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We report electron-hole plasma (EHP) lasing in hexagonal ZnO microrods and thin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser, ultraviolet whispering-gallery mode (WGM) lasing was observed from hexagonal ZnO microrods. When EHP was formed at high excitation energy density, the center wavelength of the WGM lasing band presented a redshift from 387.5 nm to 397.5 nm, and the full width of half maximum (FWHM) of the WGM lasing band increased from 2.5 nm to 7 nm. Each lasing mode showed obvious blueshift and broadening. Such lasing characteristics were attributed to the band gap renormalization (BGR) due to the high carrier concentration at the EHP condition. In addition, EHP Fabry-Perot (F-P) mode lasing from thin ZnO nanobelt was also observed and discussed. According to the phenomenological BGR calculation with including the carrier density dependent screening effect, the values of the band gap of ZnO at different excitation energy densities were obtained, which agree well with the experimental results.
© 2014 Optical Society of America
1. Introduction
For semiconductor laser material, carrier concentration effectively determines the optical properties. Generally exciton emission takes place at low carrier concentration under a low excitation energy density. When carrier concentration is beyond critical Mott transition density, electron-hole plasma (EHP) emission will be formed because of the exciton ionization, the EHP emission usually shows band gap renormalization (BGR) effect [1–4]. Correspondingly, electron-hole plasma is attributed to semiconductor laser gain when the Mott-transition is realized. Generally the room temperature semiconductor laser gain (GaN,GaAs) is mainly attributed to electron-hole plasma [2, 5]. In last ten years, ZnO micro/ nanolaser has attracted much attention for its large band gap (3.37 eV) and high exciton binding energy (60 meV) and promising applications in shortwavelength optoelectronics devices. Three kinds of ZnO lasing modes have been investigated: random lasing in ZnO powders [6–8], Fabry-Perot (F-P) modes in thin film [9] and in nanowire/rods [10, 11], as well as whispering gallery modes (WGM) in microwire/rods [12, 13] and microdisks [14, 15]. However, the gain mechanism is still unclear in the ZnO microlaser cavities. Some of the papers attribute the ZnO lasing to exciton-related processes [10, 15], and others regard that the electron-hole plasma is the main gain origination of the ZnO lasing [16–22]. Recently the EHP lasing from different ZnO microstructures were reported. J. Takeda et al. [18] and J. Fallert et al. [11] demonstrated clear EHP F-P lasing in a ZnO nanorod/wires at low temperature. M. Versteegh et al. reported the room-temperature EHP lasing from different sized ZnO nanowires under the excitation of either nanosecond pulsed laser or femtosecond pulsed laser [20, 21]. T. Nakamura observed temperature-dependent EHP lasing in ZnO random laser, and the gain spectra of ZnO at EHP lasing condition was calculated by quantum many-body theory [19]. Even so, convincible room temperature (RT) EHP lasing spectra from high quality ZnO WGM cavity have not been reported, especially the BGR effect induced by EHP in either ZnO WGM or F-P ZnO microcavities at RT have not been clearly presented. As a result, the RT EHP-related lasing phenomena are still not very clear in ZnO microlasers. In this study, RT WGM and F-P mode lasing were observed from hexagonal ZnO microrod and rectangular nanobelt, respectively. When the excitation energy density increased, the lasing spectra presented a redshift about 10 nm due to the EHP induced BGR effect. The blueshift and broadening of each lasing mode due to the EHP was also observed. The values of the energy gap of the ZnO under different excitation energy densities were estimated. And the phenomenological BGR formula was used to calculate the band gap under different carrier density at EHP condition. The experimental result agrees well with the theoretical result.
2. Experimental section
The ZnO microrods and nanobelts were fabricated using vapor phase transport methods reported by us previously [13]. The lasing spectra from the ZnO microrods and nanobelts were measured by a confocal micro-photoluminescence system (Olympus BX53), a spectrometer (SpectraPro-2500i, Acton Research Corporation) was equipped to collect and analyze the optical spectrum, the excitation light is 325 nm line of a femtosecond laser pulse (150 fs pulsed duration and 1 kHz repetition rate) generated by OPERA SOLO, the seed light is outputted from a Ti:sapphire laser (Coherent). The excitation spot has a radius of about 10 μm. The image and video of the ZnO sample under the microscopy were captured by a CCD camera.
