Open Access
Add to BibSonomy
Abstract
To reduce the threshold and achieve unidirectional lasing emission in a whispering gallery mode microcavity, we propose and demonstrate a GaN-based eccentric microring with an inner hole located off the center. Compared to microdisk with the same outer diameter, the eccentric microring structure exhibits a remarkable reduction of lasing threshold by up to 53%. The introduction of the hole disturbs and eventually suppresses the field distribution of the higher order modes. Laser emission with high unidirectionality with a far-field divergence angle of about 40° has been achieved, meanwhile the Q factor of the whispering gallery modesis remains high as 6388. Finite-difference time-domain numerical simulation is carried out to prove that the far-field profile of the eccentric microring structure can be controlled by the position and the size of the hole. The properties of the whispering gallery mode microcavities are improved greatly through a simple structure and process, which has an important guiding significance to the research and development of the microcavity lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
GaN has stable physical and chemical properties, adjustable band gap, and a spectral range covering the entire visible light band, making it the most promising material in optoelectronic devices [1,2]. In the field of optical microcavities, whispering gallery mode (WGM) microcavity [3,4] such as microdisk or microring have drawn much attention in the fields of optoelectronics [5], quantum electrodynamics [6], biosensing [7–9] and microlasers [10] for their high quality factors and extremely small mode volume. Since WGMs are usually confined to the periphery of the microdisk. The inner volume of the microdisk structure not only contributes very little to the laser, but also causes energy loss due to the absorption of cavity materials. Besides, the internal high-order modes absorb part of the pump power in competition with the lasing modes. In comparison to the microdisk, microring structure is supposed to have fewer modes and lower threshold. Wang et al. reported that the microring laser with increasing inner diameter show systematically lower value of lasing threshold [11]. When more central volume was removed, the lasing threshold decreases from 12 µJ∕cm2 in a 1 µm microdisk to 6.2 µJ∕cm2 in a microring with a 1 µm outer diameter and 500 nm inner diameter. However, the laser emission of microring is still in-plane isotropic due to the rotational symmetry of its structure, which leads to the extremely low efficiency of free space collection. The issue of the isotropic lasing emission of traditional microdisk and microring laser greatly limit their application in many aspects of detection and optical communication [12,13]. Unidirectional laser emission can be realized by breaking the rotational symmetry of microcavities through forming asymmetric/deformed cavity structures such as spiral-shaped microcavity [14–16], limacon-shaped microcavity [17,18], rounded-isosceles-triangle-shaped microcavity [19] and et al, or introducing defects such as holes in the mode field regions [20–22]. Shape deformation allows improved directionality of emission due to refractive escape, but the Q factor is significantly spoiled. Recently, Zhou et al. reported an asymmetric circle microcavity with the cross-sectional shape of circle adding a cone angle, which can obtain unidirectional lasing emission without reducing the Q factor (the Q factor is 2100 for the microdisk and 2300 for the asymmetric circle microcavity), but the threshold was increased compared to the microdisk [23]. Wiersig and Hentschel et al. studied theoretically that the high Q factor of the cavity mode can be preserved while achieving good unidirectional emission in a microdisk with a circular air hole [20]. Preu et al. experimentally realized the directional emission of annular cavities in the terahertz domain [22]. However, these were passive microcavities based on polymer material, and the effects of the hole on the threshold haven’t been studied. The unidirectional lasing emission with high Q factor and low threshold has been a great challenge in the development of microcavity laser.
In this letter, a GaN-based eccentric microring with an inner hole located off the center is proposed. The characteristics of the eccentric microring, including the lasing mode, threshold and the lasing emission direction are studied. Finite-difference time-domain (FDTD) numerical simulation is carried out to further study the field intensity distribution of the eccentric microring with different structures.
