ZnO-Nanowire-Induced Nanocavities in Photonic Crystal Disks - ACS

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ZnO-nanowire-induced nanocavities in photonic crystal disks Sylvain Sergent, Masato Takiguchi, Tai Tsuchizawa, Hideaki Taniyama, and Masaya Notomi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00286 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ZnO-nanowire-induced nanocavities in photonic crystal disks

Sylvain Sergent1,2, Masato Takiguchi1,2, Tai Tsuchizawa1,3, Hideaki Taniyama1,2 and Masaya Notomi1,2

1Nanophotonics

2NTT

Center, NTT Corp., 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

Basic Research Laboratories, NTT Corp., 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-

0198, Japan 3NTT

Device Technology Laboratory, NTT Corp., 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-

0198, Japan KEYWORDS. Nanowire, nanocavity, nanomanipulation, photonic crystal, zinc oxide, silicon nitride, ultraviolet.

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ABSTRACT. We realize a novel type of nanowire-induced optical nanocavities based on photonic crystal disks, a design outperforming its one-dimensional and two-dimensional photonic crystal counterparts in terms of light-matter coupling. We implement this unique design using ZnO nanowires and SiN photonic crystal disks for a short-wavelength operation. We detail and assess the challenges of the fabrication process, a combination of SiN top-down processing techniques and nanowire nanomanipulation. We then investigate the optical properties of fabricated cavities and show that they present confined modes in the visible and the near ultraviolet range with experimental quality factors up to Qexp = 2.4×103. We deduce from threedimensional finite-difference time-domain calculations that such confined modes can present volumes as small as Vm = 1.3 (λ/nrdisk)3, for an exceptionally high confinement factor Γ = 47%. The high intensity enhancement ratios of confined modes confirm the high performance of these nanowire-induced cavities based on photonic crystal disks.

The unique characteristics of semiconductor nanowires (NWs) have made them key buildingblocks for the realization of compact and versatile photonic devices such as photonic and plasmonic nanolasers [1,2], single photon-sources operating at room-temperature [3], bright LEDs [4] or efficient photodetectors [5]. To maximize their interaction with light and boost the device performance, NWs often need to be integrated into optical nanocavities presenting high quality factor on mode volume ratio [1,2,6], with preferably a large confinement factor of the mode into the NW. In that realm, subwavelength nanowires positioned in photonic crystals (PhCs) have recently emerged as a versatile nanophotonic platform to achieve high quality factor nanocavities from the infra-red [6] down to the visible [7] and ultra-violet ranges [8] as well as nanolasers operating at telecommunication wavelengths [9,10]. To date, such hybrid NWinduced PhC nanocavities are based on two-dimensional (2D) PhC waveguides that are not


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optimal regarding light-matter coupling: the mode volume is larger than what can be designed e.g. in one-dimensional nanobeam PhCs [11] and their confinement factor is intrinsically limited to ~20% because a large part of the electric field lies into the surrounding PhC [6-12]. We recently showed that those two issues can be addressed by a novel NW-induced nanocavity design based on PhC disks [13]. We demonstrated by three-dimensional finite-difference timedomain (3D-FDTD) calculations that a PhC disk platform can be the basis for NW-induced nanocavities presenting not only high quality factors and small mode volumes but also exceptionally high confinement factors, provided the disk material and geometry are carefully chosen [13]. The realization of PhC disks have been seldom reported in the literature [14,15] but their implementation in III-arsenides and phosphides slabs has nevertheless led to the demonstration of lasing at telecommunication wavelengths. However, the NW-induced nanocavity that we report here departs from monolithic PhC disk cavities due to the lower dimensionality of its NW emitter, its sophisticated light confinement principle, the versatility of its hybrid design and the advanced fabrication techniques that it requires. Moreover, the implemented NW-induced nanocavity design is here optimized for ultraviolet and visible wavelengths. To that end, we use SiN PhC disks that have little absorption in that spectral range and ZnO NWs presenting a bandedge emission at 375 nm, a material system we have already used for NW-induced cavities based on 2D PhCs [8,11]. In this paper, we first explain the confinement principle of NW-induced nanocavities based on PhC disks and we briefly describe the design performance in the chosen material system as calculated with the 3D-FDTD method. We then detail the fabrication process and its challenges. We finally investigate their optical properties: we demonstrate that fabricated cavities present confined modes with quality factors as high as 𝑄 = 2.4 × 103 at 𝜆 = 404 𝑛𝑚


