Ultraviolet Random Laser Based on a Single GaN Microwire - ACS

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Ultraviolet Random Laser based on Single GaN Microwire Yuhao Ren, Hai Zhu, Yanyan Wu, Guanlin Lou, Yunfeng Liang, Shuti Li, Shi Chen Su, Xuchun Gui, Zhiren Qiu, and Zikang Tang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00336 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Ultraviolet Random Laser based on Single GaN Microwire Yuhao Ren1, Hai Zhu*,1, Yanyan Wu1, Guanlin Lou1, Yunfeng Liang1, Shuti Li2, Shichen Su2, Xuchun Gui3, Zhiren Qiu1, and Zikang Tang4 1State

Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun

Yat-Sen University, Guangzhou 510275, China. 2Institute

of Optoelectronic Material and Technology, South China Normal University,

Guangzhou 510631, China. 3State

Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics

and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China. 4The

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da

Universidade, Taipa, Macau, China.

ABSTRACT Random lasing (RL) from self-construct localized cavities based on micro-pits scatters in single GaN microwire (MW) was investigated firstly. The spectra and spatial resolution of RL exhibit the lasing modes were originated from different region in MW. Temperature-dependent lasing measurement of GaN RL shows an excellent characteristic-temperature of about 52 K. In addition, the dependence of spatial localized cavities dimension on pumping intensity profile and temperature was studied by fast Fourier transform (FFT) spectroscopy. For GaN RL, the optical feedback was supported by localized paths through the scattering effect of micro-pits in MW. The scattering feedback mechanism for RL can avoid the enormous difficulty in fabrication artificial cavity structures for GaN. Hence, the results in this paper represent a low-cost technique to realize GaN-based ultraviolet laser diodes without the fabrication difficulty of cavity facets. Keyword: GaN, microwire, random laser, multiple scattering, spiky.

*E-mail: [email protected] 1

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Over the last decade, the short wavelength III-nitride laser devices have attracted extensive attention due to their superior properties and required for a wide variety of applications, such as sterilization, water or air purification, optical recording and laser display. Currently, the most popular optical resonance cavity for GaN-based laser device is stripe Fabry-Perot (F-P) type cavity. However, the cleaving process of end facet is still a challenge for F-P waveguide structure, due to the hardness of sapphire substrate.1 In order to avoid cleaving difficulty, researchers also attempted to fabricate the distributed Bragg reflector vertical cavity and microdisk cavity structure.

2-4

However, the

time-consuming and complex fabrication process within all of above methods is inevitable, which will undoubtedly result in the high manufacture cost for GaN-based laser. Besides above artificial optical cavity, the self-constructed cavity in the random disorder scattering system can also support the optical coherence feedback for lasing modes.5-7 For RL, the optical feedback is provided by strong light scattering rather than a cavity mirror, the fabrication process of random laser will be simplified dramatically.5,8 Since the coherent random lasing action in ZnO was reported by Cao et al.,9 such a study has been performed on various semiconductor materials, such as SnO2,10 GaAs,11 and ZnSe.12 Especially, the high bright and multimode operation of UV RL will has potential application in high resolution imaging and display.6 Hence, extensive attention was paid to the wide bandgap materials. Comparing with abundant literature on RL in ZnO films, microwires, powders, nanowires, rods and nanowalls,13-18 the reports about ultraviolet RL from GaN is very rare. Although RL has been observed in GaN nanocolumns,19 particles,20 and epitaxy films,21,22 the manipulation of lasing mode and output divergence is a very challenge for two and three-dimension disorder medium. Recently, the manipulation of RL mode in one-dimension MW may open a new horizon of optimization for single GaN RL significantly.23 In this paper, we firstly demonstrated the RL from single GaN MW by means of multiple scattering feedback based on self-construct disorder micro-pits 2