3. Results and discussion
3.1. WGM lasing from ZnO microrod
Figure 1(a) shows the SEM image of a ZnO microrod laying on a quartz substrate, the ZnO microrod has perfect hexagonal cross-section with a circumscribed circle diameter D of 6.55 μm. Figure 1(b) shows the optical image of the dazzling ZnO microrod excited by 325 nm fs laser under the confocal micro-PL system. The micro-PL spectra from the microrod at different excitation energy density per pulse Iexc are shown in Fig. 1(c). The attached file is the video of the optically pumped WGM lasing in the ZnO microrod under micro-PL system. The micro-PL spectrum from the microrod shows a typical UV near band edge recombination spontaneous emission band at 387.5 nm with a FWHM of about 12 nm when Iexc is only 25 μJ/cm2. When Iexc is increased to 40 μJ/cm2, two emission peaks at 389.46 and 390.7 nm appear on the emission band, this indicates that optical resonance is formed with specific modes in the ZnO microrod. When Iexc is further increased to 50 μJ/cm2, four emission peaks (388.32 nm, 389.46 nm, 390.7 nm and 392.08 nm) appear, and the broad spontaneous emission band is suppressed. The integrated intensity of the four emission peaks is about 10 times of the spontaneous emission, and the FWHM of the emission band is only 2 nm. For Iexc > 50 μJ/cm2, the spectra are dominated by sharp emission lines, more and more emission peaks appear with the increase of Iexc, and the emission intensity of the ZnO microrod exhibits a dramatically superlinear increase with Iexc, the intensity is orders of magnitude greater than the spontaneous emission background, this indicates that multi-mode lasing action is generated in the ZnO microrod. One of the remarkable lasing characteristics is that the center wavelength of the whole emission band shows an obvious redshift to 397.5 nm when Iexc is increased to 150 μJ/cm2. In addition to the redshift of the lasing emission band, the FWHM of the lasing emission band was broadened from 2 nm to 10 nm. Figure 1(d) shows the integrated emission intensity versus Iexc, the output-input curve agrees well with the Casperson model for multi-mode laser [23], the critical Iexc of 50 μJ/cm2 is estimated as the lasing threshold. The average mode spacing for the lasing peaks is about 1.2 nm, the line width for a single sharp emission peak is about 0.4 nm for Iexc < 75 μJ/cm2, it is estimated that the Q factor of this microcavity is about 1000 according to the equation Q = λ/Δλ, where λ and Δλ are the lasing wavelength and its line width, respectively. According to the previous reports, the sharp peaks observed in Fig. 1(c) correspond to WGM modes [12, 13]. The inset of Fig. 1(a) shows the hexagonal WGM optical loop, total internal reflection can easily takes place for the WGM optical loop because the critical angel is only about 25 degree. The whispering gallery mode resonance equation for the hexagonal cavity is as following [13]:
Where N is the mode number, n is the refractive index [24]. The four lasing peaks at 388.32 nm, 389.46 nm, 390.7 nm and 392.08 nm can be index to the mode number of 101, 100, 99 and 98.
Fig. 1 (a) SEM image of the ZnO microrod (D = 6.55 μm), the inset shows a hexagonal cavity with WGM optical resonance loop. (b) Microscopy image of the ZnO microrod at lasing condition. (c) PL emission spectra from the ZnO microrod for Iexc increasing from 25 μJ/cm−2 to 150 μJ/cm−2. (d) Output-input relationship of the ZnO microrod laser.
3.2. EHP induce band gap renormalization in ZnO WGM microlaser cavities
Figure 2(a) and 2(b) show the lasing spectra from other two ZnO microrods with the diameter D of 13.05 and 12.0 μm. For the ZnO microrod (D = 13.05 μm), when Iexc is increased from 80 to 300 μJ/cm2, the lasing emission band shifted from 390 nm to 401 nm, and the FWHM of the lasing band increased from 2 nm to 10 nm. As for the ZnO microrod (D = 12.0 μm), when Iexc is increased from 75 to 125 μJ/cm2, the lasing emission band shifted from 390 nm to 395 nm, and the FWHM of the lasing band increased from 2 nm to 7 nm. Obviously these two ZnO microrods present the same lasing properties as that shown in Fig. 1(c). Comparing the WGM lasing spectra from the three different microrods, it can be found that the larger ZnO microrod has smaller lasing mode spacing, and has a larger Q factor at early stage of the lasing [13]. If the excitation is strong enough to result in a carrier concentration higher than Mott density, the excitons will transit to the electron-hole plasma states. The reported Mott density of ZnO thin film varies from 1017 cm−3 to 1020 cm-3. According to the absorption coefficient β ≈1.6 × 105 cm−1 [7, 25], the carrier density can be estimated by np = β⋅Iexc/hωexc. When Iexc is increased from 25 to 175 μJ/cm2, the carrier concentration np in the ZnO microrod increases from 0.8 × 1019 to 4 × 1019 cm-3. Such a high carrier concentration definitely results in electron-hole plasma lasing rather than the exciton lasing in the ZnO microrod. A typical effect of electron-hole plasma is the band-gap renormalization, which causes a downshift of the conduction-band edge and an upward of the valence-band edge, so the bandgap of the ZnO microrod will be decreased due to the EHP induced BGR effect [9, 19, 26].
