1. Introduction

Wide bandgap semiconductors, such as ZnO, have been extensively developed as the most promising laser materials due to their extraordinary optical gain, superior optical and electronic properties [1–4]. Due to well-defined geometric construction, affordability and ease of preparation, various kinds of micro/nanostructured ZnO, such as nanowires, microwires, micro/nanoplates, microcubes, microspheres and so on, have been prepared to construct miniaturized coherent light sources for fascinating applications in photonic integrated systems [5–10]. Lasting for decades, great efforts have been made to achieve lasing actions in Fabry-Perot (F-P) microresonators [11–13], whispering-gallery (WG) microresonators [3,10,14], random media and distributed feedback (DFB) microresonators under light or electrical excitation [15,16]. As the excitons existed in ZnO with binding energy ($\sim$ 60 meV) larger than the thermal energy, the corresponding lasing mechanisms in mostly reported laser devices are assigned to excitonic-state related stimulated emission [3,10,13,17]. Besides, the lasing mechanisms originated from electron-hole plasma (EHP) and exciton-polariton quasiparticles, have also been reported [18–20]. Despite great progress has been made in developing coherent light sources based on ZnO micro/nanostructures and studying their lasing mechanisms, the understanding of lasing mechanisms, which is related to the carrier density, is not clear; and the related investigations are still ongoing [21–25].

For the ZnO-based laser materials and devices, the gain working principle is principally determined by the carrier concentration. By comparison, exciton-related lasing actions can occur at low carrier concentration, and can be achieved at a low pumping energy density [10,26–30]. Increasing the carrier density in semiconductors beyond Mott$^\prime$s density, a new electron-hole collective state, called EHP state, will eventually replace the quasiparticle state of excitons. Thus, EHP lasing actions could be achieved due to the exciton ionization, resulting in bandgap renormalization (BGR) effect [19–21,31]. Generally, achieving carrier concentration higher than Mott$^\prime$s density is commonly obtained under higher excitation energy density. However, the higher pumping power density may lead to irreversible damages to the semiconductor micro/nanostructures; while, the usage of higher pumping power density in achieving EHP lasing defeats the purpose of developing high-performance coherent light sources with low lost, lower power consumption and easy maintenance [17,26,32]. When semiconductors are in contact with nanostructured metals, nonradiative decay of metal plasmons produces hot electrons in metal nanostructures that can be injected into neighboring semiconductor micro/nanostructures [33–36]. In addition to the strong pulsed excitation, the injection of hot electrons into the conduction band edge in the semiconductors may provide an alternative approach to increase exciton density above the critical Mott$^\prime$s density, to further achieving EPH lasing upon low pulsed excitation [22,23,25,37].

In this study, Ga-incorporated ZnO (ZnO:Ga) microbelts were prepared using carbothermal reduction method. Upon pulse excitation, F-P lasing was realized in a ZnO:Ga microbelt, with the bilateral facets working as optical feedback mirrors. Incorporating Ag nanoparticles (AgNPs), especially for the diameter ($d$) $\sim$ 200 nm, is utilized to optimize the lasing features of the as-grown ZnO:Ga microbelt. Apart from significantly enhanced lasing characters, containing lower threshold and higher output, the increased features also exhibit a distinct redshift of each lasing band, and the lasing line width broadening towards the longer wavelength shoulder, indicating that the EHP lasing is achieved in a ZnO:Ga microbelt covered by large size AgNPs. To study EHP lasing, the lasing mechanism is attributed to energetic bandgap renormalization arising from injection of hot electrons, which are generated by quadrupole plasmons resonances of deposited large size AgNPs through nonradiative decay. The modulation of large size AgNPs on carrier characteristics and lasing dynamics, were studied; the corresponding transition between the excitonic and EHP states is also demonstrated. The results can provide a deeper understanding of carrier characteristics and their role in studying lasing mechanisms.

2. Experimental section

2.1 Preparation of individual AgNPs@ZnO:Ga microbelt

The ZnO:Ga microbelt was prepared by chemical vapor deposition via carbothermal reduction method, as previously reported literature [12,13,36]. A microbelt was placed on a clean quartz substrate, immobilised with indium (In) particles featuring as electrodes. Thus, individual ZnO:Ga microbelt based filament-type emitter is designed [36,38,39]. Ultra-pure Ag was evaporated on the microbelt using magnetron sputtering equipment, forming Ag quasiparticle nanofilms covered ZnO:Ga (Ag@ZnO:Ga) microbelt [35,40]. An incandescent-like lamp made of an individual Ag@ZnO:Ga microbelt was also constructed. When illuminated electrically, bright incandescent-type radiation is observed, and the light-emitting area is closing to the center of the microbelt. Due to Joule heating effect, isolated AgNPs are created on the microbelt via self-annealed process. AgNPs are uniformly distributed in the light-emitting area, thus forming an AgNPs@ZnO:Ga microbelt [35,38,40]. Further, increasing the sputtering time can increase the size of AgNPs, which prepared on ZnO:Ga microbelts.

