Random Lasing in ZnO Nanopowders Based on Multi-photon

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Random Lasing in ZnO Nanopowders Based on Multiphoton Absorption for Ultrafast Upconversion Application Jiao Tian, Guo-En Weng, Youyang Wang, Xiaobo Hu, Shaoqiang Chen, and Jun-Hao Chu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02300 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Nano Materials

Random Lasing in ZnO Nanopowders Based on Multi-photon Absorption for Ultrafast Upconversion Application Jiao Tian, Guoen Weng, Youyang Wang, Xiaobo Hu, Shaoqiang Chen* and Junhao Chu Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China *Corresponding author: [email protected]

ABSTRACT: This study reports ultraviolet picosecond random lasing (RL) based on multiphoton-absorption in Zinc Oxide (ZnO) nanopowders with a mean size of around 100 nm under femtosecond optical excitation. Upconversion random lasing is achieved with an excitation wavelength increasing from 600 nm to 1800 nm corresponding to a five-photon excitation. The RL threshold has a slight positive relationship with excitation wavelength, except for a significant increase with four-photon excitation at 1200 nm~1300 nm, demonstrating promising upconversion lasing properties of ZnO nanopowders. Remarkable lower RL threshold is observed at long excitation wavelengths ranging from 1500 nm~1800 nm than that for fourphoton pumping (1200 nm~1300 nm) processes. Defect states in ZnO are suggested to be the reason that results in the excitation wavelength dependence of random lasing. Transient RL characteristics are investigated using time-resolved photoluminescence measurements, with the

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RL pulse delay time and pulse width observed to decrease with increasing excitation power, demonstrating typical gain-switching lasing characteristics. The 2.6-ps ultrashort pulse width indicates potential for ZnO in ultrafast upconversion random lasing devices.

KEYWORDS: ZnO nanopowders, random lasing, multi-photon absorption, upconversion, ultrafast pulse, photoluminescence INTRODUCTION Random lasers (RLs), which have potential applications in display, medical diagnostics and imaging,1,2 have been intensely studied in recent decades due to their mirror-less structure.1,3 Unlike conventional lasers, where the gain medium occurs inside a precisely designed mirrorbased optical cavity, the RL optical phenomenon is caused by multiple scattering in a disordered gain medium.4 However, some lasing properties in RLs are similar to those of a regular laser, such as linewidth reduction and nonlinear increase with increasing pump power in emitting intensity concomitant with a pump power threshold. Based on the feedback mechanisms, RLs can be classified into two categories: 1) incoherent RLs with incoherent feedback and 2) coherent RLs with coherent feedback,3 both of which have been demonstrated in combinations of gain media and scatters that consist of random lasing materials, such as titanium dioxide (TiO2) nanoparticles-doped dye solution,5,6 polymers,7,8 semiconductors,9-11 powdered laser crystals,12 biological media,13,14 etc.15 Generally, coherent feedback occurs due to light interference, which presents several narrow spikes in the emission spectrum, while the corresponding lasing characteristic of incoherent feedback is a smooth spectrum caused by a diffused propagation of light.1,3,9 Numerous RL mediums can emit specific lasing wavelengths, such as the ultraviolet (UV) wavelength,9 which is relatively difficult to obtain when using conventional lasers.


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Therefore, RLs emitting such wavelengths have a greater development prospects in particular applications. A promising UV laser material, ZnO has drawn widespread attention due to its direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV. Its high gain and strong potential for scattering make it suitable for random lasing in the near-UV region. Although room temperature (RT) UV lasing in bulk ZnO crystals has been demonstrated by C. Klingshirn in 1975,16 RT random lasing behaviour under optical pumping was not reported until 1998.17 Following that, RL behaviours were widely investigated in various ZnO micro/nanostructures, such as micro/nanodisks, micro/nanowires, microneedle, powders and polycrystal films.18-23 Although most UV random lasing actions in ZnO have been obtained using one-photon excitation, efficient two- and three-photon-excited stimulated emissions have also been demonstrated in different ZnO structures, such as ZnO-on-Si nanostructured films and ZnO powders.21,22 In general, the absorption efficiency of single-photon-pumped light is higher than multi-photon excitation. While the upconversion process must employ multiple photons to obtain light emission, downconversion only requires a single photon. Nevertheless, there are several major advantages to upconversion excitation, including large penetration depth and high spatial resolution, giving it potential in practical applications in multi-photon-absorption fluorescence microscopy,24 three-dimensional optical data storage,25 optical limiting,26 and imaging.27 Moreover, upconversion processes make it possible to obtain high-frequency light using lowfrequency light. For example, UV laser emission can be obtained by directly pumping with a visible or near-infrared excited source. These advantages have motivated this intensive research into ZnO random lasing behaviours with multi-photon excitation, especially with two- and threephoton excitation. Recently, H. Zhu et al. comprehensively illustrates four-, five-, six-, and


