1. INTRODUCTION

Rapid development of integrated photonics based on an AlInGaN material system has been witnessed in recent years due to the stable physical and chemical properties and wide bandgap of the material system [1–3]. Because of the high-quality factor ($Q$-factor) and low lasing threshold inherited from the small mode volume, whispering gallery mode (WGM) microcavities based on GaN quantum well (QW) structure have drawn much attention [4–6]. Single-mode lasing with good monochromaticity, excellent stability, and high beam quality is more conducive to many practical applications such as microlasers, integrated photonics, and biological sensing [7,8]. However, most WGM microcavities support multiple lasing modes with small free spectral range (FSR) because their dimensions are typically much greater than optical wavelengths [9,10]. Several approaches have been proposed to achieve single-mode operation such as reducing the size of the microcavity [11,12] or coupling two cavities based on the Vernier effect and the parity-time symmetry effect [13–19]. However, the increased optical bending loss accompanied with sharp increase in the threshold is unavoidable in ultrasmall WGM lasers. Further, mode selection strategy from two coupling cavities includes sophisticated micro fabrication. Due to the rotational symmetry structure, for a conventional WGM microdisk, the light emission direction is isotropic [20,21]. Even anisotropic WGM structures with certain deformations or deliberately caused defects are troubled by light emission limited to 2D planes [22–24]. This leads to extremely low coupling and emitting efficiency, making it unfavorable for building multifunctional coherent light sources in optoelectronic integration. Therefore, it is of great significance to explore WGM microcavities with unidirectional laser emission in 3D.

In this paper, we fabricated a GaN-based warped microring from strained III-nitride-based QWs multilayers with an embedded off-centered hole. The optical properties involving mode, threshold, and light emitting direction of the warped microring were studied and compared with the warped microdisk without a hole. Finite-difference time-domain (FDTD) simulation was used to analyze the mode selection mechanism and 3D far-field distribution. An optimized WGM microcavity with vertical emitting direction, single-mode, and reduced threshold lasing was realized.

2. EXPERIMENTS

A. Fabrication Methods

The warped microring was fabricated from strained III-nitride multilayers grown by metal organic vapor phase epitaxy on $c$-plane-(0001) oriented sapphire substrates. The whole structure, as shown in Fig. 1(a), includes a 2 μm thick undoped GaN (u-GaN) followed by a 2 μm thick regularly doped n-GaN current-spreading layer ($3\times {10}^{18}{\mathrm{cm}}^{-3}$) and a 500 nm thick heavily doped ${\mathrm{n}}^{+}$-GaN sacrificial layer ($3\times {10}^{19}{\mathrm{cm}}^{-3}$), which was supposed to be selectively etched away by electrochemical (EC) etching to realize the floating structure to enhance the vertical light confinement. Then, a strained multilayer includes a 20 nm thick ${\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ layer (bottom), three pairs of MQWs (3 nm thick ${\mathrm{In}}_{0.15}{\mathrm{Ga}}_{0.85}\mathrm{N}$ quantum wells separated by 12 nm thick GaN barriers), and a 50 nm thick u-GaN capping layer (top) grown sequentially, which eventually forms the warped microring with a subwavelength thickness of 115 nm. The preparation process of the warped GaN microring is illustrated in Fig. 1(c). A microring pattern with an off-centered hole was initially formed by photolithography. Inductive coupled plasma (ICP) etching was then conducted to transfer the pattern onto the wafer until the side wall of the bottom sacrificial layer was exposed. The ICP etching process is carried out for 90 s under the atmosphere of ${\mathrm{BCl}}_3/{\mathrm{Cl}}_2$ (22 sccm/3 sccm) (sccm, standard cubic centimeter per minute) and the etching power of 900 W/400 W for the upper and lower electrodes. After removing the photoresist residual, EC etching in ${\mathrm{HNO}}_3$ solution at a voltage of 24 V was applied for 40 s to etch the ${\mathrm{n}}^{+}$-GaN sacrificial layer from the exposed edge until a small underneath pedestal was formed. During this process, the strained multilayer above began to detach and roll away from the substrate, eventually forming a warped microring. For comparison, the same method was used to fabricate a warped microdisk without a hole.