2.2 Sample characterization

Electrical characterization is used to obtain the current-voltage ($I$-$V$) curves of the samples via a Keysight B1500A sourcemeter system. The ANDOR detector (CCD-13448) and Omni-$\lambda$ 500 Spectrograph are employed to obtain the electroluminescence (EL) spectra of the single microbelt based incandescent-like bulb. An optical microscope was used to collect the EL images of the single microbelt based incandescent-type light sources. Photoluminescence (PL) of a single ZnO:Ga microbelt not covered, and covered by AgNPs, are performed by using He-Cd laser (excitation wavelength of 325 nm) via LabRAM-UV Jobin-Yvon spectrometer. Lasing characterization of the single microbelt was pumped by using a femtosecond pulsed laser (excitation wavelength of 355 nm, repetition rate of 1 kHz, pulse length of 100 fs). The extinction spectra of AgNPs prepared on sapphire substrate were measured using a Shimadzu UV-3101PC scanning spectrophotometer.

3. Results and discussions

Using carbothermal reduction reaction, individual ZnO:Ga microbelts with controlled size, high-yield, highly-crystallized quality, were successfully prepared [12,13]. A scanning electron microscope (SEM) was used to observe surface morphology and structures of the as-grown samples. Shown in Fig. 1(a), quadrilateral cross section of a ZnO:Ga microbelt is obtained, and the width of the microbelt is measured to approximately 85 $\mu$m. The thickness of the microbelt is approximately measured to 5 $\mu$m, which can be acquired in Fig. 1(b). The crystal phase of as-prepared ZnO:Ga microbelts was examined by an X-ray diffraction (XRD) spectrometer. XRD pattern shown in Fig. 1(c) exhibits a strongest (002) peak located at $34.3^\circ$, indicating that the as-grown ZnO:Ga microbelts have a relatively good $c$-axis preferred orientation [7,17,29]. Further, it is observed that the full width at half maximum (FWHM) of the (002) peak is $0.06^\circ$, suggesting that the as-synthesized samples possess good crystalline quality. Optical properties of a single ZnO:Ga microbelt were tested and the PL spectrum is shown in Fig. 1(d). The figure shows that the main PL peak emits at 378 nm and the line width of the peak is evaluated to 12.5 nm. This peak is probably originated from typical near-band-edge (NBE) emission [10,12,38]. Once again, it suggests that the synthesized microbelts possess higher crystalline quality [10,12,25]. Besides, we also observed a much weak visible light emission emitting at around the wavelength of 500 nm, which may result from intrinsic defects (such as zinc interstitials ($Zn_{i}$), zinc anti-site ($Zn_{O}$), oxygen vacancies ($V_{O}$)), or impurity levels associated with Ga-incorporating [7,35].

 

Fig. 1. (a) SEM image showing the ZnO:Ga microbelts. (b) SEM image of a cross-section of the ZnO:Ga microbelt having a thickness of approximately 5 $\mu$m. (c) XRD pattern of a single microbelt. (d) PL spectrum of an individual microbelt. (e) $I$-$V$ characteristic curve of an individual ZnO:Ga microbelt. (f) EL spectra of a ZnO:Ga microbelt based fluorescent filament emitter. (g) Optical microscopic EL image of a single ZnO:Ga microbelt where the voltage is applied at both ends of the microbelt, and bright green radiation from the microbelt can be observed.

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Employing the In particles as electrodes, we built a incandescent-type light bulb using an individual ZnO:Ga microbelt, whose incandescent-type radiation properties are assigned to the substitution of Ga for Zn induced Ga-related impurity levels [36,38–40]. The $I$-$V$ characteristic curve is measured. Illustration in Fig. 1(e), the $I$-$V$ curve is close to linear, indicating that the electrode and the microbelt has formed a relatively good ohmic contact [3,38]. Increasing the input current reached a certain value, bright green radiation from the body of the microbelt is observed. The emitted photons were recorded. EL spectra as a function of the current passing through the microbelt is shown in Fig. 1(f). The main EL wavelengths emit at around 535.0 nm, with a significant increase in intensity when the current varies from 43 to 52 mA. Optical microscopic EL images were captured when the current varying from 42 to 50 mA, and shown in Fig. 1(g). Clearly, a significant increase in the brightness of the emission can be seen.

