The working mechanism of our proposed UCNCs-based microlaser is schematically displayed in Fig. 1, which integrates two key advances involving gain material and cavity design to endow dynamically tunable single-mode laser. In this design, light manipulation at nanoscale (Fig. 1a) is achieved through dispatching different Ln3+ ions into separated layers of multishelled UCNCs, which thus enable two distinct upconverting processes under the excitation of λ1 and λ2 lasers, respectively. Upon pumping at different wavelengths, the integration of those UCNCs and microdisk resonator (Fig. 1b) from patterned substrate[30] would lead to two groups of multicolour lasing emission through the formation of whispering gallery modes (WGMs). Such an arrangement usually suffers from inherently isotropic output as well as multiple modes feature arising from simple microdisk laser. According to the following equation[16, 30]:
FSR = λ02/neffL (1)
where FSR is the free spectral range, λ0 is the wavelength of resonant peak, neff is the group refractive index, and L is the optical length, single-mode operation could be achieved by decreasing the size of cavity down to several or a dozen of microns[16, 30]. However, this strategy is quite limited since it usually gives rise to a highly increased lasing threshold while the multicolour output problem cannot be effectively addressed. To overcome these obstacles, the UCNCs-based size-mismatched PM structure with the cavity size at the submillimeter scale can be exploited to attain single-mode laser under asymmetric excitation (Fig. 1c). Once PM laser is established by withholding the excitation from one of two tangent microdisks, a single-mode laser emerges from two coupled microdisks cavity, a stark contrast to microdisk lasers. In this regard, other competing modes would be highly suppressed. Most importantly, through external excitation manipulation, this on-chip PM device offers robust dynamic mode-switching functionality without the need for any other intricate components.
As a proof of concept, NaGdF4:Yb/Nd@NaGdF4:Yb/Ho@NaYF4:Ca@NaYbF4:Tm@NaYF4:Ca core-multishelled UCNCs were synthesized via a modified literature procedure [16]. Generally, NaGdF4:Yb/Nd core nanocrystals with a diameter of ~ 16 nm were firstly prepared followed by successive deposition of four epitaxial shells of NaLnF4 (Ln = Gd, Yb, Ho, Tm, Y). Note that, the NaYF4:Ca inner layer with a thickness of 5 nm is used to prohibit the Nd3+→Yb3+→Tm3+ energy transferring path (Figure S2 and Figure S3) under 808 nm excitation, while the outmost layer of NaYF4:Ca is employed to suppress surface quenching and protect upconverting processes. The Yb3+ clusters were conducted in the neighboring layer to mediate the energy transfer from Nd3+ to Ho3+ ions and avoid concentration quenching effect. The successful synthesis of the core-multishelled UCNCs was inferred from the results of transmission electron microscopy (TEM, Fig. 2a and Figure S1), X-ray powder diffraction (Figure S4) and photoluminescence (PL, dotted lines in Fig. 2e) results. It is interrogated that the monodispersed UCNCs, with uniform size at ~ 40 nm in diameter, reveal a single-crystalline nature with the Bragg diffraction lines expected for the hexagonal-form NaYF4 (JCPDS #16–0334). As plotted in Fig. 2b, high-resolution TEM image and the Fourier transform diffraction pattern permit resolution of lattice fringes of {100} with a d-spacing of 0.52 nm, which matches with that of β-NaYF4 [16, 30]. For optical characterization, the UCNCs solution was measured under the excitation of continue-wave (CW) NIR laser. The Nd3+/Yb3+ sensitizer-pair clearly leads to two groups of characteristic peaks of Ho3+ and Tm3+ ions while being optically pumped at the wavelengths of 808 nm (Fig. 2e, top PL) and 980 nm (down PL), respectively. Typically, for 808 nm pumped case, the excitation energy would be harvested by Nd3+ ions incorporated in the core layer, then migrating to Ho3+ ions in the first shell layer through Yb-sublattice, while Tm3+ ions would dominantly exhaust the excitation energy collected by highly doped Yb3+ ions under conditions of 980 nm excitation. As shown in Fig. 2e, these emitting peaks can be assigned to 5F3→5I8 (487 nm), 5S2→5I8 (542 nm), and 5F5→5I8 (646 nm) transition of Ho3+ ions, and 1I6→3F4 (346 nm), 1D2→3H6 and 3F4 (362 and 451 nm), and 1G4→3H6 and 3F4 (476 and 648 nm) transition of Tm3+ ions, respectively. The weak peak at 542 nm and 648 nm under 980 nm pumping may arise from Yb3+→Ho3+ upconversion process. This unique behaviour of such UCNCs allows it to be a promising candidate for constructing dynamic switchable microlaser.
