Three-dimensional anisotropic microlaser from GaN-based self- bent

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Letter

Three-dimensional anisotropic microlaser from GaN-based self-bent-up microdisk Yufeng Li, Lungang Feng, Feng Li, Peng Hu, Mengqi Du, Xilin Su, Dongxu Sun, Haijun Tang, Qiang Li, and Feng Yun ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01061 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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Three-dimensional anisotropic microlaser from GaN-based selfbent-up microdisk Yufeng Li,†,‡ Lungang Feng,†,‡ Feng Li,*,§,⊥ Peng Hu, †,‡ Mengqi Du, †,‡ Xilin Su,†,‡ Dongxu Sun,†,‡ Haijun Tang§, Qiang Li,†,‡ and Feng Yun*,†,‡ †

Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Shaanxi Provincial Key Laboratory of Photonics & Information Technology, Xi’an Jiaotong University, Xi’an, 710049, China ‡ Solid-State Lighting Engineering Research Center, Xi’an Jiaotong University, Xi’an, 710049, China § Institute of Physical and Optoelectronics Technology, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China ⊥ Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK

ABSTRACT: Microcavities with whispering gallery modes (WGM), usually formed by twodimensional (2D) circular structures, are significant elements in integrated optics, quantum information and topological photonics. We report three-dimensional (3D) WGM from selfbent-up microdisks consisting of strain-released AlGaN/GaN bilayers, which provide extra degree of freedom of the WGM photons in the vertical dimension, in contrast with the 2D WGM whose field mainly distributes in the horizontal plane. Despite the ultrathin and deformed cavity laser, the 3D WGM shows a reasonably high quality factor for GaN-based microdisks (~1300) and exhibits single mode lasing due to the anisotropic feature of the bentup disk, a unique advantage over the conventional planar microdisks of the same material and size. Such devices provide altitude dependence of emitting direction and are promising for applications in multilevel integrated photonics circuits. KEYWORDS: whispering gallery mode, microdisk, self-bent-up device, single mode lasing, wide bandgap microcavity TOC:

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Microcavities supporting whispering gallery mode (WGM), which induces optical confinement by successive total internal reflections, have attracted much attention for both fundamental science and realistic applications, such as micro-lasers,1 light-matter interaction,2-4 parity-time symmetric optics,5-7 and enhanced sensing at exceptional points.8,9 The WGM microdisk or microtoroid structures in early stages provide isotropic light confinement along their circular periphery, while anisotropic WGM structures10,11 are further designed to improve their practical functions as optoelectronic devices, such as unidirectional emission,12-14 single-mode lasing,15,16 and chaos-assisted broadband coupling.17 Nevertheless, the output of such optical devices are usually along the WGM ray direction at the shape boundaries, making it unfavorable for vertical emitting devices, and thereby, getting WGM microcavities that combine the advantages of easy fabrication (compared to VCSELs) and vertical light emission has drawn significant interest of research in recent years. Up to now, there are mainly three approaches reported to achieve vertically emitting microdisk lasers, including azimuthal gratings along the disk/ring circumference,18,19 upward light coupling via a metal antenna,20 and inclined mirrors.21 These methods, nevertheless, involve a series of complicated and technical-demanding fabrication processes, and are limited by the wavelength-dependent coupling efficiency for specific designs. On the other hand, self-rolledup structures from strained bilayers help to start a new way for fabricating on-chip vertical emitting devices, i.e., microtube cavities in which the WGM resonates in the vertical plane.22,23 Especially, such devices have been very recently realized in III-nitride materials,24 adding an alternative approach to achieve GaN-based micro/nano lasers.25-26 In this letter, we fabricate self-bent-up microdisks from strained bilayers of III-nitride materials, which exhibit high quality WGM emission in three dimensions despite the ultrathin and deformed cavity layer. Such WGM exhibits spatially-varying vertical wavevector component along the microdisk periphery, and is therefore azimuthally anisotropic. Single mode lasing is observed due to extra mode-selection mechanism induced by the asymmetric 2 ACS Paragon Plus Environment

