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.

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 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]:RoC=16(t1+t2+t3)3ε1t1(t2+t3)+ε2(t1+t2)t3,(1)where t1, t2, and t3 stand for the thicknesses of the AlGaN, MQWs, and GaN layers, respectively; ε1 is the strain caused by lattice mismatch between MQWs and AlGaN; and ε2 accounts for the strain between the GaN layer and MQWs. When t3=0, Eq. (1) reduces to the well-known formula for a two-layer film [26,27]. The RoC of the warped microdisk calculated from Eq. (1) is 50.4 μm. The positive sign of RoC indicates that the microdisk is bent upward. If the size of the strain multilayer is large enough to be rolled up one round, a microtube is formed [28,29].

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 60μm×60μm by a 15× 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).

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 2.45MW/cm2, and a single sharp mode appears at 441.3 nm and 2.79MW/cm2. 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 3.96MW/cm2 and 4.83MW/cm2, 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 m4 and m5 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.

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−Y plane with different θ [θ 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.

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 2.79MW/cm2 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.

Acknowledgment. The SEM work was done at the International Center for Dielectric Research (ICDR), Xi’an Jiaotong University. The authors also thank Y. Dai for help.