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defect. The ultraviolet RL was realized through the random localized resonance cavities in MW. Moreover, the dimension of spatial localized lasing modes was investigated by using the FFT method. Finally, the polarization characteristics and high-temperatures performance of the GaN RL were studied. RESULTS AND DISCUSSIONS The gain media GaN MWs arrays were grown on Si substrate through metalorganic chemical vapor deposition (MOCVD). The detailed crystalline characteristics of as-growth GaN MWs were given in Figure S1. After growth, the sample was released by selective wet etching and ultrasonic process, then single MW was separated and transferred onto silicon substrate. As shown in Figure 1a, the GaN MW has a uniform diameter in the lengthwise direction. The trapezoidal cross-section MW can be seen clearly from SEM image (inset of Figure 1a). It is worth pointing out that the maximum length of GaN MWs can approach several hundred micrometers with ~3 μm hight and bottom (upper) width of about 5 μm (2 μm). Remarkable, the SEM photo shows many micro-pits distribute on the surface of GaN MW randomly (top of Figure 1b), and the size of these micro-pits is about several hundred nanometers (bottom of Figure 1b). These micro-pits may be originated from the local imperfect screw-type crystal dislocation during growth process.24 Although these defect-related micro-pits were considered destructive for electron transport,

3,25

it has a strong scattering feedback effect

for the light propagation in MW. Especially, the scattering feedback from micro-pits can localized light field and form localized paths to construct RL action.23 Schematic diagram of localized paths about RL in MW was illustrated in Figure 1c. Through such self-formed scattering centers, many localized cavities can be constructed in single MWs. In addition, the excellent optical characteristics of GaN MWs have been investigated comprehensively through the micro-photoluminescence /Raman measurement (Figure S2). Schematic diagram of the measurement set-up for RL from single MW is shown in Figure 2a. Here, the pump light (266 nm) was focused onto MW using 3


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an objective and the emission from sample was collected by the same objective and then coupled into a spectrometer. As the displayed emission spectra (Figure 2b), a weak spontaneous emission (SPE) band dominates in the spectrum at low excitation power, which corresponds to the typical near band-edge emission of GaN. When the pump intensity exceeded 200 kW cm-2, impressive narrow emission spikes emerged and superimposed on the broad SPE band. With increasing of pump intensity furtherly, more strong emission spikes were observed nearby 375 nm. The light-out versus light-in (L-L) curve of sample exhibits a kink behavior (inset of Figure 2b). The emergence of sharp emission spikes as the excited power above the kink point confirms that the single MW supports lasing action, and the lasing threshold was extracted to be about 202.9 kW cm-2 from L-L curve. Moreover, the positions of lasing modes were changed randomly from one pulse to another pulse, which behavior is known as a typical characteristic of RL.26 In the case of mean free path of photons is not too short with respect to sample size, RL could also be observed due to Lévy-type fluctuations.27 For our sample, no artificial optical resonance cavity was fabricated, hence the multiple-modes lasing emission can be attributed to the multiple

scattering

feedback

via

self-formed

micro-pits.

The

output

characteristics of single-MW RL display distinct lasing spikes in different directions (Figure S3). Moreover, the evolution of emission spectra with the changing of excitation area is given in Figure S5. Actually, the scattering feedback for the RL through pits defect was also obtained in GaN and ZnO film.8,21 The Q-factor of cavity constructed by the scattering is expected to be ν/δν=1376, where ν and δν is the frequency and width of lasing mode. It is noted that the lasing threshold is about one order of magnitude lower than that of GaN nanoparticle RL (2.5 MW cm-2).20 This indicates the RL threshold from the micro-pits scattering is lower than that strong localized mode, which is consistent with the theory calculation results by D. S. Wiersma.28 Figure 2c present the near-field patterns under different excitation powers. Under low excitation power (0.7Pth), a uniform spontaneous 4