Fig. 2 (a,b) WGM lasing spectra from the ZnO microrods with D = 13.05 and 12.0 μm, respectively. (c) The relationship between band gap and the carrier concentration for the three ZnO microrod and a ZnO nanobelt discussed in the next paragraph, the solid line is the theoretical result.
According to the phenomenological BGR formula [19], the band gap energy Eg induced by the BGR effect is expressed as Eg = Eg0-E0a0/λs(n), where E0 and a0 denote the exciton binding energy and Bohr radius, respectively. Eg0 is the intrinsic band gap without taking into account the BGR effect at temperature. λs(n) denotes the screening length of electron-hole plasma. λs(n) depends on the excited carrier density n and the temperature. Here we calculated the dependence of band gap on the carrier density, as shown in Fig. 2(c), the calculation was performed by using parameters in [19]. The experimental band gap values of the ZnO microcavities at different carrier density were obtained from the center wavelength of the lasing spectrum, i. e. Eg = 1240/λcenter. We can find that the experimental result agrees well with the theoretical result calculated by many-body method.
In Fig. 1(c), it is worth noting that when Iexc is increased from 50 μJ/cm2 to 150 μJ/cm2, the lasing modes show blue shift. As shown in Fig. 1(c), the lasing mode (N = 98) shifted from 392.08 nm to 390.05 nm when Iexc increased from 50 μJ/cm2 to 125 μJ/cm2, and then disappeared at 150μJ/cm2. As the previous report pointed, the blueshift of each lasing mode is related to the EHP process [11]. There is a small reduction of the refractive index when the carrier density increases, such effect can be described by the Drude model [11]. This reduction of the refractive index with the increase of the carrier density can definitely cause a blue shift of the lasing modes. Although the lasing modes at the high energy side blueshifted and disappeared finally, more lasing modes appeared successively on the low energy side. Because the lateral lasing modes consume a certain fraction of the available electron-hole pairs, the appearance of the lasing mode at low energy side is accompanied by a quenching of those lasing modes at high energy side [11], such phenomenon can be clearly seen in Fig. 1(c). In addition, the line width of each lasing mode is broadened with the increase of Iexc, which is caused by the interaction of atoms and phonons in the ZnO microrod or the heating effect under intensive excitation.
