Electrically pumped single-mode microlasers with omnidirectional radiation for optical broadcasting communication

Abstract

Semiconductor whispering gallery mode (WGM)–visible microlasers are promising compact, on-chip light sources for high-speed visible-light communication due to their small footprints, high Q factors, and in-plane emission. However, achieving fabrication robustness, precise mode selection, narrow linewidths, and low thresholds for WGM-visible microlasers remains challenging. Here, we report a scalable strategy to fabricate continuous-wave, electrically pumped WGM blue microlasers based on III-nitride semiconductors with low threshold current densities and high slope efficiencies across diverse diameters from 10 to 160 micrometers. This high performance was achieved through effective vertical optical confinement by the waveguide structures and minimal sidewall scattering from atomically smooth surfaces. We further converted the laser cavity from a microdisk to a microring, which suppresses high-order WGMs and allows for single-mode operation with a high Q factor of 17,066. The omnidirectional in-plane radiation of WGM microlasers demonstrates angle-independent, high-modulation bandwidths, enabling synchronous broadcasting communication with high data transmission rates.

A compact on-chip blue light source with angle-independent radiation.

INTRODUCTION

The exponential growth of artificial intelligence applications and big data services is straining the information capacity of communication networks. Beyond 5G, high-speed visible-light communication (VLC) is increasingly positioned as a key element of advanced wireless roadmaps due to its low cost and large available bandwidth. Meanwhile, optical links are also being explored for inter- and intrachip data exchange (1–3). Within the whole visible spectrum, the blue emission region, where III-nitride semiconductors are usually chosen, is especially compelling because blue emitters exhibit a high differential gain and subnanosecond carrier lifetime and enable multigigahertz direct modulation for VLC. Moreover, blue emitters can also serve as an efficient pumping source to excite down-conversion materials with green/red emission for full-color displays or to excite fluorophores and tissue autofluorescence for biosensing (4, 5). All these advantages make blue emitters more favorable compared to green and red emitters.

Previous works prove that blue laser emitters, for example, edge-emitting lasers (EELs), present superior communication and display performance compared to light-emitting diodes (LEDs). However, EELs are not easy to miniaturize for compact integration, for which microlasers are considered as more promising candidates. The most popular kind of microlaser, vertical cavity surface–emitting laser (VCSEL), however, comprises distributed Bragg reflectors and presents vertical light emission, which makes the planar integration complex. By contrast, whispering gallery mode (WGM) microlasers use thin epitaxial layers similar to EELs and provide lateral out-coupling, simplifying in-plane photonic integration (6–9).

WGM microlasers typically use a microdisk or microring cavity resonator, which possesses a high Q factor due to minimal radiation loss (10, 11). Nevertheless, the small difference in refractive index between III-nitride heterostructures limits the vertical optical confinement, impeding the coupling of the emission with the gain medium for lasing (12). Consequently, prevalent methods often rely on sophisticated undercut fabrication techniques, such as selective chemical etching, to produce mushroom-shaped microdisks with edge regions enclosed by air (13, 14). These mushroom-shaped microdisk lasers are mostly optically pumped, but electrically pumped lasing has recently been achieved successfully (15). However, the mushroom morphology requires complex processing steps and is susceptible to cracking from external forces, posing challenges to reliable electrically pumped operation (16, 17). Recently, with the maturation of GaN-based EELs, WGM microdisk lasers, which use similar epitaxial layers, appear to be more competitive than mushroom-shaped microdisk lasers due to their simpler fabrication process and superior consistency (18, 19). However, these microdisk lasers still face issues with high thresholds and multimode lasing, which lead to increased power consumption and limit their practical use for data transmission in optical communications (20).

Here, we demonstrate continuous-wave (CW) electrically pumped GaN-based WGM blue microdisk lasers with diameters ranging from 10 to 160 μm by a scalable fabrication strategy. The vertical optical confinement and sidewall surfaces of the microdisks were carefully treated to reduce optical losses, leading to remarkably low threshold current densities and high slope efficiencies across all microdisk lasers. Furthermore, we modulated the microlaser’s morphology by converting microdisk to microring to suppress high-order modes, successfully achieving single-mode operation with a Q factor of 17,066, much higher than previous reported values. These single-mode microlasers also demonstrated an identical high modulation bandwidth for diverse routing directions, showing high-speed data transmission rates up to 6.25 gigabits/s (Gbps), which are not available in previous works.