Ag quasiparticle nanofilms was sputtered on an individual ZnO:Ga microbelt using magnetron sputtering equipment [38,40,41]. Varying sputtering time can result in different thicknesses of Ag quasiparticle nanofilms, which deposited on the microbelt. Another single ZnO:Ga microbelt was utilized to design an incandescent-like bulb [38,39]. First, electrical characterization of the ZnO:Ga microbelt not covered, and covered by Ag quasiparticle nanofilms via various thickness, were measured. Figure 2(a) shows the variation of the $I$-$V$ characteristic curves as the sputtering time is varied from 1 to 5 minutes. It is observed that the conductivities of the single microbelt increases significantly with increasing sputtering time. EL characterization of the as-constructed incandescent-like bulb was checked, and EL spectra of a bare ZnO:Ga microbelt in terms of injection current is shown in Fig. 2(b). Clearly, the bulb emits at the wavelength of about 543.5 nm. By depositing Ag quasiparticle nanofilm via the sputtering time of 1 min, the EL features can be modulated, especially for the main EL wavelengths, showing a little blue shift (See Fig. 2(c)) [35,36,38]. By increasing the sputtering time ranged from 1–5 min, the output light was obtained. After normalization, the modulation of the emission by Ag quasiparticle nanofilms can be viewed in Fig. 2(d). Increasing sputtering time, the main wavelengths of EL peaks illustrates an observable blue shift [38,40].

 

Fig. 2. (a) $I$-$V$ characteristic curves of a ZnO:Ga microbelt not covered, and covered by Ag quasiparticle nanofilms. The sputtering time varies from 0 to 5 min. (b) EL spectra of the single ZnO:Ga microbelt based incandescent-type bulb versus different input current. (c) By depositing Ag quasiparticle nanofilm via sputtering time $\sim$ 1 min, the EL spectra versus different input current. (d) Normalized EL spectra of the as-constructed incandescent-type bulbs, which composed of the ZnO:Ga microbelt covered by Ag quasiparticle nanofilms, the sputtering time varied from 1 min to 5 min.

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According to the operating principle of blackbody radiation, the visible radiation corresponds to the calculated temperature of about 3000 K. In particular, the more blue shift of the main emission peaks are, the higher the temperature located in the incandescent-like light-emitting area reaches [42,43]. However, the as-synthesized ZnO:Ga micro/nanostructures can only withstand a temperature of about 1000 K [38,44]. Therefore, a blackbody radiation operating principle, which induced by the Joule heating effect, is incompatible with the incandescent radiation mechanism of electric lighting based on a typical filament structure, which is constructed by an individual ZnO:Ga microbelt. The Joule heating effect plays an important role in incandescent radiation, but does not govern the corresponding radiation properties [36,38,40]. The incompatibilities are summarised as follows: (i) the wavelength-tunable emission depends strongly on the concentration of the incorporated Ga-dopant, rather than the temperature caused by the Joule heating effect; (ii) there is little variation of the main peak wavelengths and the line width with an increase of injection current; and (iii) a decrease in ambient temperature leads to a significant increase in incandescent-type radiation. There is still insufficient scientific evidence that the incandescent radiation properties, including the main EL wavelength, spectral linewidth and light output intensity, are directly related to the temperature produced by the Joule heating effect [38,40].

