The Lasing-mode Switch of Hexagonal ZnO Pyramid Driven by

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

The Lasing-mode Switch of Hexagonal ZnO Pyramid Driven by Pressure within Diamond Anvil Cell Yadong Huang, Liu Yang, Chang Liu, Xinxia Liu, Junsong Liu, Xiaoping Huang, Pinwen Zhu, Tian Cui, Cheng Sun, and Yongjun Bao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03748 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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The Journal of Physical Chemistry Letters

The Lasing-mode Switch of Hexagonal ZnO Pyramid Driven by Pressure within Diamond Anvil Cell Yadong Huang1, Liu Yang1, Chang Liu1, Xinxia Liu1, Junsong Liu1, Xiaoping Huang2, Pinwen Zhu1, Tian Cui1, Cheng Sun3 and Yongjun Bao1, * 1. State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China 2. School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, China 3. Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Rd, Evanston, Illinois 60208, USA

E-mail: [email protected]

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The Journal of Physical Chemistry Letters 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

Abstract Nanolasers are expected to be integrated on chip as miniaturized coherent light source and their application is dependent on their lasing behavior strongly. In this work, the lasing behavior of a single hexagonal ZnO pyramid (HZOP) is tailored by tuning the electronic bandgap with pressure. The lasing of HZOP nanolaser is dominated by a helical whispering-gallery-like mode and the lasing threshold varies little with the increment of pressure. All lasing peaks of HZOP is limited in a spectral prescreen window on the right shoulder of the fluorescence emission and blueshift gradually accompanying with several abrupt hops with pressure increasing. This feature of spectral prescreen window originates from the strong coupling between excitons and the coupling is described with a dispersive complex refractive index. These results provide us a new perspective to tune and switch the lasing mode of nanolaser in precise by pressure induced bandgap broaden of semiconductor. TOC image

KEYWORDS: nanolaser, hexagonal ZnO pyramid, helical whispering-gallery-like mode, pressure, diamond anvil cell


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Semiconductor nanolasers are expected to speed up information communication between inter or intra optoelectrical devices on chip as coherent light source. It is very important to design and engineer a semiconductor nanolaser of desired mode profile to improve the coupling efficiency between optoelectrical devices at the lasing wavelength.1-4 Because the lasing behaviors of nanolaser are a joint effect of geometry configuration and intrinsic optical properties, several methods have been utilized to tailor the lasing of semiconductor nanolasers. The geometric configuration of nanolaser is performed by fabricating semiconductor nanocavity of various shape or deforming these nanocavities from its original shape.5, 6 In past two decades, a lot of semiconductor nanostructures of regular geometry have been fabricated successfully, such as nanorods, nanotetrapods, nanowires, nanorings, and nanobelts, where the lasing behaviors are tuned by various shaped semiconductor nanostructure cover all visible spectral range. Meanwhile, the lasing peaks have also been tuned within several nanometers by curving a nanowire to a loop of varying diameter or changing the nanowire length to control the intrinsic of self-absorption of nanowire. The intrinsic optical property of semiconductor is another adaptive parameter to tune the lasing peaks of nanolaser by engineering the electronic band gap of semiconductor directly. The lasing peaks is reported to be tune in whole visible wavelength range by varying the materials composite of CH3NH4PbX3 (Cl, Br, I) perovskite system.7 A 30 nm red shift of nanolaser is realized by the intrinsic absorption along the long axis of the CdS nanowire originating from the strong coupling between exciton and polariton.8 The fluorescence emission is blueshifted by doping massive electron to conducting band from the metallic structure nearby under high pump fluence, which is ascribed to Bustein-Moss effect.9 The lasing of micro-rod is regulated under the help of Vernier effect by tuning the coupling between micro-rods.10 Once the nanolaser is manufactured, the geometric shape or the intrinsic optical properties could not be changed anymore in an actual application. It becomes a critical challenge to study the lasing behavior of a specific nanolaser at different resonance wavelength. Pressure is proposed as a physical method to tailor the intrinsic optical properties of semiconductor nanolaser continuously without introducing additional composite.11-16 The lasing behavior of GaN nanowire laser has



