Selective pump focusing on individual laser modes in microcavities

Jun 20, 2018 - ... Jeong , Min-Kyo Seo , Wonshik Choi , and Hong-Gyu Park. ACS Photonics , Just Accepted Manuscript. DOI: 10.1021/acsphotonics.8b00648...
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Selective Pump Focusing on Individual Laser Modes in Microcavities Jae-Hyuck Choi,†,‡ Sehwan Chang,†,‡ Kyoung-Ho Kim,†,‡ Wonjun Choi,§,† Soon-Jae Lee,† Jung Min Lee,† Min-Soo Hwang,† Jungkil Kim,† Seungwon Jeong,§,† Min-Kyo Seo,∥ Wonshik Choi,§,† and Hong-Gyu Park*,†,⊥ †

Department of Physics, Korea University, Seoul 02841, Korea Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science, Seoul 02841, Korea ∥ Department of Physics and Institute for the NanoCentury, KAIST, Daejeon 34141, Korea ⊥ KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 23, 2018 at 09:23:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: We demonstrate selective pump focusing for highly isolated single-mode lasers in microdisk and microring cavities, and achieve lasing action from a microdisk cavity underneath a scattering medium. The spatial profile of the pumping light evolves by an iterative feedback process and is optimized to maximize the field overlap with a selected cavity mode. The high order of mode selectivity and high resolving power are obtained in a multimode cavity in the presence of significant modal overlaps. As a result of the adaptive optical pumping, we successfully achieve the efficient energy transfer to a microdisk underneath a random scattering medium and observe lasing action through the scattering medium. We believe that our selective pumping procedure will pave the way for the development of low-threshold, single-mode nanolasers embedded in various materials. KEYWORDS: semiconductor laser, single-mode laser, laser beam shaping, scattering medium, energy transfer fficient allocation of a given optical gain to a desired resonant mode in a microcavity is key to achieving stable single-mode laser operation.1,2 In general, an optical gain shared by multiple modes causes spectral/spatial instability in the laser emission and degraded spectral purity and emission beam quality.3−9 For example, microdisks and microrings, which are excellent platforms for on-chip light source applications, suffer from multimodal emission instability because of densely populated whispering-gallery modes (WGMs) in a small spectral range. Advances in nanofabrication techniques and additional photonic structures in cavities enable efficient manipulation of the optical gain in a cavity. Microdisks with a metal grating,10 parity-time symmetric coupled cavities,11−13 and parity-time symmetric microrings with a balanced gain/loss grating14 have successfully demonstrated single-mode lasing operation. However, these microcavities generate unwanted distortions in the target laser modes, preclude active switching of the desired modes, and require complex design and fabrication processes. Recently, several studies have suggested that selective pump focusing can be a promising approach to excite only a desired resonant mode.15−20 Optical pumping with a nonuniform spatial profile allows active mode selection without degrading the modal characteristics.17 The idea of the excitation scheme with engineered beam pattern for single mode laser operation17 and the adaptive optimization of the pump

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pattern19 have been discussed in previous reports. In this work, we develop a new approach in the selective pump focusing method, by introducing an iterative feedback processes to maximize the field overlap with a selected cavity mode. We demonstrate highly isolated single-mode lasing operation in microdisk and microring cavities, without a preliminary knowledge of the mode characteristics. In addition, we exploit selective pump focusing to transfer the pump power to a microdisk underneath a random scattering medium and demonstrate lasing action from the microdisk.