To characterize the influence of Ag nanostructures on the incandescent-type luminescence features, the characteristics of surface morphology and structure were checked using SEM. Figure 3(a) shows an optical microscopic EL image based on a microbelt via Ag nanostructures deposition. In this case, the non-radiating area is labelled as region $\mathbf {I}$, the transition area between the incandescent-type radiating area, or not, is labelled as region $\mathbf {II}$, and the incandescent-type radiating area is labelled as region $\mathbf {III}$. It is worth noting that the brightest emitting region (i.e. region $\mathbf {III}$) is always located toward the center region of the microbelt, which located in the incandescent-type emission region. The optical features as well as the incandescent-type emission regions are located in the region $\mathbf {III}$, corresponding to the well-crystallized region of the ZnO:Ga microbelt. It is concluded that the EL can be ascribed to the Ga-related impurity level rather than to the intrinsic donor defect energy level [38–40]. Figure 3(b) shows the SEM image of a bare microbelt, showing clean and well-faceted structure. SEM image of the transition from region $\mathbf {I}$ to region $\mathbf {III}$ (i.e. in and around region $\mathbf {II}$) is shown in Fig. 3(c), where region $\mathbf {II}$ is aligned with the position of the transition from quasiparticle nanofilms to isolated nanoparticles, which is consistent with Fig. 3(a). A SEM image of the incandescent-type luminescent region (i.e. region $\mathbf {III}$) is shown in Fig. 3(d), and Fig. 3(e) clearly shows an enlarged SEM image of the AgNPs within region $\mathbf {III}$. Thus, AgNPs were prepared on the microbelt, indicating that individual AgNPs@ZnO:Ga microbelt were successfully prepared [35,40]. Due to the temperature in the brightest region caused by the Joule heating effect, the evaporated metal quasiparticle nanofilms on ZnO:Ga microstructures, can be annealed into physically isolated nanoparticles. Especially, varying the thickness of metal quasiparticle nanofilms, metal nanoparticles with controlled sizes can be realized [25,39–41].

 

Fig. 3. (a) Optical microscopic EL image of fluorescence-emission from a microbelt covered by Ag quasiparticle nanofilm at a sputtering time of 5 min. (b) SEM image of a bare microbelt. (c) SEM image of Ag nanostructures in the region $\mathbf {II}$ along the $x$-direction, which corresponding to the transition from region $\mathbf {I}$ to region $\mathbf {III}$. (d) SEM image of isolated AgNPs in the incandescent-type luminescent area of a typical filament structure (i.e. region $\mathbf {III}$). (e) Enlarged SEM image of these AgNPs along the $y$-direction.

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Accordingly, the variation of EL characteristics of single ZnO:Ga microbelt based incandescent-like bulb can be assigned to the deposited AgNPs, which created by Joule heating effect [39,40]. The AgNPs plasmon-mediated EL mechanism was found that nonequilibrium distribution of hot electrons in ZnO:Ga microbelt, generated by the deposited AgNPs, can be responsible for the shift of EL wavelengths [36,40]. By varying the sputtering time of Ag nanofilms, AgNPs with various sizes can be prepared on the ZnO:Ga microbelt [25,40]. As the diameter of AgNPs lower than certain value, the plasmonic behavior can be attributed to dipole plasmons [45–47]. Increasing the diameter well above 100 nm, another resonance peaks appeared in the shorter wavelength band, contributing to the quadrupole plasmons. By increasing the size of the Ag particles beyond 100 nm, the profile curves of the quadrupole plasmons became stronger and stronger, and then dominated primary character in the plasmonic features of AgNPs [48]. Compared with the dipole plasmonic behavior of small size AgNPs, the quadrupole plasmonic peaks remained unchanged, which could be attributed to the reduced decaying length. Therefore, increasing the size of AgNPs can result in tunable plasmons, especially for the higher-order plasmons. Upon electrical excitation, AgNPs plasmons induced the generation and injection of nonequilibrium hot electrons can be responsible for the wavelength-tunable incandescent-type lighting features of the single ZnO:Ga microbelt [36,39,40].

Lasing characterisation of the microbelt was carried out using a fs pulsed laser, and the PL spectra operated at different pump energy densities $I_{exc}$ were collected, as shown in Fig. 4(a). When the excitation intensity reaches 12.3 mW/$\mu$m$^{2}$, a series of sharp lines appear in the spontaneous radiation spectra, and the average FWHM of the sharp lines $\delta \lambda$ was examined to $\sim$ 0.10 nm. The microbelt-based optical microcavity $Q$ is calculated to $\sim$ 3900 according to the equation $Q$ = $\lambda$/$\delta \lambda$, in which $\lambda$ is the emission wavelength [10]. When the pumping fluence is further increased, a range of sharp emission peaks dominate the PL spectra, accompanied by a distinct suppression of the spontaneous radiation. In particular, the peak intensity of the PL spectra increases rapidly with an increase of the pumping fluence, as well as their corresponding increase in the lasing numbers of oscillating sub-peaks. The experimental results show that the cross-section of an individual ZnO:Ga microbelt forms an optical microresonant cavity with an average mode spacing $\Delta \lambda$ of about 0.20 nm for the sharp peaks of the PL spectra. The mode spacing of the F-P microresonant cavity is given by $\Delta \lambda = \lambda ^{2}/[2L_c(n-\lambda dn/d\lambda )]$, where $L_c$ is the width of the microbelt, i.e. the cavity length of the resonant cavity, the refractive index $n_{ZnO:Ga}$ = 2.35 and $dn /d\lambda = -0.010$ nm$^{-1}$ represents the dispersion relation of the refractive index at 395 nm [10,12,28]. The resonant cavity length $L_c$ can be determined to about 80 $\mu$m, and this is in general agreement with the SEM measurements. Thus, individual ZnO:Ga microbelts can provide potential candidates to construct F-P microlasers, and the corresponding lasing mechanism orignates from excitonic state [13,49,50].