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seven-photon absorption upconversion lasing in ZnO.28,29 Efficient multi-photon absorption was reported in these studies. Whereas, the RL research on four- and five-photon excitation is still rare and the high threshold required for lasing is always one of the main obstacles limiting potential applications of upconversion RLs devices. This study obtains room-temperature upconversion UV RL emission with incoherent feedback in ZnO nanopowders using femtosecond (fs) optical pumping. Efficient two-, three-, four- and five-photon-pumped (2PP, 3PP, 4PP, 5PP, respectively) UV random lasing from ZnO powders was demonstrated and discussed. Lower threshold at 5PP as well as 4PP (1500 nm) processes than that for 4PP (1200 nm~1300 nm) case occurred. With multi-photon excitation, the PL spectra showed a near band edge exciton RL emission band belonging to the UV region, and an absence of spikes in the emission spectra indicates incoherent feedback. The RL dynamics were further investigated via time-resolved photoluminescence (TRPL) measurements based on a Kerr-gate method. Ultrashort RL pulses with a pulse width as short as 2.6 ps were obtained for application in ultrafast upconversion lasing devices. EXPERIMENTAL SECTION ZnO Nanopowders Preparation: ZnO nanopowders were prepared following a low-cost simple synthesis route exploiting a sol-gel. To begin, citric acid (9.78 g) was dissolved in deionized water (15 mL) and stirred for 20 min (Figure 1a and 1b). Next, the precursor sol (35 mL) was prepared by mixing the above citric acid solutions and zinc nitrate hexahydrate (1.78 g) in deionized water at room temperature and magnetically stirred for 3 h (Figure 1c and 1d). The resulting sol was subsequently dried at 80 °C for 4 h, and then the temperature was increased to 100 °C until most of water had evaporated (about 12 h) to obtain the gel (Figure 1e). Then, the gel was cooled to room temperature and ground manually into fine powders. The fine powders


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were calcined at 700 °C for 2 hours in a high-temperature furnace to form the precursor powders (Figure 1f). The final ZnO powders were acquired by manually grinding the precursor product.

Stirring

Adding

20 min

raw material

(a)

(f)

H 2O

ZnO

Zn(NO3)2·6H2O

(c) Stirring, 3h

(b) Sintering

Drying

700℃, 2h

80℃, 4h; 100℃,12h (e)

(d)

Figure 1. Illustrative Diagram for the Synthetic Process of the ZnO nanopowders. Characterization and Random Lasing Measurements: The microstructure of the ZnO nanopowders was characterized using SEM (XL30FEG, Philips) and XRD (D8 Advance, Bruker AXS; Cu Kα) at room temperature. A Ti: sapphire pulse laser operating at 800 nm, 35 fs pulse duration, and 1 kHz repetition rate acted as the excitation source for RL emission. The outputs of an optical parametric amplifier (OPA) with tunable wavelength have the same pulse duration and repetition rate. Then, the ZnO nanopowders were pressed into a thin disk that had a diameter of 6 mm and thickness of 1 mm. The excitation beam was focalized to a diameter of 500 μm with a confocal system. Next, the PL signals were detected with an electrically-cooled CCD (CCD: charge-coupled device) -coupled triple-grating spectrometer (SR303, Andor). Fundamental laser


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pulses (800 nm) from the Ti: sapphire laser acted as gating pulses and an excitation source for the TRPL measurements. All measurements were completed at room temperature. RESULTS AND DISCUSSION The ZnO powders were prepared following a low-cost simple sol-gel synthesis method. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements were performed to confirm the morphology and crystallographic orientation of the ZnO grains. SEM images with low and high magnifications, as shown in Figure 2a and 2b, show that the stacked ZnO grains have a polyhedral shape with a mean size of about 100 nm. The XRD pattern in Figure 2c confirms the ZnO powder crystalline phases. The diffraction pattern has a dominant sharp peak at 36.27°, which is indexed to the diffraction peak from 101 surfaces, and other relatively weak peaks are also shown in Figure 2c. The results reveal that ZnO grain growth prefers an orientation along the (101) direction under our experimental conditions.

Figure 2. (a), (b) SEM images with different magnifications (scale bars: 500 nm (a); 400 nm (b) and (c) X-Ray diffraction pattern of ZnO powders synthesized at 700 °C. (d) Optical images of


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the emissions from ZnO nanopowders with various pump powers under 800-nm excitation. Figure 2d shows optical images of the sample with 800-nm optical excitation with an excitation spot size of about 500 μm and various pump powers. As pump power increases, emission brightness increased abruptly, displaying generation of UV random lasing in the ZnO nanopowders. A corresponding supporting video is attached, showing the optical emission process.

Figure 3. Two-photon excited ZnO RL characteristics. (a) Emission spectra at various pump powers under 2P (λexc = 600 nm) excitation. Inset shows the proposed mechanism for 2PP random lasing. (b) Peak intensity (red spheres) and FWHM (blue squares) versus excitation intensity for excitations at 600 nm. (c) Emission spectra and (d) peak intensity and FWHM