2. Experiments
The microdisks and eccentric microrings were fabricated from a wafer grown by Metal Organic Chemical Vapor Deposition (MOCVD). A 2 µm undoped GaN (u-GaN) is first grown on a c-plane sapphire substrate. Then 2 µm n-GaN (doping concentration of 3 × 1018 cm-3) and a 500 nm highly doped n+-GaN layer (3 × 1019 cm-3) was grown sequentially. The n-GaN layer serves as the current spreading function in the electrochemical (EC) etching process during which the highly doped n+-GaN sacrificial layer was conductivity-selectively etched away. Then it is followed by a layer sequence which forms a microcavity eventually. The layer sequence consists of 20 nm Al0.2Ga0.8N etching stop layer, 5 pairs of MQWs (3 nm In0.15Ga0.85N/12 nm GaN) and 5 nm u-GaN top layer. The fabrication process starts with a regular photolithographic patterning. The eccentric microrings pattern was formed on the photoresist mask during the photolithography process. After that, inductive coupled plasma (ICP) etching was used to etch the wafer to expose the bottom of the sacrificial layer. BCl3 and Cl2 with a ratio of 22/3 was selected as the etching gas, and the etching power of the upper and lower electrodes is 500 W / 50 W, respectively. Then, an EC etching using HNO3 was adopted to laterally remove the n+-GaN sacrificial layer, creating a floating GaN microdisk or eccentric microring with an undercut structure. In this step, the etching voltage and time has to be precisely controlled so as to form an appropriate pillar to support the upper microdisk or microring. The optical properties of the microdisk and the eccentric microring were characterized using a micro-photoluminescence (µ-PL) test system at room temperature. In this system, the samples were placed on a moving stage consist of a X-Y-Z electric stage and a hand rotating platform. A pulsed excimer laser (248 nm, 30 Hz, 5 ns) was used as the excitation light source to pump the samples, and the excitation laser was focused on a single microdisk or eccentric microring through a microscope objective (15X). The emitting light was collected by the same objective lens or an optical fiber, then dispersed by a 500-cm spectrometer with 2400 lines/mm grating, offering optical resolutions of 0.02 nm. The spot size for the microdisk and eccentric microring is both 60 µm × 60 µm, which is larger than the size of microdisk or eccentric microring, so the position of the laser spot hardly impacts the lasing characteristics.
3. Results and discussion
3.1 Morphology of the eccentric microring
The top and side view scanning electronic microscope (SEM) images show the morphology of the microdisk and the eccentric microring, as illustrated in Fig. 1. The diameter of the microdisk is 40 µm while the eccentric microring has an outer diameter of 40 µm and an inner diameter of 20 µm. To break the symmetry of the structure, the inner hole is located off the center to form an eccentric microring and the minimal distance of the hole to the microring’s boundary is 5 µm. The floating GaN microdisk or microring has smooth sidewalls and surface, which are critical to generating the WGM lasing with high quality factor and low threshold. The eccentric microring is slightly deformed/bent, which is due to the larger size and thin thickness of the microring as well as the support ring pillar is not in the center. Theoretically, the Q factor is barely affected by this slight deformation since it is mainly determined by material absorption loss and scattering loss caused by rough surface of microcavities. The suspended structure of the microcavity can be clearly seen from side viewing SEM image. For the eccentric microring, the highly doped n+-GaN sacrificial layer is exposed in the etching solution both for the inner wall and the outer wall, therefore the electrochemical etching can from inside and outside edges of the microring at the same time. As a result, the outer edge and inner edge are etched into suspended structure, leaving an eccentric ring pillar supports the floating microring. The air gap between the suspended microstructure and the substrate beneath improves the light confinement along the vertical direction.
Fig. 1. (a) Top and (b) side view SEM images of a 40 µm diameter microdisk and (c) (d) an eccentric microring with outer diameter of 40 µm, inner diameter of 20 µm and minimal edge-to-hole spacing of 5 µm.