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and intensity enhancement ratios as high as 56, which stems from the significant light-matter coupling taking place in this unique cavity design. PhC disks are constituted of a circular array of air holes into a dielectric microdisk of refractive index 𝑛𝑑𝑖𝑠𝑘 (Figure 1a). If the holes are positioned at the antinodes of whispering-gallery mode 𝑟 TEn,1, n being the azimuthal number, the mode degeneracy is lifted and a photonic bandgap (PBG) is created for the whispering-gallery mode. By introducing a defect into the circular symmetry, the PBG can be leveraged to further confine the light. In PhCs, defects are typically formed by locally tuning the radius or position of air holes [15]. Instead, the defect is here created by inserting a single subwavelength NW into the PhC disk (Figure 1a), which locally modulates the refractive index. This modulation shifts the bandedges of the PBG and creates a donor-like cavity mode (Figure 1b) that overlaps well with the NW [13]. We apply this confinement principle to the material system introduced earlier with 𝑛𝑁𝑊 = 2.4 and 𝑛𝑑𝑖𝑠𝑘 = 2.0, 𝑟 𝑟 the refractive indices of the NW and the disk respectively. For an azimuthal number n = 36 and a given disk radius rdisk, we set phole = 0.94rdisk as the radial position of the holes, a = π.rdisk.phole/n the lattice constant, t = 0.8a the disk thickness and dhole = 0.5a the hole diameter. The bended NW subtended by an angle θNW = 90° is placed inside the disk at the radial position pNW = 0.82rdisk. In order to be as realistic as possible, we actually consider a NW with a circular section placed into a groove presenting a square section. The NW diameter is δNW = 0.73a and hgroove = wgroove = δNW are the groove width and depth. According to 3D-FDTD simulations, this design leads to a donor-like nanocavity mode at a/λ = 0.332 with λ the operating wavelength. The quality factor is as high as Qth = 1.6×104. For an operation in the near-UV and violet range, it translates into a disk radius rdisk comprised between 1.7 and 2 μm. Considering a circular NW in a square groove leads to a confinement factor Γ = 50%, slightly lower than our initial square-


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NW-section design [13] because part of the field is confined in the air around the circular NW (see inset of Figure 1c). It is nevertheless significantly higher than what is found in all reported plasmonic, 2D PhC and 1D PhC nanocavities integrating subwavelength nanowires [1,2,6-12]. More specifically, this confinement factor is 2.6 times larger than for 2D PhC nanocavities based on the same material system [8]. The cavity has a mode volume Vm = 1.8 (λ/nrdisk)3 (Figure 1c). It is smaller than what is typically found in NW-induced nanocavities based on 2D PhCs with similar materials [8,11]. Moreover, for a given NW geometry and lattice constant, the donor-like mode can be further shrunk by reducing both the azimuthal number n and the disk radius rdisk. For n = 18 (Figure 1d), the donor-like mode at a/λ = 0.355 has a smaller volume Vm = 1.3 (λ/nrdisk)3, while the confinement factor and the quality factor remain as high as Γ = 47% and Qth = 5.8×103 respectively. In addition to the donor-like cavity mode, there exists an acceptor-like cavity mode [13] presenting limited overlap with the NW and a larger mode volume (Figure 1e). Similarly to a plain microdisk, our system also supports sets of whispering-gallery-like modes, i.e. whisperinggallery modes distorted by the presence of the PhC holes and the NW (Figure 1f). Such modes also present a poor overlap with the NW as well as a much larger mode volume. One would thus expect limited light-matter coupling with both acceptor-like and whispering-gallery-like modes. We implement the nanocavity design using ZnO NWs and SiN PhC disks. The fabrication process flow is similar to the one described in Reference 8 and is schematically represented in Figure 2a. The PhC disks are first defined by electron-beam lithography and dry etching in a 108-nm-thick SiN-on-silicon layer. Disks are fabricated for azimuthal numbers n = 36, 30, 24, and 18 with disk radii rdisk = 2, 1.67, 1.33, and 1 μm respectively. We also fabricate sets of disks presenting an identical design but adding a scale factor sf = 0.85, 0.9 and 0.95. Subsequently,