RESULTS

Low-threshold electrically pumped microdisk laser

The GaN-based blue laser structure was epitaxially grown on a GaN substrate using metal-organic chemical vapor deposition. As shown in the scanning transmission electron microscopy (STEM) image in Fig. 1A, the InGaN/GaN multiple quantum wells (MQWs), serving as the active region, are sandwiched between the InGaN waveguide layers and AlGaN cladding layers on both the n- and p-sides. These epitaxial layers, also identified by the elemental mappings of Al (in red) and In (in blue) (Fig. 1B), were designed to effectively provide strong optical confinement in the active region along the vertical direction. The active region denoted by a black rectangle in Fig. 1A was further examined by high-resolution STEM, as shown in Fig. 1C, and the elemental mappings of In as shown in fig. S1. The clear and sharp interfaces were observed between the InGaN QWs, barriers, and their prestrained layers, implying the high crystal quality of the active region. Comprehensive details about the epitaxial structure are provided in Materials and Methods.

Fig. 1. GaN-based blue laser structure.

(A) Schematic cross section of a GaN laser epitaxial structure. The inset shows a TEM image of the laser structure, highlighting the n-InGaN and p-InGaN waveguide layers, n-AlGaN and p-AlGaN cladding layer, p-GaN and indium tin oxide (ITO) p-contact layer. Scale bar, 200 nm. (B) Elemental mapping of Al (red) in the AlGaN cladding layer and In (blue) in the InGaN waveguide and ITO layers. Scale bars, 200 nm. (C) High-resolution TEM images of the MQW region [black rectangle in (A)] and (D) ITO/p-GaN interface [red rectangle in (A)]. Scale bar, 5 nm. a.u., arbitrary units. (E) Simulated internal optical field losses as a function of thickness for different metals (5 to 50 nm) and ITO (20 to 200 nm) as the p-contact layer. (F) Vertical profiles of simulated optical field intensity and refractive index for the blue laser structure with and without a 100-nm ITO layer.

In addition, a 100-nm-thick indium tin oxide (ITO) layer, evidenced by the pronounced indium content in Fig. 1B, was deposited onto the p-GaN surface. This ITO layer served as a current spreading layer after rapid thermal annealing, which was a prevalent fabrication process designed to reduce both bulk and contact resistances. The high-resolution STEM image (Fig. 1D) of the red rectangle region in Fig. 1A reveals a smooth and homogeneous interface between the annealed ITO and p-GaN layers, which indicates that the interfacial scattering losses remain minimal after the deposition and annealing of the ITO layer.

To further evaluate the optical performance of this additional ITO layer, we used a transfer matrix method to simulate the intracavity optical losses for various candidate p-contact materials, including ITO, Ag, Pt, Pd, and Ni (Fig. 1E). Obviously, ITO and Ag exhibit the lowest internal losses compared to the other p-contact metals. Considering that ITO has far lower p-contact resistances than Ag, we lastly chose a 100-nm-thick ITO layer as the optimal contact for the microdisk laser. Figure 1F presents a comparison of the optical field intensity and refractive index distributions along the vertical direction with and without the ITO layer. Compared to the bare laser structures, the presence of the optimized ITO layer could notably reduce absorption losses and enhance the optical field intensity within the waveguide region by ~43%. This higher optical field intensity enhances the Purcell effect, thereby lowering thresholds of electrically pumped microdisk lasers.

Then, we defined circular device mesas to fabricate microdisk lasers, as shown in Fig. 2A (see Materials and Methods and fig. S2 for details). Figure 2B shows a scanning electron microscope (SEM) image of a microdisk laser with a diameter of 40 μm. The ITO contact and p-electrode were directly formed on top of the microdisk laser, while the n-electrode surrounded the microdisk mesa, ensuring good current spreading for both the n- and p-sides (fig. S3).

Fig. 2. Morphology and emission characteristics of GaN-based blue microdisk lasers.

(A) Schematic of a GaN-based microdisk laser device. (B) SEM image of a microdisk laser with a diameter of 40 μm. Scale bar, 10 μm. (C) SEM images of microdisk sidewalls produced by conventional (left) (blue rectangle) and optimized (right) (red rectangle) fabrication processes. EL intensity of 40-μm-diameter microdisk laser fabricated via conventional and optimized processes as a function of (D) current density and (E) CW operation time at 2 I_th, on a thermoelectric cooler (TEC) set to 23°C. (F) Far-field emission patterns were recorded at a blank panel located 20 cm away from the microdisk laser that was pumped at 0.5 I_th and 1.5 I_th. (G) Emission spectra of a microdisk laser with a diameter of 40 μm at injection currents from 10 to 50 mA. (H) Light output power versus injection current and (I) external quantum efficiency (EQE) versus current density for a 40-μm-diameter microdisk laser.