We subsequently fabricated UCNCs-based microdisk array by simply spin-coating a mixture of UCNCs and silica resin (6.5 wt%) onto the preformed SiO2 substrate [30]. Figure 2c and 2d show the scanning electron microscopy (SEM) images of UCNCs-on-SiO2 microdisk array and an isolated one (d = ~ 100 µm, t = ~ 300 nm), respectively. The UCNCs-based microdisk well inherits the pattern of underneath SiO2 pillar. For its laser characterization, a NIR pulsed laser (808/980 nm, pulsewidth 6 ns, repetition rate 10 Hz, Φ8 mm) is focused onto the top surface of the microdisk, with the emission light from the boundary of the cavity being collected by an optical fiber coupled to a monochromator. Figure S5 and S6 summarize the lasing actions in UCNCs-based microdisk under NIR excitation. For instance, we observed that several periodic sharp peaks centered at 345.8 nm emerge from the broad emission band once above the transparent threshold (i.e., Pa = 47.02 mJ cm− 2, as reflected by the first kink value in Figure S5c), and then quickly dominate the emission spectra as the increase of pumping fluences. This experimental FSR reads as ~ 0.24 nm, which matches the calculated one from the formula (1) (i.e., neff, ~ 1.58, and L are the group refractive index and the perimeter of UCNCs-based microdisk) [30]. Concurrently, the transition from spontaneous emission through amplification to lasing oscillation, as reflected by three regions with distinct slopes, is clearly visible from the corresponding light-light curves[16, 30]. Both this power-dependent behaviour and the well-defined mode spacing values confirm the lasing action along WGMs. Similar laser behaviour has also been observed in other characteristic peaks (Sect. 3 in Supporting Information) except for the resonance peaks at 542 nm and 648 nm under the excitation of 980 nm laser, since there is no sufficient gain to support efficient population inversion. As anticipated, the profile of the observed multicolour WGMs lasers exhibit pronounced differences while being excited by 808 nm and 980 nm lasers respectively.
WGMs lasing actions derived from Ln3+-doped materials have been well understood in many similar systems [16, 30–32]. Notably, single-mode laser can be realized in precisely regulated resonators with the diameter down to 4 µm or 20 µm [16, 32]. Moreover, an enhanced 290 nm (i.e., 1I6→3H6 of Tm3+) lasing was reported in a microdisk laser through suppressing competitive emission at 345 nm (i.e., 1I6→3F4 of Tm3+) via keeping the thickness of UCNCs-based microdisk at a “cutoff” value (around 130–140 nm)[30]. However, other characteristic peaks including 362 nm, 451 nm, 476 nm, 545 nm and 648 nm of Tm3+ ions can hardly be simultaneously quenched at a given cutoff thickness or through other chemical approaches. Consequently, the presence of such multicolour, multimode and isotropic output feature in Ln3+-based microdisk lasers leads to temporal and spatial fluctuations of the light source[33], thus is destructive to spectral purity and beam quality.
