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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 11 — May. 21, 2012
  • pp: 12457–12462
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Lasing action in gallium nitride quasicrystal nanorod arrays

Shih-Pang Chang, Kuok-Pan Sou, Chieh-Han Chen, Yuh-Jen Cheng, Ji-Kai Huang, Chung-Hsiang Lin, Hao-Chung Kuo, Chun-Yen Chang, and Wen-Feng Hsieh  »View Author Affiliations


Optics Express, Vol. 20, Issue 11, pp. 12457-12462 (2012)
http://dx.doi.org/10.1364/OE.20.012457


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Abstract

We report the observation of lasing action from an optically pumped gallium nitride quasicrystal nanorod arrays. The nanorods were fabricated from a GaN substrate by patterned etching, followed by epitaxial regrowth. The nanorods were arranged in a 12-fold symmetric quasicrystal pattern. The regrowth grew hexagonal crystalline facets and core-shell multiple quantum wells (MQWs) on nanorods. Under optical pumping, multiple lasing peaks resembling random lasing were observed. The lasing was identified to be from the emission of MQWs on the nanorod sidewalls. The resonant spectrum and mode field of the 12-fold symmetric photonic quasicrystal nanorod arrays is discussed.

© 2012 OSA

1. Introduction

There have been great research interests in gallium nitride material due to its promising applications in UV to blue optoelectronic devices. Conventionally, the devices are built in two-dimensional thin film structure, where emission sources are from the planar quantum wells. Recently, devices with one-dimensional nanostructure have gained substantial attention for their interesting properties and potential applications [1

1. F. Qian, S. Gradečak, Y. Li, C.-Y. Wen, and C. M. Lieber, “Core/Multishell nanowire heterostructures as multicolor, high-efficiency light-emitting diodes,” Nano Lett. 5(11), 2287–2291 (2005). [CrossRef] [PubMed]

3

3. H. Sekiguchi, K. Kishino, and A. Kikuchi, “Emission color control from blue to red with nanocolumn diameter of InGaN/GaN nanocolumn arrays grown on same substrate,” Appl. Phys. Lett. 96(23), 231104 (2010). [CrossRef]

]. The one-dimensional structure can be fabricated by top-down patterned etching or bottom-up self assembled growth. The fabricated nanostructures have shown quantum confinement effect and interesting light emission properties [4

4. H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, “Structural and optical properties of GaN nanocolumns grown on (0001) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy,” J. Cryst. Growth 300(1), 259–262 (2007). [CrossRef]

6

6. Y. Kawakami, S. Suzuki, A. Kaneta, M. Funato, A. Kikuchi, and K. Kishino, “Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns,” Appl. Phys. Lett. 89(16), 163124 (2006). [CrossRef]

]. Stimulated emission from single free standing lying nano wire and 2-D periodic nanorod arrays have been observed [7

7. S. Gradečak, F. Qian, Y. Li, H.-G. Park, and C. M. Lieber, “GaN nanowire lasers with low lasing thresholds,” Appl. Phys. Lett. 87(17), 173111 (2005). [CrossRef]

,8

8. T. Kouno, K. Kishino, K. Yamano, and A. Kikuchi, “Two-dimensional light confinement in periodic InGaN/GaN nanocolumn arrays and optically pumped blue stimulated emission,” Opt. Express 17(22), 20440–20447 (2009). [CrossRef] [PubMed]

], where lasing modes are the Fabry-Perot modes of the wire end surfaces or photonic crystal band edge modes. Recently, lasing action due to the feed back from the multiple scatterings of optically pumped disordered nanorods was also reported [9

9. M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, and K. Kishino, “Random laser action in GaN nanocolumns,” Appl. Phys. Lett. 97(15), 151109 (2010). [CrossRef]

]. This phenomenon is called random lasing [10

10. H. Cao, “Review on latest developments in random lasers with coherent feedback,” J. Phys. A 38(49), 10497–10535 (2005). [CrossRef]

], which is often analyzed in terms of statistics.

