to the distance between (0002) planes.

The lasing characteristics of a single ZnO nanocrystal were examined as follows. The ZnO nanocrystals were taken out of the substrate by ultrasonic rinsing in ethanol, and then dispersed on a silica glass substrate. The ZnO nanocrystals were excited by the third-harmonic of a Q-switched Nd:YAG laser (λ = 355 nm, τ = 5 ns), which was 5 mm in diameter and injected through the substrate. The resultant fluorescence from a single nanocrystal was collected through an optical microscope with an objective lens with a magnification factor of 100, and then coupled to a spectrometer (focal length 25 cm, Lambda Vision, TC-2000) through a light fiber. The fluorescence spectrum was acquired by a charge coupled device (CCD) camera. The observation area of the fluorescence measurement system was about 15 μm in diameter.

3. Lasing characteristics of ZnO nanocrystals

3.1 Lasing in a ZnO nanowire

First, lasing in a single ZnO nanowire was examined. Figure 2(a)
Fig. 2 (a) CCD image and (b) AFM image of a single ZnO nanowire lying on a silica glass substrate. (c) CCD image of the optically pumped nanowire, and PL from the red dotted circle was observed by the spectrometer. (d) PL spectra from the single ZnO nanowire for the different excitation power densities. (e) Plotted-peak intensities as a function of the excitation power density at 389.3 nm in Fig. 2 (d). The lasing threshold was estimated to be about 150 kW/cm2.
shows an optical microscope image of a single ZnO nanowire on a silica glass substrate. The nanowire was observed by an atomic force microscope (AFM) (KEYENCE, VN-8000M/8010M) as shown in Fig. 2(b). The diameter and the length were 400 nm and 15 µm, respectively. Figure 2(c) shows the nanowire excited by the third-harmonic of the Q-switched Nd:YAG laser beam when the excitation power density was 450 kW/cm2. The fluorescence light from one end of the nanowire, as marked with a red dotted circle in Fig. 2(c), was observed by the spectrometer.

These observations clearly indicate that the lasing took place within the single ZnO nanowire, due to its micro-cavity effect. The mode spacing in a FP cavity is given by Δλ = λ2[2L(n-λ∙dn/dλ)]−1, where L is the cavity length, n is the refractive index at the wavelength of λ, and dn/dλ indicates the dispersion of light [18

18. A. E. Siegman, Lasers (University Science Books, 1986).

]. The refractive index and the length of the ZnO nanowire are n = 2.4 at λ = 389 nm and L = 15 µm, respectively. With Δλ = 0.78 nm as shown in Fig. 2 (d), the dispersion of light is calculated to be dn/dλ ≈-0.010 nm−1, which is in reasonable agreement with the value of dn/dλ ≈-0.012 nm−1 reported by M. A. Zimmler et al. [11

11. M. A. Zimmler, F. Capasso, S. Muller, and C. Ronning, “Optically pumped nanowire lasers: invited review,” Semicond. Sci. Technol. 25(2), 024001 (2010). [CrossRef]

]. Therefore, it is concluded that the cavity is formed by the FP cavity formed by two ends of the nanowire in the present experiment.

From the AFM result, the diameter of the nanowire was about 400 nm. When the nanowire is regarded as a cylindrical waveguide surrounded by air for simplicity, the normalized frequency V which is an indicator to determine the propagation modes inside the nanowire is given by V = 2πa(nZnO2-nair2)1/2/λ, where a is the radius of the nanowire. The refractive indices of air and ZnO at λ = 389 nm are nair = 1.0 and nZnO = 2.4 [19

19. S. Adachi, Optical Constants of Crystalline and Amorphous Semiconductors: Numerical Data and Graphical Information (Kluwer Academic, 1999), Chap. D2.