 

Fig. 4. Lasing characterization of a single ZnO:Ga microbelt not covered, and covered by large size AgNPs ($d$ $\sim$ 200 nm) using fs pulsed laser. (a) PL spectra of the bare ZnO:Ga microbelt versus pump energy density varying from 12.3 to 24.7 mW/$\mu$m$^{2}$. (b) PL spectra of the same ZnO:Ga microbelt covered by large size AgNPs, by varying the pump energy density from 4.0 to 21.5 mW/$\mu$m$^{2}$. (c) Comparison of lasing spectra of the ZnO:Ga microbelt not decorated, and decorated by AgNPs, their PL measurements was performed at a pump energy density of 20.0 mW/$\mu$m$^{2}$. (d) Comparison of integrated PL intensity obtained from the ZnO:Ga microbelt not decorated, and decorated by AgNPs.

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The same ZnO:Ga microbelt covered by large size AgNPs ($d$ $\sim$ 200 nm), was further pumped optically by varying pump energy density $I_{exc}$ varied in the range of 4.0 to 21.5 mW/$\mu$m$^{2}$. PL spectra as a function of excitation fluences are plotted in Fig. 4(b). When the pump fluence further increases to well above the excitation fluence of 11.5 mW/$\mu m^2$, sharp emission peaks occur superimposed to the broadband spontaneous radiation with a superlinear power dependence. That is, the lasing intensity increases rapidly with an increase of the pump energy, while additional sub-peaks emerging at the longer wavelength shoulders. In particular, a monotonous redshift of each lasing mode toward longer wavelengths can be explicitly observed with increasing excitation energy intensity. Varying the pump fluence, the average mode spacing extendes from 0.50 to 0.75 nm; while, the FWHM of the lasing peaks also show a clear broadening ranged from 0.10 to 0.30 nm. The cavity length of an individual Ag NPs@ZnO:Ga microbelt based F-P microcavity is evaluated to about 71.5 $\mu$m, calculated for a specific PL spectra at a pump energy density of 21.5 mW/$\mu$m$^{2}$, suggesting that the large size AgNPs can influence the lasing charateristics of the single ZnO:Ga microbelt. It indicates that the origination of F-P lasing can be assigned to EHP state in the stimulated emission process, instead of the excitonic state or exciton-polariton quasiparticles [11,19,20]. Compared with bare ZnO:Ga microbelt, the calculated cavity length illustrates a distinct variation, which may be caused by the reduction of the refractive index with higher carrier concentration. Additionally, it is also deduced that the mode spacing at high pump fluence is much broader than that at low pump level, which is commonly originated from the carrier scattering and heat effect [19,23].

Comparison of PL intensity of the single ZnO:Ga microbelt not covered, and covered by AgNPs was examined at a pump energy density of 20 mW/$\mu$m$^{2}$, and the PL spectra are plotted in Fig. 4(c). Clearly, the lasing intensity is significantly enhanced by comparing with that of the bare ZnO:Ga microbelt [27,45,51]. In particular, a significant red shift of the entire gain profile was also observed. The integrated PL intensity of the ZnO:Ga microbelt not covered, and covered by AgNPs, as functions of pumping fluence were checked, and the results are shown in Fig. 4(d). In addition to the increased lasing intensity, the lasing threshold of the AgNPs@ZnO:Ga microbelt is extracted to be 8.5 mW/$\mu$m$^{2}$, and this value is significantly lower than that of the bare microbelt based laser (the lasing threshold is about 15.7 mW/$\mu$m$^{2}$). To ensure that the EHP emission of the microbelt was caused due to the deposited large size AgNPs, rather than the small size particle, another individual ZnO:Ga microbelt not decorated, and decorated by small size AgNPs (the diamter $d$ is about 100 nm) were also pumped optically. Unfortunately, the excitation energy was increased as high as possible, the emission spectra showed almost no shift in each lasing band in addtion to the lower theshold and increased lasing output [11,26,27]. As the average diameter of the AgNPs prepared on ZnO:Ga microbelt less than 200 nm, the lasing characteristics can be further increased. In addition to the continually reduced laser threshold, there is little variation of the entire lasing profile. That is, AgNPs with lower size can be used to improve the lasing characteristics of a single ZnO:Ga microstructures, but cannot increase the carrier concentration beyond the Mott$^\prime$s density. Therefore, increasing the pulsed excitation power is not sufficient to produce electron concentration above the critical Mott$'$s density in a bare ZnO:Ga microbelt, or the ZnO:Ga microbelt covered by small size AgNPs [20,24].