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grooves subtended by an angle θNW are defined inside each disk. The grooves are subtended by angles θNW = 90°, 108°, 135° and 180° for azimuthal numbers n = 36, 30, 24, and 18 respectively. The disk membrane is then partially released by KOH wet etching of the silicon. Finally, ZnO NWs with diameters lower than 100 nm and various lengths are transferred by dry contact onto the SiN PhC disks, manipulated with an atomic force microscope tip and inserted into the groove. A full description of the nanomanipulation and insertion of the NW into the groove is available in Supporting Information. Let us point out that this last manipulation step is significantly more challenging than for NW-induced nanocavities in 2D PhCs because of the necessity to bend the NW inside the groove. It is nevertheless rendered possible by the fact that ZnO NWs are indeed bendable [16] and we confirmed that curvature radii as small as 0.8 μm can be achieved. Such a small curvature radius is however close to the NW brittle failure limit for δNW = 100 nm, as can be derived from the previously measured ultimate strength of ZnO NWs [17]. Smaller curvature radii would thus be difficult to achieve. As detailed in Supporting Information, despite the ZnO NWs bendability there remain some technical challenges, namely (i) the precise matching of the groove and NW ends at such a nanoscale, and (ii) the necessity to cleave the ZnO NW when it is longer than the groove length. Regardless of such challenges, cavities are successfully realized for all azimuthal numbers n = 36, 30, 24, and 18 and various scale factors sf. Figure 2b shows scanning electron microscope (SEM) images of such fabricated cavities for each azimuthal number. Each cavity features a silicon post supporting the center of the disk. The post diameter is small enough for the light to be efficiently confined at the periphery of the SiN membrane where lies the donor-like nanocavity mode. One can observe the presence of NWs inserted in the grooves but there always exists a mismatch between their respective lengths due to the nanomanipulation challenges


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mentioned earlier. Finally, let us note that a bended ZnO NW inserted into a square membrane is also fabricated as a reference for optical investigations (Figure 2b). The optical properties of NW-induced nanocavities are investigated by room-temperature microphotoluminescence, using an excitation source emitting at 266nm in a confocal configuration. The emission spectrum of the NW placed into a square membrane presents the typical room-temperature ZnO band-edge emission at 375nm. There is no observable effect of the NW bending due to the large phonon broadening at room temperature (Figure 3a). In addition, because it is a subwavelength NW with a diameter lower than 100 nm, light is not guided in that spectral range and no Fabry-Perot mode appears. By contrast, a NW inserted into a PhC disk with n = 36 and sf = 0.9 presents a series of sharp peaks in the investigated spectral range corresponding to donor-like, acceptor-like and whispering-gallery-like modes (Figure 3a and 3b). While other modes present intensity enhancement ratios lower than S/B = 2, the mode at 𝜆 = 404.5 nm stands out by an intensity enhancement ratio as high as S/B = 12. To gain more insight into this matter, we calculate far-field patterns of the various modes supported by the PhC disk (Figure 3c). They show that cavity modes emerging from NW-induced PhC disks are mostly emitted in-plane, so that in our confocal configuration a very limited part of the far field intensity can be collected from the top: respectively ξ = 1.1%, 3.3% and 0.5% for the donor-like, acceptorlike and whispering-gallery-like modes. The collection efficiencies of these modes are comparable, with a slight advantage for the acceptor-like mode. When there exists a mismatch between the groove and NW lengths, the air gap scatters part of the light out-of-plane. Collection efficiencies thus increase up to ξ = 5.0%, 4.4% and 5.3% respectively for the donor-, acceptorand whispering-gallery-like modes, keeping the efficiencies comparable. Collection efficiencies


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thus play a minor role in the relative measured mode intensities. As a result, the high intensity enhancement ratio observed at 𝜆 = 404.5 nm is in good agreement with the donor-like mode, which is expected to present a significantly higher Purcell factor owing to its smaller mode volume and larger confinement factor, as highlighted in Figure 1c to 1f. The microphotoluminescence mapping of the cavity confirms that the donor-like mode originates from the inserted NW with the maximum of intensity detected in the vicinity of the NW center (Figure 3d). We also observe that the donor-like mode is linearly polarized with a polarization direction close from the radial direction at the NW center and with a 95% degree of linear polarization (lower panel of Figure 3e, pink dots). In contrast, the NW positioned in a square membrane has a similar polarization orientation but a significantly lower degree of linear polarization 20% (lower panel of Figure 3e, gray dots). This is in good agreement with the inplane components of the electric field extracted from 3D-FDTD calculations (upper panel of Figure 3e). The donor-like mode presents an experimental linewidth Δλ = 190 pm, which translates into a quality factor Qexp = 2.4×103 when accounting for the Gaussian broadening of the measurement setup (Figure 3b). This constitutes the highest quality factor obtained in this work and is very similar to the highest experimental quality factors obtained in NW-induced 2D PhC nanocavities realized in the same material system [8]. It shows that scattering losses due to fabrication imperfections are of the same order in both types of cavities despite additional constraints, namely the NW to groove matching. This is actually in agreement with 3D-FDTD calculations showing the resilience of the PhC disk design to the mismatch between the NW and the groove: quality factors Qth remain larger than 2×103 for mismatch angles as large as 2θa, θa = π / n being the angle between two adjacent holes. It is worth noting that this experimental quality factors is