], respectively. Thus, the normalized frequency of the ZnO nanowire in Fig. 2 (b) is calculated to be V = 7.0, and it indicates that a number of modes exist inside the nanowire [11

11. M. A. Zimmler, F. Capasso, S. Muller, and C. Ronning, “Optically pumped nanowire lasers: invited review,” Semicond. Sci. Technol. 25(2), 024001 (2010). [CrossRef]

,20

20. M. K. Barnovski, Fundamentals of Optical Fiber Communications (Academic, 1981).

]. However, since the mode structure in Fig. 2 (d) was regular, the lasing took place only on the fundamental mode, and the effective refractive index (neff) is close to the refractive index of the material.

3.2 Lasing in a ZnO nanosheet

For the sake of simplicity, the nanosheet can be regarded as a slab waveguide with a thickness of t = 150 nm surrounded by air due to a gap between the nanosheet and the substrate because the nanosheet was simply dispersed without any adhesions. In the slab waveguide, the relation between t/λ and neff is expressed as t/λ = [2 tan−121) + mπ]/(2πα1), where m is an integer, α1 = (nZnO2- neff2)1/2 and α2 = (neff2-nair2)1/2. TEm and TMm modes exist inside the slab where the mode number corresponds to the integer m in the above equation. Since the thickness-wavelength ratio was estimated to be about t/λ = 0.39, multi-mode oscillation of TE0, TE1, TM0, and TM1 can propagate inside the nanosheet.

For further consideration, electrical-field propagations inside the nanosheet were simulated by a high-frequency structure simulator (Ansoft, HFSS ver. 11). A single ZnO nanosheet was prepared on the x-y plane as shown in Fig. 4
Fig. 4 Simulation on electrical-field propagation inside a ZnO nanosheet placed on the x-y plane. The nanosheet has a thickness of 150 nm and the tip size of 100 nm. Incident light was planar wave with the wavelength of 385 nm linearly polarized in the z-axis. The light absorption was neglected for the observation of the propagation.
, which corresponded to the nanosheet in Fig. 3. The nanosheet has the size of 150 nm in thickness and 100 nm at the tip according to the AFM result. Incident light was planar wave with the wavelength of 385 nm linearly polarized in the z-axis, and those were injected from the bottom side to the tip side of the nanosheet. The absorption of the light was neglected for the observation of the propagation inside the nanosheet, surrounded by air.

According to the simulation results, the incident light propagates inside the nanosheet by the total reflection at the boundary of ZnO and air, and then, the reflection at the tip due to the narrow waveguide behavior was also observed. Therefore, it is considered that oscillation routes inside the nanosheet would be intricately formed by a FP type resonator between both ends with the help of total reflection at the lateral sides. It is also worth while noting that the present triangular-shaped nanosheet will be very useful as a UV nano-light source, which can be coupled from the tip.

4. Summary

Acknowledgments

The authors would like to thank Dr. T. Daio in the research laboratory for high voltage electron microscopy in Kyushu Univ. for his assistance in the experiments. A part of this work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS, No. 20360142) and Special Coordination Funds for Promoting Science and Technology from Japan Science and Technology Agency are also acknowledged.

References and links

1.

R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process. 93(4), 843–847 (2008). [CrossRef]

2.

R. Q. Guo, M. Matsumoto, T. Matsumoto, M. Higashihata, D. Nakamura, and T. Okada, “Aligned growth of ZnO nanowires by NAPLD and their optical characterizations,” Appl. Surf. Sci. 255(24), 9671–9675 (2009). [CrossRef]

3.

J. H. Park and J. G. Park, “Synthesis of ultrawide ZnO nanosheets,” Curr. Appl. Phys. 6(6), 1020–1023 (2006). [CrossRef]

4.

L. Xu, Y. Guo, Q. Liao, J. Zhang, and D. Xu, “Morphological control of ZnO nanostructures by electrodeposition,” J. Phys. Chem. B 109(28), 13519–13522 (2005). [CrossRef] [PubMed]

5.

F. Wang, R. Liu, A. Pan, L. Cao, K. Cheng, B. Xue, G. Wang, Q. Meng, J. Li, Q. Li, Y. Wang, T. Wang, and B. Zou, “The optical properties of ZnO sheets electrodeposited on ITO glass,” Mater. Lett. 61(10), 2000–2003 (2007). [CrossRef]

6.