been tuned with pressure up to 7.1 GPa.17 In this work, the lasing behavior of a hexanal ZnO pyramid (HZOP) is studied under pressure with a diamond anvil cell (DAC). ZnO is an II-VI semiconductor of wurtzite crystal structure with direct wide bandgap of 3.37eV and exciton binding energy of 60 meV under ambient pressure. The exciton binding energy of ZnO is larger than the thermal energy at room temperature (26 meV) and ZnO optoelectrical device can work at room temperature or even high temperature.3, 18-29 The large exciton binding energy potentiates the ZnO optoelectrical devices integrated on chip to conquer the overheat shortcoming of chip. Intensive attention has been focused on manufacturing ZnO miniaturized structure to work as ultra-violet nanolaser. As the semiconductor nanostructures not only amplify light as gain media but also resonate as optical cavity, diversities of method have been performed to fabricate ZnO cavities in micro & nano scale, such as thermal evaporation, metal–organic vapor phase epitaxy (MOVPE), laser ablation, hydrothermal synthesis3, 19-24.

There are three kinds of resonance mechanism, Fabry-Perot mode, whispering

gallery mode and random mode, are reported to dominate the lasing behaviors of various ZnO nanolaser. A helical whispering-gallery-like mode is responsible for the lasing behavior of a hexagonal ZnO pyramid (HZOP) nanolaser within a spectral prescreen window.30-34 Then pressure is expected to explore the evolution of the lasing mode of HZOP nanolaser gradually by shifting this prescreen window with pressure induced electronic bandgap broaden below 10 GPa, where the wurtzite to saltrock phase transition has not yet happed yet. The pressure related lasing spectrum of HZOP nanolaser is investigated by measuring their photoluminescence with a home-made micro photoluminescence spectrometer (μ-PL) for high pressure as shown in fig. 1 (a). An Almax Plate DAC (Almax easylab) is used to ensure the conical aperture about 85° to impose the pump fluence high enough to excite the stimulated emission of the investigated HZOP as shown in fig. 1 (b), where the high numerical aperture allows to observe a single HZOP within a DAC in darkfield mode. The HZOPs of high optical quality are manufactured on a sapphire substrate with quartz-tube chemical vapor deposition (CVD) and the geometric shape of HZOP is presented as shown in fig. 1 (c). The bottom side of


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pyramid is about 0.5 µm, the upper side is about 0.1 µm and the height is about 2 µm. The high-pressure sample chamber is composed of two opposite culets and a stainless steel (T301) gasket of 200 µm hole inserted between these two diamond anvils. The diameter of culet is 450 μm and the thickness of gasket is pre-indented to 73 µm from the original thickness 250 µm. Liquid nitrogen is used as pressure transmitting medium to ensure the refractive variation of surrounding media varies little under pressure. The pressure on the specimen is calibrated by the fluorescence emission R1 line of ruby sphere placed in the sample chamber excited by a pulsed 532 nm laser. In this experiment, a tungsten needle prepared with electrochemical corrosion is used to

Fig. 1 The schematic diagrams of optical experimental setup. (a) The illumination of the home-made micro spectrometer to measure the PL spectrum of a single HZOP nanolaser. The 532 nm laser (green) is used to calibrate the pressure within DAC with the R1 line emission of ruby sphere. The 355 nm laser (purple) is used to pump the HZOP. White light (yellow) is used for optical inspect the samples within DAC. The camera is used to take the optical images and PL images. The spectrograph is used to measure the (PL) spectra of HZOP within DAC. (b) The schematic is the Diamond Anvil Cell (DAC). The sample cell is composed with two opposite diamond facets and gasket drilled a hole. A ruby sphere is used to mark the pressure. (c) The schematic of a single HZOP loaded in DAC.