3.2 Optical properties of the eccentric microring
Figure 2 shows the PL spectrum of the microdisk [Fig. 2(a)] and the eccentric microring [Fig. 2(b)] under different pumping power densities. The emission light is collected from the top of the samples by the same objective lens which passes the excitation laser beam. For the microdisk, weak sharp peak emerge when the power density is relatively low (425 kW/cm2) as shown in the inset to Fig. 2(a). The center of the lasing peak is located at 438.5 nm with the full width at half-maximum (FWHM) of 0.084 nm. The Q factor of the microcavity can be roughly calculated according to the equation: Q=λ/Δλ., where λ and Δλ represent the center wavelength and the FWHM of the lasing peak respectively. Therefore, the Q factor of the microdisk is approximately 5220. When increase the pumping power density, more lasing modes appear and their intensity increase simultaneously while the initial emerging peak remains the highest intensity.
Fig. 2. PL spectrums of (a) microdisk and (b) eccentric microring under different pumping power densities. Inset to (a) shows the PL spectrum of the microdisk at an excitation power density of 425 kW/cm2 while inset to (b) shows the PL spectrum of the eccentric microring at an excitation power density of 200 kW/cm2.
In contrast, the emission spectrum of the eccentric microring shows different characteristics. From the inset to Fig. 2(b), it can be clearly seen that a lasing peak appears at 440.8 nm at the low power density (200 kW/cm2). The FWHM of the lasing mode reaches 0.069 nm, corresponding to a Q factor of 6388. As the power density increases, the lasing intensity increase rapidly accompanied by other modes emerging. However, the emerging modes are suppressed greatly and their intensities are far below the intensity of the lasing at 440.8 nm. This is because the introduction of hole in the eccentric microring disturbs the field distribution of the higher order modes whose fields spatially overlap with the hole, which increase the loss of those modes [24,25]. Since the field distribution of the low order mode is close to the edge of the microring thus it was not affected much by the hole.
Figure 3 shows the relationship of the excitation power and the laser intensity of microdisk and eccentric microring. Threshold was determined to be the excitation density under which the FWHM suddenly decreases. The lasing threshold is estimated around 425 kW/cm2 for microdisk and 200 kW/cm2 for eccentric microring. This result actually matches the power density when the first laser peak appears, as shown in Fig. 2. The threshold of the eccentric microring is reduced by 53% compared to the microdisk, which is attributed to two reasons. Firstly, part of the inner material is removed for the eccentric microring structure, thus the absorption of the internal cavity material is decreased and the pump volume is reduced. Secondly, the introduction of the hole reduce the photon lifetimes for the higher order mode in the eccentric microring [26]. Since the amplification of spontaneous emission (ASE) is depended on the photon lifetimes of the corresponding modes, thus decrease the ASE in these higher order mode and make more carriers available for the lasing modes.
Fig. 3. Laser intensity and FWHM of microdisk (red lines) and eccentric microring (black lines) as a function of the excitation power density.
To analyze the unidirectional emission characteristics of the GaN-based eccentric microring, PL spectrums under different angles are measured at an excitation power density of 250 kW/cm2. A fixed optical fiber was used to collect the emission light from the side of the sample while the samples were placed at the center of the moving stage consist of a X-Y-Z electric stage and a rotating platform. By rotating the stage 360° in the horizontal plane at 10° intervals, we measure the angular distribution of the PL spectra of the eccentric microring. Figure 4(a) illustrates a polar coordinate image of the PL intensity of the eccentric microring as a function of measurement angle. From the intensity distribution pattern, it is clear that the eccentric microring exhibits an obvious unidirectional emission property with a far-field divergence angle of about 40°. The emission intensity is far stronger at the direction of 270° in which the hole located. The inset shows the CCD-captured PL image of the eccentric microring. The highest intensity was observed where the distance of the hole to the cavity edge is minimal. It’s worth noting that the lasing emission direction of the eccentric microring is contrary to what was reported in the paper [24]. Figure 4(b) shows the PL image of the microdisk, and it’s obvious that the lasing emission of the microdisk is isotropic.