E. S. Jang, X. Chen, J. H. Won, J. H. Chung, D. J. Jang, Y. W. Kim, and J. H. Choy, “Soft-solution route to ZnO nanowall array with low threshold power density,” Appl. Phys. Lett. 97(4), 043109 (2010). [CrossRef]

7.

E. S. P. Leong, S. F. Yu, and S. P. Lau, “Directional edge-emitting UV random laser diodes,” Appl. Phys. Lett. 89(22), 221109 (2006). [CrossRef]

8.

S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett. 93(18), 181106 (2008). [CrossRef]

9.

M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]

10.

L. K. van Vugt, S. Rühle, and D. Vanmaekelbergh, “Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire,” Nano Lett. 6(12), 2707–2711 (2006). [CrossRef] [PubMed]

11.

M. A. Zimmler, F. Capasso, S. Muller, and C. Ronning, “Optically pumped nanowire lasers: invited review,” Semicond. Sci. Technol. 25(2), 024001 (2010). [CrossRef]

12.

G. P. Zhu, C. X. Xu, J. Zhu, C. G. Lv, and Y. P. Cui, “Two-photon excited whispering-gallery mode ultraviolet laser from an individual ZnO microneedle,” Appl. Phys. Lett. 94(5), 051106 (2009). [CrossRef]

13.

C. Czekalla, T. Nobis, A. Rahm, B. Cao, J. Zúñiga-Pérez, C. Sturm, R. Schmidt-Grund, M. Lorenz, and M. Grundmann, “Whispering gallery modes in zinc oxide micro- and nanowires,” Phys. Status Solidi B 247(6), 1282–1293 (2010). [CrossRef]

14.

S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G.-C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004). [CrossRef]

15.

E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater. (Deerfield Beach Fla.) 18(13), 1685–1688 (2006). [CrossRef]

16.

Y. T. Shih, C. Y. Chiu, C. W. Chang, J. R. Yang, M. Shiojiri, and M. J. Chen, “Stimulated emission in highly (0001)-oriented ZnO films grown by atomic layer deposition on the amorphous glass substrates,” J. Electrochem. Soc. 157(9), H879–H883 (2010). [CrossRef]

17.

J. C. Ryan and T. L. Reinecke, “Band-gap renormalization of optically excited semiconductor quantum wells,” Phys. Rev. B Condens. Matter 47(15), 9615–9620 (1993). [CrossRef] [PubMed]

18.

A. E. Siegman, Lasers (University Science Books, 1986).

19.

S. Adachi, Optical Constants of Crystalline and Amorphous Semiconductors: Numerical Data and Graphical Information (Kluwer Academic, 1999), Chap. D2.

20.

M. K. Barnovski, Fundamentals of Optical Fiber Communications (Academic, 1981).

OCIS Codes
(140.3610) Lasers and laser optics : Lasers, ultraviolet
(140.5960) Lasers and laser optics : Semiconductor lasers
(160.6000) Materials : Semiconductor materials
(300.6540) Spectroscopy : Spectroscopy, ultraviolet
(160.4236) Materials : Nanomaterials

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: July 6, 2011
Revised Manuscript: September 21, 2011
Manuscript Accepted: September 21, 2011
Published: October 3, 2011

Citation
Kota Okazaki, Daisuke Nakamura, Mitsuhiro Higashihata, Palani Iyamperumal, and Tatsuo Okada, "Lasing characteristics of an optically pumped single ZnO nanosheet," Opt. Express 19, 20389-20394 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-21-20389