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cavity structure, providing a major advantage over conventional microdisk cavities. The spatial-dependence of emission direction makes it useful for multifunctional coherent light sources on photonics chips. Moreover, considering the outstanding exciton properties of IIInitride materials at room temperature,27 the bent-up microdisk also provides a platform promising for 3D-distributed polariton condensate and quantum light.28 The microdisk consists of a strained AlGaN/GaN bilayer with an InGaN single quantum well (SQW) embedded as active media, showing a total thickness of ~50 nm (see Methods for detail). 3 sets of structures with different AlGaN/GaN layer thickness ratio and Al composition were used to fabircate microdisks for optical properties comparison. The detail of the sample structure information were listed in Table S1. (Supporting Information). The initial lateral diamter of the microdisk from each set of structure varies from 5 to 12 μm before bending up. When released from the substrate, the microdisk with different layer structures and Al compositions bends up due to the relaxation of strain with different radius of curvature (RoC), whilst the 200 nm-width n+-GaN rod underneath the microdisk is solid enough to mechanically support the upper layer. Figure 1a-f show SEM images from different viewing angles of bent-up microdisks of structure 1 with diameters of 5 μm and 12 μm respectively, both exhibiting an almost smooth bent surfaces with a radius of curvature (RoC) of 3.75um and 2.5 um, respectivly. The rolling mechanism of the system is shown in Figure S2 (Supporting Information). After the selective etching process is initiated, the AlGaN/GaN bilayer begins to detach from the substrate. During this process, the expansion of the compressively strained GaN layer is resisted by the AlGaN layer. As a result, a pair of opposing forces created between the two layers gives rise to a net moment which acts as the driving force necessary to roll up. Eventually, a bend-up micro disk is formed. If the lift off bilayer is large enough to complete one or more circles, a rolled-up cylindrical micro-tube is formed.22-24 Such rolled-up cylindrical micro-tubes or micro disk have certain RoC, which depends on the ratio of thickness and on strain difference in upper and lower layer. 3 ACS Paragon Plus Environment

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The classical formula for the RoC of a thin bi-metallic strip29 can be used to roughly illustrate the dependence of tube diameter on system parameters. Figure S3 shows the calculated RoC/tGaN (described as a ratio of RoC to the thickness of lower GaN layer) as a function of the thickness ratio of upper AlGaN layer to lower GaN layer (tAlGaN/tGaN) at different Al composition. Plotted together is the experimentally measured RoC of each sample of different structure. The simulation predicts a strong dependence on the Al composition, because higher Al introduces larger lattice constant mismatch thus higher strain. It also indicates a strong dependence on the thickness ratio of AlGaN to GaN layer. The RoC approaches to infinity for pure GaN or pure AlGaN film, and its minimum was reached between thickness ratio value of 0.25 and 0.5. It is found that the measured RoC of the micro disk is less than the predicted value while that of the microtube is even smaller. This implies that the size of the disk can play an important role. When the dimension of the bend-up film is not large enough to form a complete tube24 the gravity force acts as a counter force to the strain and prevents the membrane from further bending. Consequently, the RoC very slightly increases from the disk center to the upper edge. Such behavior is more dominant in smallsized microdisks than large ones. Thus the RoC of the small microdisks made from the same structure and Al composition is slightly larger than the predict value. On the contrary, if the diameter of the disk is sufficiently large (>20 μm), the membrane rolls up right above the supporting rod and in this case the gravity helps it to roll into a microtube with a uniform RoC.24 Thick samples (GaN/MQWs) without AlGaN/GaN bilayers form flat microdisks without any bending or rolling phenomenon, as can be seen in Figure S1a and b (Supporting Information).


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Figure 1. Morphology and linear optical properties of the bent-up microdisk cavities. Top and side view SEM images of (a-c) 5 μm and (d-f) 12 μm diameter bent-up microdisks based on strained structure 1. (g)PL spectra of the 12 μm bent-up microdisk below lasing threshold, under pumping power from 8.8 mW (brown) to 153.7 mW (black). WGMs are labeled as m 1 to m6, located at 449.42 nm, 451.48 nm, 453.72 nm, 455.92 nm, 458.32 nm, and 460.68 nm. The inset shows the enlarged PL spectra from 457 nm to 463 nm.

The linear optical property of the bent-up microdisk cavities is investigated using μphotoluminescence (PL) under continuous excitation of a 405 nm laser (see Methods for detail), and the measured spectra under a series of pumping powers are shown in Figure 1c. The measured microdisk made from structure 1 with the highest Al composition and largest tAlGaN/tGaN ratio has a diameter of 12 μm so that its upper edge almost faces straight up, making it convenient to collect vertical light emission from the top. Microdisks made from structure 2 and 3 has larger RoC due to less Al composition and smaller tAlGaN/tGaN ratio and thus do not form straight up edges. The WGMs, shown as narrow peaks labeled by m1 to m6 in Figure 1c, are superimposed on the broad PL envelop centered at ~460 nm from the 5 ACS Paragon Plus Environment