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about an order of magnitude higher than what has been recently reported in bulk ZnO PhC nanocavities [18]. This highlights the advantage of our hybrid approach to limit scattering losses when dealing with materials such as ZnO that are especially difficult to process. Note that it is possible to achieve higher quality factors in some ZnO-based resonators fabricated through bottom-up methods that do not require any top-down processing: for example whispering-gallery modes with quality factors close to 4000 can be obtained in ZnO microwires, but at the expense of a larger mode volume [19]. Donor-like modes can be found in nanocavities presenting azimuthal numbers n = 18, 24, 30, 36 and various scale factors (Figure 4). The mode wavelength (Figure 4a) tends to shorten as scale factors and azimuthal numbers decrease, in good agreement with 3D-FDTD calculations. The experimental quality factors tend to increase as the mode wavelength lengthens, which stems from a reduction in both scattering and absorption losses in the ZnO NW (Figure 4b). Scattering losses are expected to follow a 1/λ2 trend for a given level of fabrication imperfections [20]. However, such a trend is not visible due to large variations in the disorder level especially because of multidimensional mismatches between the groove and the NW. The high level of absorption losses at short wavelength [21] (see dashed line in Figure 4b) also plays a significant role and is a direct consequence of the large confinement factor of this unique nanocavity design. One can conversely observe that experimental quality factors of whispering-gallery-like and acceptor-like modes can be high even at short wavelengths (Figure 4b), owing to their poor confinement and thus low NW absorption. The cavity with the smallest mode volume Vm = 1.3 (λ/nrdisk)3 is achieved at λ = 392 nm for an azimuthal number n = 18 and an experimental quality factor Qexp = 200. Remarkably, such a mode volume is 2.6 times as small as hybrid NW-induced 2D PhC nanocavities based on the same material system [8]. It is also significantly smaller than


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the bulk ZnO PhC cavities that we mentioned earlier [18] and it is of the same order as what is typically obtained in monolithic III-nitride nanobeam PhC cavities operating at similar wavelengths [22,23]. The intensity enhancement ratios of the donor-like modes (Figure 4c) are significantly higher than what has been obtained in ZnO NW-induced nanocavities in 2D PhCs: we here find ratios as high as S/B = 56 in a PhC disk with n = 36 and sf = 0.95, as compared with a maximum of S/B = 3.5 in 2D PhCs [8]. Such a large difference is obtained despite similar experimental quality factors and far-field collection efficiencies. In 2D PhC cavities, ξ = 7.7% of the field intensity is typically collected by the objective i.e. larger than the collection efficiency of the PhC disk donor-like mode with or without groove-NW mismatch. The high intensity ratios are thus evidence of the higher performance of the PhC disk design, namely a greater Purcell effect driven by a smaller mode volume and a larger confinement factor. However, as the mode wavelength shortens and experimental quality factors drop, measured intensity enhancement ratios decrease together with the Purcell factor. The confinement factor may also play a role here as it varies from cavity to cavity due to small mismatches between the groove width and the NW radius. For an 8% mismatch, the confinement factor typically drops down to Γ = 38% because a greater part of the electric field lies in the air groove rather than the NW. In conclusion, we have implemented a novel type of NW-induced nanocavity based on PhC disks. Such hybrid nanocavities exhibit exceptionally high confinement factors, going beyond plasmonic, 2D PhC and 1D PhC nanocavities integrating subwavelength nanowires [1,2,6-12]. It more specifically outperforms previously reported ZnO-NW-induced nanocavities based on 2D SiN PhCs: our design can show smaller mode volumes by a factor 2.6 and larger confinement factors by a factor 2.6. As a result, this NW-induced nanocavity design shows good prospects for