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versus excitation intensity for excitations at 700 nm. The symbol ‘mJ cm-2/pulse’ is the unit of emission threshold (Eth) in (b) and (d). The measured 2PP excitation-power-dependent emission spectra following excitations of 600 nm and 700 nm are shown in Figure 3a and 3c, respectively. For clarity, the curves have been vertically shifted, and some are magnified 5-fold, 25-fold, etc. Under low excitation intensity, only a broad spontaneous emission spectra centred at near 390 nm (the black curves in Figure 3a and 3c) occurred, which is assigned to near band edge emission.18,30,31 As excitation intensity increases, the broad photoluminescence band narrows, and emitted intensity high nonlinear growth can be observed in Figure 3b and 3d. The log plot of PL intensity versus excitation intensity exhibits a representative kink behaviour and the emission threshold (Eth) is derived to be about 0.31 mJ cm-2/pulse and 0.33 mJ cm-2/pulse for excitations at 600 nm and 700 nm, respectively. The full width at half maximum (FWHM) decreased dramatically from ~18 nm to ~4 nm as the excitation intensity rises above the threshold for 600 nm pumping, and it decreases from ~21 nm to ~4 nm for 700 nm pumping. The figures also show that the spectra broaden and redshift with increasing pump power when the excitation intensity exceeds the threshold, which likely results from the exciton-exciton interactions enhanced electron-hole plasma (EHP) emissions at higher excitation intensities.32-35 To gain further insight into multi-photon absorption (MPA) behaviour, the fitting curve of PL intensity versus pumping intensity below the threshold has been plotted, which can discern the number of photons absorbed in the excitation process. According to the formula IM ∝ (IL)n, where the emission intensity IM through MPA is proportional to the incident laser light intensity (IL)n, the slope (n) of the fitting curve corresponds to the absorbed number of photons n. Figure 3b and 3d show that the value of the slope is about 1.62 and 1.91, respectively, which


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corroborates the two-photon absorption process. As illustrated in the inset of Figure 3a, the energy of one photon at 600 nm and 700 nm is smaller than that of an emission photon, however, the total energy of two photons is enough to excite near band edge transition. Upconversion emission for two-photon absorption (2PA) has two possible mechanisms, including band-to-band transitions via virtual states by simultaneous absorption of two photons and transitions via defects or interstitial states within the band gap by stepwise absorption of two photons.36,37 Furthermore, typical spikes for RLs with coherent feedback cannot be observed at any excitation intensity, meaning that the random lasing in our nanostructured ZnO powders is incoherent. As mentioned before, diffused propagating light through the randomly distributed ZnO nanopowders gain volume creates incoherent feedback in the system.

Figure 4. Three-photon excited ZnO RL characteristics. (a), (c) Evolutions of the emission spectra and (b), (d) peak intensity and FWHM with increasing excitation intensity for excitation


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at 800 nm and 1000 nm, respectively. The symbol ‘mJ cm-2/pulse’ is the unit of emission threshold (Eth) in (b) and (d). Inset in (a) illustrates the proposed mechanism for 3PP random lasing. Emission spectra at different excitation intensities for three-photon absorption (3PA) at 800 nm and 1000 nm are plotted in Figure 4a and 4c, respectively. The emission wavelength (390 nm~396 nm) is shorter than half and longer than one-third of the excitation wavelength (800 nm and 1000 nm). This indicates that photoluminescence (PL) emission is induced by 3PA as shown in the inset of Figure 4a. We observe an interesting spectra optical behaviour for 800 nm pumping, where a UV band from ~392 nm to ~396 nm coexists with a violet band centred at about 401 nm. A similar result was reported by C. T. Dominguez et al.21,22 At weak excitation intensity, the UV band is broad and is barely detectable. However, its emission intensity increases much faster than that of the violet band as the pump power increases, resulting in a dominant emission for the UV band at high excitation intensities. Here, note that the violet band emission is due to second-harmonic generation (SHG)

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since the 401 nm band is not present

under the excitations of 600 nm, 700 nm and 1000 nm. FWHM of the UV emission is obtained by Lorenz fitting of the spectra. The linewidth narrowing and threshold behaviour of the emission intensity are evidence of RL in the ZnO nanopowders, as shown in Figure 4b and 4d. The RL threshold is estimated to be about 0.59 mJ cm-2/pulse (pumped by 800 nm) and 0.87 mJ cm-2/pulse (pumped by 1000 nm). The fitting of the PL intensity versus pumping energy below the threshold gives slope value of 2.73 and 2.88 for excitation at 800 nm and 1000 nm, respectively. This confirms that the PL emission resulted from 3PA. Note that these thresholds are slightly larger than those for 2PA due to the nonlinear optical absorption process,21,38 where


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the 3PA and 2PA correspond to fifth order and third order nonlinear effects, and the transition probability of nonlinear phenomenon decreases with increasing nonlinear order.19,39

Figure 5. Four-photon excited ZnO RL characteristics. (a), (c), (e) Emission spectra and (b), (d), (f) peak intensity and FWHM at various pump energies under excitation at 1200 nm,1300 nm and 1500 nm, respectively. The symbol ‘mJ cm-2/pulse’ is the unit of emission threshold (Eth) in (b), (d) and (f). Inset in (a) illustrates the energy diagram showing the proposed mechanism for 2PP random lasing. Compared to the 2P and 3P induced RL UV emission processes, reports on 4PP and 5PP RL processes with ZnO nanopowders are rare29. Figure 5 shows the power dependence of PL


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characteristics under excitations of 1200 nm, 1300 nm and 1500 nm. The sum energy of the corresponding three photons is not enough to overcome the band gap, so the transition from valence band to conductor band must absorb four photons. The slope value of about 4 indicates a four-photon absorption (4PA) process. For 4PA, Figure 5 demonstrates obvious linewidth narrowing and the threshold requirement for generating random lasing. The RL thresholds are estimated to be 3.21 mJ cm-2/pulse, 6.27 mJ cm-2/pulse and 1.24 mJ cm-2/pulse with 1200 nm, 1300 nm and 1500 nm pumping, respectively. Here, note that the RL threshold for 4PA is higher than that for 3PA, owing to the lower efficiency of a nonlinear optical absorption process, whereas the threshold for excitation at 1500 nm exhibits a considerable decrease compared with those at 1200 nm and 1300 nm. Moreover, the 5PP upconversion RL is dramatically more effective than that of the 4PP process except at 1500 nm pumping. As shown in Figure 6, the thresholds (2.47 and 2.17 mJ cm-2/pulse at 1600 and 1800 nm pumping, respectively) of 5PP RL actions are smaller.

