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
Zinc oxide (ZnO) has a wide band-gap energy of approximately 3.37 eV at room temperature and a significantly larger exciton binding energy of 60 meV than the thermal energy at room temperature (26 meV). Furthermore, ZnO is a naturally abundant and eco-friendly material. Thus, ZnO is one of the most attractive semiconductor materials for optoelectronic applications in the ultraviolet (UV) region. ZnO nanocrystals especially have been paid great attention as the building blocks for the UV applications. ZnO nanocrystals can be synthesized by several methods, including nanoparticle-assisted pulsed laser deposition (NAPLD) [1,2], chemical vapor deposition (CVD) , electrodeposition process [4,5] and a solution route . There are many reports on various kinds of ZnO nanocrystals such as the nanowires, nanorods, nanosheets and nanoparticles.
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) [7,8]. 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 [1,2,9–13], nanosheets [5,6] and random lasing by nanorod arrays , nanoparticles  and thin films . 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.
Regarding the laser action in a single ZnO nanowire, the lasing mechanisms due to micro-cavity effect have been discussed and can be considered by two mechanisms of Fabry-Perot (FP) cavity effect [10,11] or whispering-gallery mode (WGM) [12,13]. In the FP cavity theory, both edges of the nanowire serve as mirrors. On the other hand, in the WGM theory, the hexagonal side walls of the ZnO nanowire play the role of mirrors by total internal reflection. In both theories, the light is confined inside the nanowire and amplified by stimulated emissions.
Compared to the ZnO nanowire, there are only a few reports on lasing from ZnO nanosheets [5,6]. 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 . 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.
In this study, we report lasing characteristics from a single ZnO nanosheet excited by the third-harmonic of a Q-switched Nd:YAG laser (λ = 355 nm, τ = 5 ns). A single ZnO nanowire was also examined at the same conditions for comparison. As a result, a clearer lasing spectra than the previous reports due to the modal structure from the single ZnO nanosheet were observed, and it is demonstrated that the ZnO nanosheet has a lower threshold power density (50 kW/cm2) than that of the ZnO nanowire (150 kW/cm2). The lasing mechanisms are discussed based on the lasing spectra and a simulated wave-propagation inside the nanosheet.
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 . 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) 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.
The observed spectra were plotted in Fig. 2(d) for different excitation power densities. When the excitation power density was increased, the modal structure became observable. The spectral width of each mode was limited by the spectral resolution of the detection system, which was 0.34 nm. The mode spacing was observed to be about 0.78 nm. The peak intensities for the mode at 389.3 nm in Fig. 2(d) were plotted as a function of the excitation power density as shown in Fig. 2(e), where the threshold behavior was clearly observed. The threshold power density for lasing was estimated to be about 150 kW/cm2. The peaks due to lasing at the longer wavelength appeared as the excitation power densities were increased. This is probably due to the band-gap renormalization caused by the interaction between the high density electrons and holes created by the optical excitation .
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 . 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. . 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 , 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 [11,20]. 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
Next, lasing in a single ZnO nanosheet was examined as well. The optical microscope image of the tested single nanosheet on a silica glass is shown in Fig. 3(a) , and the corresponding AFM image is shown in Fig. 3(b). As a result, the size of the nanosheet with a triangular shape was about 150 nm in thickness and 15 µm in length. The CCD image of the nanosheet excited by the third-harmonic of the Q-switched Nd:YAG laser beam at 450 kW/cm2 is shown in Fig. 3(c). PL from the red dotted circle in Fig. 3 (c) was observed by the spectrometer. The observed PL spectra are shown in Fig. 3(d) for different excitation power densities. The spectra showed the modal structure as well as the spectra from a single ZnO nanowire at a higher excitation power of 60 kW/cm2, indicating the onset of lasing. The red shift of the peak wavelength was also observed in this case, due to the band-gap renormalization under high excitation power density. The peak intensities at the red-broken line in Fig. 3(d) were plotted as a function of the excitation power density, as shown in Fig. 3(e). The threshold power density for lasing was estimated to be about 50 kW/cm2.
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−1(α2/α1) + 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.
Concerning ZnO nanosheets, there are only a few reports on lasing. F. Wang et al. reported the stimulated emission based on the narrowing of PL spectra from ZnO nanosheets excited by a laser beam excitation with a Ti: Sapphire laser (λ = 266 nm, τ = 120 fs). Those ZnO nanosheets were grown by cathodic electrodeposition . In addition, E. S. Jang et al. have also reported that lasing based on the narrowing of PL spectra from ZnO nanosheets excited by a Q-switched YAG laser (λ = 355 nm, τ = 6 ns) was observed. Those ZnO nanosheets were synthesized by a solution process, and the oscillation mechanism of a WGM type phenomenon inside the nanosheet were suggested . In these studies, a large number of ZnO nanosheets on a substrate were totally examined by the PL method. However, we investigated detailed PL characteristics from a single ZnO nanosheet, and then, the obviously clearer lasing spectra due to modal structure were obtained as well as the lasing spectra from a single ZnO nanowire. The threshold power density for lasing of the ZnO nanosheet (50 kW/cm2) was lower than that of the ZnO nanowire (150 kW/cm2) as well as the above report (25 kW/cm2) . It indicates that a ZnO nanosheet can be a superior laser medium compared to a ZnO nanowire.
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 , 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.
Lasing characteristics from a single ZnO nanosheet and a single ZnO nanowire as a reference were examined by photoluminescence (PL) method with the third-harmonic of a Q-switched Nd:YAG laser (λ = 355 nm, τ = 5 ns). Those ZnO nanocrystals were synthesized by carbothermal chemical vapor deposition (CVD) method. Lasing spectra due to modal structure were observed from both the ZnO nanosheet and the nanowire, and the threshold power densities of the ZnO nanosheet and the nanowire were 50 kW/cm2 and 150 kW/cm2, respectively. It indicates that the ZnO nanosheet can be a superior laser medium compared to the ZnO nanowire, and this is the first time that the lasing characteristics from a single ZnO nanosheet were observed. The lasing mechanisms were discussed by the observation of electrical-field propagations inside the nanosheet with a simulation software (Ansoft, HFSS ver. 11). As a result, oscillation routes would be formed by the total reflection at the lateral sides and the narrow-waveguide reflection at the tip.
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.
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