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RESULTS AND DISCUSSION Figure 1a (left) presents a schematic illustration of selective pump focusing with a spatially nonuniform profile that overlaps well with the desired cavity mode. The selective pump focusing discriminates a target laser mode (μth mode) from nonselected modes (νth modes) by the efficient delivery of pump power to the μth mode and the suppression of gain supply to the νth modes. This scheme is highly distinguishable from Gaussian-shaped pumping, which excites multiple resonant modes (Figure 1a, right).9 Note that the pump overlap factor, defined by the spatial overlap between the Received: May 15, 2018 Published: June 20, 2018 A

DOI: 10.1021/acsphotonics.8b00648 ACS Photonics XXXX, XXX, XXX−XXX

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substrate (Figure 1b, inset). The peak intensity of the target laser mode is maximized by iterative shaping of the pump laser with a fixed pump power (see Methods).22−24 The iteration is finished when the peak intensity is saturated. Sufficient iteration is necessary to excite a desired mode. Then, we measure optical characteristics of the solely excited target lasing mode. We note that this adaptive feedback process is highly distinguishable from previous ones maximizing the extinction ratios.19,20 We perform experiments for selective pump focusing using a 5 μm diameter microdisk on a sapphire substrate (Figure 2a− c). As the feedback process continues, the pump laser changes from the initial speckle pattern to a tightly focused ring-shaped pattern on the microdisk (Figure 2a). In addition, the peak intensity at a wavelength of 1562.2 nm rapidly increases as the number of feedback steps increases from 500 to 6000 (Figure 2b). The image of a WGM is observed from the microdisk at a step of 6000 (Figure 2b, inset) as a result of the adaptive optical pumping. Furthermore, we measure the output peak intensity of the microdisk as a function of the incident pump power (L−L curve), with varying feedback steps (Figure 2c). As the feedback step increases, a notable decrease in lasing threshold power is observed. Therefore, the highly focused ring-shaped pump laser (Figure 2a) leads to both an enhanced peak intensity (Figure 2b) and a reduced lasing threshold (Figure 2c). These results strongly indicate that our optimization scheme for increasing the intensity of a target laser mode is equivalent to maximizing the pump overlap factor of the target mode.17 In addition, we demonstrate positioning control of the adaptive optical pumping. In two microdisk cavities (Disks 1 and 2) with a center-to-center distance of 22 μm (Figure 2d, left), only the desired microdisk is selectively pumped. First, we select the peak emission wavelength of Disk 1 as the target wavelength. After a sufficient number of feedback steps, the pump laser is positioned onto Disk 1 (Figure 2d, middle). Next, when the peak emission wavelength of Disk 2 is selected as the target wavelength, the pump laser is positioned onto Disk 2 (Figure 2d, right). Thus, the PL spectra measured for positioning control of the optical pumping exhibit single-mode lasing for each microdisk cavity (Figure 2e). Taken together, these results demonstrate that the adaptive shaping of the pump beam distinguishes two independent modes from two different cavities by forming the desired shapes of the pump laser at the intended locations. We further investigate selective pump focusing on a microcavity supporting multimodes in a small spectral range. We examine a microdisk cavity with a diameter of 13.4 μm (Figure 3a, left, inset). With conventional Gaussian-shaped optical pumping, three different peaks are observed at wavelengths of 1573.0, 1579.9, and 1583.7 nm (Figure 3a, e, and i, insets). Among these peaks, a desired single peak is selectively excited by adaptive optical pumping. First, the feedback process is performed with a target wavelength of 1573.0 nm. As a result, the lasing mode with a wavelength of 1573.0 nm is solely excited (Figure 3a). In this selective pump focusing, we observe the ring-shaped image of the pump laser (Figure 3b) and a whispering-gallery lasing mode image from the microdisk cavity (Figure 3c). In particular, as shown in the L−L curve (Figure 3d), only the selected mode demonstrates a superlinear increase in output intensity above the lasing threshold of ∼1.5 mW. The other two modes, at wavelengths of 1579.9 and 1583.7 nm, exhibit nonlasing behavior. Second,

Figure 1. (a) Schematic diagrams of selective pump focusing with a spatially nonuniform profile (left) and Gaussian-shaped optical pumping (right). (b) Experimental setup. The shape and position of the 980 nm CW pump laser are optimized for efficient optical pumping. The feedback procedure is conducted to increase the peak intensity of the target cavity mode. Inset, SEM image of a fabricated InGaAsP microdisk cavity on a sapphire substrate. Scale bar, 2 μm.