Figure 6. Five-photon excited ZnO RL characteristics. (a), (c) Evolution of the emission spectra


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and (b), (d) peak intensity and FWHM with increasing excitation intensity at 1600 nm and 1800nm, respectively. The symbol ‘mJ cm-2/pulse’ is the unit of emission threshold (Eth) in (b) and (d). Inset in (a) illustrates the energy diagram showing the proposed mechanism for 5PP random lasing. In general, similar to proposed upconversion mechanisms for 2PP emission, there are two major mechanisms in multiphoton pump process. The first occurs when multiple photons are simultaneously absorbed by a virtual level and the second occurs when some photons are absorbed by a metastable excited level followed by further photon absorption or excitation via energy transfer.40,41 The second mechanism is more efficient because the intermediate states involved are not virtual but real states.36 Taking into account everything mentioned above, it is likely that the phenomenon of lower thresholds can be attributed to real metastable state absorption at 1500 nm~1800 nm and virtual intermediate state absorption at 600 nm~1300 nm. To investigate the optical quality of ZnO nanopowders, a linear-absorption-induced random lasing action at 355 nm was also observed in the UV region. As shown in Figure 7a, 7b and 7d, important lasing characteristics, including lasing threshold, FWHM, and polarization, were studied. The threshold of 0.13 mJ cm-2/pulse for one-photon absorption (1PA) decreases about 3~49 times compared with multi-photon absorption. FWHM narrows from about 20 nm to 5 nm. The slope of the best-fit curve for spontaneous emission is about 1, which is consistent with the 1PP process (Figure 7c). For the polarization measurements, a polarizer is placed in the signal collection path, whose direction is rotated to measure the signal intensity for various polarization angles. Figure 7d shows the emitted RL polarization behaviour at different polarization angles. Under RL conditions, the emission shows a dumbbell angular dependency that exhibits clear characteristics of high polarization and obvious orientation of the RL polarization. Figure 7e


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shows the excitation-wavelength-dependent thresholds of random lasing in ZnO nanopowders induced by one- to five-photon absorption. In contrast to 1P, 2P, and 3P excitations, the RL threshold for 4P excitation (1200~1300 nm pumping) exhibits a considerable increase. Whereas, the threshold shows a dramatic decrease when the excitation wavelength is more than 1300 nm, showing that the ZnO nanopowders have a promising upconversion RL property, even with fivephoton absorption (5PA). When going from 1PA to 4PA, it can be speculated that linear single– photon and nonlinear multi-photon absorption schemes are responsible for the threshold increment phenomenon. For the remarkably low threshold at long excitation wavelengths, it is reasonable if the intermediate states involved in photon absorption are real in 5PA and 4PA (1500 nm) process but the intermediate states in photon absorption are virtual in 4PA (1200 nm~1300 nm) and 1PA-3PA process. Some of rich defect levels in ZnO reported in the literature,37,42-46 such as vacant Zn (VZn), vacant O (VO), interstitial Zn (Zni), interstitial O (Oi), and complex of VO and Zni (VOZni), as shown in Figure 7f, can provide real intermediate states. Defect states of VO and Zni are respectively located at about 1.75 eV and 2.77~3.0 eV above the valence band maximum (VBM). The energy interval from VBM to complex for the two defect states ranges from 2.2~2.4 eV. The energetic position of defect states in the ZnO band differs slightly with different preparations and theoretical calculation methods, so the complex defect states of VO and Zni should be located in the range of 1.75~3.0 eV above the VBM. In our work, the continuous-wave PL spectrum of ZnO nanopowders (Figure S1 in Supporting information) shows a broad emission band in the visible range from 500 nm to 800 nm, which is generally attributed to defect-related luminescence. Combined with the photon energies in Table 1, there should be some suitable positions of the VO and Zni (VO Zni) complexes that can be regarded as real intermediate states for excitation at 1500~1800 nm. In this case, three pump photons


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(photons energies: 2.067~2.49 eV for 1800~1500 nm) are first absorbed into the real states of VOZni, followed by absorption of two additional photons (or one photon at 1500 nm) to the valence band. This is consistent with the fact that VO and Zni are the predominant defect types for samples annealed in air.44,45 Thus, the phenomenon of lower thresholds can be attributed to the real metastable state absorption at 1500 nm~1800 nm. It’s well known that high threshold is always one of the main obstacles limiting potential applications of upconversion RL. So it is always desired to obtain low threshold in the upconversion process. Meanwhile, more photons involved in the upconversion process can provide more diverse nonlinear optical applications. In present, although 2P, 3P, 4P, 5P, 6P and 7P lasing have been achieved in semiconductors,19,23,28,29 reports on the above 3-photon excited random lasing are still very rare. The RL thresholds of ZnO nanopowders via 2P and 3P excitation in our cases are about two orders of magnitude higher than that of the ZnO nanostructured films and powders21,22,34, and smaller than those reported in CdSe/CdS/ZnS coremulti-shell QDs,47 FAPbBr3/PEO thin film,48 and MAPbBr3 thin film11 as well as some other material systems.49-51 Notably, 4P- and 5P-inducted RL were also systematically explored in our case. The much lower thresholds of the ZnO nanopowders unambiguously indicate their more superior nonlinear optical characteristics. Table1. Excitation wavelengths (nm) and corresponding photon energies (eV) under 1- to 5photon pumping. One-photon 355 (3.49)