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light-matter coupling with deterministically positioned emitters and novel nanophotonic devices such as nanolasers. In order to achieve such devices however, limiting the nonradiative carrier recombinations at the NW surfaces would be key, e.g by protecting the ZnO NW core in a ZnMgO shell. Higher performance can also be achieved by using NWs with controlled dimensions in order to solve the groove-to-NW mismatch issue and limit subsequent scattering losses. One can consider more refined PhC disk designs as well: greater performance can be achieved by reducing the index difference 𝑛𝑁𝑊 ― 𝑛𝑑𝑖𝑠𝑘 𝑟 𝑟 , for example by using AlGaN as the disk material [13]. Furthermore, let us note that the hybrid nature of this novel cavity design opens possibilities for implementations with other NW materials that can be easily manipulated and bended such as GaN/InGaN [24], CdSe [25], CdS [26] or even GaAs [27] and InP [28]: such types of NWs would allow addressing various spectral ranges for applications in information processing or biosensing. Finally, the recent development of the deterministic assembly of inplane curved NWs [29,30] gives NW-induced PhC disk designs exciting perspectives for large scale integration. In particular, the in-plane emission of the donor-like mode (Figure 3c) is well suited for integration in in-plane photonic circuitry as it could be easily harnessed and routed through bus waveguides, similarly to what is done e.g. in the visible range with monolithic InGaN/GaN microdisk lasers [31].

METHODS 3D-FDTD

calculations.

Three-dimensional

finite-difference

time-domain

(3D-FDTD)

calculations are carried out using our home-built algorithm (see References 11-13 for details) and confirmed with the Lumerical FDTD toolbox. The spatial resolution is set to σ = 8.5nm. The extent of the calculation domain is 4rdisk and 10t or more in the in-plane and out-of-plane


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directions respectively. Far-field patterns are derived from the near-field calculated above the cavity slab with the Lumerical toolbox. Nanomanipulation. The NW manipulation is carried out in an atomic force microscope with silicon cantilevers presenting an 8 nm tip radius and a 42 N/m spring constant. The NWs are manipulated in contact mode with the cantilever tip moving with a 500 nm/s speed. The force applied to the NW by the tip is controlled through the cantilever deflection setpoint. It is minimized in order to limit damages to the tip, avoid the brittle failure of the NW and prevent the collapse of the SiN photonic membrane. However, in order to cleave the NW end when there exists a mismatch between the NW and groove lengths, the deflection setpoint often needs to be increased. Optical measurements. The optical properties of NW-induced nanocavities are investigated by room-temperature microphotoluminescence, using an 80MHz pulsed laser emitting at 266 nm as the excitation source. A Mitsutoyo ultraviolet microscope objective (N.A. = 0.42, 50-fold magnification) is used to focus the laser on the ZnO NWs, resulting in a 1 μm spot radius and an average 0.16 kW.cm-2 excitation density. The emitted light is collected in a confocal configuration, with the objective oriented perpendicularly to the disk plane. After passing through a 30-μm slit, the collected light is dispersed by a 1200 grooves/mm grating on an ultraviolet-enhanced coupled camera device. Polarization properties are investigated by adding a polarization analyzer on the signal collection path. The resolution of the setup is 74 pm.

Figure 1. a, Schematic representation of a NW-induced nanocavity in a PhC disk. b, Confinement principle of the NW-induced nanocavity. The local PBG is highlighted in pink


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while the purple line represents the donor-like cavity mode. c, Field energy of the donor-like mode for θNW = 90° and n = 36. The central inset is a vertical cross section at the center of the NW. d, Donor-like mode for θNW = 180° and n = 18. e, Acceptor-like mode and f, whisperinggallery-like mode for θNW = 90° and n = 36.

Figure 2. a, Fabrication process flow: 1. E-beam lithography of PhC disks in resist, 2. Transfer of PhC disks into a SiN on silicon layer by dry etching, 3. E-beam lithography of circular grooves, 4. Transfer of grooves into the SiN, 5. Wet underetching of the silicon substrate, 6. ZnO NW transfer, 7. Nanomanipulation and insertion into groove. b, SEM images of ZnO-NWinduced nanocavities in PhC disks presenting azimuthal numbers n = 36, 30, 24, and 18. The bended NWs are highlighted with white arrows for clarity. The scale factors are sf = 0.9, 0.95, 0.95 and 1. This translates into respective disk radii rdisk = 1.8, 1.59, 1.26, and 1 μm. The SEM image of a bended ZnO NW inserted into a 4 μm2 square membrane is also displayed. The horizontal scale bars correspond to 1 μm. The color-code of each azimuthal number will be used in following Figures.