pump and mode profiles, is inversely proportional to the lasing threshold of the mode (Figure S1 and Supplementary Text). By maximizing the pump overlap factor for a desired mode (Figure S1d), one can lower its lasing threshold (Figure S1e) and increase thresholds of nonselected modes (Supplementary Text Figure 1). We consider two cases in selective pump focusing. First, if the spatial overlap between the μth and νth modes is weak, we should maximize the pump overlap factor for the μth mode, fμ (Figure S1 and Supplementary Text), to reduce the lasing threshold of the μth mode.17 The pump overlap factor for the νth mode, fν, is then automatically reduced, which increases the threshold of the νth mode. Accordingly, threshold discrimination enables efficient selection of only the target mode. Next, if the spatial overlap between the μth and νth modes is strong, fν will increase with increasing fμ. In this case, the threshold difference between the μth and νth modes is small; however, mode competition occurs between the modes to occupy the same optical gain.1,18 Then, excitement of the μth mode, with maximized fμ, is forced by weighted gain feeding. Therefore, the spatial pump profile will be optimized toward maximizing the pump overlap factor of the target mode.17,18 To realize selective pump focusing, we construct an experimental setup for adaptive optical pumping with a feedback process (Figure 1b and Figure S2). The spatial shape of a continuous-wave (CW) pump laser is controlled by a digital micromirror device (DMD; see Methods),21 and photoluminescence (PL) measurements are carried out for InGaAsP microdisk or microring cavities on a sapphire B

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Figure 2. (a) Captured images of the pump laser at feedback steps of 1, 500, 1000, and 6000, when the microdisk cavity (yellow dashed circle) in Figure 1b is used. The pump laser is evolved to optimize the target lasing mode of the microdisk. Scale bars, 5 μm. (b) Measured lasing spectra of the microdisk as the feedback step varies from 500 to 6000. The peak intensity at a wavelength of 1562.2 nm increases with increasing feedback steps. The incident pump power is fixed to 2.0 mW in all cases. Inset, measured lasing mode image at 6000 steps. Scale bar, 5 μm. (c) Measured output peak intensities of the microdisk laser versus the incident pump power, as the number of feedback steps varies. The threshold pump powers are reduced from 2.7 to 0.6 mW as the feedback steps increase from 500 to 6000. We note that the threshold pump power is 1.0 mW with conventional Gaussian-shaped pumping. (d) Individual optical pumping of two microdisk cavities with a center-to-center distance of 22 μm. The left panel presents an SEM image of the two microdisks, Disks 1 and 2, on a sapphire substrate. The diameters of Disks 1 and 2 are 5.4 and 4.9 μm, respectively. Scale bar, 5 μm. Captured images of the pump laser focused only on Disks 1 (middle) and 2 (right). The yellow dashed circles indicate Disks 1 and 2. (e) Measured lasing spectra of the microdisk as the pump laser is focused only on Disks 1 (top) and 2 (bottom). The measured lasing wavelengths of Disks 1 and 2 are 1563.3 and 1551.3 nm, respectively. The incident pump power is fixed to 3.5 mW in both cases.

and 1583.7 nm, respectively, for a pump power of 4 mW. These values are comparable or higher than those in previous reports19,20 as a result of successful adaptive pumping of the target mode. Second, we note that the pump images differ from the corresponding target lasing mode images. The lasing mode images are captured as Poynting vectors a few microns above the microdisk, which are strongly affected by the cavity geometries (Figure S3). In contrast, the pump image of the selective pump focusing is determined toward maximizing the pump overlap factor of the target lasing mode. Third, the measured L−L curves clearly show mode competition between the selected and nonselected modes and the resultant gain suppression of nonselected modes. For example, a significant mode overlap between the modes at 1573.0 and 1579.9 nm leads to similar lasing thresholds (Figure 3h). However, only the target mode at 1579.9 nm survives as a result of mode competition to occupy the same optical gain.18 A similar single-mode selection at 1573.0 nm can be observed in Figure 3d. In contrast, the target mode at 1583.7 nm is solely excited due to enhanced threshold discrimination between the selected and nonselected modes (Figure 3l).17