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emission light was observed from the CCD image. As the excitation power exceeded the threshold (1.5 Pth), some bright spots emerged from the MW surface which corresponding to the radiation loss from localized cavity. In order to investigate the UV RL properties from GaN MW detailly, the measurement about spatially resolved lasing emission spectra were performed (Figure S4). The emission light from sample was irradiated onto the entrance slit of spectrometer, and then the dispersed beam was recorded by CCD with 2 μm real-space resolution in vertical direction. The spatially (vertical axis) and spectrally (horizontal axis) resolved intensity (SSRI) of RL emission profiles under different pumping powers were summarized in Figure 3. Only a relatively weak light emission (spontaneous emission) can be seen from the SSRI at low excited power (Figure 3a). By increasing the exciting power, a few discrete bright spots emerge obviously as shown in the images of Figure 3b. It should be noted that the bright spots in SSRI profile are responsible for the sharp spikes in RL spectra. Furthermore, the number of lasing spikes was increasing at higher power condition (1.5 Pth). Meanwhile the imaging of RL confirms that the lasing modes in spectra were originated from a series of localization spatial region (Figure 3c). The temporal SSRI of GaN RL that collected at different times with same pumping power indicates that the random multiple lasing modes at distinct wavelengths originated from different spatial localized regions, and this behavior in one of the typical features of RLs (Figure 4a and b). This spatial intensity fluctuation behavior of RL has also been observed in other disorder gain media system.29 Considering that the refractive index of active media will be changing slightly follow the fluctuation of pumping pulse at high pumping power. So, we simulated the localized random lasing mode profile in MW with identical random distributed micro-pits (with diameter of 100 nm) using the finite-difference time-domain FDTD method (Figure 4c).30,31 Comparing to the spatial localized optical field in MW with refractive index of 2.4 (top), the profile of optical field intensity vary dramatically as refractive index increasing to 2.6 (bottom). Above 5


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results indicate the slight fluctuation of refractive index for scatters will result into the lasing mode jitter. The polarization characteristic of MW RL was studied with excited power of 1.5 Pth (Figure 5a). The polar plot displays the emission intensity of sample versus rotation angle φ of the linear polarizer. At φ=0°(φ=90°), the electric field polarized parallel (perpendicular) to MW will transmit. The plot diagram indicates that RL light from MW is unpolarized, which is caused by multi-mode lasing from different localized resonance cavities. The fast Fourier transform (FFT) of lasing spectra has been demonstrated to be an efficient method to analyze the formation of localized random cavities.32,33 The spatial dimension can be obtained from the formula d=mnLc/2π, where d is Fourier component, m is an integer denoting the FFT harmonic, Lc is localized cavity dimension, and n represents the refraction index of lasing medium.34 The FFT of lasing spectra with different pumped intensity were shown in Figure 5b. The localized cavity length Lc was extracted to be about 32 to 35 μm (d=14 μm) at low excitation power (1.1 Pth and 1.2 Pth). However the average value of cavity length will decrease with increasing of the pump intensity at higher levels (1.5 Pth). The formation of the random cavities depends mainly on the profile of the optical gain. The optical excitation has a Gaussian profile, which means the optical gain at the middle of pump spot is highest.32 For the dependence of Lc on pump intensity can be explained using the real space profile of pumping light field in sample (Figure 5c). Generally, the lasing threshold can be described by Ith∝e-(g-α)L, which g is gain coefficient, α means mode loss factor and L is cavity dimension. For a give lasing threshold, the cavity length is inversely proportional to optical gain coefficient of media. As a result, a relatively large cavity length is required to sustain the lasing oscillation for small g value under low-pumping power condition. Along with the excitation intensity increasing, the optical gain in central spot area will be enhanced dramatically than the boundary of active 6