Figure 3. a, Microphotoluminescence spectra of the NWs inserted in a square membrane (black line) and in a PhC disk with n = 36 and sf = 0.9 (pink line). The investigated NWs are displayed in Figure 2b. The upper panel shows the intensity enhancement ratio for each mode of the nanocavity disk. b, Close-up of the donor-like mode observed in panel a. The black line is a Voigt fitting to the experimental data (pink dots) taking into account the Gaussian broadening of


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the experimental setup. c, Projections of the far-field intensity of the donor-like, acceptor-like and whispering-gallery-like modes presented in Figure 1c, 1e and 1f. The thick inner circle indicates the collection cone of the microscope objective used in the optical experiments (N.A. = 0.42). Collection efficiencies are respectively 1%, 3.3% and 0.5%. d, Microphotoluminescence intensity map of the donor-like mode featured in panels a and b. The step distance is 0.5 μm. e, The upper panel represents the in-plane components of the electric field as calculated by 3DFDTD. The lower panel displays the polarization measurements of the donor-like mode (pink dots) and the NW positioned in a square membrane (gray dots).

Figure 4. a, Scaling factor, b, quality factor and c, intensity enhancement ratio of donor-like modes as a function of wavelength and as observed by microphotoluminescence for various azimuthal numbers. Open circles in panel b represent the quality factor of whispering-gallerylike and acceptor-like modes for all cavities. The gray spectrum in panel b corresponds to the emission of the NW inserted in a square membrane and displayed in Figure 2b. The dashed line highlights the expected quality factor of the donor-like mode, taking into account (i) intrinsic losses as calculated by 3D-FDTD, (ii) absorption losses as derived from measured absorption in bulk ZnO [20] and (iii) no scattering losses. Open circles in panel c represent typical intensity enhancement factor obtained in ZnO-NW-induced nanocavities in 2D SiN PhCs extracted from [8].

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information provides additional fabrication details. The following files are available free of charge via the Internet at http://pubs.acs.org. Nanomanipulation data (.AVI) AUTHOR INFORMATION Corresponding author *Email: [email protected] Author Contributions S.S. carried out FDTD calculations with the assistance of H.T.. Fabrication of PhC disks was realized by S.S. and T.T.. Optical experiments were conducted by S.S. with the support of M.T.. S.S. carried out nanomanipulation experiments analyzed the data and wrote the manuscript through contributions of all authors. M.N. supervised the entire project. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. Funding Sources This work has been supported by the JSPS KAKENHI Grant Number 15H05735. ABBREVIATIONS NW, nanowire; PhC, photonic crystal; 3D-FDTD, three-dimensional finite-difference timedomain calculations. REFERENCES


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(8) Sergent, S.; Takiguchi, M.; Tsuchizawa, T.; Yokoo, A.; Taniyama, H.; Kuramochi, E.; Notomi, M. Nanomanipulating and tuning ultraviolet ZnO-nanowire-induced photonic crystal nanocavities. ACS Photon. 2017, 4, 1040. (9) Yokoo, A.; Takiguchi, M.; Birowosuto, M. D.; Tateno, K.; Zhang, G.; Kuramochi, E.; Shinya, A.; Taniyama, H.; Notomi, M. Subwavelength nanowire lasers on a silicon photonic crystal operating at telecom wavelengths. ACS Photon. 2017, 4, 355. (10) Takiguchi, M.; Yokoo, A.; Nozaki, K.; Birowosuto, M. D.; Tateno, K.; Zhang, G.; Kuramochi, E.; Shinya, A.; Notomi, M. Subwavelength nanowire lasers on a silicon photonic crystal operating at telecom wavelengths. APL Photon. 2017, 2, 046106. (11) Sergent, S.; Takiguchi, M.; Taniyama, H.; Kuramochi, E.; Notomi, M. Design of nanowireinduced nanocavities in grooved 1D and 2D SiN photonic crystals for the ultra-violet and visible ranges. Opt. Express 2016, 24, 26792. (12) Birowosuto, M. D.; Yokoo, A.; Taniyama, H.; Shinya, A.; Kuramochi, E.; Takiguchi, M.; Notomi, M. Design for ultrahigh-Q position-controlled nanocavities of single semiconductor nanowires in two-dimensional photonic crystals. J. Appl. Phys. 2012, 112, 113106. (13) Sergent, S.; Taniyama, H.; Notomi, M. Design of nanowire-induced nanocavities in photonic crystal disks. Opt. Lett. 2017, 42, 5121. (14) Fujita, M.;. Baba, T. Microgear laser. Appl. Phys. Lett. 2002, 80, 2051. (15) Zhang, Y.; Hamsen, C.; Choy, J. T.; Huang, Y.; Ryou, J.-H.; Dupuis, R. D.; Lončar, M. Photonic crystal disk lasers. Opt. Lett. 2011, 36, 2704.