In general, the device mesa is formed by a single inductively coupled plasma (ICP) dry etching step using a positive photoresist mask. We also used this conventional approach to creating the microdisk mesa but observed rough and uneven sidewalls (see blue rectangle image in Fig. 2C), which, we believed, would severely compromise performance in WGM resonators that require total internal reflection from sidewalls.

To eliminate this issue, we optimized the dry-etching process by performing a cyclic etching procedure using a Ni metal mask instead of the photoresist mask (see Materials and Methods for details). This optimized procedure could mitigate the thermal deformation of the mask and reduce sidewall etching damage from prolonged exposure to high-energy plasma. Besides, we also carried out a KOH wet etching to further repair surface defects on device mesa sidewalls, which has been proven effective in performance improvements of micro- and nano-LED (21, 22). The red square image in Fig. 2C shows the optimized sidewall morphology of a microdisk laser mesa. The surface was observed to be very smooth and uniform, which was expected to substantially enhance the resonance of the WGMs in microdisk lasers.

Figure 2 (D and E) shows the CW electroluminescent (EL) intensity of the microdisk lasers fabricated via conventional and optimized dry-etching processes as a function of current density and CW operation time, respectively. As anticipated, the smoother sidewalls led to a significant reduction in the threshold current density (J_th), dropping from 5.53 kA/cm² in the conventional microdisk laser to 2.32 kA/cm² in the optimized one. This reduction in J_th resulted in less heat generation during the CW operation, contributing to the improved emission stability of the optimized microdisk lasers compared to the conventional ones (fig. S4A). However, the output power of the optimized microdisk lasers with a thermoelectric cooler (TEC) still decreases by 21% under CW operation for 7200 s at 2 I_th, although the forward voltage remains at about 4 V (Fig. 2E). This power reduction was possibly due to the increased defect-assisted nonradiative recombination at the unpassivated microdisk sidewalls.

Moreover, we measured far-field emission patterns of the microdisk lasers at the injection currents of 0.5 I_th and 1.5 I_th in Fig. 2F. Clear interference fringes could be observed in the far-field pattern when the microdisk laser was operated above the threshold current, implying the lasing behavior of the device. The emission spectra of the microdisk laser operated below and above the threshold current are shown in Fig. 2G. A single pronounced lasing peak first emerged at a wavelength of around 445 nm at 20 mA. In addition, as the injection current increased, some additional lasing peaks were excited at the shorter-wavelength region. These lasing peaks indicated that multiple WGMs existed in our microdisk lasers. Last, the light output power of a microdisk laser was measured at various injection currents, exhibiting a rapid increase beyond the threshold with a fitted slope efficiency of ~0.41 W/A (Fig. 2H). Then, the external quantum efficiency (EQE) was calculated and plotted as a function of injection current density in Fig. 2I. The peak EQE was obtained as high as 13%. To our best knowledge, these values have been reported, providing essential benchmarks for GaN-based microdisk lasers.

Size scalability of microdisk lasers

After establishing the optimized fabrication process, we investigated the size scalability of microdisk lasers by varying a wide range of microdisk diameters from 10 to 160 μm. The differential resistance obtained from the current-voltage curves would slightly increases, as the laser diameter decreases (fig. S5). All these microdisk lasers with different diameters could be electrically pumped to lase above the threshold, and their typical EL emission images when operated at 1.2 I_th are shown in Fig. 3A. When the microdisk diameter increases from 10 to 160 μm, the threshold current density of the lasers decreases significantly from 4.6 to ~0.9 kA/cm² (Fig. 3B). The size-dependent threshold current densities of microdisk lasers fabricated by a conventional process are shown in fig. S6 for comparison. The variation in threshold current density for optimized microdisk lasers is much smaller than that for conventional microdisk lasers, reflecting the advancement of our optimized fabrication technique, particularly for small microdisk lasers. The J_th values derived from linear scaling with the diameter (D) can be fitted by the equation of J_th ≈ J₂ + 4j₁/D, where J₂ denotes the bulk, size-independent recombination current and j₁ quantifies the surface nonradiative recombination (23). This equation decouples the contribution of surface recombination from the overall J_th. Process optimization reduces j₁ from 2.8 to 1.04 A/cm, demonstrating effective suppression of surface recombination (fig. S6). Notably, this j₁ value is markedly lower than that reported for GaAs QW–based WGM lasers (~4.5 A/cm) (21), indicating a weaker size dependence in GaN-based WGM lasers.