 

Fig. 1. (a) Schematic of the epitaxial layer structure. (b) Formation of strain-induced warped microring. (c) Fabrication process of the warped microring.

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B. Mechanism of Preparation

The strain-induced rolling mechanism of the warped membrane is illustrated in Fig. 1(b). We define the strained membrane into three sections: the bottom 20 nm AlGaN etching stop layer, the middle MQWs (combine InGaN and GaN into one layer according to the components), and the top 50 nm u-GaN layer. When the MQWs are epitaxially grown on the crystalline AlGaN, the upper MQWs layer suffers from compressive strain, while the lower AlGaN layer suffers from tensile strain and certain stress gradient buildup in order to fit the lattice constant at the interface. Similarly, the upper GaN layer would be stretched when deposited on the top of the MQWs. When the ${\mathrm{n}}^{+}$-GaN sacrificial layer was selectively etched away, the strained multilayer began to separate from the substrate, and each strain layer tended to restore its inherent lattice constant, resulting in the detached AlGaN/ MQWs/GaN film, which subsequently exhibited upward warping. The membrane seems to have enough mechanical flexibility such that the integrity of the film was kept without crack or defects. We calculate the radius of the curvature (RoC) of the warped microdisk based on a macroscopic continuous mechanical model assuming similar Young’s modulus of all three layers. The RoC for a warped structure can be estimated by the following equation [25]:

C. Experimental Configuration

The lasing characteristics of the samples were tested by micro-photoluminescence (μ-PL) spectroscopy using a pulsed 337 nm laser (repetition rate of 20 Hz, pulse width of 3.5 ns) as the excitation source. The samples were fixed on a 3D moving platform composed of an electric translational stage and a manual rotating platform. The pump laser was focused to a spot size of $60\mathrm{\mu m}\times 60\mathrm{\mu m}$ by a $15\times$ objective lens to cover the whole area of a single microring. The emission spectra were collected from the top of the sample through the same objective lens, or from the side of the sample through a slightly inclined optical fiber, and then detected by a spectrometer equipped with a cooled charge coupled device (CCD).

3. RESULTS AND DISCUSSION

A. Morphology

Figure 2 shows the scanning electron microscope (SEM) images of the warped membrane. The initially designed outer diameters of the microdisk and microring before warping are both 40 μm. An off-centered hole is clearly seen near the peripheral of the warped microring in Fig. 2(b). The diameter of the hole is 10 μm and its edge is 3 μm from the edge of the microring. The smooth top surfaces and vertical sidewalls of both samples are essential for producing WGM lasing with high $Q$-factor and low threshold. The RoC of the warped microdisk measured from the SEM image is 47.5 μm, which fits well with the calculated value of 50.4 μm. The RoC of the warped microring is 48.2 μm, which is slightly larger than that of the warped microdisk due to certain strain release introduced by the hole. The bending depth was therefore estimated to be 4.1 μm for the warped microring, offering adequate optical isolation from the substrate. Compared with the microdisk, the bending of the microring is directional, and the two warped sides are symmetrical with respect to the hole. The height of the supporting pedestal is too small (500 nm) relative to the size of the upper microcavity (40 μm) to be seen. The suspended microstructure creates an air gap between the warped membrane and the underlying GaN substrate, thereby reducing the optical leakage from the substrate and improving light confinement. The thickness of subwavelength of the warped membrane provides a large overlap between the MQWs gain material region and the confined optical mode.

 

Fig. 2. SEM images of (a) warped 40 μm microdisk and (b) warped 40 μm microring.

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B. Optical Properties

The micro-PL spectra were measured under various excitation power densities. Here, we collected the stimulated emission through the objective lens above the sample. A broad spectrum attributed to the spontaneous emission was observed under the pumping power density of $3.01{\mathrm{MW}/\mathrm{cm}}^2$ for the warped microdisk [Fig. 3(a)]. A weak sharp peak emerges at $3.24{\mathrm{MW}/\mathrm{cm}}^2$, with the peak center located at 439.6 nm and a full width at half maximum (FWHM) of 0.105 nm. The $Q$-factor of 4186 can be roughly estimated by

 

Fig. 3. PL spectra of (a) warped microdisk without a hole and (b) warped microring with an off-centered hole under different pumping power densities.