3.3. EHP induce band gap renormalization in ZnO F-P mode nanobelt lasing cavity
Except for the hexagonal WGM lasing, EHP Fabry-Perot lasing from an ultrathin ZnO nanobelt is also presented here, which is not reported previously. As shown in the Fig. 3(a), ZnO nanobelt was dispersed on a quartz substrate, the ZnO nanobelt has a width of 20 μm, the length of the nanobelt can be up to 1 mm. The inset of the Fig. 3(a) shows the SEM image of the end of the nanobelt, we can clearly see that the nanobelt has an ultrathin thickness of about 200 nm. Figure 3(b) shows the optical image of the ZnO nanobelt at lasing condition. The video of the lasing from the ZnO nanobelt is attached as a supplemental material. We can clearly find the ultraviolet light emits out at two edges of the nanobelt, and the central circular spot is the excitation light spot. The optical microscopy image of the lasing indicates that the F-P mode lasing between two parallel edges formed in the ZnO nanobelt. Because the ZnO nanobelt only has a thickness less than the lasing wavelength, the lasing mode can’t be formed between bottom and top surfaces for the extremely low optical confinement, which results in the formation of F-P modes between two edges as shown in the Fig. 3(a). Figure 3(c) shows the lasing spectra when Iexc is increased from 50 to 150 μJ/cm2, the F-P mode lasing spectra gradually shift from 390 nm to 397 nm and the FWHM of the lasing emission band increases from 2 nm to 12 nm. The lasing properties (emission band redshift and broadening) of the ZnO nanobelt are similar to the EHP lasing characteristics observed from the ZnO microrods, so we can conclude that the EHP F-P mode lasing are realized in the ZnO nanobelt. The lasing spectrum (Iexc = 75 μJ/cm2) is shown in the inset of Fig. 3(c), 11 lasing emission peaks can be found between 388 nm to 393 nm, the average mode spacing is about 0.44 nm. The strongest lasing mode is at 390 nm, and the FWHM of this lasing mode is about 0.2 nm, the Q factor is about 1950 for the cavity. To confirm the F-P mode, the experimental mode spacing was compared with the theoretical result. The mode spacing for the F-P modes are expressed as following [10]:
Where n ≈2.4 is the refractive index, and λ is the lasing wavelength, dn/dλ = −0.012 nm−1, L = 25.5 μm is the cavity length which is equal to the distance between the two edges. The calculated mode spacing for the F-P mode is 0.42 nm, which agree well with our experimental result. So the EHP F-P mode lasing was formed in the cavity. The values of the band gap at different carrier concentrations are also shown in Fig. 2(c), which are very close to the theoretical band gap curve at EHP condition.Fig. 3 (a) SEM image of the ZnO nanobelt (W = 25.5 μm), the inset shows the SEM image of the end. (b) Microscopy image of the ZnO nanobelt at lasing condition (Media 1). (c) PL emission spectra from the ZnO nanobelt for Iexc increasing from 50 μJ/cm2 to 150 μJ/cm2. (d) Output-input relationship of the ZnO nanobelt laser.
4. Conclusion
In summary, EHP lasing properties were investigated in ZnO hexagonal microrods and ultrathin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser with high density, UV WGM lasing was observed from ZnO hexagonal microrods, the lasing spectra redshift from 387.5 nm to 397.5 nm, and the FWHM of the lasing spectra band increases from 2.5 nm to 7 nm. The similar lasing spectra redshift and broadening of the F-P mode lasing from an ultrathin ZnO nanobelt were also observed. Such lasing characteristics were attributed to the EHP effect. The experimental band gap of the ZnO microcavity at EHP lasing condition agree well with the theoretical calculation with taking into account the excited carrier density dependent screening effect.
Acknowledgments
This work was supported by NSFC (11104119, 61275054, 61475035), 973 Program (2013CB932903 and 2011CB302004), Open fund of state key laboratory of bioelectronics, China postdoctoral science foundation (2014M551485) and QingLan project.
References and links
1. P. A. Wolff, “Theory of the band structure of very degenerate semiconductors,” Phys. Rev. 126(2), 405–412 (1962). [CrossRef]
2. G. Göbel, “Recombination without k‐selection rules in dense electron‐hole plasmas in high‐purity GaAs lasers,” Appl. Phys. Lett. 24(10), 492–494 (1974). [CrossRef]
3. D. C. Reynolds, D. C. Look, and B. Jogai, “Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN,” J. Appl. Phys. 88(10), 5760–5764 (2000). [CrossRef]
4. F. S. Al-Ajmi, “Stimulated Emission and Laser Action from Gallium Nitride, Aluminium Gallium Nitride, Aluminium Gallium Nitride/Gallium NitrideQuantum Wells and Heterostructures,” Ph. D. Dissertation, North Carolina State University (2007).