Fig. 3. Size-dependent lasing characteristics of GaN-based blue microdisk lasers.

(A) Optical emission images of microdisk lasers under 1.2 I_th (diameters, 10 to 160 μm), fabricated via the optimized process. Scale bar, 40 μm. (B) Threshold current density versus diameter for GaN-based microdisk lasers in this work and other reports (15, 18–20, 24–26). (C) Simulated in-plane distribution of the fundamental mode in a 10-μm-diameter microdisk laser, with an inset showing a magnified distribution pattern for the partial edge region. (D) Cross-sectional fundamental mode profiles for microdisk lasers with diameters of 5, 10, and 20 μm by three-dimensional finite-difference time-domain (3D-FDTD) simulation. (E) Sidewall overlap factors (defined as the ratio of the optical field integrated intensity across the region within 50 nm from the sidewall to total integrated intensity) for microdisk lasers with different diameters. (F) L-I curves, (G) slope efficiency, and (H) WPE at 1 I_th and 3 I_th for microdisk lasers with diameters of 20 to 60 μm.

We also presented the previously reported threshold current densities of the microdisk lasers with different diameters in Fig. 3B for comparison. Because of the careful design of the ITO capping layer and the optimized smooth sidewall surfaces, our microdisk lasers exhibited lower threshold current densities than those of most of the previous microlasers in similar device structures. However, we noticed that the mushroom-shaped microdisk lasers, depicted as a hollow circle in Fig. 3B, exhibited a lower threshold current density. This is due to the microdisk edge regions, which are suspended in air, offering superior vertical optical confinement for WGM resonance within the microdisk. Despite this advantage, the complex fabrication process required for mushroom-shaped microdisk lasers and their susceptibility to cracking at the microdisk edge regions make them unsuitable for mass production or heterogeneous integration in practical applications.

To explain the size-dependent behavior of the threshold current density in our microdisk lasers, we used a three-dimensional finite-difference time-domain (3D-FDTD) method to simulate the electric field intensity distribution of the fundamental WGM (Fig. 3, C and D). The magnified inset in Fig. 3C reveals that the fundamental WGM is predominantly situated at the microdisk periphery, indicating its significant sensitivity to sidewall morphology. Meanwhile, we extracted the cross-sectional profiles of the fundamental WGMs for microdisks with diameters of 5, 10, and 20 μm in Fig. 3D. As the microdisk diameter increases, the fundamental WGM shifts gradually away from the edge of the microdisk.

In general, the extent to which the microdisk lasing is influenced by the sidewall morphology is determined by the coupling between the fundamental WGM and sidewall surface scattering. To quantify this coupling effect, we defined a sidewall overlap factor as the ratio between the integrated intensity within 50 nm of the microdisk edge (corresponding to typical GaN sidewall roughness) and the total intensity of the electric field of the fundamental WGM. The effect of the sidewall surface scattering was considered identical for all microdisk lasers. Figure 3E plots the sidewall overlap factors for microdisk lasers with diameters ranging from 2 to 40 μm. The microdisk lasers with larger diameters exhibited lower sidewall overlap factors, indicating less coupling of the fundamental WGM to sidewall surface scattering and stronger resonance of the WGMs within the microdisks. Therefore, the larger microdisk lasers have lower threshold current densities, which agrees well with the result in Fig. 3B.