The statistical nature of random lasing is interesting. However, it puts a limitation to device applications due to the lack of control on the lasing modes and frequency locations from sample to sample. Recently, there are great interests in studying photonic quasicrystals, which may look random at first glance yet have well defined patterns. Their optical properties are fascinating and lie somewhere between those of periodic and random structures [11

11. Y. Lai, Z.-Q. Zhang, C.-H. Chan, and L. Tsang, “Anomalous properties of the band-edge states in large two-dimensional photonic quasicrystals,” Phys. Rev. B 76(16), 165132 (2007). [CrossRef]

14

14. K. Nozaki and T. Baba, “Lasing Characteristics of 12-Fold Symmetric Quasi-periodic Photonic Crystal Slab Nanolasers,” Jpn. J. Appl. Phys. 45(8A), 6087–6090 (2006). [CrossRef]

]. It therefore may provide a way to design a pseudo random laser with deterministic properties, which could be important for practical applications. Here we report the observation of lasing action from room temperature optically pumped GaN quasicrystal nanorod arrays. The lasing phenomenon resembles that of a random laser.

The quasicrystal nanorod array sample was fabricated from a GaN epitaxial wafer by nano imprint patterned etch, followed by epitaxial regrowth. The imprint was a 12-fold symmetric quasicrystal pattern [12

12. M. E. Zoorob, M. D. B. Charlton, G. J. Parker, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature 404(6779), 740–743 (2000). [CrossRef] [PubMed]

]. The regrowth formed hexagonal facets on the nanorod sidewalls and hexagonal pyramids on the top. The regrowth also grew InGaN/GaN MQWs on the sidewalls and pyramid facets. The use of MQWs allows us to investigate the optical properties of QPC at a designed wavelength. Under optical pumping, multiple lasing peaks were excited. The lasing was identified to be from the MQWs on the nanorod sidewalls. The peak distribution did not show obvious regularity and behaved like a random lasing action. The linewidth of laser peaks was in the range of 0.2-0.3 nm, indicating a strong resonant feedback oscillation. The lasing threshold pump intensity decreases with increasing pump area. The lasing mechanism is attributed to the feedback of close loop multiple scatterings among nanorods.

2. Fabrication and measurements

The fabrication process of nanorods is as follows. A 0.5 μm SiO2 thin film was deposited by plasma-enhanced chemical-vapor deposition on a 3 μm GaN substrate grown by metal organic chemical vapor deposition (MOCVD) on a c-plane sapphire template. The SiO2 thin film was fabricated into circular disks arranged in a 12-fold symmetric quasicrystal pattern by nano imprint lithography. The quasicrystal pattern is a two-dimensional pattern filled with squares and equilateral triangles whose edges have the same length (superlattice constant a), as shown in Fig. 1(a)
Fig. 1 (a) 12 fold symmetric quasicrystal pattern. (b) SEM plane-view image of the fabricated GaN quasicrystal nanorod arrays. The upper right inset image shows the crystalline facets.
. The distance a between every two nearest neighboring points is the same. The circular disks were placed at vortices. Such a pattern does not have translational symmetry but has long-range order and scaling similarity. The SiO2 disks were used as hard masks in inductively coupled plasma reactive ion etching (ICP-RIE), which etched the exposed GaN and formed GaN nanorods. The height of nanorods was about 1 μm. The SiO2 disks were subsequently removed by buffer oxide etching. The fabricated GaN nanorods were put back to MOCVD regrowth. Crystalline facets were formed on the etched nanorod surfaces. Multiple InGaN/GaN QWs were subsequently grown on the crystalline facets, leading to a core-shell nanorod structure. The plane-view SEM image of the fabricated nanorods is shown in Fig. 1(b), which shows the 12-fold symmetric quasicrystal pattern of nanorod arrays with superlattice constant a ~800 nm. The sidewall length of hexagonal nanorod is ~360 nm. The quasicrystal pattern shown in Fig. 1(a) is overlaid on the SEM image to show its quasicrystal pattern. Despite the original cylindrical shape of nanorods created by ICP-RIE, crystalline facets were formed on the regrown nanorods, as shown in Fig. 1(b). The side walls are {10-10} M-planes and the facets of the hexagonal pyramid are {10-11} semi-polar planes, as shown in the upper right inset of Fig. 1(b).

The SEM cross-section image of nanorods is shown in Fig. 2(a)
Fig. 2 (a) GaN nanorod SEM side view. (b) Spatially integrated CL spectrum. (c)-(h) Spectrally resolved CL images showing the location dependent emission wavelength.
, which shows the pyramid tips and the crystalline sidewall facets. The total nanorod height is about 1.5 um. The SEM scattering electron detection was switched to cathodoluminescent (CL) detection under the same magnification. The spatially integrated CL spectrum of the cross section image is shown in Fig. 2(b). The first peak near 365 nm is the band edge emission of GaN. The large broad spectrum on the right is due to the emission from the InGaN/GaN core-shell MQWs surrounding the nanorod facets. The full width at half maximum linewidth (FWHM) is almost 100 nm. To analyze the emission locations of the broad spectrum, the spectrally resolved CL images were taken at several wavelengths, as shown in Fig. 2(c)-2(h). They show that the emission wavelength red shifts as the scanned location moves from the bottom toward top of nanorods. This red shifted emission is attributed to the increase of In incorporation in MQWs toward the top portion of nanorods. The CL cross-section images also show that the gain is mainly distributed over the wavelength range 460-480 nm, where CL intensity is high.