Two-photon

Three-photon

Four-photon

Five-photon

500 (2.48)

800 (1.55)

1200 (1.03)

1600 (0.78)

600 (2.07)

900 (1.38)

1300 (0.95)

1800 (0.69)

700 (1.77)

1000 (1.24)

1400 (0.89)


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1100 (1.12)

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1500 (0.83)

Figure 7. (a) Evolutions of the emission spectra and (b) peak intensity and FWHM with increasing excitation intensity for one-photon absorption at 355 nm. The symbol ‘mJ cm-2/pulse’ is the unit of emission threshold (Eth) in (b). (c) Energy diagram of the proposed mechanism for 1PP random lasing. (d) Polarization measurement of the RL emission at 355 nm. (e) RL threshold under 1P~5P excitation at 355 nm~1800 nm. (f) Draft of the calculated and experimental defect levels in ZnO.


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Although various lasing thresholds are dependent on excitation wavelengths, similar phenomena can still be observed for all pump wavelengths in our case. As pump intensity increases, the FWHM of the emission band decreases first and then increases due to the formation of an electron-hole plasma state at high excitation intensities.34 A peak shift toward a longer wavelength (redshift) due to a band gap renormalization can also be observed for all excitation situations (see Supporting information).33 In addition, the typical spikes for RLs with coherent feedback are all absent. Consequently, the RL in our nanostructured ZnO powders is actually incoherent under both multi- and single-photon excitation. As for the lasing mechanism, as mentioned above, the lasing behaviour in our ZnO nanopowders results from multiple light scattering. Moreover, the ZnO nanoparticles act as both gain medium and scatter. Light is efficiently multiply scattered due to particle randomness, eventually resulting in random lasing. To further investigate the dynamics of ZnO nanopowder upconversion random lasing we measured the excitation-intensity-dependent time-resolved photoluminescence spectra under the excitation of an 800-nm fs laser using an optical Kerr-gate method52 at room temperature. For the 0.611 mJ cm-2/pulse excitation intensity below threshold, as shown in Figure 8a and 8e, a broad emission spectrum with a central wavelength of around 402 nm and a duration time of 2.5 ps was measured. The highly dispersed spectrum (Figure 8a) and narrow pulse width further indicate that the violet band emission is indeed the pump pulse SHG signal, and there is no random lasing emission from the sample. As shown in Figure 8b and 8f, when excitation power increases to 0.611 mJ cm-2/pulse, an emission of around 392 nm with a pulse width of 4.0 ps was measured. In Figure 8c, 8d, 8g and 8h, when excitation intensity increases to 0.917 mJ cm-2/pulse, an RL emission at 392 nm dominates the PL emission, and pulse width reduces from 4.0 ps to 2.6 ps.


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Figure 8. TRPL images under excitation of 800 nm from ZnO nanopowders at excitation intensities of 0.611 mJ cm-2/pulse (a), 0.713 mJ cm-2/pulse (b), 0.764 mJ cm-2/pulse (c) and 0.917 mJ cm-2/pulse (d). (e), (f), (g) and (h) Linear plots of the pulse waveforms at 0.611 mJ cm2/pulse,

0.713 mJ cm-2/pulse, 0.764 mJ cm-2/pulse and 0.917 mJ cm-2/pulse excitation intensity,


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respectively. The waveform on the left and right respectively correspond to the waveform of the SHG pulse and RL pulse. The arrows indicate pulse widths.

Figure 9. (a) Linear plot and (b) the corresponding log plot of the pulse waveforms at different pump intensities. Compared to the pump pulse SHG signal, RL pulses have a long delay and the delay time decreases with increasing pump intensity. (c) Log plot of the waveform of RL pulse at pump intensity of 0.917 mJ cm-2/pulse. An exponential model (solid lines) fits well to the experimental data, from which the decay lifetime (τdecay) and rise time (τrise) can be extracted. (d) Delay time, pulse width, decay time and rise time of RL pulses with various excitation intensities. Figure 9a and 9b show that, compared to pump pulse SHG signal, RL pulses have a long delay and the delay time decreases with increasing pump intensity. Furthermore, an exponential model fits well to the experimental data for all excitation cases. At excitation intensity of 0.917 mJ cm2/pulse,

as shown in Figure 9c, the decay lifetime (τdecay) and rise time (τrise) of the RL pulse are 1.9 ps

and 1.4 ps, respectively. The pulse width, rise time, decay time and delay time of these RL pulses


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at different excitation intensities are summarized in Figure 9d. The narrow emission spectra with an ultrafast lifetime confirm that emissions from the ZnO nanopowders are indeed lasing. Meanwhile, decreased pulse width and delay time with increasing excitation intensity as well as exponential rise and decay are typical characteristics of gain-switched pulses.53 Gain switching is an important method for short pulse generation in semiconductor lasers. The results demonstrate that gain switching can occur even with multiple photon absorption, and the obtained 2.6-ps ultrashort pulses are comparable to advanced semiconductor pulse lasers,54 indicating that ZnO nanopowders have great potential for application in ultrafast upconversion lasing devices.