Moreover, the light output power-current (L-I) characteristics of these microdisk lasers with diameters from 20 to 60 μm were measured, and the typical curves are shown in Fig. 3F. The slope efficiency could be derived from these curves and statistically plotted in Fig. 3G for comparison. The average slope efficiency remains nearly constant for microdisk lasers smaller than 40 μm, yet it decreases for those with larger diameters exceeding 40 μm. As previously discussed, an increase in diameter leads to a smaller sidewall overlap factor for microdisk lasers, which signifies less coupling of the WGMs to sidewall scattering loss. Although this limited scattering loss benefits the lasing behavior, it also results in restricted light extraction of the WGMs, thereby diminishing the output power of the devices. Therefore, when the sidewall scattering loss is balanced for lasing and light extraction, the slope efficiency becomes nearly constant, as observed for microdisk lasers smaller than 40 μm; however, when the sidewall scattering loss is too small, leading to insufficient extraction of the WGMs, the slope efficiency begins to decrease, as seen in microdisk lasers larger than 40 μm. This is also the reason for the similar behaviors of the measured wall-plug efficiency (WPE) of the microdisk lasers with diameters from 20 to 60 μm at currents of I_th and 3 I_th in Fig. 3H. Notably, we realized a remarkable WPE of as high as ~7.2%, which sets the benchmark for GaN-based microdisk lasers. The observed dispersion in slope efficiency and WPE (Fig. 3, G and H) mainly arises from material inhomogeneity and diverse sidewall losses, which affect the performance consistency of microlasers and require further optimization.

In general, the scaling-down of microlasers is a trade-off between size reduction and lasing performance. On the one hand, the small-sized microdisk lasers have a low absolute threshold current (fig. S7A), high slope efficiency (Fig. 3, F and G), and high WPE (Fig. 3H), implying lower power consumption during operation. On the other hand, these small-sized microdisk lasers exhibit high threshold current density (Fig. 3B), high sidewall scattering loss (Fig. 3E), and faster power decay (fig. S7B), indicating that the instability issue of small-sized microdisk lasers becomes more severe. Therefore, the microdisk laser size should balance its efficiency and reliability, two factors that are critical for practical applications.

Single-mode lasing enabled by the conversion of microdisk to microring

Microdisk lasers exhibit multimode lasing as evidenced by the spectra in Fig. 2G, which is attributed to the resonance of multiple WGMs within the microdisk. We simulated the spatial distribution of these resonant WGMs corresponding to wavelengths between 440 and 450 nm. One fundamental WGM (q = 1) is observed hugging the disk periphery, and several high-order WGMs (q > 1) distribute in circles inside the fundamental WGM in sequence (Fig. 4A). To suppress the high-order WGMs, we introduced a cylindrical air hole at the center of the microdisk, converting it into a microring. The air hole disrupted the resonance of the high-order WGMs due to scattering on the inner sidewalls of the microring, resulting in only the fundamental WGM being able to resonate and propagate within the microring, as simulated in Fig. 4B. Notably, the high-order WGMs were not completely suppressed and would be excited at much higher pumping levels than the threshold of the fundamental WGM.

Fig. 4. Modal engineering and emission characteristics of GaN-based microring lasers.

Simulated WGM in-plane distribution of a GaN-based (A) microdisk and (B) microring laser corresponding to the wavelengths between 440 and 450 nm. (C) Extracted Q factors at different resonant wavelengths (λ) for the microdisk and microring resonators with a diameter of 50 μm and an inner diameter of 30 μm. (D) SEM image of a microring laser with an outer diameter of 50 μm and an inner diameter of 30 μm. Scale bar, 10 μm. The inset shows EL image. (E) EL spectra of the microring laser with an outer diameter of 50 μm and an inner diameter of 30 μm at varying injection currents (0.5 to 4 I_th). (F) High-resolution spectrum at 2 I_th for the microring laser with laser with an outer diameter of 50 μm and an inner diameter of 30 μm. The inset displays the EL intensity at a logarithmic scale (in decibel-milliwats). (G) Comparison of Q factors measured from the microring laser in this work and from the previous literature of GaN-based WGMs lasers (15, 18–20, 27, 28), VCSEL (29–32), photonic crystal surface-emitting laser (PCSEL) (33–35), and distributed-feedback (DFB) lasers (36–39). (H) Side-mode suppression ratio (SMSR) versus current level for microdisk and microring lasers. (I) SMSR and (J) L-I curves of the microring lasers with an inner diameter from 0 to 50 μm.

Figure 4C depicts the calculated Q factors at different resonant wavelengths for microdisk and microring lasers. For microdisk lasers, the coexistence of three high–Q factor WGMs makes the microdisk favor multimode lasing. While for microring lasers, only one high–Q factor WGM stands out, leading to the single-mode operation of microring lasers. In addition, the Q factor of this WGM for microring lasers was observed to be even larger than that of microdisk lasers in Fig. 4C. We presumed that this was due to the superior radial confinement of the resonant WGM, which is determined by the refractive index at the inner region of the microring and the ring width. Increasing the refractive index contrast or decreasing the ring width could notably facilitate the single-mode operation. Detailed design guidelines of single-mode microring lasers are discussed in Materials and Methods.