Discussion

We investigated the resonant modes of this pseudo random lasing by performing two-dimensional (2D) finite difference time domain (FDTD) simulation. The 2D model was chosen for simplicity and believed to be a reasonable approximation because the gain was predominantly distributed in 2D direction perpendicular to nanorod axis. The lasing mode field would therefore mainly follow the gain in the 2D direction. The finite nanorod length in the vertical direction will certainly modify the mode frequency and profile, but we consider it as a secondary effect. The simplified model is meant to explain qualitatively the observed lasing behavior. Figure 5(a)
Fig. 5 (a) The quasicrystal model used in FDTD simulation. (b)-(d) The resonant spectra obtained by launching broad band pulses at locations labeled correspondingly in Fig. 5(a)
shows the quasicrystal model used for simulation. The nanorod and quasicrystal pattern dimensions measured from SEM image were used to construct the model. The over all pattern size is about 16 μm across. The finite pattern size was used due to the limit of computation power and time. An optical pulse with a center wavelength at 470 nm and a linewidth of 30 nm was launched at a randomly chosen location near nanorod sidewall to simulate the emission from MQWs. The pulse spectral width was chosen according to the observed lasing spectral range. Both transverse electric and magnetic field were calculated. After a long enough propagation time, the shape of spectrum became steady. The spectrum contained distinct resonant peaks, which were the resonant modes excited by the broad-band pulse. The same calculation was repeated for different pulse launching locations. The quality factors of these modes are in the range of 500. The spectrum varied from location to location and had many or just a few resonant peaks. All these resonant peaks can be regarded as the potential lasing modes. For illustration purpose, three spectra were shown in Fig. 5(b)-5(d). The corresponding launching locations are the points labeled (b), (c), and (d) in Fig. 5(a). Points (b) and (c) are deliberately at locations away from center. Point (d) is at the center rod.

The calculated resonant spectra show irregular peak locations, similar to what was observed in the experiment. The calculated resonant peaks do not match to the observed spectrum in peak to peak locations. This can be attributed to the fact that the model used in simulation is an overly simplified approximation. The lasing peaks should be ideally deterministic and can be engineered by design since the QPC pattern is deterministic. This could be important in practical applications.

We then calculated the resonant mode profiles. It was calculated by choosing one of the peaks in the resonant spectrum and launching it at the same location. After propagating for a reasonable long time, a distinctive pattern was developed. We regarded it as the mode profile of the specific peak. Ideally, one can repeat the calculation for all the peaks to obtain all the mode profiles. We have calculated a few of them. The mode profiles could be rather irregular or have certain distinctive feature. For illustration purpose, the mode profiles of the resonant peaks labeled 6(a) and 6(b) in Fig. 5(b), and 6(c)
Fig. 6 (a)-(c) Mode field patterns correspond to the resonant peaks labeled as 6(a), 6(b), and 6(c) in Fig. 5.
in Fig. 5(c) are shown in Fig. 6(a)-6(c), respectively. The mode field in Fig. 6(a) shows that certain part of nanorods can couple together to provide guiding effect and form various propagation loops. Figure 6(b) on the other hand shows a more scattering like coupling pattern. Figure 6(c) is the field pattern for a source launched at the center rod. The mode field propagates outward and forms triangular shape resonant loops. These patterns show that the intricate property of quasicrystal structure has varieties of mode field patterns, which require a further investigation.

Summary

We have observed a lasing action in an optically pumped 12-fold symmetric quasicrystal nanorod arrays. The sample was fabricated from a GaN epitaxial substrate by nano patterned etching and epitaxial regrowth. The regrowth grew core-shell MQWs and crystalline facets on nanorods. Under optical pumping, multiple lasing peaks emerged from broad emission background. The irregular multiple lasing wavelengths and the inverse dependence of threshold pump intensity on pump spot area resembles the characteristics of random lasing. The irregularity of resonant peaks is qualitatively explained by a simplified FDTD simulation.