transfer the sample from sapphire substrate onto the culet of one diamond anvil. The samples are stirred for many times with the needle to disperse the HZOPs sparsely enough and isolate a single HZOP from each other. The lasing behavior of HZOP nanolaser is characterized by measuring the spontaneous and stimulated emission of a single HZOP nanolaser with variable pump fluence under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. The photoluminescence (PL) spectra under a specific pressure are plotted in a map of PL intensity as a function of wavelength and pump fluence as shown in fig. 2. The lasing of HZOP nanolaser is confirmed by the emergence of narrow peaks in the PL maps as the pump fluence increases over the lasing threshold. While pressure increases gradually, the spontaneous emission corresponding to the band edge emission of HZOP presents a continuous blueshift with pump fluence below the lasing threshold and the number of lasing modes vary with pressure the pump fluence above the lasing threshold. There is only one lasing peak at 389.63 nm under 0.30 GPa as shown in fig. 2 (a). However, there are two lasing peaks with the major peak at 388.70 nm and the minor peak at 383.89 nm under 0.63 GPa as shown in fig. 2 (b). Furthermore, there are two peaks in comparable intensity at 381.84 nm and 386.56 nm under 1.37 GPa as shown in fig. 2 (c). While pressure is increased to 2.7 GPa, 3.72 GPa and 4.61 GPa, single mode lasing is found at 378.87 nm, 377.09 nm, 375.98 nmas shown in fig. 2 (d), fig. 2 (e) and fig. 2 (f), respectively. With pressure increased further to 5.76 GPa, even three narrow peaks appear at 373.62 nm, 372.06 nm, and 369.37 nm in the PL map as shown in fig. 2 (g). There are two tiny lasing peaks at 371.62 nm and 367.36 nm under 6.73 GPa, as shown in fig 2 (h). No lasing peak can be found in the PL maps anymore. The sharp lasing peaks of HZOP originating from the stimulated emission shows a complicated pressure dependence relative to the fluorescence peak corresponding to spontaneous emission.


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Fig. 2 The pseudo-color PL spectral maps of a single HZOP nanolaser under pressure. (a)-(i) The PL maps are excited with incremental pump fluences under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. The scale bar on the right is a logarithmic color scale in arbitrary value. In order to investigate the pressure dependence of PL emission in detail, the PL spectra of HZOP nanolaser at low pump fluence and high pump fluence are analyzed individually. The spontaneous emission of HZOP is measured with the pump fluence of 3.2 mJ/cm2 under 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa and 4.61 GPa as shown in fig. 3 (a). An abrupt drop marked with a dashed line is found at 367.72 nm in all the PL spectra of HZOP and ascribed to the detectability of the spectrograph used in our lab. The fluorescence peaks under different pressure are identified from these PL spectra at 384.43 nm, 382.58 nm, 379.02 nm, 375.16 nm, 372.93 nm and 371.14 nm as plotted with the black half circles as shown in fig. 3 (c), while the fluorescence peaks under 6.73 GPa and 8.24 GPa can’t be extracted from the PL spectra anymore. The pressure parameter is used to describe the pressure dependence of spontaneous or stimulated emission on pressure as shown in fig. 3 (c), which are the slopes of the fitted



lines of resonance wavelength to pressure. The pressure parameter of the spontaneous emission of HZOP decreases with pressure increasing. The gradual blueshift of florescence peak originates from the pressure-induced bandgap broaden of ZnO before a wurtzite- rocksalt phase transition occurring. Meanwhile, the stimulated emission of HZOP nanolaser is measured with pump fluence of 12.3 mJ/cm2 under 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa as shown in fig. 3 (b), respectively. The lasing modes of HZOP nanolaser is confirmed by the narrow peak appearance in PL spectrum with pump fluence over the lasing threshold. The electronic bandgap, geometric shape and refractive index of HZOP change gradually with pressure increment below 10 GPa, while the cavity resonances of HZOP are discrete and provide the feedback mechanism for the lasing mods. All the lasing peaks are marked with color half circle and fitted with a line as a function of pressure as shown in fig. 3 (c). The abrupt change of the pressure dependence of lasing peak could be used the criterion to identify the switch of lasing mode with pressure increasing. These lasing peaks are divided into four groups marked with red, green, blue and magenta colors. The red mode is at 389.63 nm, 388.7 nm and 386.51 nm under 0.30 GPa, 0.63 GPa and 1.37 GPa. The green mode is at 381.67 nm, 378.87 nm and 377.09 nm under 1.37 GPa, 2.70 GPa and 3.72 GPa. The blue mode is at 375.98 nm, 373.65 nm and 371.56 nm under pressure of 4.61 GPa, 5.76 GPa and 6.73 GPa. The magenta mode is at 369.37 nm and 367.38 nm under pressure of 5.76 GPa and 6.73 GPa. During the process of pressure increasing, the red mode blueshifts gradually with a pressure parameter of 2.92 nm/GPa till to the emergence of a new lasing peak, green mode at 1.73 GPa. The green mode blueshifts gradually with a pressure parameter of 1.96 nm/GPa till to 3.72 GPa. Once the pressure is over 3.72 GPa, the abrupt change of pressure parameter could be used to judge the emergence of new lasing modes, blue mode and magenta mode with the pressure parameters of 2.08 nm/GPa and 2.05 nm/GPa, respectively. It is obvious that all of the lasing modes resonance at the wavelengths longer than the fluorescence peak and the lasing dependence that the pressure parameter of fluorescence peak is greater than the pressure parameter of lasing peak greatly at low pressure and the discrepancy of pressure parameter between