Fig. 4. (a) Polar coordinate image of the PL intensity of the laser peak at 440.8 nm as a function of measurement angle. The inset shows the CCD-captured PL image of the eccentric microring. (b) The CCD-captured PL image of the microdisk.
3.3 Numerical simulation
Finite-difference time-domain (FDTD) numerical simulation is carried out to study the field intensity distribution of the eccentric microring. A dipole source with a broadband of 400-450 nm is added at the edge area of the structure to excite all the optical modes in the cavity, which matches with the actual emission wavelength of the sample. Figure 5(a) shows the near field intensity distribution of the first-order WGM in linear coordinates (the white line represents the hole. It confirms that most of the light is confined within the eccentric microring and transmits along the interface between the microcavity and air, thus giving a high Q factor. To show the details of the light emission from this cavity, we plot the light intensity distribution in a logarithmic scale in Fig. 5(b). It can be clearly seen that a unidirectional beam emits from the same side of hole, thereby suggesting that the existence of the off-center hole can scatter the emission light and break its isotropy distribution. Then we calculated the far field pattern to study the far-field divergence angle of the eccentric micoring. As shown in Fig. 5(c), the emission intensity is the strongest at the direction of the side of the hole. The far-field divergence angle is about 45°, which is consistent with the experimental results demonstrated in Fig. 4.
Fig. 5. Near-field intensity pattern of the eccentric annulus (a) in linear coordinates and (b) in logarithmic coordinates in 2D-FDTD simulation. (c) Far-field pattern of the eccentric annulus.
Figure 6 shows the far-field pattern of the eccentric microring with different inner diameter (d) and hole-to-edge spacing (s). According to the simulation results, with constant hole-to-edge spacing, the lasing emission direction shows an interesting difference with the change of the size of the hole [as shown in Fig. 6(a)–(c)]. Specifically, obvious unidirectional emission is demonstrated when the hole is either very small (d=5 µm) or very big (d=20 µm), but the lasing emission direction of those two structure is opposite, such as Figs. 6(a) and 6(d). For the eccentric microring with a medium size hole (d=10 µm), the direction of light emission is not clear. Those simulation results is consistent with the experimental results of the pierced microdisk reported [24] and our eccentric microring. On the other hand, as the hole moves toward the center the unidirectional emission property becomes poor [as shown in Figs. 6(c)–6(f)], and eventually shows in-plane isotropic for the concentric microring. These calculations prove that the far-field profile of such an eccentric microring structure can be controlled by designing the position and the size of the hole.
Fig. 6. Far-field pattern of the eccentric microring with different inner diameter (d) and hole-to-edge spacing (s). The outer diameters are all 40 µm. The insets show the schematic diagram of the eccentric microring with different size.
The small hole close to the edge can be thought of as a point scatterer. As shown in Fig. 7(a), the plane waves are focused on a bright spot near the boundary of the cavity, and the spot size is about several wavelengths. For the same reason, scattering light from a point scatterer near the boundary can also be collimated to a plane wave by the microcavity body, and the emission is unidirectional predominantly in the opposite direction of the scatterer [13,21]. Thus, the laser emits from the opposite to the hole when it is small. For the eccentric microring with a bigger hole, the off-center hole is too large to be regarded as a point scatterer. As shown on Fig. 7(b), the existence of the hole disturbs the field distribution of the higher order modes whose fields spatially overlap with the hole, and leads to unidirectional light emission from the same side of the hole due to refractive escape [7]. In order to show this phenomenon clearly, the simulated eccentric microring in Fig. 7(b) is 20 times smaller than that in the experiment. For the eccentric microring with a medium size hole, the scatter of the hole and refractive escape of the high-order modes both happen at the same time. As a result, the direction of light emission is not obvious.
Fig. 7. (a) Schematic demonstrating the collimation effect of a microcavity. (b) Near field intensity pattern of the eccentric microring whose size is 20 times smaller than what in experiment.