CONCLUSION In conclusion, single- and multi-photon-pumped RL emissions have been demonstrated in ZnO nanopowders under femtosecond laser pulse excitation at various wavelengths ranging from 355 to 1800 nm. The absence of spikes in lasing spectra indicates incoherent feedback of multiple light scattering during random lasing. As excitation wavelengths increase, the RL threshold induced by 1P, 2P, 3P and 4P absorption increases due to linear single–photon and nonlinear multi-photon absorption schemes. Meanwhile, compared to 4P absorption from 1200 nm~1300 nm, a higher efficiency upconversion RL can be achieved with 5P pumping and 4P pumping at 1500 nm despite a lower absorption coefficient for higher-order nonlinear absorption processes. This phenomenon can be attributed to real metastable state absorption at 1500 nm~1800 nm but a virtual intermediate state absorption at 600 nm~1300 nm. Excepting cases from 1200 nm~1300 nm, the slight lasing threshold increase with increasing excitation wavelength reveals that ZnO nanopowders have a promising upconversion RL property. Furthermore, the clearly observed time- and wavelength-resolved 2-D image and the exponential profiles of the pulse with an


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ultrafast decay lifetime of 1.9 ps and an ultrashort pulse width of 2.6 ps indicate high quality upconversion random lasing. The efficient room-temperature multi-photon-absorption RL emission in the UV region of ZnO nanopowders can be used in designing ultrafast upconversion lasing devices for applications in information storage, biomedical imaging, as well as biologic and medical theranostics. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge at the ACS Publications website. CW-PL spectrum, redshift phenomenon, optical video under an 800-nm optical excitation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Guoen Weng: 0000-0001-7786-2133 Shaoqiang Chen: 0000-0002-0458-3894 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 61604055, 61704055, 61874044), and the Program of Shanghai Science and Technology Committee (No. 17142202500).


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REFERENCES (1) Wiersma, D. S. The Physics and Applications of Random Lasers. Nat. Phys. 2008, 4,359367. (2) Redding, B.; Choma, M. A.; Cao, H. Speckle-free Laser Imaging Using Random Laser Illumination. Nat. Phys. 2012, 6, 355-359. (3) Cao, H. Lasing in Random Media. Waves Random Media 2003, 13, R1-R39. (4) Ramachandran, H. Mirrorless Lasers. Pramana 2002, 58, 313-323. (5) Lawandy, N. M.; Balachandran, R. M. A.; Gomes, S. L.; Sauvain, E. Laser Action in Strongly Scattering Media. Nature 1994, 368, 436-438. (6) de Matos, C. J.; De, S. M. L.; Brito-Silva, A. M.; Gámez, M. A. M.; Gomes, A. S. L.; de Araújo, C. B. Random Fiber Laser. Phys. Rev. Lett. 2007, 99, 153903. (7) Tulek, A.; Polson, R. C.; Vardeny, Z. V. Naturally Occurring Resonators in Random Lasing of π-conjugated Polymer films. Nat. Phys. 2010, 6, 303-310. (8) Lisinetskii, V.; Ryabchun, A.; Bobrovsky, A.; Schrader, S. Photochromic Composite for Random Lasing Based on Porous Polypropylene Infiltrated with Azobenzene-Containing Liquid Crystalline Mixture. ACS Appl. Mater. Interfaces. 2015, 7, 26595–26602. (9) Cao, H.; Zhao, Y. G.; Ho, S. T.; Seelig, E. W.; Wang, Q. H.; Chang, R. P. H. Random Laser Action in Semiconductor Powder. Phys. Rev. Lett. 1999, 82, 2278-2281. (10) Gollner, C.; Ziegler, J.; Protesescu, L.; Dirin, D. N.; Lechner, R. T.; Fritz-Popovski, G.; Sytnyk, M.; Yakunin, S.; Rotter, S.; Amin, A. A. Y.; Vidal, C.; Hrelescu, C.; Klar, T. A.; Kovalenko, M. V.; Heiss, W. Random Lasing with Systematic Threshold Behavior in Films of CdSe/CdS Core/Thick-Shell Colloidal Quantum Dots. ACS Nano. 2015, 9, 9792-9801. (11) Weng, G. E.; Xue, J. J.; Tian, J.; Hu, X. B.; Bao, X. M.; Lin, H. C.; Chen, S. Q.; Zhu, Z. Q.;