Then, we designed and fabricated microring lasers with an inner air hole, following the simulation described above. As shown in the SEM image in Fig. 4D, the diameter of the air hole is 30 μm. The inset of Fig. 4D displays an emission image of a microring laser operated at 1.2 I_th. The EL spectra recorded at various injection currents in Fig. 4E demonstrate that the microring mesa enables single-mode lasing until the current increases up to 4 I_th, at which point an additional lasing peak corresponding to a high-order WGM emerges. Possible strategies for suppressing these high-order WGMs could follow the general design guidelines of single-mode microring lasers. Figure 4F shows the EL spectrum of the microring lasers pumped at 2 I_th, with a central peak at 443.73 nm and a full width at half maximum (FWHM) of 26 pm, as determined by Lorentzian fitting. Consequently, the corresponding Q factor of the microring lasers achieved 17,066, significantly surpassing previously reported values for electrically pumped GaN-based lasers (Fig. 4G). For other laser architectures, such as distributed-feedback (DFB) lasers or VCSEL, pursuing a high Q factor typically comes at the cost of a higher lasing threshold; by contrast, the intrinsic resonant WGMs, combined with our tailored cavity design, allow our single-mode microring laser to attain the highest Q factor reported to date at a low threshold current, even when compared with optically pumped GaN lasers (fig. S8). The side-mode suppression ratio (SMSR) of the microring lasers is around 15.8 dB (see the inset in Fig. 4F), marking a 26.3-fold increase compared to that of the microdisk lasers (Fig. 4H).

Furthermore, we investigated microring lasers with an identical outer diameter of 50 μm but varying inner diameters ranging from 10 to 40 μm (Fig. 4, I and J). Microdisk lasers lacking an inner air hole (with an inner diameter of 0 μm) were also presented for comparison. As the inner diameter increases, the SMSR of the microlasers at 2 I_th continues to rise and begins to saturate when the inner diameter exceeds 30 μm. The saturation of the SMSR indicates that high-order WGMs have been almost suppressed for microring lasers with an inner diameter larger than 30 μm. Besides, although the inner emission area was removed for these microring lasers, the total output power remained unchanged, with a maximum variation of 9.6% at the same currents (Fig. 4J). Thus, all the microring and microdisk lasers demonstrated similar threshold current density (fig. S9) and slope efficiency, implying little current performance degradation for the single-mode operation of microring lasers. However, above the threshold current density, the microring lasers with a smaller inner diameter exhibited higher output powers (fig. S10). This behavior is reasonable because the microring lasers with smaller inner diameters have a larger active emission area, which could produce additional light output.

Omnidirectional real-time signal transmission from a microlaser

Because of the continuous rotational symmetry of the resonant WGMs, the radiation from microlasers with a disk or ring shape should be uniformly distributed in all in-plane omnidirectional angles. This omnidirectional radiation makes microlasers attractive for optical broadcasting communication. To investigate this proof of concept, we established a visible light communication system that uses a microlaser as the transmitter (Fig. 5A). The photodetector (PD), serving as the receiver, collected real-time signals at various angles ranging from 0° to 90°, as illustrated in Fig. 5B (see details in Materials and Methods).

Fig. 5. Angle-resolved real-time signal transmission performance of a single-mode microlaser.

(A) Schematic of a real-time signal transmission system using a single-mode microlaser. (B) Experimental setups for the PD located at different angles of 0°, 60°, and 90° for signal collection. (C) Angle dependence of −3-dB modulation bandwidth for the single-mode microlaser with an outer diameter of 50 μm and an inner diameter of 30 μm at 70 mA. (D) Comparison of maximum −3-dB bandwidth and corresponding output power among WGMs lasers, micro-LED (40–46), and EEL (47–50). (E) Eye diagrams at a data rate of 5 Gbps with different collected angles. (F) Front and side photographs of the microdisk laser and fiber edge coupling setup. (G) Eye diagrams of fiber edge coupling signal transmission system at different data rate with a current of 70 mA.