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region. Consequently the more short length cavities will satisfy the lasing resonance condition, hence the average value of cavity dimension was reducing. The high-temperatures operation property of laser device is a critical factor for practice application. The performances of single GaN MW RL at high temperatures were measured (Figure 6a). Here, all of the lasing spectra were obtained at excitation power of ~1.5 Ith, where Ith represents the pump threshold at the corresponding temperature. Noticeable, the lasing peaks redshift from 374 to 387 nm as temperature increased from 300 to 450 K, which is resulted from the shrinkage of band gap. Although the number lasing spikes was reduced under high-temperature, the lasing emission can be sustained even at 450 K. Moreover, the dependence of lasing threshold versus temperature exhibits an exponential relationship (Figure 6b). The experimental data can be fitted very well using the empirical formula: Ith(T)=I0 exp(T/TC), where I0 is a constant and TC is the characteristic temperature.35 The value of TC was determined to be about 52 K according to the fitting, which approach to the value of ZnO RL.36 To further investigate the evolution of localized dimension at high temperature, FFT of lasing spectra is used to explore the changing of cavities dimension LC,37 which is an important factor in retaining the RL at high T. Figure 6c shows the FFT plots of lasing spectra of sample at 300, 330, 360, 390, and 420 K under an excitation density of 1.5 Ith. In view of the fundamental harmonic peak, the typical cavities dimension for five temperatures was determined to be ~15.7, ~14.1, ~13.9, ~10.6, and ~11.4 µm, respectively. Inset of figure 6c plots the evolution about LC values versus T. It is found that the length of the cavities reduced gradually as temperature increases. According to Yu et al. illustration,32,35 the scattering path near the middle of pump spot (with a small LC) will undergo more higher optical gain at high excitation intensity (as shown in figure 5c). Consequently, at high-temperature region, only the inner cavities with abundant gain under strong pumping power have the ability to sustain the lasing.38 In other words, the optical gain of scattering path at out-ring position (large Lc) is not large enough to overcome the scattering loss under 7


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high-temperature condition. CONCLUSIONS In summary, the single MW ultraviolet GaN RL was realized at room-temperature in virtual of the self-constructed cavity based on micro-pits. The micro-pits at surface of MW can be acted as light scatters to form random localized cavities and sustain the optical scattering feedback. The localized modes were studied by the spatially/spectrally resolved lasing profiles and the RL characteristics with different pump intensity and temperature were investigated. It was shown that the multiple scattering feedback by micro-pits have the excellent ability to realize RL, which can avoid the enormous difficulties in fabricating artificial cavity structures. Hence, our result has an important implication value for other wider bandgap materials (AlN, BN), which provides a feasible route to ultraviolet laser by using the defect-related scatters. METHODS The morphology of GaN MWs was examined using field emission scanning electron

microscopy

(FESEM,

Hitachi

S-4300).

To

perform

the

photoluminescence (PL) measurement，the sample was excited by a He−Cd laser (325 nm) through a micro-photoluminescence (µ-PL) system (Horiba HR). The lasing characteristics of GaN MW were investigated by a quadruplet Q-switched Nd:YAG laser (266 nm) in pulsed operation (~20 ns, 50 Hz). A pumping spot with a diameter of 60 μm was focused onto the top surface of single MW by an objective lens (20×). The lasing emission from sample was collected by the same objective lens and coupled into a spectrometer (Princeton Instruments Acton SP2750), then recorded by a liquid nitrogen cooled CCD (PyLoN:2K). The spectral resolution of our spectrometer system is 0.05 nm. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the website. Experimental details about the crystalline and optical properties of GaN MWs. 8