22

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chu, J. H. Picosecond Random Lasing Based on Three-Photon Absorption in Organometallic Halide CH3NH3PbBr3 Perovskite Thin Films. ACS Photonics 2018, 5, 2951-2959. (12) Noginov, M. A. In Solid-State Random Lasers; Rhodes, W. T., Ed., Springer, New York, 2005; Chapter 1, pp 1-7. (13) Song, Q. H.; Xiao, S. M.; Xu, Z. B.; Liu, J. J.; Sun, X. H.; Drachev, V.; Shalaev, V. M.; Akkus, O.; Kim, Y. L. Random lasing in bone tissue. Opt. Lett. 2010, 35, 1425-1427. (14) Zhang, D. K.; Kostovski, G.; Karnutsch, C.; Mitchell, A. Random Lasing from Dye Doped Polymer within Biological Source Scatters: The Pomponia Imperatorial Cicada Wing Random Nanostructures. Org. Electron. 2012, 13, 2342-2345. (15) Viola, I.; Ghofraniha, N.; Zacheo, A.; Arima, V.; Conti, C.; Gigli, G. Random Laser Emission from a Paper-based Device. J. Mater. Chem. C 2013, 1, 8128-8133. (16) Klingshirn, C. The Luminescence of ZnO under High One- and Two-Quantum Excitation. Phys. Status Solidi B 1975, 71, 547-556. (17) Tang, Z. K.; Wong, G. K.; Yu, L. P.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Room-temperature Ultraviolet Laser Emission from Self-assembled ZnO Microcrystallite Thin Films. Appl. Phys. Lett. 1998, 72, 3270-3272. (18) Dai, J.; Xu, C. X.; Li, J. T.; Lin, Y.; Guo, J.; Zhu, G. Photoluminescence and Two-Photon Lasing of ZnO:Sn Microdisks. J. Phys. Chem. C 2014, 118, 14542-14547. (19) Zhu, G. P.; Xu, C. X.; Zhu, J.; Lv, C. G.; Cui, Y. P. Two-photon Excited Whisperinggallery Mode Ultraviolet Laser from an Individual ZnO Microneedle. Appl. Phys. Lett. 2009, 94, 051106. (20) Que, M. L.; Wang, X. D.; Peng, Y. Y.; Pan, C. F. Flexible Electrically Pumped Random Lasing from ZnO Nanowires Based on Metal–insulator–semiconductor Structure. Chin. Phys. B


23


Page 24 of 28

2017, 26, 067301. (21) Dominguez, C. T.; Gomes, M. A.; Macedo, Z. S.; de Araújo, C. B.; Gomes, S. L. Multiphoton Excited Coherent Random Laser Emission in ZnO Powders. Nanoscale 2015, 7, 317-323. (22) Dominguez, C. T.; Vieira, M. S.; Oliveira, R. M.; Ueda, M.; de Araújo, C. B.; Gomes, A. S. L. Three-photon Excitation of an Upconversion Random Laser in ZnO-on-Si Nanostructured Films. J. Opt. Soc. Am. B 2014, 8, 1975-1980. (23) Ghosh, P.; Sharma, A. K. Two-photon Induced Photoluminescence and Lasing in Pulsedlaser Deposited ZnO Nanostructures Pumped by Continuous Wave He-Ne Laser. Opt. Mater. 2017, 66, 651-658. (24) Fischer, M. C.; Wilson, J. W.; Robles, F. E.; Warren, W. S. Invited Review Article: Pumpprobe Microscopy. Rev. Sci. Instrum. 2016, 87, 031101. (25) Jiu, H. F.; Zhang, L. X. Ce(DBM)3Phen Doped Poly(methyl methacrylate) for Threedimensional Multilayered Optical Memory by Femtosecond Laser. J. Rare Earths. 2009, 27, 786-789. (26) Liberman, V.; Rothschild, M.; Bakr, O. M.; Stellacci, F. Optical Limiting with Complex Plasmonic Nanoparticles. J. Opt. 2010, 12, 065001. (27) Zhou, J.; Liu, Z.; Li, F. Y. Upconversion Nanophosphors for Small-animal Imaging. Chem. Soc. Rev. 2012, 41, 1323-1349. (28) Chen, A. Q.; Zhu, H.; Wu, Y. Y.; Yang, D. C.; Li, J. Y.; Yu, S. F.; Chen, Z. Y.; Ren, Y. H.; Gui, X. C.; Wang, S. P.; Tang, Z. K. Low-Threshold Whispering-Gallery Mode Upconversion Lasing via Simultaneous Six-Photon Absorption. Adv. Optical. Mater. 2018, 6, 1800407. (29) Zhu, H.; Chen, A. Q.; Wu, Y. Y.; Zhang, W. F.; Su, S. C.; Ji, X.; Jing, P. T.; Yu, S. F.; Shan, C. X.; Huang, F. Seven-Photon-Excited Upconversion Lasing at Room Temperature. Adv.


24

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Optical Mater. 2018, 6, 1800518 (30) Lin, B. X.; Fu, Z. X.; Jia, Y. B. Green Luminescent Center in Undoped Zinc Oxide Films Deposited on Silicon Substrates. Appl. Phys. Lett. 2001, 79, 943-945. (31) Yu, W. D.; Li, X. M.; Gao, X. D. Catalytic Synthesis and Structural Characteristics of HighQuality Tetrapod-Like ZnO Nanocrystals by a Modified Vapor Transport Process. Cryst. Growth Des. 2005, 5, 151-155. (32) Djurisić, A. B.; Leung, Y. H. Optical Properties of ZnO Nanostructures. Small 2006, 2, 944961. (33) Yamamoto, A.; Kido, T.; Goto, T.; Chen, Y. F.; Yao, T. Bandgap Renormalization of ZnO Epitaxial Thin Films. Solid State Commun. 2002, 122, 29-32. (34) Chelnokov, E. V.; Bityurin, N. Two-photon Pumped Random Laser in Nanocrystalline ZnO. Appl. Phys. Lett. 2006, 89, 171119. (35) Lee, S. H.; Goto, T.; Miyazaki, H.; Yao, T. Waveguide Lasing from V-shaped ZnO Microstructure. Opt. Lett. 2013, 38, 2413-2415. (36) Chen, S. L.; Stehr, J.; Reddy, N. K.; Tu, C. W.; Chen, W. M.; Buyanova, I. A. Efficient Upconversion of Photoluminescence via Two-photon Absorption in Bulk and Nanorod ZnO. Appl. Phys. B 2012, 108, 919-924. (37) Ghosh, P.; Sharma, A. K. Two-photon Induced Photoluminescence and Lasing in Pulsedlaser Deposited ZnO Nanostructures Pumped by Continuous Wave He-Ne laser. Opt. Mater. 2017, 66, 651-658. (38) Zheng, Q. D.; Zhu, H. M.; Chen, S. C.; Tang, C. Q.; Ma, E.; Chen, X. Y. Frequencyupconverted Stimulated Emission by Simultaneous Five-photon Absorption. Nat. Photonics 2013, 7, 234-239.


