Ultra-thin curved visible microdisk lasers with three-dimensional whispering gallery modes

1 Introduction

Microcavities with the whispering gallery modes (WGMs) have triggered intense research interest in both realistic applications and fundamental science, such as linear and nonlinear optics [1], [2], strongly coupled cavity quantum electrodynamics [3], as well as compact microlasers [4]. Light is strongly trapped in-plane by successive total internal reflections at the circular periphery of the cavity surface within various geometric configurations, including microspheres [2], [4], microtoroids [1], [5], [6], microrings [7], [8], as well as microdisks [9], [10]. In particular, semiconductor microdisk lasers have attracted considerable attention and are presently significant elements of photonic integrated circuits (PICs) [11], [12], [13], benefiting from their intrinsic small footprints, wafer-scale fabrication and chip-integration. Usually, the WGMs show a two-dimensional (2D) spatial intensity distribution in the horizontal plane of mechanically inflexible 2D planar microdisk lasers, owing to the high intrinsic stiffness. While, 3D WGMs can provide an extra degree of freedom for the confined photons compared with 2D WGMs, which can be realized by directly growing strained semiconductor active layers based on the self-rolling-up mechanism [14].

Here, we demonstrated ultra-thin curved visible microdisk lasers with a thickness ∼80 nm, of which the 3D curved architecture is formed by strain induced rolling mechanism [14], [15], [16]. Compared with directly epitaxial growth of stained active layers for self-rolling microcavities [14], [16], the strain relaxation of active materials was achieved by wet-etching the Ⅲ–V cladding layers from a generally thick planar structure. The method used here makes it possible to achieve strain relaxation of grown thick Ⅲ–V slab structures for a wide range of material systems. Single-mode lasing emission was observed from the ultra-thin curved visible microdisk lasers with a high quality factor (Q-factor) of ∼17,000. The ultra-thin curved microdisk lasers with a 3D spatial profile of WGMs enable many potential applications, such as integrated flexible and multilevel photon sources for the PICs [17].

2 Structural design and device fabrication

The epitaxial layer structure for the microdisk laser is shown in Figure 1a, of which the active region is composed of compressively strained InGaP/InGaAlP quantum wells (QWs) with whole thickness ∼180 nm [8], [9], [18].

Figure 1:

(a) Epitaxial layer structure consisting of compressively strained InGaP/InGaAlP QWs (two 7 nm InGaP QWs separated by 10 nm InGaAlP barriers) with the whole thickness of active region ∼180 nm. (b) PL spectrum of the epitaxial structure, which is centred at ∼670 nm with a linewidth of ∼34 nm.

To fabricate much thinner and curved microdisk lasers, firstly, a layer of SiO₂ with a thickness of ∼120 nm was deposited on the epitaxial wafer by plasma-enhanced chemical vapor deposition (PECVD) as a hard mask for dry etching. Electron beam resist ZEP520 was spin coated on the surface of the hard mask. Subsequently, electron beam lithography was used to define the microdisk pattern in ZEP520. The microdisk pattern was transferred from ZEP520 into the hard mask by using reactive ion etching (RIE). Then further transferred by chlorine-based inductively coupled plasma RIE (ICP-RIE) through the active region and the sacrificial layer to obtain the microdisks. After that, a time-controlled oxidation treatment of Al_0.96GaAs by water vapor was performed inside a tube furnace at 400 °C for 30 min to enhance its resistence to a mixture of concentrated sulfuric acid and hydrogen peroxide. The residual SiO₂ hard mask was removed in diluted hydrofluoric acid. Finally, both top and bottom cladding layers were wet-etched with reduced thickness by using the mixture of concentrated sulfuric acid and hydrogen peroxide (30 wt.% in H₂O) at ∼50 °C for 5 s, during which the bending force was induced by strain relaxation to form the self-bending-down ultra-thin curved microdisk cavities. The whole thickness of the active region was ∼80 nm after the wet-etching process as determined from the scanning electron microscope (SEM) images.