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge financial support from National Natural Science Foundation of China (NO. 61504172, NO. 51232009, NO. 61574063). Gratefully acknowledge partial supported by Guangdong Natural Science Funds for Distinguished Young Scholars (NO. 2016A030306044), and Funding from University of Macau to Z. K. Tang (SRG2016-00002-FST). REFERENCES (1) Nakamura, S.; Pearton, S.; Fasol, G. The Blue Laser Diodes 2nd edn (Berlin: Springer) 2000 (2) Someya, T.; Werner, R.; Forchel, A.; Catalano, M.; Cingolani, R.; Arakawa, Y. Room Temperature Lasing at Blue Wavelengths in Gallium Nitride Microcavities. Science 1999, 285, 1905-1906. (3) Tamboli, A. C.; Haberer, E. D.; Sharma, R.; Lee, K. H.; Nakamura, S.; Hu, E. L. Room-temperature continuous-wave lasing in GaN/InGaN microdisks. Nature photonics 2007, 1 (1), 61-64. (4) Choi, H. W.; Hui, K. N.; Lai, P. T.; Chen, P.; Zhang, X. H.; Tripathy, S.; Teng, J. H.; Chua, S. J. Lasing in GaN microdisks pivoted on Si. Appl. Phys. Lett. 2006, 89 (21), 211101. (5) Wiersma, D. S. The physics and applications of random lasers. Nature Physics 2008, 4, 359-367. (6) Redding, B.; Choma, M. A.; Cao, H. Speckle-free laser imaging using random laser illumination. Nature photonics 2012, 6, 355-359. (7) Wiersma, D. S. Disordered photonics. Nature photonics 2013, 7 (3), 188-196. (8) Kalusniak, S.; Wunsche, H. J.; Henneberger, F. Random semiconductor lasers: scattered versus Fabry-Perot feedback. Phys. Rev. Lett. 2011, 106 (1), 013901. (9) Cao, H.; Zhao, Y. G.; Ho, S. T.; Seelig, E. W.; Wang, Q. H.; Chang, R. P. H. random laser action in semiconductor power. Phys. Rev. Lett. 1999, 82, 2278-2281.. (10) Yang, H. Y.; Yu, S. F.; Lau, S. P.; Tsang, S. H.; Xing, G. Z.; Wu, T. Ultraviolet coherent random lasing in randomly assembled SnO2 nanowires. Appl. Phys. Lett. 2009, 94 (24), 241121. (11) Noginov, M. A.; Zhu, G.; Fowlkes, I.; Bahoura, M. GaAs random laser. Laser Physics Letters 2004, 1 (6), 291-293. (12) Takahashi, T.; Nakamura, T.; Adachi, S. Blue-light-emitting ZnSe random laser. 9


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Y. H. Ren et al. Figure Caption Figure 1. (a) SEM topography of single GaN MW. Inset, the cross-section image indicates the trapezoid shape of MW, where the scale-bar is 5 μm. (b) The surface morphology of single MW, which shows many pits were random distributed on the surface of MW (top). Noticeable, the disorder micro-pits with the dimension of about 0.5 µm can be observed from the enlarged SEM image (Bottom). (c) Schematic of localized paths about random lasing in GaN MW. Figure 2. (a) Schematic of the setup for single MW lasing measurements. (b) Room temperature lasing spectra of sample with various pumping power. The inset shows output intensity as a function of excitation power density. (c) The images about near-field emission pattern of GaN RL with different excited power. Figure 3. CCD images of the spatially/spectrally resolved intensity (SSRI) of RL profile plotted as slit height versus wavelength with different pumping power. 1.02 Pth (a), 1.2 Pth (b) and 1.5 Pth (c). Here, the spatial resolution in vertical direction is about 2 μm. Figure 4. The CCD images of spatially and spectrally resolved RL intensity profiled at two different recording moments t1 and t2, respectively (a) and (b). (c). Simulated results about the spatial localized RL mode profile in GaN MW with different refractive index n=2.4 (top) and n=2.6 (bottom). Figure 5. (a) polarization behavior of random laser observed in GaN MW. The polarization of Random laser in GaN is not obvious. (b) FFT of the spectra with different pumped intensity. (c) The formation of localized random cavities inside the GaN microwire at low and high pump intensity/temperature. Figure 6. (a) Lasing emission spectra at different temperatures from MW, the emission spectra were obtained under an excitation density of 1.5Ith. (b) The dependence of lasing threshold versus temperature for GaN MW RL. (c) FFT of lasing spectra that obtained from different measured temperature. Inset, the scale of cavity versus temperature for GaN MW.

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figure-1 91x33mm (600 x 600 DPI)


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ACS Photonics

figure-3 132x43mm (300 x 300 DPI)


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ACS Photonics

figure-5 126x39mm (300 x 300 DPI)


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ACS Photonics

TOC figure 39x19mm (600 x 600 DPI)


Ultraviolet Random Laser Based on a Single GaN Microwire - ACS

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