25


Page 26 of 28

(39) Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Nonlinear Multiphoton Processes in Organic and Polymeric Materials. Rep. Prog. Phys. 1996, 59, 1041-1070. (40) Luan, F.; Gu, B. B.; Gomes, A. S. L.; Yong, K. T.; Wen, S. C.; Prasad, P. N. Lasing in Nanocomposite Random Media. Nano Today 2015, 10, 168-191. (41) Dong, Y. J.; Xu, J.; Zhou, G. Q.; Zhao, G. J.; Jie, M. Y.; Yang, L. Y.; Su, L. B.; Qiu, J. R.; Feng, W. W.; Lin, L. H. Blue Upconversion Luminescence Generation in Ce3+:Gd2SiO5 Crystals by Infrared Femtosecond Laser Irradiation. Opt. Express 2006, 14, 1899-1904. (42) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. Mechanisms behind Green Photoluminescence in ZnO Phosphor Powders. J. Appl. Phys. 1996, 79, 7983-7990. (43) Xu, P. S.; Sun, Y. M.; Shi, C. S.; Xu, F. Q.; Pan, H. B. The Electronic Structure and Spectral Properties of ZnO and Its Defects. Nucl. Instrum. Methods Phys. Res. Sect. B 2003, 199, 286290. (44) Ischenko, V.; Polarz, S.; Grote, D.; Stavarache, V.; Fink, K.; Driess, M. Zinc Oxide Nanoparticles with Defects. Adv. Funct. Mater. 2005, 15, 1945-1954. (45) Zeng, H. B.; Duan, G. T.; Li, Y.; Yang, S. K.; Xu, X. X.; Cai, W. P. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561-572. (46) Alkauskas, A.; Pasquarello, A. Band-edge Problem in the Theoretical Determination of Defect Energy Levels: The O Vacancy in ZnO as a Benchmark Case. Phys. Rev. B 2011, 84, 125206. (47) Wang, Y.; Ta, V. D.; Gao, Y.; He, T. C.; Chen, R.; Mutlugun, E.; Demir, H. V.; Sun, H. D. Stimulated Emission and Lasing from CdSe/CdS/ZnS Core-Multi-Shell Quantum Dots by


26

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Simultaneous Three-Photon Absorption. Adv. Mater. 2014, 26, 2954-2961. (48) Xu, L.; Meng, Y.; Xu, C. X.; Chen, P. Room Temperature Two-Photon-Pumped Random Lasers in FAPbBr3/Polyethylene Oxide (PEO) Composite Perovskite Thin Film. RSC Adv. 2018, 8, 36910-36914. (49) Liu, M.; Quah, H. S.; Wen, S. C.; Li, Y.; Vittal, J. J.; Ji, W. Multiphoton Absorption and Two-Photon-Pumped Random Lasing in Crystallites of a Coordination Polymer. J. Phys. Chem. C 2018, 122, 777-781. (50) Xu, X. H.; Zhang, W. F.; Jin, L. M.; Qiu, J. B.; Yu, S. F. Random Lasing in Eu3+ Doped Borate Glass-Ceramic Embedded with Ag Nanoparticles under Direct Three-Photon Excitation. Nanoscale 2015, 7, 16246-16250. (51) Zhang, W. F.; Ni, Y. Q.; Xu, X. H.; Lu, W.; Ren, P. P.; Yan, P. G.; Siu, C. K.; Ruan, S. C.; Yu, S. F. Realization of Multiphoton Lasing from Carbon Nanodot Microcavities. Nanoscale 2017, 9, 5957-5963. (52) Takeda, J.; Nakajima, K.; Kurita, S.; Tomimoto, S.; Saito, S.; Suemoto, T. Time-resolved Luminescence Spectroscopy by the Optical Kerr-gate Method Applicable to Ultrafast Relaxation Processes. Phys. Rev. B 2000, 62, 10083-10087. (53) Chen, S. Q.; Yoshita, M.; Ito, T.; Mochizuki, T.; Akiyama, H.; Yokoyama, H.; Kamide, K.; Ogawa, T. Analysis of Gain-Switching Characteristics Including Strong Gain Saturation Effects in Low-Dimensional Semiconductor Lasers. Jpn. J. Appl. Phys. 2012, 51, 098001. (54) Asahara, A.; Chen, S. Q.; Ito, T.; Yoshita, M.; Liu, W. J.; Zhang, B. P.; Suemoto, T.; Akiyama, H. Direct Generation of 2-ps Blue Pulses from Gain-switched InGaN VCSEL Assessed by Up-conversion Technique. Scientific Reports 2014, 4, 6401.


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Table of Contents Graphic


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Random Lasing in ZnO Nanopowders Based on Multi-photon

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