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The emission spectra of the warped microring with the off-centered hole show different characteristics. In Fig. 3(b), a broad spontaneous emission is observed at $2.45\mathrm{MW}/{\mathrm{cm}}^2$, and a single sharp mode appears at 441.3 nm and $2.79\mathrm{MW}/{\mathrm{cm}}^2$. The FWHM of the stimulated emission was 0.11 nm, and the $Q$-factor is calculated to be 4011. Such a lasing mode was considered the same mode as that of the warped microdisk at 441.4 nm. Their wavelengths are a little different since the RoC of the warped microring is slightly smaller than that of the warped microdisk, which affects the optical path in the cavity. When the excited power is improved to $3.96{\mathrm{MW}/\mathrm{cm}}^2$ and $4.83{\mathrm{MW}/\mathrm{cm}}^2$, new modes emerge at 439.3 and 440.7 nm, respectively. For the same reason, they correspond to the modes at 439.6 and 440.4 nm of the warped microdisk. The presence of the hole seemed to introduce a lasing mode selection function since two modes at 439.3 and 440.7 nm were significantly suppressed, and the other two modes [corresponding to ${\mathrm{m}}_4$ and ${\mathrm{m}}_5$ in Fig. 3(a)] disappeared completely. Eventually, only one peak was left, whose intensity is high and can almost be considered as single mode. The mode selection of the hole is due to its different effects on the modes corresponding to different radial field distributions. The lasing mode (441.3 nm) is assumed to be the first-order ($n=1$) WG mode, whose electrical field is concentrated to the edge of the microring. In contrast, other modes (439.3 and 440.7 nm and two disappeared modes) were assumed to be high order modes with electrical fields inside. The introduction of hole disturbed the high order modes and increased their loss but did not affect the first order mode near the periphery of the microcavity and therefore provides an effective mode selection to achieve single mode operation.

Figure 4 illustrates the relationship between the excitation power density and the emission intensity of warped microdisk and microring, respectively. Together, the plot is the variation of the FWHM with the excitation power density. It shows that, when excitation power density increases beyond the lasing thresholds, the PL intensity rises significantly along with the sudden narrowing of the FWHM for both samples. This implies that the emission spectra of the warped membrane switch from spontaneous emission to stimulated radiation. Threshold was defined as the power density when the FWHM suddenly decreases and the PL intensity increases simultaneously. The lasing threshold is estimated around $3.24{\mathrm{MW}/\mathrm{cm}}^2$ for the warped microdisk and $2.79{\mathrm{MW}/\mathrm{cm}}^2$ for the warped microring. We have measured a total of 50 samples of the warped microdisk and the warped microring. The threshold power density of the warped microdisk varies from 3.16 to $3.32{\mathrm{MW}/\mathrm{cm}}^2$, while that of the warped microring varies from 2.70 to $2.88{\mathrm{MW}/\mathrm{cm}}^2$, which presents good homogeneity among different samples. The lowering of the threshold is mainly due to the presence of holes, which removes some material from the cavity and reduces the energy loss because of the absorption of cavity materials and the pump volume. In addition, the higher-order modes are significantly suppressed, which reduce the energy distributed by those modes, and more carriers become available for the lasing mode. The pump energy is concentrated in the marginal low-order mode, thus obtaining fewer modes and a lower threshold.

 

Fig. 4. FWHM and PL intensity of the warped microdisk without hole (red triangle) and with an off-centered hole (black square) along the excitation power density.

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Far-field distribution of the light emission of the warped microring was measured at $3.5{\mathrm{MW}/\mathrm{cm}}^2$ to analyze the directionality of the emission. The sample was placed on a 360° rotating platform to test the PL spectra under different azimuthal angles $\phi$ [$\phi$ is defined in Fig. 5(b)]. An optical fiber was fixed at about 15° to the horizontal plane to collect the PL light. As shown in Fig. 5(a), the lasing emission shows obvious anisotropic characteristics since light emitted from the highest altitude of the warped microring contains the largest vertical component while that from the lowest altitude is mostly inside the horizontal plane. Figure 5(c) shows the PL images of the warped microring captured by the CCD. The white dotted line represents the outline of the microring. One can observe that the warped sides exhibit the highest luminous intensity because of the large vertical component of light, which can be efficiently collected through the objective lens above the samples.