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For Table of Contents Use Only.


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Figure 1. a, Schematic representation of a NW-induced nanocavity in a PhC disk. b, Confinement principle of the NW-induced nanocavity. The local PBG is highlighted in pink while the purple line represents the donorlike cavity mode. c, Field energy of the donor-like mode for θNW = 90° and n = 36. The central inset is a vertical cross section at the center of the NW. d, Donor-like mode for θNW = 180° and n = 18. e, Acceptorlike mode and f, whispering-gallery-like mode for θNW = 90° and n = 36. 141x190mm (300 x 300 DPI)


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Figure 2. a, Fabrication process flow: 1. E-beam lithography of PhC disks in resist, 2. Transfer of PhC disks into a SiN on silicon layer by dry etching, 3. E-beam lithography of circular grooves, 4. Transfer of grooves into the SiN, 5. Wet underetching of the silicon substrate, 6. ZnO NW transfer, 7. Nanomanipulation and insertion into groove. b, SEM images of ZnO-NW-induced nanocavities in PhC disks presenting azimuthal numbers n = 36, 30, 24, and 18. The bended NWs are highlighted with white arrows for clarity. The scale factors are sf = 0.9, 0.95, 0.95 and 1. This translates into respective disk radii rdisk = 1.8, 1.59, 1.26, and 1 μm. The SEM image of a bended ZnO NW inserted into a 4 μm2 square membrane is also displayed. The horizontal scale bars correspond to 1 μm. The color-code of each azimuthal number will be used in following Figures. 106x190mm (300 x 300 DPI)


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Figure 3. a, Microphotoluminescence spectra of the NWs inserted in a square membrane (black line) and in a PhC disk with n = 36 and sf = 0.9 (pink line). The investigated NWs are displayed in Figure 2b. The upper panel shows the intensity enhancement ratio for each mode of the nanocavity disk. b, Close-up of the donor-like mode observed in panel a. The black line is a Voigt fitting to the experimental data (pink dots) taking into account the Gaussian broadening of the experimental setup. c, Projections of the far-field intensity of the donor-like, acceptor-like and whispering-gallery-like modes presented in Figure 1c, 1e and 1f. The thick inner circle indicates the collection cone of the microscope objective used in the optical experiments (N.A. = 0.42). Collection efficiencies are respectively 1%, 3.3% and 0.5%. d, Microphotoluminescence intensity map of the donor-like mode featured in panels a and b. The step distance is 0.5 μm. e, The upper panel represents the in-plane components of the electric field as calculated by 3DFDTD. The lower panel displays the polarization measurements of the donor-like mode (pink dots) and the NW positioned in a square membrane (gray dots). 216x190mm (300 x 300 DPI)


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Figure 4. a, Scaling factor, b, quality factor and c, intensity enhancement ratio of donor-like modes as a function of wavelength and as observed by microphotoluminescence for various azimuthal numbers. Open circles in panel b represent the quality factor of whispering-gallery-like and acceptor-like modes for all cavities. The gray spectrum in panel b corresponds to the emission of the NW inserted in a square membrane and displayed in Figure 2b. The dashed line highlights the expected quality factor of the donorlike mode, taking into account (i) intrinsic losses as calculated by 3D-FDTD, (ii) absorption losses as derived from measured absorption in bulk ZnO [20] and (iii) no scattering losses. Open circles in panel c represent typical intensity enhancement factor obtained in ZnO-NW-induced nanocavities in 2D SiN PhCs extracted from [8]. 179x190mm (300 x 300 DPI)


ZnO-Nanowire-Induced Nanocavities in Photonic Crystal Disks - ACS

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