we select the single lasing mode at a wavelength of 1579.9 nm using a similar feedback procedure (Figure 3e). In this case, a square-shaped adaptive pump laser (Figure 3f) leads to laser mode emission with intensity maxima at the boundary of the microdisk (Figure 3g). The lasing behavior of this selected mode is clarified by the measured L−L curve with a lasing threshold of ∼1.6 mW (Figure 3h). Third, we select the single lasing mode at a wavelength of 1583.7 nm (Figure 3i). The imperfect ring-shaped adaptive pump laser (Figure 3j) excites the lasing mode with multiple bright spots and ring patterns (Figure 3k). In the measured L−L curve (Figure 3l), only the selected lasing mode is dominantly observed, showing a lasing threshold of ∼1.8 mW. The measurements reveal several key features of the effective laser mode manipulation. First, the selective pump focusing successfully enhances the intensity of the desired laser mode and suppresses other cavity modes (Figure 3a, e, and i). The mode selectivity of a selected laser mode (μth mode) can be assessed by the extinction ratio Rμ, which is defined by Iμ/Iν0, where Iμ is the peak intensity of the desired mode and Iν0 is the highest peak among undesired modes.19,20 Rμ is measured to be ∼12, ∼55, and ∼99 for the laser modes at 1573.0, 1579.9, C

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Figure 3. CW pump laser (980 nm) with a power of 4 mW is adapted to excite only the microdisk cavity mode with a wavelength of 1573.0 (a−d), 1579.9 (e−h), and 1583.7 nm (i−l). Among the three lasing peaks obtained by Gaussian-shaped conventional optical pumping with a spot size of ∼8 μm (insets of a, e, and i), the desired single lasing peak is solely excited by the adaptive optical pumping (a, e, and i): the colored peak in each inset is selected for single-mode lasing. When the lasing spectra of (a), (e), and (i) are measured, the corresponding images of the pump laser (b, f, and j) and the target lasing mode (c, g, and k) are captured at feedback steps of 4000, 6000 and 2000, respectively. Scale bars, 10 μm. (d, h, l) Measured output peak intensities of the cavity modes with wavelengths of 1573.0 (red), 1579.9 (blue), and 1583.7 nm (green) as a function of the incident pump power. The graphs in (d), (h), and (l) are obtained as the pump lasers of (b), (f), and (j) are used, respectively. (l) We do not plot the curves of the modes with wavelengths of 1573.0 and 1579.9 nm because the corresponding output intensities are too small. Left inset in (a) provides an SEM image of the 13.4 μm diameter InGaAsP microdisk cavity on a sapphire substrate. Scale bar, 10 μm.

laser (Figure 4b) and the corresponding lasing mode of the microring cavity (Figure 4c). A pentagon-shaped pump laser and circular lasing mode images are clearly observed. Second, we select the peak with a wavelength of 1577.2 nm as the target cavity mode. After a sufficient number of adaptive feedback steps, we observe a sharp single-mode lasing peak at the target wavelength (Figure 4d). The captured images of the adaptive pump laser (Figure 4e) and the target lasing mode (Figure 4f) are quite similar to the images in Figure 4b and c, respectively. However, a π/5 rotation between the two pentagon-shaped pump lasers are observed in Figures 4b and e. Third, we select the peak with a wavelength of 1574.8 nm as the target mode (Figure 4g). The single-mode lasing peak is obtained by the adaptive pump laser with the ring shape, not the pentagon shape (Figure 4h), and the corresponding lasing mode image is observed (Figure 4i). Thus, due to the high resolving power of our selective pump focusing, we successfully