 

Fig. 5. (a) Relationship of PL intensity of the laser mode at 441.3 nm with the detection angle. The angle ($\phi$) is defined in (b). (b) Illustration of warped microdisk in 3D spherical coordinate system. (c) CCD image of the warped microring. (d) Far-field pattern of the warped microring calculated by FDTD. The angle (θ) is defined in (b). Inset: top view of the simulation model.

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C. Numerical Simulation

Three-dimensional finite-difference time-domain (FDTD) numerical simulation was performed to study the far-field intensity distribution of the warped microring. We use a 1 m far-field hemisphere to calculate its far-field pattern (the warped microring is set in the center of the hemisphere). The 3D hemisphere surface is represented by a 2D plan view, and each circle represents a plane parallel to the $X\text{−}Y$ plane with different $\theta$ [$\theta$ is defined in Fig. 5(b)]. As shown in Fig. 5(d), the far field of the warped microring is concentrated in the range of 0°–30°, which indicates that the warped microring has a small far-field divergence angle in the vertical direction. Thus, the warped microring was proved to provide directional light emission in three dimensions.

Then, we analyzed the effect on different order of modes by the off-centered hole through FDTD. Four electric field monitors were placed at various heights of the warped membrane to simulate the field intensity distribution of the first-order WGM and the high-order WGM in the microcavity, as shown in Fig. 6(a). Each monitor can calculate the electric field intensity (${\mathit{E}}^2$) distribution in the corresponding plane. We extracted the calculated ${\mathit{E}}^2$ field profiles of the four monitors, respectively, and summed the values of the corresponding points to obtain the overall distribution of the electric field [Fig. 6(b)]. The first-order WGM in both structures distributes along the edge, indicating that the introduction of the off-centered hole hardly affects the first-order WGM. In contrast, there was spatial overlap of the field distribution with the off-center hole for the higher-order mode (indicated schematically by the white dotted line). Photons were lost from higher-order modes, causing the electric field strength of these modes to decrease significantly. Figure 6(c) shows the calculated resonance spectrum of the warped membrane. The mode spacing between the lower-order WGMs is around 7 nm, which is much larger than the FSR estimated from a flat microdisk with a diameter of 40 μm. We attribute such increase to the greatly altered light propagating path introduced by the warping structure. The peak of the high-order modes (green color) shown in the warped microdisk (the upper picture) disappears in the warped microring (the lower picture) and only the lower-order modes (red color) remain. Although the simulation model does not completely reflect the bending geometry of the experimental samples, it still shows the distribution of the electric field in the warped membrane and proves the suppression effect of the higher-order modes by the introduction of the hole.

 

Fig. 6. (a) Illustration of the simulation model with four individual planes placed at different heights on the warped microring. (b) Calculated electric fields distribution of the first-order WGM and higher-order WGM in the warped membrane, respectively. (c) Calculated resonance wavelength of the warped membrane.

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4. CONCLUSION

In summary, we have demonstrated single-mode WGM lasing in a warped microring with an off-centered hole based on a III-nitride MQWs membrane with subwavelength thickness. The warped structure is obtained by selectively releasing the strained multilayer heterostructure formed by lattice mismatch. Single mode has been achieved based on the additional mode selection capabilities provided by warped structure and high-order mode suppression caused by the off-centered hole. Moreover, the introduction of the embedded hole reduces the laser threshold from 3.24 to $2.79{\mathrm{MW}/\mathrm{cm}}^2$ while maintaining a high $Q$-factor of more than 4000. This is owing to the reduction of the absorption loss as well as the perturbation of the high order modes, which increase the energy available for lasing modes. Since the warped structure provides the vertical component of the light, and the far-field divergence angle is small in the vertical direction, the warped microring is proved to have directional light emission in 3D space. This work provides a promising way for achieving single-mode GaN QW-based WGM lasing with a high $Q$-factor and low threshold. The directional emission characteristics in 3D space give it great application potential in multifunctional coherent light sources for integrated photonics.