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fluorescence peak and lasing peak becomes little. This spectral phenomenon indicates the mechanism dominating the spontaneous and stimulated emission tend to be in consistent. The pressure related radiation of HZOP nanolaser under pressure is characterized by taking the PL images and measuring the corresponding PL spectrum under high pump fluence. The PL images and spectra of HZOP are excited with 0.93 mJ/cm2, 4.89 mJ/cm2, 6.90 mJ/cm2, 7.85 mJ/cm2, 10.04 mJ/cm2 and 13.58 mJ/cm2 under 4.61 GPa 0.

Fig. 3 The PL spectra of a single HZOP nanolaser are excited with low pump fluence and high pump fluence far from the lasing threshold under pressure. (a) The PL spectra of HZOP nanolaser are excited with about 3.2 mJ/cm2 under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. (b) The PL spectra of HZOP nanolaser are excited with the pump fluence about 12 µJ/cm2 under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. (c) The peaks of spontaneous emission (black half-filled circle) and stimulated emission (color half-filled circle) under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. The color lines of black, red, green, blue, cyan, magenta, yellow, dark yellow and navy in fig. (a) and fig. (b) present the PL spectra under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa. The black half-filled circles and the color half-filled circles present the peaks of spontaneous emission and stimulated emission in fig. (c), respectively. The peaks are separated into four lasing modes of HZOP nanolaser marked with red, green, blue and magenta half-filled circle.



as shown in fig. 4, respectively. There is a ZnO fragment close to the investigated HZOP and its emission is excluded from our measured PL spectra by adjusting the width of the input slit of monochromator and setting the interest region of CCD detector. A lasing peak of HZOP nanolaser emerges in PL spectrum as the pump fluence increase over 6.90 mJ/cm2 as shown in fig. (c), (d), (e) and (f). The lasing peak of HZOP doesn’t shift with the increment of pump fluence and it proves that the exciton density is far from the density leading to strong exciton coupling. The far-field PL image of HZOP nanolaser looks like three bright spots configured in triangle and it corresponds to the cross section of hexagonal pyramid. This radiation character of three spots indicates that light is reflected by the faces of pyramid and scattered by the apexes and base margins of pyramid. Furthermore, one of the intrinsic property of HZOP lasing is confirmed by the obvious coherenct interference fringes surrounding HZOP in consistent with the lasing spectrum under 13.58 mJ/ cm2 as shown in fig. 4 (f), where only the coherent light source scattered from the different parts of HZOP present a far-

Fig. 4 The PL spectra and fluorescence images of the HZOP under 4.61 GPa. (a) –(f) The PL spectra and fluorescence images are excited by a 355 nm pulsed laser with pump fluence of 0.93 mJ/cm2, 4.89 mJ/cm2, 6.90 µJ/cm2, 7.85 µJ/cm2, 10.04 mJ/cm2 and 13.58 mJ/cm2, respectively.


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field interferent pattern. The lasing performance of a single HZOP nanolaser under pressure is invested by measuring the lasing threshold under various pressure. The lasing threshold is extracted from the slope abrupt change of the plots between the PL intensity and the pump fluence at 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.7 GPa, 3.72 GPa, 4.61 GPa and 5.76 GPa as shown in fig. 5. The lasing threshold of a single HZOP nanolaser is the pump fluence corresponding to the cross point of the fitted red lines under a specific pressure as shown in fig. 5 (a)- (g). In order to exclude the effect of pressure-induced mode shift and switch, the PL intensity of each lasing mode is only integrated within a wavelength range covering the strongest lasing peak. The lasing thresholds of HZOP nanolaser are plotted as a function of pressure as shown in fig. 5 (h), which are marked with half-filled color circle in consistent with those used for the lasing mode as shown in fig. 3 (c). In general, the lasing thresholds of HZOP nanolaser varies little with pressure increasing and each lasing mode has its individual dependence of lasing threshold on pressure. The lasing threshold of red mode decreases with pressure increment, while the lasing thresholds of green and blue mode increase with the increment of pressure. This result is far from that of GaN nanowire nanolaser dominated by Fabry-Perot mode that become large greatly with pressure increasing. 17 There great discrepancy originates from the detailed resonance mechanisms of helical whispering-gallery-like mode and Fabry-Perot. The resonance of Fabry-Perot mode between two parallel reflective surfaces is more sensitive to the pressure effect of surrounding media, while the resonance of helical whispering-gallery-like mode working in total-internal-reflection is not sensitive to the pressure effect of surrounding media.