Next, we examine the selection of cavity modes in a microring cavity with a diameter of 14.7 μm (Figure 4a, right inset). This cavity will be suitable to assess the resolving power of our selective pump focusing, because it can excite dominantly WGMs with similar mode shapes and similar resonant wavelengths. Indeed, three closely located peaks are simply excited in the microring cavity when conventional Gaussian-shaped optical pumping is used (Figure 4a, d, and g, insets). In particular, the two peaks with wavelengths of 1578.2 and 1577.2 nm are observed showing a wavelength difference of only 1 nm. Similar to the adaptive optical pumping shown in Figure 3, the desired single lasing peak is selectively excited by the feedback process. First, we select the peak with a wavelength of 1578.2 nm as the target cavity mode and increase the output intensity. As the number of feedback steps increases, a singlemode lasing peak is solely excited at that target wavelength (Figure 4a). We also record the images of the adaptive pump D

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discrimination of the pump overlap allows mode selection with a high resolving power, even for doubly degenerate modes. To take full advantage of the adaptive optical pumping in a potential biological application, we examine lasing action from a microdisk cavity underneath a scattering medium (Figure 5a). Biological tissues can be modeled as a scattering medium that induces multiple wave scattering and wavefront distortion.25 In fact, the disturbed wavefront cannot efficiently optically pump the cavity when an unmodified pump laser is injected through the scattering medium (Figure 5b, left).22,24 The adaptive optical pumping, however, can deliver sufficient energy to the cavity by shaped light focusing (Figure 5b, right).25,26 In the experiment, we use a microdisk cavity with a diameter of 5.2 μm and ZnO powder as the scattering medium (see Methods). ZnO powder is chosen because it is nonabsorptive in the infrared lasing wavelength, but scatters light significantly.27 The output intensities from the microdisk cavity are then measured as a function of the incident pump power under three different pumping conditions (Figure 5d−f). First, no scattering medium is introduced, and unmodified Gaussianshaped pump laser is used (Figure 5d). A single-mode lasing peak is observed at a wavelength of 1561.8 nm (Figure 5d, inset), and the lasing threshold is measured to be ∼0.8 mW. Second, the scattering medium is introduced, but the unmodified pump laser is still used (Figure 5e). In this case, no lasing is observed due to significant scattering of the pumping light. Third, the scattering medium is introduced, and the adaptive pump laser is used (Figure 5f). We select the cavity mode at a wavelength of 1561.8 nm as the target mode, as was observed in the first case with no scattering medium. After a sufficient number of adaptive feedback steps, a singlemode lasing peak is observed at the target wavelength, even with the introduction of a scattering medium. The measured L−L curve shows a lasing threshold of ∼2.4 mW. In this case, we also observe the images of the adaptive pump laser (Figure 5c, top) and the corresponding lasing mode of the microdisk through the scattering medium (Figure 5c, bottom). Furthermore, we carry out an experiment to show scattering medium thickness versus a feedback step that saturates the laser peak intensity (Figure S5). Taken together, these results indicate that the adaptive optical pumping serves as a robust method to obtain shaped light focusing through a scattering medium. By overcoming obstacles such as the disturbance of light absorption/emission, the pump energy is efficiently transferred to a desired mode without preliminary knowledge of the mode characteristics.

Figure 4. CW pump laser (980 nm) with a power of 4 mW is adapted to excite only the microring cavity mode with a wavelength of 1578.2 (a−c), 1577.2 (d−f), and 1574.8 nm (g−i). Among the three lasing peaks obtained by Gaussian-shaped conventional optical pumping (insets of a, d, and g), the desired single lasing peak is solely excited by the adaptive optical pumping (a, d, and g): the colored peak in each inset is selected for single-mode lasing. When the lasing spectra of (a), (d), and (g) are measured, the corresponding images of the pump laser (b, e, and h) and the target lasing mode (c, f, and i) are captured at feedback steps of 5000, 8000, and 6000, respectively. Scale bars, 10 μm. The right inset in (a) presents an SEM image of the 14.7 μm diameter InGaAsP microring cavity on a sapphire substrate. Scale bar, 10 μm. The colored figures shown in the insets of (b), (e), and (h) illustrate the spatial distribution of the intensity maxima of the pump laser.