Acknowledgments

We thank the National Science Council of Taiwan for the support of this research under contract NSC 100-2112-M-001-021-MY3. We also would like to thank Prof. M. D. B. Charlton at University of Southampton, UK, for providing insightful discussions.

References and links

1.

F. Qian, S. Gradečak, Y. Li, C.-Y. Wen, and C. M. Lieber, “Core/Multishell nanowire heterostructures as multicolor, high-efficiency light-emitting diodes,” Nano Lett. 5(11), 2287–2291 (2005). [CrossRef] [PubMed]

2.

R. Chen, H. D. Sun, T. Wang, K. N. Hui, and H. W. Choi, “Optically pumped ultraviolet lasing from nitride nanopillars at room temperature,” Appl. Phys. Lett. 96(24), 241101 (2010). [CrossRef]

3.

H. Sekiguchi, K. Kishino, and A. Kikuchi, “Emission color control from blue to red with nanocolumn diameter of InGaN/GaN nanocolumn arrays grown on same substrate,” Appl. Phys. Lett. 96(23), 231104 (2010). [CrossRef]

4.

H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, “Structural and optical properties of GaN nanocolumns grown on (0001) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy,” J. Cryst. Growth 300(1), 259–262 (2007). [CrossRef]

5.

Y. Sun, Y.-H. Cho, H.-M. Kim, and T. W. Kang, “High efficiency and brightness of blue light emission from dislocation-free InGaN/GaN quantum well nanorod arrays,” Appl. Phys. Lett. 87(9), 093115 (2005). [CrossRef]

6.

Y. Kawakami, S. Suzuki, A. Kaneta, M. Funato, A. Kikuchi, and K. Kishino, “Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns,” Appl. Phys. Lett. 89(16), 163124 (2006). [CrossRef]

7.

S. Gradečak, F. Qian, Y. Li, H.-G. Park, and C. M. Lieber, “GaN nanowire lasers with low lasing thresholds,” Appl. Phys. Lett. 87(17), 173111 (2005). [CrossRef]

8.

T. Kouno, K. Kishino, K. Yamano, and A. Kikuchi, “Two-dimensional light confinement in periodic InGaN/GaN nanocolumn arrays and optically pumped blue stimulated emission,” Opt. Express 17(22), 20440–20447 (2009). [CrossRef] [PubMed]

9.

M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, and K. Kishino, “Random laser action in GaN nanocolumns,” Appl. Phys. Lett. 97(15), 151109 (2010). [CrossRef]

10.

H. Cao, “Review on latest developments in random lasers with coherent feedback,” J. Phys. A 38(49), 10497–10535 (2005). [CrossRef]

11.

Y. Lai, Z.-Q. Zhang, C.-H. Chan, and L. Tsang, “Anomalous properties of the band-edge states in large two-dimensional photonic quasicrystals,” Phys. Rev. B 76(16), 165132 (2007). [CrossRef]

12.

M. E. Zoorob, M. D. B. Charlton, G. J. Parker, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature 404(6779), 740–743 (2000). [CrossRef] [PubMed]

13.

L. Mahler, A. Tredicucci, F. Beltram, C. Walther, J. Faist, H. E. Beere, D. A. Ritchie, and D. S. Wiersma, “Quasi-periodic distributed feedback laser,” Nat. Photonics 4(3), 165–169 (2010). [CrossRef]

14.

K. Nozaki and T. Baba, “Lasing Characteristics of 12-Fold Symmetric Quasi-periodic Photonic Crystal Slab Nanolasers,” Jpn. J. Appl. Phys. 45(8A), 6087–6090 (2006). [CrossRef]

15.

A. L. Burin, H. Cao, and M. A. Ratner, “Understanding and control of random lasing,” Physica B 338(1-4), 212–214 (2003). [CrossRef]

16.

Y. Ling, H. Cao, A. L. Burin, M. A. Ratner, X. Liu, and R. P. H. Chang, “Investigation of random lasers with resonant feedback,” Phys. Rev. A 64(6), 063808–063815 (2001). [CrossRef]

17.