Fig. 5 The lasing threshold of a single HZOP nanolaser under pressure. (a)-(g) The plots of PL intensity integration as a function of pump fluence under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.7 GPa, 3.72 GPa, 4.61 GPa and 5.76 GPa, respectively. (h) The plot of lasing thresholds under pressure of 0.30 GPa, 0.63 GPa, 1.3 7 GPa, 2.7 GPa, 3.72 GPa, 4.61 GPa and 5.76 GPa. Red lines in (a)- (g) are the linear fitted PL spectra of spontaneous emission and stimulated emission. The color of half-filled circles in (g), red, green and blue, present the corresponding lasing modes as described in fig. 4 (c).

To summarize, the lasing behaviors of nanolaser are determined by the geometric configuration of resonant cavity and the intrinsic optical properties of semiconductor. In our experiment, the lasing peaks of HZOP nanolaser blueshift gradually with the increment of pressure accompanied by several abrupt lasing mode switches at some specific pressure points. As the volume of ZnO crystal changes less 5% below 10GPa, it would not affect the lasing mode obviously by geometric re-configuration. The gradual shift of lasing peak could be ascribed to pressure induced continuous variation of HZOP before the wurtzite-saltrock phase transition around 10 GPa, such as volume, bandgap and refractive index. Meanwhile, all the lasing peaks are found to exist only on the right shoulder of fluorescence peak and the shoulder works like a prescreen window to choose the lasing mode to work. This spectral character of spectral prescreen window on the right shoulder confirm that the lasing of HZOP originates from the


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recombination of exciton. It is well known that the exciton energy levels are below the conducting band. Once electron is pumped to the band higher than conducting band (left shoulder), there is no steady state for the excited electron to occupy and electrons must dissipate their energy by scattering with high energy phonons. While electron is pumped to the band lower that conducting band (right shoulder), there exist exciton levels for electron to occupy and the electron will recombine with the holes and radiate in fluorescence. This mechanism is consistent with the dispersive refractive index measured by Postava et. al,35 where the large imaginary part of refractive at short wavelength corresponds to the dissipation of exciton pumped to high energy excited state. Then the decreases of pressure parameters for fluorescence peak and lasing peaks can be ascribed to the strong coupling among excitons. A helical whispering-gallerylike mode is proposed to dominate the optical resonance of pyramid cavity with an optical feedback of total-internal-reflection. This resonant mechanism makes the loss non-sensitive to the refractive variation of ZnO and surrounding media induced by pressure, which make the HZOP nanolaser achieve a high lasing performance under pressure. As we all know, the lasing of nanolaser cannot be tuned anymore by changing geometric configuration or varying the material components. In this work, pressure is used as an efficient method to tailor the lasing behavior of a single HZOP nanolaser in a continuous and precious way. Although pressure is hard to be applied in an actual situation to tune a semiconductor nanolaser, it provides us a clear physical picture to understand the underlying mechanism dominating the lasing behavior of the semiconductor nanolaser. The blueshift and mode switch are be realized by varying the refractive index and electric bandgap with pressure or other methods in an actual application. The pressure related tunability of nanolaser is an important method to help us design and optimize semiconductor nanolaser with a specific mode profile. Methods: The Boehler-Almax design diamond anvils are used for the optical spectroscopy measurements of HZOP, where the angles of the apertures are large up to 85°. Because