5. Y. Higuchi, K. Omae, H. Matsumura, and T. Mukai, “Room-temperature CW lasing of a GaN-based verticalcavity surface-emitting laser by current injection,” Appl. Phys. Express 1, 121102 (2008). [CrossRef]
6. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
7. J. Fallert, R. J. B. Dietz, J. Sartor, D. Schneider, C. Klingshirn, and H. Kalt, “Co-existence of strongly and weakly localized random laser modes,” Nat. Photonics 3(5), 279–282 (2009). [CrossRef]
8. H. K. Liang, S. F. Yu, and H. Y. Yang, “ZnO random laser diode arrays for stable single-mode operation at high power,” Appl. Phys. Lett. 97(24), 241107 (2010). [CrossRef]
9. Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films,” Appl. Phys. Lett. 72(25),3270–3272 (1998). [CrossRef]
10. M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science 292(5523), 1897–1899 (2001). [CrossRef] [PubMed]
11. J. Fallert, F. Stelzl, H. J. Zhou, A. Reiser, K. Thonke, R. Sauer, C. Klingshirn, and H. Kalt, “Lasing dynamics in single ZnO nanorods,” Opt. Express 16(2), 1125–1131 (2008). [CrossRef] [PubMed]
12. C. Czekalla, C. Sturm, R. Schmidt-Grund, B. Q. Cao, M. Lorenz, and M. Grundmann, “Whispering gallery mode lasing in zinc oxide microwires,” Appl. Phys. Lett. 92(24), 241102 (2008). [CrossRef]
13. J. Dai, C. X. Xu, K. Zheng, C. G. Lv, and Y. P. Cui, “Whispering gallery-mode lasing in ZnO microrods at room temperature,” Appl. Phys. Lett. 95(24), 241110 (2009). [CrossRef]
14. D. J. Gargas, M. C. Moore, A. Ni, S. W. Chang, Z. Y. Zhang, S. L. Chuang, and P. D. Yang, “Whispering Gallery Mode Lasing from Zinc Oxide Hexagonal Nanodisks,” ACS Nano 4(6), 3270–3276 (2010). [CrossRef] [PubMed]
15. R. Chen, B. Ling, X. W. Sun, and H. D. Sun, “Room temperature excitonic whispering gallery mode lasing from high-quality hexagonal ZnO microdisks,” Adv. Mater. 23(19), 2199–2204 (2011). [CrossRef] [PubMed]
16. D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, M. Y. Shen, and T. Goto, “High temperature excitonic stimulated emission from ZnO epitaxial layers,” Appl. Phys. Lett. 73(8), 1038–1040 (1998). [CrossRef]
17. C. Klingshirn, R. Hauschild, J. Fallert, and H. Kalt, “Room-temperature stimulated emission of ZnO: Alternatives to excitonic lasing,” Phys. Rev. B 75(11), 115203 (2007). [CrossRef]
18. S. Mitsubori, I. Katayama, S. H. Lee, T. Yao, and J. Takeda, “Ultrafast lasing due to electron-hole plasma in ZnO nano-multipods,” J. Phys. Condens. Matter 21(6), 064211 (2009). [CrossRef] [PubMed]
19. T. Nakamura, K. Firdaus, and S. Adachi, “Electron-hole plasma lasing in a ZnO random laser,” Phys. Rev. B 86(20), 205103 (2012). [CrossRef]
20. M. A. M. Versteegh, T. Kuis, H. T. C. Stoof, and J. I. Dijkhuis, “Ultrafast screening and carrier dynamics in ZnO: Theory and experiment,” Phys. Rev. B 84(3), 035207 (2011). [CrossRef]
21. M. A. M. Versteegh, D. Vanmaekelbergh, and J. I. Dijkhuis, “Room-temperature Laser Emission of ZnO Nanowires Explained by Many-Body Theory,” Phys. Rev. Lett. 108(15), 157402 (2012). [CrossRef] [PubMed]
22. C. Klingshirn, Semiconductor Optics, 3rd ed., (Springer, Berlin, 2007).
23. L. W. Casperson, “Threshold Characteristics of Multimode Laser Oscillations,” J. Appl. Phys. 46(12), 5194–5201 (1975). [CrossRef]
24. R. Schmidt-Grund, P. Kühne, C. Czekalla, D. Schumacher, C. Sturm, and M. Grundmann, “Determination of the refractive index of single crystal bulk samples and micro-structures,” Thin Solid Films 519(9), 2777–2781 (2011). [CrossRef]
25. J. F. Muth, R. M. Kolbas, A. K. Sharma, S. Oktyabrsky, and J. Narayan, “Excitonic structure and adsorption coefficient measurements of ZnO single crystal epitaxial films deposited by pulsed laser deposition,” J. Appl. Phys. 85(11), 7884–7887 (1999). [CrossRef]
26. H. D. Li, S. F. Yu, S. P. Lau, E. S. P. Leong, H. Y. Yang, T. P. Chen, A. P. Abiyasa, and C. Y. Ng, “High-Temperature Lasing Characteristics of ZnO Epilayers,” Adv. Mater. 18(6), 771–774 (2006). [CrossRef]