4. Conclusion
In summary, we have fabricated a GaN-based eccentric microring with a hole located off the center. Compared with the microdisk with the same outer diameter, the threshold of eccentric microring is decreased by up to 53%. This threshold decrease is attributed to the reduction of the pump volume, the decrease of the cavity material absorption, and the suppression of the internal high-order modes which makes more energy available for lasing modes. Unidirectional laser emission has been achieved in the eccentric microring with a far-field divergence angle of about 40°, thereby suggesting that the existence of the off-center hole can break the isotropy distribution of the emission light. Simultaneously, the Q factor remains high as 6388. FDTD simulation demonstrates that unidirectional characteristics of the eccentric microring can be optimized by designing the position and size of the hole. The successful realization of these simple-structured microcavities through standard photolithographic fabrication makes low-threshold directional light source possible, which makes great is achieved through such an eccentric microring structure, which makes great significance for many applications such as photonic integrated circuit and biological sensing.
Funding
National Key Research and Development Program of China (2016YFB0400801); Fundamental Research Funds for the Central Universities (XJJ2017011, Z201805198); National Natural Science Foundation of China (61574114, 61774121).
Acknowledgments
The SEM work was done at International Center for Dielectric Research (ICDR), Xi'an Jiaotong University; the authors also thank Yanzhu Dai for help.
Disclosures
The authors declare no conflicts of interest.
References
1. F. A. Ponce and D. P. Bour, “Nitride-based semiconductors for blue and green light-emitting devices,” Nature 386(6623), 351–359 (1997). [CrossRef]
2. S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku, “Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes,” Appl. Phys. Lett. 70(7), 868–870 (1997). [CrossRef]
3. K. J. Vahala, “Optical microcavities,” Nature 424(6950), 839–846 (2003). [CrossRef]
4. L. He, Ş. K. Özdemir, and L. Yang, “Whispering gallery microcavity lasers,” Laser Photonics Rev. 7(1), 60–82 (2013). [CrossRef]
5. J. Hryniewicz, P. Absil, B. Little, R. Wilson, and P.-T. Ho, “Higher order filter response in coupled microring resonators,” IEEE Photonics Technol. Lett. 12(3), 320–322 (2000). [CrossRef]
6. P. Michler, A. Kiraz, C. Becher, W. Schoenfeld, P. Petroff, L. Zhang, E. Hu, and A. Imamoglu, “A quantum dot single-photon turnstile device,” Science 290(5500), 2282–2285 (2000). [CrossRef]
7. J. Wiersig, “Enhancing the sensitivity of frequency and energy splitting detection by using exceptional points: application to microcavity sensors for single-particle detection,” Phys. Rev. Lett. 112(20), 203901 (2014). [CrossRef]
8. F. Vollmer, S. Arnold, and D. Keng, “Single virus detection from the reactive shift of a whispering-gallery mode,” Proc. Natl. Acad. Sci. U. S. A. 105(52), 20701–20704 (2008). [CrossRef]
9. L. Shao, X. F. Jiang, X. C. Yu, B. B. Li, W. R. Clements, F. Vollmer, W. Wang, Y. F. Xiao, and Q. Gong, “Detection of single nanoparticles and lentiviruses using microcavity resonance broadening,” Adv. Mater. 25(39), 5616–5620 (2013). [CrossRef]