The −3-dB modulation bandwidth of the single-mode microring laser with an outer diameter of 50 μm and an inner diameter of 30 μm at 70 mA was measured to be around 3.85 GHz for all in-plane angles from 0° to 180° (Fig. 5C). The constant modulation bandwidth across all in-plane angles demonstrates the omnidirectional radiation of our single-mode microlasers. Moreover, we used modulation bandwidth and output power as the two primary metrics to benchmark light communication sources, as depicted in Fig. 5D. Although GaN EELs can deliver similar bandwidths and higher power, their highly directional beams complicate broadcast applications. The multimode microdisk lasers with various dimensions and single-mode microring lasers in this work achieved a high output power of 24 to 32 mW and a high −3-dB modulation bandwidths of 3 to 4.2 GHz simultaneously, surpassing the performance of state-of-the-art micro-LEDs. The frequency response curves of the microdisk and microring lasers with diverse sizes are presented in fig. S11. Figure S12 presents a 3-GHz sine-wave signal that drives the single-mode microlaser, along with the corresponding waveforms received by the PD at angles of 0°, 30°, and 90°, respectively. Both amplitude and phase remained consistent across different receiving angles, allowing for synchronous broadcasting communication.

Then, real-time eye diagrams at a data transmission rate of 5 Gbps were recorded at receiving angles ranging from 0° to 180° (Fig. 5E). The signal integrity of all the eye diagrams demonstrated identical high-speed data delivery capabilities for omnidirectional radiation of the single-mode microlasers, implying no interference for multiusers at different receiving angles. We also demonstrated nonreturn-to-zero (NRZ) 5-Gbps free space communication links with different link distances from 0.2 to 2 m (fig. S13). No quality degradation was observed in the eye diagrams, indicating that our microlasers were able to support meter-level, high-fidelity omnidirectional communications at a high data transmission rate. These metrics meet the VLC targets for gigabit links and are compatible with current VLC standards.

Last, we also investigated the performance of fiber-coupled communication links using microdisk lasers, as shown in Fig. 5F (see details in Materials and Methods). We obtained milliwatt-level coupled power to the fiber with an efficiency of 24.8 to 29.6% over the operating current range from 30 to 100 mA (fig. S14). Then, the high-speed transmissions at rates of 2.5 to 6.25 Gbps were demonstrated, and the corresponding real-time eye diagrams were recorded as shown in Fig. 5G. The 6.25-Gbps eye diagram appears partially closed because of the bandwidth limit of the detector, whereas lower rates remain clearly open. These results imply that our microlasers are also able to support milliwatt-level, high-fidelity fiber communications at a high data rate.

DISCUSSION

In summary, we report CW electrically pumped GaN-based WGM microlasers with a low current density down to 0.9 kA/cm² and a high slope efficiency above 0.4 W/A. The peak EQE and WPE are ~13 and 7.2%, respectively, serving as benchmarks for GaN-based visible WGM microlasers. We also modified the microlaser shape from a microdisk to a microring, realizing single-mode operation with a remarkable Q factor of 17,066, which is 69% larger than the highest value ever reported. Last, the concept of real-time synchronous broadcasting communication was demonstrated, revealing omnidirectional routing radiation featuring consistent amplitudes and phases, an identical modulation bandwidth exceeding 3.85 GHz, and an NRZ transmission data rate up to 6.25 Gbps. The performance of GaN-based WGM microlaser, micro-LEDs, EELs, VCSELs, and photonic crystal surface-emitting lasers (PCSELs) is summarized for comparison in table S1. Our work establishes a viable route toward compact, efficient, and single-mode electrically pumped WGM microlasers with a high Q factor. This kind of microlaser sources can be integrated by evanescent side coupling to specific designed waveguides or directly butt-coupling at the edge of microlasers, highlighting their great potential for next-generation on-chip integrated photonics and VLC applications.

MATERIALS AND METHODS

The epitaxial structure of GaN-based blue laser structure

The GaN-based blue laser structures were grown on commercially available, low-defect c-plane (0001) bulk GaN substrates by metal-organic chemical vapor deposition reactor. Trimethylgallium, trimethylindium, and trimethylaluminium served as group III precursors, while high-purity ammonia (NH₃; 99.999%) supplied nitrogen, and the p-type and n-type dopants are dicyclopentadienyl magnesium (Cp₂Mg) and silane (SiH₄), respectively. Growth began with a 160-nm Si-doped GaN buffer, followed by a 1.2-μm Si-doped AlGaN lower cladding layer and a 260-nm Si-doped InGaN optical waveguide layer. The active region comprised two pairs of InGaN/GaN MQWs (fig. S1). A 300-nm Mg-doped InGaN upper waveguide and a 330-nm Mg-doped AlGaN cladding were epitaxially grown in sequence. Last, a 10-nm heavily Mg-doped GaN contact cap was grown to minimize contact resistance.