The measured photoluminescence (PL) spectrum of the unpatterned region was centred at ∼670 nm, as presented in Figure 1b. Microdisk cavities with diameter (D) designed at 10 μm were fabricated. The D (∼1 μm) of supporting pedestal was carefully controlled during wet-etching process to facilitate the bending deformation of the curved microdisk cavity. A schematic of the fabricated ultra-thin curved microdisk cavity is depicted in Figure 2a, of whch a saddle-shaped bending down microdisk cavity is mechanically supported by a small pedestal. Two side-view images (Figures 2b and c) illustrate the bending deformation of the microdisk cavity. Figures 2d–g shows the false-color tilted SEM images (tilt angle from 70° to 85°) of the fabricated ultra-thin curved microdisk cavity. A top-view SEM image (Figure 2h) of the fabricated microdisk cavity shows the smooth wet-etched top surface, which contributes to a high Q-factor by decreasing the optical scattering losses. Surface topography of the fabricated ultra-thin curved microdisk cavity was measured by using a laser microscope, as shown in Figure 2i. The height profiles of the top surface along the two orthogonal directions are provided in Figure 2j, which reflects a degree of anisotropy in curving. More than 400 nm bending depth from the periphery to the center is obtained due to strain induced rolling mechanism.

Figure 2:

(a)–(c) Schematics of the fabricated ultra-thin curved microdisk laser. (d)–(g) False-color tilted SEM images with the tilt angle at 70°, 75°, 80° and 85°, respectively. (h) A top-view SEM image of the fabricated ultra-thin curved microdisk laser. The blue region indicates the microdisk cavity. (i) Height profile of the fabricated ultra-thin curved microdisk laser obtained from a laser microscope. (j) Height profiles along the two orthogonal directions as indicated by the two dashed-line arrows (purple and blue) in (i).

3 Results and discussion

The ultra-thin curved microdisk lasers were optically pumped at room temperature with a micro-photoluminescence (μ-PL) measurement system in a surface-normal pump configuration, using a 405 nm laser (8 ns pulses with 40 μs periods) as the excitation source. The pump beam was focused on the microdisk region with a spot size of ∼3 μm through 50X objective lens, and the pumping position was controlled by using piezo electric nanopositioners. Laser emission was observed when the pumping positions located near the edge of microdisk cavities. The emission spectra were collected from the top by using the same objective lens and analyzed by a monochrometer with a liquid nitrogen cooled charge coupled device (CCD) detector. A 500 nm longpass filter was used to block the excitation light from reaching the detector. The near-field intensity profiles were collected by using a CCD camera.

Single-mode lasing emission was observed from the ultra-thin curved microdisk lasers. Figure 3a presents the collected PL spectra of a fabricated curved microdisk laser under various incident pump powers, indicating single-mode lasing operation. The corresponding measured light-out/light-in (L–L) curve and linewidth (Δλ) as a function of incident pump powers are demonstrated in Figure 3b, which exhibit the evidence of the lasing emission with a clear kink of L–L curve and the spectral linewidth narrowing effect. The lasing threshold is estimated to be ∼1.13 μW by fitting the L–L curve (red line is the fitting data). Benefiting from the smooth wet-etched surface and the side wall of the fabricated microdisk cavity, an intrinsic linewidth of ∼0.04 nm measured near below the threshold was achieved, corresponding to a high Q-factor (Q = λ/Δλ) of ∼17,000. The inset in Figure 3b shows an optical microscope image of the fabricated curved microdisk laser, of which a faint dot at the centre implies the size of the supporting pedestal. Figure 3c displays a blue-shift of the measured lasing peak with increasing incident pump powers mainly attributed to the carrier plasma effect, and the discontinuities are due to the limited spectral measurement resolution. Figure 3d presents a near-field intensity image of the ultra-thin curved microdisk laser collected below the lasing threshold. By increasing the pump powers above the threshold, strong speckle patterns appear as shown in Figure 3e, resulting from the high degree of coherent emission.

Figure 3:

Optical characterization of the fabricated ultra-thin curved microdisk lasers. (a) Measured spectra under various input pump powers. (b) Collected L–L curve and linewidth of the lasing peak at ∼696.7 nm. The inset depicts an optical microscope image of the curved microdisk laser. The scale bar represents 5 μm. (c) Lasing wavelengths under various input pump powers. (d), (e) Near-field intensity images collected below (d) and above (e) the lasing threshold, respectively. The blue line indicates the boundary of the curved microdisk laser and scale bars in (d) and (e) represent 5 μm.