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spontaneous emission of the SQW. The nearly equally spaced WGMs show a free spectrum range (FSR) of ~2.25 nm. Estimating from a flat GaN microdisk of the same size with FSR=λ2/(2πneffR), where neff =2.5 is the effective refractive index of the bilayer calculated for the SQW wavelength and R the radius of the microdisk, we get a FSR of 2.3 nm which is in very good agreement with the experimental result. The full width at half maximum (FWHM) of the narrowest mode reaches 0.35 nm corresponding to a quality factor (Q) of ~1300, evidencing the efficient optical confinement despite the ultrathin disk thickness (50 nm) and the deformed disk shape. Interestingly, each WGM mode contains a group of 3 to 4 submodes that can be identified in the enlarged spectra in the inset of Figure 1c. This might be related to the etching-induced thickness gradient and imperfections along the disk radial direction, as well as the non-uniformity of the strain residual distribution. We perform finite-difference time-domain (FDTD) simulations to understand the 3D feature of the WGMs in the bent-up microdisk. First of all, we start from simulating the field distribution in a flat microdisk using a 2D-FDTD model, as shown in Figure S4 (Supporting Information). In the wavelength range of SQW emission, the microdisk exhibits azimuthal mode order m of about 245, which is large enough to make the approximation that at each point the wavevector of WGMs is along the tangent line of the disk periphery. When the microdisk is bent up, we perform 3D-FDTD simulation to confirm that the WGM still distributes along the bent edge, as demonstrated in Figure 2b. Meanwhile, the wavevector of the WGM becomes anisotropic azimuthally, since its vertical component continuously varies due to the bent shape. As illustrated in Figure 2a, while P1 and P3 still exhibit horizontal propagation but at different altitudes, P2 exhibits a large angle with the horizontal plane XY and thereby has a significant vertical light component along Z. The 3D feature of the WGM is well demonstrated experimentally in Figure 2c, where a spectral-integrated real-space image is taken with the collecting objective lens above the bentup microdisk (see Methods). The pump spot is deliberately aligned at one side of the 6 ACS Paragon Plus Environment

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microdisk (upper part of the image) so that we can observe the WGM field distribution from the other half of the device. P2 exhibits the highest intensity as the light emission, which contains a large vertical component, is very efficiently collected by the objective lens. On the contrary, P1 and P3 show very low intensities as the light emission is mostly inside the horizontal plane and barely collected. Such anisotropy provides a useful platform of getting unidirectional lasers in three dimensions, with the capability to choose the exact emission direction by adding defects at specific positions of the disk periphery.

Figure 2. Spatial field distribution of the WGMs. (a) Schematic diagram of a bent-up microdisk. P1 to P3 label three points on the disk periphery. Three purple rectangles which are electric field monitors in the FDTD software, are chosen to be placed at three different altitudes of the bent microdisk. (b) The electric field distribution of the mode at 439.08 nm in the three electric field monitors in (a), showing that in each monitor, the field mainly distributes at the positions of the disk edges. (c) PL image of 12 μm-diameter bent-up microdisk under the excitation of 405 nm laser, the highest intensity at the upper part of the image corresponds to the laser directly-pumping area, and P2 part shows higher intensity than P1 and P3.

As the heating effect under CW pumping prevents further increase of the output power, the investigation of the nonlinear optical properties was taken under a 337 nm pulsed laser (pulse width 3.5 ns, repetition rate 20 Hz). Above a pumping threshold of ~239 nJ shown in Figure 3a, only one shape peak at 432.28 nm (Figure 3c) corresponding to the lasing mode was observed for the 12 μm bent-up microdisk, showing a FWHM of 0.03 nm. The large blue-shift (~30 nm) of the lasing mode compared to the WGM below threshold shown in Figure 1c is due to band-filling effect and screening of the quantum confined stark effect (QCSE) at increased pumping power,30 which is proved by comparing the spectra of the asgrown sample under increasing pumping power of pulsed laser, with details in Figure S5 of


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the Supporting Information. The spontaneous emission coupling factor β was estimated to be ~0.013 from the ratio of PL intensities below and above the threshold.31 We perform spatial dependent PL measurements to verify there is no other coexisting mode that emits beyond our detection range, and thereby confirm that the lasing mode we detected is indeed single. First, we image the lasing microdisk from the top view, as shown in Figure 3b and c. The field distribution shows similar features as below threshold, anisotropic around the disk periphery. We took spectra at different parts of the field, as shown in Figure 3c, and obtained lasing peaks at the same wavelength at all measured positions with the same FWHM. Furthermore, we measured the WGM from the side view by collecting the emission with an optical fiber pointing to the disk edge, as illustrated in the inset of Figure 4. Again, the measured peak position and FWHM agree perfectly with the top view measurement, showing that all over the space there is only one lasing mode from the bent-up microdisk. Another three different microdisks have been measured and showed the same single-mode or a dominating mode result (Figure S6 in Supporting Information). The single mode lasing is a key feature and advantage compared to the conventional microdisks. Indeed, we have measured numerous flat III-nitride microdisks with either the same diameter or similar FSR as the bent-up ones, as well as bent-up disks with much smaller curvature and size (the size determines how long the curvature actually take effect around one optical path of the WGM) from layer structures 2 and 3, and all of them exhibit multi-mode lasing, with an example shown in Figure S7 of the Supporting Information. Therefore, the single mode lasing from the bent microdisk is undoubtedly related to the curvature of the structure, which can induce enhancement or extra mechanism of the mode competing effect. Nevertheless, the exact physical process of such mechanism may involve too complicated modeling and analysis and is beyond the scope of this work in which we focus on demonstrating devices.