10. A. C. Tamboli, E. D. Haberer, R. Sharma, K. H. Lee, S. Nakamura, and E. L. Hu, “Room-temperature continuous-wave lasing in GaN/InGaN microdisks,” Nat. Photonics 1(1), 61–64 (2007). [CrossRef]
11. D. Wang, E. L. Hu, R. A. Oliver, and T. Zhu, “Ultra-low-threshold InGaN/GaN quantum dot micro-ring lasers,” Opt. Lett. 43(4), 799 (2018). [CrossRef]
12. S. Yang, Y. Wang, and H. Sun, “Advances and Prospects for Whispering Gallery Mode Microcavities,” Adv. Opt. Mater. 3(9), 1136–1162 (2015). [CrossRef]
13. X. F. Jiang, C. L. Zou, W. Li, Q. Gong, and Y. F. Xiao, “Whispering-gallery microcavities with unidirectional laser emission,” Laser Photonics Rev. 10(1), 40–61 (2016). [CrossRef]
14. G. D. Chern, H. E. Tureci, A. D. Stone, R. K. Chang, M. Kneissl, and N. M. Johnson, “Unidirectional lasing from InGaN multiple-quantum-well spiral-shaped micropillars,” Appl. Phys. Lett. 83(9), 1710–1712 (2003). [CrossRef]
15. T. Ben-Messaoud and J. Zyss, “Unidirectional laser emission from polymer-based spiral microdisks,” Appl. Phys. Lett. 86(24), 241110 (2005). [CrossRef]
16. M. Kneissl, M. Teepe, N. Miyashita, N. Johnson, G. Chern, and R. Chang, “Current-injection spiral-shaped microcavity disk laser diodes with unidirectional emission,” Appl. Phys. Lett. 84(14), 2485–2487 (2004). [CrossRef]
17. J. Wiersig and M. Hentschel, “Combining directional light output and ultralow loss in deformed microdisks,” Phys. Rev. Lett. 100(3), 033901 (2008). [CrossRef]
18. C. Yan, J. W. Qi, L. Diehl, M. Hentschel, J. Wiersig, N. Yu, C. Pflügl, F. Capasso, M. A. Belkin, and T. Edamura, “Directional emission and universal far-field behavior from semiconductor lasers with limaçon-shaped microcavity,” Appl. Phys. Lett. 94(25), 251101 (2009). [CrossRef]
19. M. Kurdoglyan, S.-Y. Lee, S. Rim, and C.-M. Kim, “Unidirectional lasing from a microcavity with a rounded isosceles triangle shape,” Opt. Lett. 29(23), 2758–2760 (2004). [CrossRef]
20. J. Wiersig and M. Hentschel, “Unidirectional light emission from high-Q modes in optical microcavities,” Phys. Rev. A 73(3), 031802 (2006). [CrossRef]
21. Q. J. Wang, C. Yan, N. Yu, J. Unterhinninghofen, J. Wiersig, C. Pflügl, L. Diehl, T. Edamura, M. Yamanishi, and H. Kan, “Whispering-gallery mode resonators for highly unidirectional laser action,” Proc. Natl. Acad. Sci. U. S. A. 107(52), 22407–22412 (2010). [CrossRef]
22. S. Preu, S. I. Schmid, F. Sedlmeir, J. Evers, and H. G. Schwefel, “Directional emission of dielectric disks with a finite scatterer in the THz regime,” Opt. Express 21(14), 16370–16380 (2013). [CrossRef]
23. G. Zhu, F. Qin, J. Guo, C. Xu, and Y. J. A. P. L. Wang, “Unidirectional ultraviolet whispering gallery mode lasing from floating asymmetric circle GaN microdisk,” Opt. Lett. 111(20), 202103 (2017). [CrossRef]
24. Z. N. Tian, F. Yu, Y. H. Yu, J. J. Xu, Q. D. Chen, and H. B. Sun, “Single-mode unidirectional microcavity laser,” Opt. Lett. 42(8), 1572 (2017). [CrossRef]
25. X.-W. Ma, X.-M. Lv, Y.-Z. Huang, Y.-D. Yang, J.-L. Xiao, and Y. Du, “Mode characteristics for unidirectional-emission microring resonator lasers,” J. Opt. Soc. Am. B 31(11), 2773–2778 (2014). [CrossRef]
26. S. Backes, J. Cleaver, A. Heberle, J. Baumberg, and K. Köhler, “Threshold reduction in pierced microdisk lasers,” Appl. Phys. Lett. 74(2), 176–178 (1999). [CrossRef]