Boehler-Almax Design DAC has advantage to project and collect light at a wide angle through the sample chamber, a transmitted darkfield method is performed to observe the specimen in sub-micrometer scale clearer under pressure produced with a DAC. Here a bright-darkfield objective (Olympus, LMPLFLN20XBD) is used as a darkfield illuminator with a white light source, while an ultra violet objective (Mitutoyo, Plan Apo Nuv 50X) is used to project the incident laser beam and collect the back-scattering optical signal. A 355nm pulsed laser of a 1.3 ns pulse width and 3 KHz repetition rate is used to pump the photo luminescence of the HZOP located in the chamber of the DAC and a 532 nm laser is used to excite the ruby sphere to calibrate the pressure of the sample chamber of the DAC. Once the specimen in the sample chamber is excited by laser, the backscattering signal will be split into two orthogonal directions by a beam splitter. The fluorescence image of the sample is captured by a CCD camera (Pixera, Pro 600ES) at backward direction and the spectrum of the pumped photoluminescence is taken by a monochromator (Princeton Instruments, SpectraPro2500) with a CCD detector (Princeton Instruments, ProEM:16002) and a grating of 1200 slits/mm at the vertical direction. ACKNOWLEDGMENT This research was supported by Nature Science Foundation of Jilin Province Grant No. 20180101279JC. Supporting Information. Details of the schematic of the SEM image of HZOPs on the substrate and the darkfield image of the investigated HZOP nanolaser within a DAC, the pressure parameters of ZnO nanocavities structured with various shapes, the PL spectra and images of HZOP are taken under pressure excited with incremental pump fluence.


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The image for TOC 50x50mm (600 x 600 DPI)


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The schematic diagrams of optical experimental setup. (a) The illumination of the home-made micro spectrometer to measure the PL spectrum of a single HZOP nanolaser. The 532 nm laser (green) is used to calibrate the pressure within DAC with the R1 line emission of ruby sphere. The 355 nm laser (purple) is used to pump the HZOP. White light (yellow) is used for optical inspect the samples within DAC. The camera is used to take the optical images and PL images. The spectrograph is used to measure the (PL) spectra of HZOP within DAC. (b) The schematic is the Diamond Anvil Cell (DAC). The sample cell is composed with two opposite diamond facets and gasket drilled a hole. A ruby sphere is used to mark the pressure. (c) The schematic of a single HZOP loaded in DAC. 272x208mm (300 x 300 DPI)



The pseudo-color PL spectral maps of a single HZOP nanolaser under pressure. (a)-(i) The PL maps are excited with incremental pump fluences under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. The scale bar on the right is a logarithmic color scale in arbitrary value. 104x78mm (600 x 600 DPI)


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The PL spectra of a single HZOP nanolaser are excited with low pump fluence and high pump fluence far from the lasing threshold under pressure. (a) The PL spectra of HZOP nanolaser are excited with about 3.2 mJ/cm2 under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. (b) The PL spectra of HZOP nanolaser are excited with the pump fluence about 12 µJ/cm2 under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. (c) The peaks of spontaneous emission (black half-filled circle) and stimulated emission (color half-filled circle) under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa, respectively. The color lines of black, red, green, blue, cyan, magenta, yellow, dark yellow and navy in fig. (a) and fig. (b) present the PL spectra under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.70 GPa, 3.72 GPa, 4.61 GPa, 5.76 GPa, 6.73 GPa and 8.24 GPa. The black half-filled circles and the color half-filled circles present the peaks of spontaneous emission and stimulated emission in fig. (c), respectively. The peaks are separated into four lasing modes of HZOP nanolaser marked with red, green, blue and magenta half-filled circle. 101x67mm (600 x 600 DPI)



The PL spectra and fluorescence images of the HZOP under 4.61 GPa. (a) –(f) The PL spectra and fluorescence images are excited by a 355 nm pulsed laser with pump fluence of 0.93 mJ/cm2, 4.89 mJ/cm2, 6.90 µJ/cm2, 7.85 µJ/cm2, 10.04 mJ/cm2 and 13.58 mJ/cm2, respectively. 127x67mm (600 x 600 DPI)


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The lasing threshold of a single HZOP nanolaser under pressure. (a)-(g) The plots of PL intensity integration as a function of pump fluence under pressure of 0.30 GPa, 0.63 GPa, 1.37 GPa, 2.7 GPa, 3.72 GPa, 4.61 GPa and 5.76 GPa, respectively. (h) The plot of lasing thresholds under pressure of 0.30 GPa, 0.63 GPa, 1.3 7 GPa, 2.7 GPa, 3.72 GPa, 4.61 GPa and 5.76 GPa. Red lines in (a)- (g) are the linear fitted PL spectra of spontaneous emission and stimulated emission. The color of half-filled circles in (g), red, green and blue, present the corresponding lasing modes as described in fig. 4 (c). 127x86mm (600 x 600 DPI)


The Lasing-mode Switch of Hexagonal ZnO Pyramid Driven by

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