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Figure 3. Single mode lasing of the bent-up microdisk. (a) Relation between PL intensity and the excitation energy in a log-log plot for the lasing mode of a 12 μm bent-up microdisk, (b) PL image of half of the bent-up microdisk in lasing status. (c) The lasing spectra correspond to different parts of the sample. The right insets show the emission pattern of the sample, the white dashed lines were the middle location of the entrance slit, PL signals were measured using a slit of 0.01 mm.

Figure 4. PL spectra of one microdisk measured from top and side direction under 337 nm pulsed laser, the inset shows the simplified schematic diagram of the measurement setup. As the side fiber is not a tapered fiber and is placed far from the disk (~3 mm), the collection efficiency from the side is much lower, resulting in a lower detected intensity.

In summary, we have demonstrated 3D WGMs in a self-bent-up microdisk based on IIInitride materials, which allows the vertical degree of freedom of the WGM photons and enables single-mode lasing with a large portion of vertical light emission. Such 3D WGM has an altitude distribution that allows one to couple light in and out at different altitudes on a photonic chip containing multilevel components, and could enable power output towards a customer-chosen angle with respect to the chip surface by defining a defect at the corresponding point on the disk periphery. Furthermore, noticing the application of nitride based devices in room temperature polaritonics, the bent-up microdisk is a promising platform for studying exciton-polariton condensates and quantum light that travel in three-dimensional 9 ACS Paragon Plus Environment

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trajectories, which would enable hybrid cavity-waveguide polaritonic devices possibly working at a range of height levels, combining the advantages of polariton condensation and high polariton velocity. 32-34

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METHODS

The bent-up microdisk structures are made from strained GaN layers grown by metal organic vapor phase epitaxy (MOVPE) on c-plane sapphire substrate. Firstly, a 2 μm n-GaN and a ~500 nm depth highly-doped n+-GaN layer (doping 3×1019 cm-3) were grown sequentially. The n-GaN layer serves as current spreading function and the n+-GaN as sacrificial layer during electrochemical (EC) etch process. The active layers consisted of AlGaN/GaNbilayers and InGaN single quantum well (SQW, Indium composition of 15%) (~5 nm) were then grown above the n+-GaN layer. The Aluminum composition in AlGaN layer was varied from 20% to 53%. For comparison, 3 sample structures with different AlGaN/GaN thickness ratio and Al composition were grown. The detail sample information was list in Table S1 (Supporting Information). 5 μm-diameter SiO2 spheres and 12 μm-diameter photolithographic mask were used to form the corresponding circular structure, followed by inductive coupled plasma (ICP) dry-etching membranes down to the n+-GaN layer. It has been proved in our previous experiments that SiO2 spheres as hard mask brings a smoother periphery, which decreased the scattering loss and in turn improved the cavity quality. EC wet etching of the samples in HNO3 electrolyte was then conducted for several seconds to polish the n+-GaN layer from the exposed edge to middle part. Detailed EC etching process can be found elsewhere. Once the bottom wet-etching of n+-GaN happened, the detached active layers began to roll. The final curvature mainly depends on the initial strain within the membranes, so it can be flexibly controlled by adjusting the membrane thickness of each layer and Al concentration in the topmost AlGaN layer. The optical properties of the bent-up microdisks were characterized by micro-photoluminescence (μ-PL) spectroscopy at room temperature 10 ACS Paragon Plus Environment

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(RT) by using a CW 405 nm laser and a pulsed 337 nm laser (pulse width 3.5 ns and repetition rate 20 Hz) for excitation. The vertically emitting light was collected by the same 60× objective and reflected to the Horiba iHR550 spectrometer. And the horizontal emission was collected by a slightly tilted fiber (~3 mm to the sample). The spectrometer at 1800 lines/mm grating supplies a spectral resolution of 0.02 nm.

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ASSOCIATED CONTENT

Supporting Information Related experimental and simulated results of the planar microdisk for comparison can be found in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

This research was supported by the National Key Research and Development Program of China (2016YFB0400801), National Natural Science Foundation of China (61574114, 11804267), and Natural Science Basic Research Plan in Shaanxi Province of China (2016JM6019, 2018JQ6041). The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University; the authors also thank Yanzhu Dai for her help in using SEM.

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Three-dimensional anisotropic microlaser from GaN-based self- bent

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