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CONCLUSION In summary, we have successfully demonstrated adaptive optical pumping of microdisk and microring lasers. An optimally shaped pump laser efficiently delivers the pump energy to a selected lasing mode, which results in an enhanced laser emission and a reduced lasing threshold. In a multimode laser system, a desired lasing mode is selected to achieve singlemode operation under a pump profile that is well overlapped with the lasing mode profile. Efficient control of the pump laser is particularly useful for optical pumping of a microcavity through a random scattering medium. We note that our DMDbased adaptive optical pumping takes the order of hundreds milliseconds and thus, faster wavefront shaping will be necessary to respond to transient motions of biological materials:31 for example, ferroelectric liquid crystal based

achieve mode selection with a wavelength difference of only 1 nm. We note that the bright spots in the lasing mode images are in good agreement with the strong pumping positions. In particular, the π/5 rotation between pump profiles in Figures 4b and e causes different pump overlap factors. For example, the 1578.2 nm lasing mode in Figure 4c is well overlapped with the pump profile in Figure 4b, but less overlaps with the pump profile in Figure 4e (Figure S4). Similarly, the 1577.2 nm lasing mode in Figure 4f is well overlapped only with the pump profile in Figure 4e. Although highly similar wavelengths and mode images are shown in these two lasing modes, the E

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Figure 5. (a) Schematic illustration of an optically pumped microdisk laser underneath a scattering medium. (b) Schematic diagrams showing the working principle of adaptive optical pumping through a scattering medium. Whereas the unmodified pump laser is disturbed by the scattering medium (left), the adaptive pump laser can efficiently pump the microdisk cavity through the scattering medium (right). (c) Captured images of the adapted pump laser through the scattering medium (top) and the lasing mode of the microdisk (bottom) at a feedback step of 9000. Scale bars, 10 μm. A 980 nm CW pump laser is used to optically excite a microdisk cavity with a diameter of 5.2 μm. ZnO powder is used as the scattering medium. (d−f) Measured output intensities of the microdisk laser as a function of the incident pump power under three different pumping conditions. (d) No scattering medium is introduced. The lasing threshold of the microdisk is 0.8 mW when the unmodified Gaussian-shaped pump laser is used. Inset, measured spectrum at 1 mW. (e) Scattering medium with a thickness of 194 μm is introduced, but the unmodified Gaussianshaped pump laser is used. No lasing action is observed. Inset, measured spectrum at 3.2 mW. (f) Scattering medium with a thickness of 194 μm is introduced, and the adaptive pump laser is used. The lasing threshold of the microdisk is 2.4 mW. Inset, measured spectrum at 3.2 mW. The lasing wavelength is 1561.8 nm, which is the same as that in (d).

the infrared wavelength. The scattering medium is placed 2−3 mm above the microdisk laser. Shaping of Optical Pumping. A 980 nm CW laser diode is used to optically pump the microdisk/microring cavities at room temperature. To realize selective pump focusing, a computer-driven DMD (DLP Discovery Kit 4100, Texas Instruments, 1024 × 768 pixels, 13.68 × 13.68 μm2 per pixel) is used (Figure S2). To shape the pump laser, micromirrors with 768 × 768 micropixels are grouped into 32 × 32 macropixels. The pump profile on a microcavity is determined by superposition of the reflected light from each macropixel. A binary grating pattern is written in each macropixel, and the pattern is laterally translated to manipulate the phase retardation (Δϕ) of the first-order diffracted light.21 We use only the first diffraction order to shape the pump laser, because the first order delivers higher pump power than other nonzero diffraction orders. The phase retardation cannot be manipulated in the zeroth order by changing the grating pattern, and thus the zeroth order is excluded. The other diffraction orders except the first order are blocked using an iris diaphragm. Then, the shape of the pump laser is changed by controlling Δϕ from each macropixel with varying grating patterns. Our method can generate a shaped pump laser with a spatial resolution of ∼2.8 μm, which corresponds to the angular range of numerical aperture of 0.42. Adaptive Feedback Process. The pump profile is optimized using an iterative feedback process that maximizes

spatial light modulator (FLC-SLM) can be used instead of DMD. We believe that our adaptive optical pumping method will pave the way for the realization of selective single-mode lasing operation with a low lasing threshold, as well as the injection of microlasers into biological tissue for efficient optical stimulation of biomaterials.25,26,28−31