As previously reported, lasing action of ZnO-based materials and devices is mostly attributed to exciton-related emission behaviour [1,3,10]. By incorporating large size AgNPs, the exciton can become EHP state, resulting in EHP excitation [21,24]. Thus, it is believed that the EHP F-P lasing with a lower threshold can be achieved using an individual ZnO:Ga microbelt via large-size AgNPs deposition [11,27]. It’s worth noting that the incorporation of large size AgNPs can achieve a transition from the excitonic state to the EHP state at the Mott density in the as-grown ZnO:Ga microbelts. In particular, the EHP lasing action can be achieved at lower excitation energy densities [22,23,26,37].

AgNPs via small sizes have been widely used to improve optical and electrical properties of semiconductor materials and optoelectronic devices due to the excitation of dipole plasmons [41,46,48]. In this study, varying the sputtering time can tune the size of the AgNPs deposited on the microbelt through thermal resistance effect [35,40]. SEM image of AgNPs prepared on ZnO:Ga MW according to Fig. 5(a) shows that the average diameter of the AgNPs is examined approximately to 200 nm. Thus, as the sputtering time reaches 5 min, the diameter of the AgNPs can increase up to about 200 nm. The enlarged image is shown in Fig. 5(b). By comparison, AgNPs with similar size was also prepared on sapphire substrate by using the same experimental condition. SEM image of AgNPs prepared on sapphire substrate is displayed in Fig. 5(c). By comparison, the optical characteristics of as-prepared AgNPs via the sputtering time of 1 min was studied. Generally, the exinction spectrum is plotted in Fig. 5(d), and the peak position locates at about 480 nm. Clearly, it shows a distinct dipole plasmons. The visible extinction peak is assigned to the dipole plasmons element [41,47,48].

 

Fig. 5. (a) SEM image of AgNPs, which prepared on ZnO:Ga microbelt. (b) An enlarged AgNP with $d$ $\sim$ 200 nm. (c) SEM image of AgNPs prepared on sapphire substrate, and the sputtering time is 5 min. (d) Extinction spectrum of AgNPs prepared on sapphire substrate, the sputtering time is 1 min. The diameter of AgNPs is measured to approximatively 100 nm. (e) Calculated electric field intensity $|E|^{2}$ of an AgNP with electromagnetic waves propagating in the $x$-direction along the $x$-$y$ plane (the diameter $d$ is 100 nm). (f) Calculated $|E|^{2}$ at AgNP/ZnO:Ga interface with electromagnetic waves propagating in the $x$-direction along the $x$-$z$ plane. (g) Extinction spectrum of AgNPs prepared on sapphire substrate, the sputtering time is 5 min. The diameter of the AgNPs is measured to approximatively 200 nm. (h) Calculated electric field intensity $|E|^{2}$ of an AgNP with electromagnetic waves propagating in the $x$-direction along the $x$-$y$ plane (the diameter $d$ of AgNP is 200 nm). (i) Calculated $|E|^{2}$ at AgNP/ZnO:Ga interface with electromagnetic waves propagating in the $x$-direction along the $x$-$z$ plane ($d$ $\sim$ 200 nm).

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To confirm the dipole feature of AgNPs, theoretical calculations of electromagnetic field intensity $\sim$ $|E|^{2}$ were performed using the finite-difference time-domain (FDTD) method [40,41,47]. In the simulation, the diameter of AgNP $D$ is 100 nm, the resonance wavelengths $\lambda$ is 480 nm, and the refractive indexes of ZnO:Ga and air are $n_{ZnO:Ga}$ = 2.35 and $n_{air}$ = 1.0, respectively. The dielectric function of Ag can be referred to the previous literature [41,48,52]. Figure 5(E) exhibits the calculated $|E|^{2}$ distribution of an isolated AgNP, which placed on ZnO:Ga, in $x$-$y$ plane. The strongest enhancement effect is represented by the brightest blue color. According to the calculation result, the simulated $|E|^{2}$ strongly distributes at the bilateral sides of the AgNP in $x$-$y$ plane. The spatially localized $|E|^{2}$ of an AgNP placed on ZnO:Ga was also simulated in the $x$-$z$ plane. As illustrated in Fig. 5(f), substantial enhancement in a local field is also captured at the AgNP/ZnO:Ga interfaces, which simulated at the wavelength of 480 nm. Combining the experimental and theoretical results, the localized surface plasmon resonances at the wavelength of 480 nm is assgined to dipole characteristics [41,46,48].