Fabrication of GaN-based WGM microlasers

A 100-nm-thick ITO layer was deposited on the GaN-based blue laser epitaxial wafer via magnetron sputtering, and rapid thermal annealing was performed sequentially under air and N₂ atmospheres for 5 and 10 min, respectively, to reduce the film resistivity and optical absorption. Next, a 200-nm SiO₂ layer and a patterned 20-nm Ni metal layer by standard photolithography and a lift-off process were deposited in sequence as etching masks. With the patterned Ni mask, we initially removed the excess SiO₂ by reactive ion etching in CF₄ plasma to form a secondary mask and then defined the microlaser mesas by conducting 10 cycles of ICP etching with a single-pulse duration of 20 s. Postetching samples were then treated in a KOH solution. Last, Ti/Al/Ni/Au (30/120/50/100 nm) were deposited and annealed at 300°C to form n- and p-electrodes.

Free space and fiber-coupling communication setups

For free space communication, a 20×, numerical aperture 0.42 objective coupled emission from one side of the microlaser into a PD with a lens port. For fiber-coupling communication, the chips were cleaved to form edge-couplable microlasers, the device and a 1-m multimode fiber (50-μm core) were comounted on a six-axis stage with two orthogonal cameras for simultaneous lateral/vertical alignment. The fiber’s distal end was connected to a power meter for real-time readout, which enabled us to adjust the stage position to obtain the maximum measured output power of the microlaser. We then recorded the fiber-collected light output power (P_fiber) at different currents and calculated the coupling efficiency by the equation of η = P_fiber/P_total, where P_total is the total output power measured without the fiber.

Materials and device characterization

The cross section of STEM and energy-dispersive x-ray spectroscopy mapping images were obtained by Titan Themis Z TEM with probe corrector at an accelerating voltage of 300 kV. The output power of the laser diodes was measured using an integrating sphere coupled with an optical power meter (Ocean Optics FOIS-1) and a dc power supply (Keithley 2400). Emission spectra were acquired by an optical spectrum analyzer (Yokogawa AQ6374E) with a resolution of 0.02 nm. The small-signal modulation response was characterized using a network analyzer (AV3672B-S; 26.5 GHz), a high-speed GaAs PD (Newport 818-BB-45; 12.5 GHz), and radio frequency (RF) probes (Cascade GSG200). NRZ signal transmission rates and real-time eye diagrams were evaluated via an arbitrary waveform generator (AWG; Keysight M8195A; 26.5 GHz), a high-speed Si PD (Graviton SPD-4; 3 GHz), and a high-speed oscilloscope (Keysight 86100D; 25 GHz).

General design guidelines for single-mode microring lasers

The following factors should be considered for designing single-mode microring lasers:

1) Cavity size: Smaller size of microring lasers is usually beneficial to enlarge the free spectral range, thereby reducing the number of additional longitudinal modes within the gain bandwidth. However, the diameter should not be too small, as this will increase the threshold of the fundamental mode and complicate the process. On the basis of our experiments, a diameter of 20 to 60 μm provides a practical window.

2) Ring width: Narrowing the ring width effectively suppresses higher-order lasing modes, as reflected by the increased SMSR in Fig. 4I. A microring laser with a ring width of 5 μm achieves an SMSR above 15 dB; further reduction is expected to strengthen single-mode operation. However, using the narrow ring width would also increase the fabrication difficulty, which should be considered for a trade-off.

3) Refractive-index contrast: Higher refractive-index contrast between the microring and its surrounding dielectric materials helps to confine light and reduce radiation loss within the microring structure. This will increase the Q factor of the fundamental mode and enable its low-threshold lasing compared to high-order modes. Therefore, single-mode operation and larger SMSR could be achieved. As an example in this work, we converted microdisk to microring (inner region is air, and refractive index is 1) and obtained high SMSRs in Fig. 4H. Besides, filling low–refractive index materials at the inner region of microrings is also able to realize single-mode operation, but the mode suppression is not as good as the internal air gap.

4) Differential modal loss: Engineering the modal loss is also possible to eliminate high-order modes, such as distributed feedback lasers. Mode-selective feedback (e.g., a Bragg grating along the inner wall) can achieve a single-mode resonance. However, these gratings are complicated for both design and fabrication (6).

Supplementary Materials

This PDF file includes:

Figs. S1 to S14

Table S1

References