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METHODS Device Fabrication. Microdisk/microring structures are fabricated in a 220 nm thick InGaAsP/800 nm thick InP/100 nm thick InGaAs/InP substrate wafer using electron-beam lithography and chemically assisted ion-beam etching. The InGaAsP slab includes two layers of quantum wells with a central emission wavelength of ∼1.5 μm. The sacrificial InP layer is selectively wet-etched using a dilute HCl/H2O (3:1) solution. Then, the fabricated microdisk/microring is transferred onto a sapphire substrate, which has good thermal conductivity, using a polypropylene carbonate (PPC)-coated polydimethylsiloxane (PDMS) stamp. Experiment with Scattering Media. For the scattering medium in Figure 5, a PDMS film with randomly dispersed zinc oxide particles (Sigma-Aldrich #96479) is used. The thickness of the scattering medium is 194 μm. Optical density (OD) of 0.61 and 0.35 are measured at the wavelength of pump laser (λ = 980 nm) and microdisk laser (λ = 1550 nm), respectively. The absorption coefficient of the scattering medium is nearly zero since the medium is nonabsorptive in F

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(2017R1A2B2009117). W.C. acknowledges support by IBSR023-D1.

the peak intensity of the target lasing mode (see Figure S2). First, to find the local maximum at the ith feedback step, we randomly select a portion of the macropixels.22 We then measure the peak intensity with varying phase retardation (Δϕseli) of the selected macropixels and find the optimal Δϕseli (Δϕsel,opti) satisfying the local maximum of the peak. To reduce the measurement overhead, we use the four-phase method, which finds Δϕsel,opti by changing Δϕseli only four times.23,24 While Δϕseli is changed by adding 0, π/2, π, and 3π/2, the peak intensity is measured with these four phase additions. Next, Δϕsel,opti is obtained by fitting the measured four peak intensities using the equation I(Δϕseli) = A cos(Δϕseli − Δϕsel,opti) + B, where I is the peak intensity and A and B are fitting constants (Figure S2). Finally, the iteration is finished when the peak intensity is saturated. We determine the final peak intensity as the global maximum. We note that 2432×32−1 different pump profiles can be formed using our adaptive feedback process. Optical Measurements. Photoluminescence (PL) experiments are performed by adaptive optical pumping of a 980 nm CW pump laser. The light emitted from the microdisk/ microring cavities is collected by a long working distance 50× objective lens (numerical aperture of 0.42) and focused onto either a monochromator (DK480, Spectral Products) or an InGaAs infrared camera (C10633, Hamamatsu). The image of the pump laser is captured by a CMOS camera (EO-0413M, Edmund Optics) at the position of the microdisk/microring sample. For Gaussian-shaped optical pumping, a 20× objective lens (numerical aperture of 0.5) is used to focus the 980 nm CW pump laser to a spot size of ∼8 μm. The PL from the microcavities is collected by the same objective lens and focused onto a 1D infrared array detector (PyLoN, Princeton Instruments).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00648.

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REFERENCES

Supplementary figures and Supplementary text with figures (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hong-Gyu Park: 0000-0002-6375-0314 Author Contributions ‡

J.-H.C., S.C., and K.-H.K. have contributed equally to this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Profs. S.-H. Kwon and Y.-S. No for helpful discussion. H.-G.P. acknowledges support by the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIT; Nos. 2018R1A3A3000666, 2014M3A6B3063710, and 2017R1A4A1015426). M.-K.S. acknowledges support by the NRF Grant G

DOI: 10.1021/acsphotonics.8b00648 ACS Photonics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsphotonics.8b00648 ACS Photonics XXXX, XXX, XXX−XXX