For precise mode management, we show that by harnessing notions from quasi-parity-time (PT) symmetry system[31], dynamically tunable single-mode laser can be readily realized in size-mismatched UCNCs-based PM (d = ~ 100 µm, t = ~ 300 nm, inset of Fig. 3a) device by asymmetric pumping and external excitation modulation. Such a kind of PM structure was proceeded through standard photolithography, followed by a second spin-coating round using a polymer compound containing UCNCs (6.5 wt%). Due to the mature CMOS technique, the size and shape of each microdisk can be well repeated in the system of two adjacent circular cavities, except for slight size-mismatching effect arising from fabrication inaccuracies of photolithography technique. Here, we define such PM laser as two tangent microdisks with similar radii of each component under asymmetric excitation, which can be experimentally realized through selectively pumping at only one constituent resonator by a NIR pulsed laser.
As shown in Fig. 3a under 808 nm right pumping, a broad luminescence at around 646 nm (i.e., 5F5→5I8 transition of Ho3+ ions) emerges at the very beginning with the full width at half-maximum (FWHM) of ~ 8 nm. With the increase of pumping fluences, the spectra become quite different. Only one peak at 646.2 nm ascends from the emission band and grows rapidly above Pa (i.e., 37.33 mJ cm− 2). The linewidth of the individual mode, once pumped above Pa, is less than ~ 0.07 nm, corresponding to a quality (Q)-factor of ~ 9000 (i.e., Q = λ/δλ, where λ and δλ denote the resonance peak and its FWHM respectively). Figure 3b displays the corresponding light-light curves of three characteristic peaks of Ho3+ ions. The integrated intensity of the resulting mode as a function of power density (red open circles in Fig. 3b) presents a clear S-curve, which unambiguously confirms the onset of single-mode lasing at 646.2 nm[16, 30, 34]. In this regard, we introduce the extinction ratio (E-ratio), defined as 10log(I1/I2) (where I1 and I2 are the intensity of the dominant peak and the highest side one respectively), to estimate the performance of supermode. It is observed that the corresponding E-ratio value (Fig. 3b) ascends steeply and reaches a value as high as 11 dB under 808 nm right pumping. Also, PM laser exhibits an ultraviolet single-mode operation at the wavelength of 345.6 nm (i.e., 1I6→3F4 transition of Tm3+ ions) with an E-ratio exceeding 13 dB under 980 nm right pumping (Fig. 3c and 3d). Apparently, there are no obvious kinks for other competing peaks (i.e., at 487 and 542 nm of Ho3+ ions, and 362, 451, 476 and 648 nm of Tm3+ ions), which thus act as typical spontaneous emission in our selectively excited PM structure. Hence, our PM device retains a single-mode laser in a wide pumping range with a switchable wavelength from 345.6 nm to 646.2 nm.
The observed mode-switching effect can be understood with the Vernier effect[35]. Figure 4a gives the schematic design of our PM device, where a fixed dual-mode upconversion material was purposely incorporated with the proposed PM structure. Note that, two microdisks directly contact with each other without any spacing. We numerically calculated two sets of resonance with identical gain spectrum to demonstrate mode selection phenomena. In the calculation, the radius of the left resonator was fixed while that of the right one varied in order to change the size deviation of two coupled microdisks. Following the Vernier effect, once PM is activated under NIR right excitation, most of the modes leak out to the passive cavity through the joint area except for selected modes which satisfy the following equations: mFSRl = nFSRr (i.e., where m and n should be integers). The threshold of resulting high-Q supermodes will be significantly decreased than their neighboring mode pairs. In principle, as the FSR of the multiple modes being enlarged, single-mode lasing would happen when there is only one mode emerging from the gain region of resonant peak. One unique thing to consider here is that each constituent peak of multicolour spectrum shares different FSRs and narrow linewidth owing to the intrinsic property of Ln3+ ions with plentiful energy states. Thus, together with two groups of distinct FSRs, only enhanced amplification of overlapped modes at 646.2 nm and 345.6 nm could be observed from our NIR right-pumped PM structure while other unwanted modes are remarkably reduced. Under the guidance of this principle, incorporated with specific sensitizer-pair and activator ions (i.e., 646 nm of Ho3+, and 346 nm of Tm3+), the proposed UCNCs exhibit two distinct Nd3+→Yb3+→Ho3+ and Yb3+→Tm3+ energy transfer paths, which can be effectively activated by 808 nm and 980 nm laser respectively. By virtue of material design, one of two mutually exclusive emitting peaks at the wavelengths of 646 nm and 346 nm would emerge from the dual-mode spectra. Controlled in a switch-like manner, our UCNCs-doped PM device would give an outstanding mode-switching performance.