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

OCIS Codes
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.7240) Lasers and laser optics : UV, EUV, and X-ray lasers
(160.3380) Materials : Laser materials
(160.6000) Materials : Semiconductor materials
(290.5890) Scattering : Scattering, stimulated

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 29, 2012
Revised Manuscript: May 8, 2012
Manuscript Accepted: May 12, 2012
Published: May 17, 2012

Citation
Shih-Pang Chang, Kuok-Pan Sou, Chieh-Han Chen, Yuh-Jen Cheng, Ji-Kai Huang, Chung-Hsiang Lin, Hao-Chung Kuo, Chun-Yen Chang, and Wen-Feng Hsieh, "Lasing action in gallium nitride quasicrystal nanorod arrays," Opt. Express 20, 12457-12462 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-11-12457


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References

  1. F. Qian, S. Gradečak, Y. Li, C.-Y. Wen, and C. M. Lieber, “Core/Multishell nanowire heterostructures as multicolor, high-efficiency light-emitting diodes,” Nano Lett.5(11), 2287–2291 (2005). [CrossRef] [PubMed]
  2. R. Chen, H. D. Sun, T. Wang, K. N. Hui, and H. W. Choi, “Optically pumped ultraviolet lasing from nitride nanopillars at room temperature,” Appl. Phys. Lett.96(24), 241101 (2010). [CrossRef]
  3. H. Sekiguchi, K. Kishino, and A. Kikuchi, “Emission color control from blue to red with nanocolumn diameter of InGaN/GaN nanocolumn arrays grown on same substrate,” Appl. Phys. Lett.96(23), 231104 (2010). [CrossRef]
  4. H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, “Structural and optical properties of GaN nanocolumns grown on (0001) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy,” J. Cryst. Growth300(1), 259–262 (2007). [CrossRef]
  5. Y. Sun, Y.-H. Cho, H.-M. Kim, and T. W. Kang, “High efficiency and brightness of blue light emission from dislocation-free InGaN/GaN quantum well nanorod arrays,” Appl. Phys. Lett.87(9), 093115 (2005). [CrossRef]
  6. Y. Kawakami, S. Suzuki, A. Kaneta, M. Funato, A. Kikuchi, and K. Kishino, “Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns,” Appl. Phys. Lett.89(16), 163124 (2006). [CrossRef]
  7. S. Gradečak, F. Qian, Y. Li, H.-G. Park, and C. M. Lieber, “GaN nanowire lasers with low lasing thresholds,” Appl. Phys. Lett.87(17), 173111 (2005). [CrossRef]
  8. T. Kouno, K. Kishino, K. Yamano, and A. Kikuchi, “Two-dimensional light confinement in periodic InGaN/GaN nanocolumn arrays and optically pumped blue stimulated emission,” Opt. Express17(22), 20440–20447 (2009). [CrossRef] [PubMed]
  9. M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, and K. Kishino, “Random laser action in GaN nanocolumns,” Appl. Phys. Lett.97(15), 151109 (2010). [CrossRef]
  10. H. Cao, “Review on latest developments in random lasers with coherent feedback,” J. Phys. A38(49), 10497–10535 (2005). [CrossRef]
  11. Y. Lai, Z.-Q. Zhang, C.-H. Chan, and L. Tsang, “Anomalous properties of the band-edge states in large two-dimensional photonic quasicrystals,” Phys. Rev. B76(16), 165132 (2007). [CrossRef]
  12. M. E. Zoorob, M. D. B. Charlton, G. J. Parker, J. J. Baumberg, and M. C. Netti, “Complete photonic bandgaps in 12-fold symmetric quasicrystals,” Nature404(6779), 740–743 (2000). [CrossRef] [PubMed]
  13. L. Mahler, A. Tredicucci, F. Beltram, C. Walther, J. Faist, H. E. Beere, D. A. Ritchie, and D. S. Wiersma, “Quasi-periodic distributed feedback laser,” Nat. Photonics4(3), 165–169 (2010). [CrossRef]
  14. K. Nozaki and T. Baba, “Lasing Characteristics of 12-Fold Symmetric Quasi-periodic Photonic Crystal Slab Nanolasers,” Jpn. J. Appl. Phys.45(8A), 6087–6090 (2006). [CrossRef]
  15. A. L. Burin, H. Cao, and M. A. Ratner, “Understanding and control of random lasing,” Physica B338(1-4), 212–214 (2003). [CrossRef]
  16. Y. Ling, H. Cao, A. L. Burin, M. A. Ratner, X. Liu, and R. P. H. Chang, “Investigation of random lasers with resonant feedback,” Phys. Rev. A64(6), 063808–063815 (2001). [CrossRef]
  17. S. F. Yu, C Yuen, S. P. Lau, W. I. Park, and G. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett.84(17), 3241–3243 (2004). [CrossRef]

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