As the size of AgNPs deposited on sapphire substrate reach 200 nm, the optical properties were measured. Figure 5(g) displays the corresponding extinction spectrum, in which the main peak positions locate at 360 nm and 467 nm, respectively. By a contrast, the peak at 360 nm in the ultraviolet wavelengths could be assigned to the quadrupole plasmons [41,47,48,53]. To determine the quadrupole plasmons characteristic, the distribution of the calculated electromagnetic field intensity $|E|^{2}$ of an isolate AgNP at a diameter of 200 nm was obtained (In the simulation, the resonance wavelength is 360 nm). Shown in Fig. 5(h), observably enhanced $|E|^{2}$ can be formed via quadrupole modes around the large size AgNP in the $x$-$y$ plane [45,48,53]. The spatially localized electric-field intensity of an AgNP placed on ZnO:Ga was also simulated in the $x$-$z$ plane. Shown in Fig. 5(i), the simulated $|E|^{2}$ exhibits that observably enhanced $|E|^{2}$ is distributed at the AgNPs/ZnO:Ga interface in $x$-$z$ plane. Compared with the experimental results, hybrid quadrupole plasmons can be excited on account of the large size AgNPs, which prepared on the ZnO:Ga microbelt [47,48,53].

Optical characterization of a single ZnO:Ga microbelt not covered, and covered by AgNPs ($d$ $\sim$ 200 nm) were measured by using a He-Cd laser for ultraviolet excitation. Illustration in Fig. 6(a), the ultraviolet emission of ZnO:Ga microbelt can be significantly enhanced via AgNPs deposition [25,35]. By exploiting the influence of AgNPs on the EHP lasing action, it is found that the main PL peak of ZnO:Ga does not match with neither the dipole, nor the quadrupole plasmons of the deposited AgNPs. Generally, the enhanced PL of an individual ZnO:Ga microbelt should not be attributed to the direct resonant coupling of AgNPs plasmons and ZnO:Ga excitons [27,35]. Optical time-resolved PL (TRPL) measurements of the ZnO:Ga microbelt before and after the decoration by large size AgNPs, were further performed. And the normalized TRPL decay curves were fitted appropriately with an individual exponential function. The decay time of an individua ZnO:Ga microbelt was significantly reduced from 208 ps to 68 ps by incorporating large-size AgNPs, as seen in Fig. 6(b). Thus, introducing large-size AgNPs can promote radiative recombination rate of electrons and holes, leading to enhancing ultraviolet emission of individual ZnO:Ga microbelts [25,27,35].

 

Fig. 6. (a) PL spectra of a ZnO:Ga microbelt not decorated, and decorated by large size AgNPs, the diameter of the deposited AgNPs is about 200 nm. (b) TRPL signals (dots) and fitted curves (solid lines) illustrating decay times of 208 ps for the bare microbelt (black square dots and lines), and 68 ps for the microbelt covered by large size AgNPs via $d$ $\sim$ 200 nm (red circles and lines). (c) $I$-$V$ curves of an individual ZnO:Ga microbelt not covered, and covered by AgNPs ($d$ $\sim$ 200 nm), which operated in dark, and under 360 nm illumination (0.5 mW/cm$^2$). (d) $I$-$t$ curves of an individual ZnO:Ga microbelt not covered, and covered by AgNPs ($d$ $\sim$ 200 nm) under 360 nm light switching (0.5 mW/cm$^2$). (e) Schematic view of hybrid quadrupole plasmons induced the generation and injection of hot electrons at the interface between AgNP and ZnO:Ga.