A more rigorous analysis was conducted by systematically investigating the lasing actions in our PM device under three types of configuration including left pumping, right pumping, and uniform pumping (Figure S7). The corresponding simulated field distribution patterns are illustrated in the insets of Figure S8a. Figure S8a and S8c give the normalized lasing spectra above Pa under three types of pumping configurations with the corresponding light-light curves in Figure S8b and S8d, respectively. There is a slight deviation in wavelengths (Figure S9) of selected modes between left excited PM and right excited one. More interestingly, compared with our PM device under NIR left pumping, the threshold value under right pumping is lower even though two adjacent microdisks share nearly identical size and UCNCs-doping concentration (Figure S8b and S8d). Both results confirm the size gap between two coupled microdisks. Nonetheless, observation of control data (Figure S9) bolsters this narrative: the recorded spectra exhibit stable single-mode lasing emission while only one of the constituent resonators being excited, whereas it shows multimode lasing in uniformly pumped PM. It is very important to note, however, that even our UCNCs were designed with a particular emission profile, weak spontaneous emission at wavelength away from the design still emerges from such selectively excited PM. This is understandable since there is an overlap of signals owing to selected modes from right pumping and left pumping, as well as the particular modes emerging from the coincidence of the resonance between left and right resonators under uniform pumping. In principle, the PM under asymmetric excitation would hold more loss when compared to that under uniform pumping. However the threshold values of single-mode laser in our PM device, reading as 33.27 mJ cm− 2@347.2 nm and 37.33 mJ cm− 2@647.0 nm respectively, are obviously lower than that of microdisk lasers (i.e., 47.02 mJ cm− 2@346.6 nm, and 45.28 mJ cm− 2@646.2 nm), largely owing to the highly suppressed neighboring modes. The corresponding incident energy from these competing modes would directly contribute to the improvement of resulting supermodes.
A further investigation on the uniformity and stabilization of these supermodes was performed in the short-listed PM array (inset of Fig. 5). Despite fabrication fluctuation in cavity size, a clear mode-switching phenomenon can be observed in all those neighboring PMs (Fig. 5a and b), except for slight variance in wavelengths of selected modes and the corresponding thresholds (Fig. 5c and d). Also, from the repetitive emission switching by excitation cycling between 980 nm and 808 nm in Fig. 5e, this PM laser switch shows excellent stability even though with a certain amount of intensity distinctions of supermodes. Furthermore, this PM structure can be rotated to attain the far-field pattern (Figure S11), which quantitatively reveals that our single-mode laser emitted along the direction of ΦFF = 180° with the divergence angle at around 30° under NIR right pumping. Such unidirectional emission directly results from the collimation of the unexcited cavity[31]. This situation is generally different in two structurally identical resonators, in which exact PT breaking of gain and loss symmetry only happens at an exceptional point to maximize emitting power of the specific mode without directional outputs [36–37]. Obviously, this unidirectional single-mode laser favors the subsequent integration in on-chip photonic circuits at the corresponding direction, showing remarkable potential in the field of integrated optics. All these results present a compelling evidence that our proposed strategy shows great advantages in pursuing switchable unidirectional single-mode lasing in submillimeter cavities with high tolerance in fabrication deviation and low demand in design complexity.