Position-dependent μ-PL measurement was performed to confirm the stable lasing emission of the ultra-thin curved microdisk lasers. The pumping spot was aligned near the edge of the curved microdisk lasers at various positions. Figure 4a displays the typical laser spectra while pumping four different positions, the inset in Figure 4a presents a near-field intensity image measured below the threshold with pumping positions marked as P1 to P4. The log-plot of collected lasing spectra at pumping positions from P1 to P4 under various incident pump powers are presented in Figure 4b–e. Single-mode lasing emission was obtained at pumping positions P1 and P3 (shown in Figure 4b and d). Multi-mode lasing emission occured at elevated pump powers when the pumping spot located at P2 and P4, mainly due to the gain saturation and mode switching. The corresponding near-field intensity images collected above the threshold for various pumping positions are presented in Figure 4f, showing strong speckles around the periphery.

Figure 4:

Position-dependent laser spectra of the ultra-thin curved visible microdisk lasers. (a) Typical measured laser spectra above threshold at various pumping positions from P1 to P4. The inset depicts a near-field intensity image measured below the lasing threshold. The four pumping positions are marked on the image. (b)–(e) Log plots of the measured PL spectrum under various input pump powers for the four pumping positions. (f) Corresponding near-field intensity images collected above the lasing thresholds. (g) Collected L–L curve and linewidth of the microdisk laser at pumping position P3. (h) L–L curves of the main lasing peaks measured at pumping positions P1, P2 and P4. (i) Lasing wavelengths at various pumping positions. The corresponding main laser wavelengths are 694.5 nm, 697.0 nm, 692.8 nm and 694.5 nm, respectively. The scale bars in (a) and (f) represent 5 μm.

Figure 4g depicts a clear kink of L–L curve with spectral linewidth narrowing effect at pumping position P3. L–L curves of the main lasing peaks at other pumping positions are illustrated in Figure 4h. Small difference in lasing thresholds was observed even for the same lasing mode (e.g. the mode at the wavelength of 694.5 nm for pumping positions P1 and P4). The lasing wavelengths at various pumping positions are summarized in Figure 4i. Some lasing modes are repeatable for various pumping positions. The effective overlaps of spatial profiles between the lasing mode and the pumping beam, may lead to the slight different in threshold, as well as the mode switching when pumping various positions. The measured free spectral range (FSR) of the multi-modes is 2.5 nm in Figure 4c (1.7 nm in Figure 4e), which is smaller than the calculated value of ∼4.7 nm (FSR = λ2/πDneff = 4.7 nm for taking λ = 690 nm, D = 10 μm and neff=3.2) for the first-order WGMs. We expected that the second-order WGMs could be excited, as well as the mode splitting induced by the unperfect fabrication may contribute to the difference between the observed and calculated FSR.

The first-order WGM profile for the ultra-thin curved visible microdisk lasers was calculated by using a 3D finite-difference time-domain (3D-FDTD) method. We extracted the top-view magnetic field (H_z) distributions at various heights, as shown in Figure 5a. Figure 5b depicts the corresponding calculated H_z field profiles at various heights from L1 to L6, which indicate that the WGM propagates along the 3D deformed periphery and returns to its original positions after a round-trip. A cross-sectional view of the calculated H_z field is presented in Figure 5c. Different from the conventional WGMs confined within a 2D planar slab, the demonstrated ultra-thin curved microdisk lasers provide a 3D spatial profile of WGMs.

Figure 5:

3D-FDTD simulation results. (a) Illustration of the simulation model with six horizontal slices intersecting with the curved microdisk structure at various heights marked as L1 to L6. (b) Calculated H_z field profiles at heights from L1 to L6. (c) Cross-sectional view of the calculated H_z field profile.

4 Conclusion

In conclusion, we presented ultra-thin curved visible microdisk lasers owing to the strain induced self-rolling mechanism by wet-etching method. Both single mode lasing emission and a high Q-factor of ∼17,000 were obtained for an ultra-thin curved microdisk laser with D ∼10 μm. Especially, the WGMs confined within the curved microdisk lasers display a 3D spatial intensity distribution as confirmed numerically by using a 3D-FDTD method, which may enable promising applications such as integrated multilevel photon sources for PICs.