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As we described above, the increased lasing performances of an individual ZnO:Ga microbelt was assgined to plasmon-mediated hot electron effect. To check plasmon-assisted hot electron transfer procedure, electrical characterization of an individual ZnO:Ga microbelt not covered, and covered by AgNPs ($d$ $\sim$ 200 nm) were measured under dark condition, and illuminated with 360 nm ultraviolet light ($\sim$ 0.5 mW/cm$^2$), respectively [4,36,54]. Illustration in Fig. 6(c), once ultraviolet light ($\sim$ 360 nm) is illuminated on the microbelt, significant enhancement of photocurrent can be detected because of the excitation of hybrid quadrupole plasmons, which originated from large size AgNPs ($d$ $\sim$ 200 nm). Further, photocurrent versus time ($I$-$t$) curves of the single ZnO:Ga microbelt not covered, and covered by AgNPs ($d$ $\sim$ 200 nm) were examined at a fixed bias of 2 V with repeated on/off cycles by controlling the switching interval between dark and ultraviolet light illumination (360 nm, 0.5 mW/cm$^2$). Turning-on ultraviolet light illumination, the photocurrent rises sharply to a stable value. After turning off the ultraviolet light, the photocurrent rapidly decreases and returns to the initial state. In particular, significant increase of the detected photocurrent in the ZnO:Ga microbelt covered by large size AgNPs is achieved by comparing with that of the bare microbelt. Therefore, the increased lasing characteristics of an individual ZnO:Ga microbelt should be ascribed to plasmon-assisted hot electron transfer, rather than plasmon-mediated electromagnetic field enhancement localized at the AgNPs/ZnO:Ga interface [26,27,32]

The explaining of plasmon-assisted hot electron transfer in AgNPs@ZnO:Ga microbelt was depicted, and the physical procedure is schematically described in Fig. 6(e). Under the irradiation of the 355 nm fs pulsed laser, metal plasmons of large size AgNPs deposited on the microbelt are excited, resulting in the formation of localized quadrupole plasmons. Through nonradiative decay, quadrupole plasmons of AgNPs transfers their energy to the electrons, leading to the creation of non-equilibrium hot electrons in collective states. Once the energized electron has sufficiently high energy by comparing with the contact potential at the AgNP/ZnO:Ga interface, they can be injected directly to the conduction band of the adjacent ZnO:Ga. The physical process of generation and injection of hot electron is estimated to be in picoseconds or less, resulting in the filling of conduction band [34,36,54].

In general, exciton-related radiation from semiconductors can occur at low carrier concentrations. As the carrier concentration exceeds the critical Mott$^\prime$s jump density, EHP emission occurs due to the exciton ionisation, leading to energy bandgap renormalization [22,23,37]. It is worth noting that the excellent electron transport properties of the ZnO:Ga microbelt can be obtained by incorporating Ga$_2$O$_3$ into the precursor mixture during the synthesis process. At this moment, the estimation of electrons concentration in the AgNPs@ZnO:Ga microbelt via large size was performed. The carrier concentration can be calculated according to the equation $n_p$ = $\beta \cdot I_{exc}/h\omega _{exc}$ [7,55], where the absorption coefficient $\beta$ $\approx$ 1.6 $\times$ 10$^5$ cm$^{-1}$. Increasing the pump energy density varied in the range 5.0 to 21.0 mW/$\mu$m$^{2}$, the carrier concentration $n_p$ in a ZnO:Ga microbelt via large size AgNPs deposition can be increased from 10$^{19}$ cm$^{-3}$ to 10$^{21}$ cm$^{-3}$. Thereby, an electron concentration above the Mott$^\prime$s density can be generated. And the higher carrier concentration at this amplitude would definitively lead to the downward shift of the conduction band edge [19,22,23]. Plasmon-assisted hot electron transfer can lead to Mott-transition from the excitonic state to EHP state in the stimulated emission procedure on account of an individual ZnO:Ga microbelt covered by large size AgNPs [22,23,37].

4. Conclusion

To summarise, the incorporation of large size AgNPs can be utilized to achieve the Mott-transition from the excitonic state to EHP state in the stimulated emission procedure based on an individual ZnO:Ga microbelt, which operated under low pulsed laser excitation. The influence of AgNPs on carrier features and dynamics of the stimulated emission in an individual AgNPs@ZnO:Ga microbelt, especially for F-P lasing in the EHP regime, are studied. Compared with exciton-related lasing behavior of the bare ZnO:Ga microbelt, enhanced lasing performances including a significant increase in optical output and a reduced threshold are obtained. In particular, the lasing characteristics also illustrate a distinct redshift of each lasing spectrum, accompanied by an extension of the spectral line width towards the longer wavelength shoulder, contributing to the EHP lasing. To exploit the EHP properties, nonradiative decay of hybrid quadrupole plasmons generates hot electrons in large size AgNPs that can be injected into neighboring ZnO:Ga microbelt, leading to bandgap renormalization. The experimental findings can supply novel insight into the origination and identification of lasing in the excitonic and EHP states, and also can provide an workable scheme to achieve the transition from excitonic to EHP lasing.