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References

  1. R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process.93(4), 843–847 (2008). [CrossRef]
  2. R. Q. Guo, M. Matsumoto, T. Matsumoto, M. Higashihata, D. Nakamura, and T. Okada, “Aligned growth of ZnO nanowires by NAPLD and their optical characterizations,” Appl. Surf. Sci.255(24), 9671–9675 (2009). [CrossRef]
  3. J. H. Park and J. G. Park, “Synthesis of ultrawide ZnO nanosheets,” Curr. Appl. Phys.6(6), 1020–1023 (2006). [CrossRef]
  4. L. Xu, Y. Guo, Q. Liao, J. Zhang, and D. Xu, “Morphological control of ZnO nanostructures by electrodeposition,” J. Phys. Chem. B109(28), 13519–13522 (2005). [CrossRef] [PubMed]
  5. F. Wang, R. Liu, A. Pan, L. Cao, K. Cheng, B. Xue, G. Wang, Q. Meng, J. Li, Q. Li, Y. Wang, T. Wang, and B. Zou, “The optical properties of ZnO sheets electrodeposited on ITO glass,” Mater. Lett.61(10), 2000–2003 (2007). [CrossRef]
  6. E. S. Jang, X. Chen, J. H. Won, J. H. Chung, D. J. Jang, Y. W. Kim, and J. H. Choy, “Soft-solution route to ZnO nanowall array with low threshold power density,” Appl. Phys. Lett.97(4), 043109 (2010). [CrossRef]
  7. E. S. P. Leong, S. F. Yu, and S. P. Lau, “Directional edge-emitting UV random laser diodes,” Appl. Phys. Lett.89(22), 221109 (2006). [CrossRef]
  8. S. Chu, M. Olmedo, Z. Yang, J. Kong, and J. Liu, “Electrically pumped ultraviolet ZnO diode lasers on Si,” Appl. Phys. Lett.93(18), 181106 (2008). [CrossRef]
  9. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science292(5523), 1897–1899 (2001). [CrossRef] [PubMed]
  10. L. K. van Vugt, S. Rühle, and D. Vanmaekelbergh, “Phase-correlated nondirectional laser emission from the end facets of a ZnO nanowire,” Nano Lett.6(12), 2707–2711 (2006). [CrossRef] [PubMed]
  11. M. A. Zimmler, F. Capasso, S. Muller, and C. Ronning, “Optically pumped nanowire lasers: invited review,” Semicond. Sci. Technol.25(2), 024001 (2010). [CrossRef]
  12. G. P. Zhu, C. X. Xu, J. Zhu, C. G. Lv, and Y. P. Cui, “Two-photon excited whispering-gallery mode ultraviolet laser from an individual ZnO microneedle,” Appl. Phys. Lett.94(5), 051106 (2009). [CrossRef]
  13. C. Czekalla, T. Nobis, A. Rahm, B. Cao, J. Zúñiga-Pérez, C. Sturm, R. Schmidt-Grund, M. Lorenz, and M. Grundmann, “Whispering gallery modes in zinc oxide micro- and nanowires,” Phys. Status Solidi B247(6), 1282–1293 (2010). [CrossRef]
  14. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G.-C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett.84(17), 3241–3243 (2004). [CrossRef]
  15. E. S. P. Leong and S. F. Yu, “UV random lasing action in p-SiC(4H)/i-ZnO–SiO2 nanocomposite/n-ZnO:Al heterojunction diodes,” Adv. Mater. (Deerfield Beach Fla.)18(13), 1685–1688 (2006). [CrossRef]
  16. Y. T. Shih, C. Y. Chiu, C. W. Chang, J. R. Yang, M. Shiojiri, and M. J. Chen, “Stimulated emission in highly (0001)-oriented ZnO films grown by atomic layer deposition on the amorphous glass substrates,” J. Electrochem. Soc.157(9), H879–H883 (2010). [CrossRef]
  17. J. C. Ryan and T. L. Reinecke, “Band-gap renormalization of optically excited semiconductor quantum wells,” Phys. Rev. B Condens. Matter47(15), 9615–9620 (1993). [CrossRef] [PubMed]
  18. A. E. Siegman, Lasers (University Science Books, 1986).
  19. S. Adachi, Optical Constants of Crystalline and Amorphous Semiconductors: Numerical Data and Graphical Information (Kluwer Academic, 1999), Chap. D2.
  20. M. K. Barnovski, Fundamentals of Optical Fiber Communications (Academic, 1981).

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