Nanomanipulating and Tuning Ultraviolet ZnO ... - ACS Publications

Letter pubs.acs.org/journal/apchd5

Nanomanipulating and Tuning Ultraviolet ZnO-Nanowire-Induced Photonic Crystal Nanocavities Sylvain Sergent,*,†,‡ Masato Takiguchi,†,‡ Tai Tsuchizawa,†,§ Atsushi Yokoo,†,‡ Hideaki Taniyama,†,‡ Eichi Kuramochi,†,‡ and Masaya Notomi*,†,‡ †

Nanophotonics Center, ‡NTT Basic Research Laboratories, and §NTT Device Technology Laboratory, NTT Corp., 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan S Supporting Information *

ABSTRACT: We report on the fabrication, nanomanipulation, and optical properties of ZnO-nanowire-induced nanocavities in grooved SiN photonic crystals. We show that subwavelength ZnO nanowires supporting intrinsically no Fabry−Pérot mode in the violet and near-ultraviolet range can induce optical confinement when introduced in a grooved twodimensional photonic crystal waveguide. Despite fabrication challenges arising at such short wavelengths, this hybrid approach leads to fundamental nanocavity modes with resolution-limited quality factors larger than Qexp = 2.1 × 103 3 SiN 3 at λ = 403 nm for a mode volume Vm = 5.9(λ/nNW r ) = 3.4(λ/nr ) , as deduced from three-dimensional finite-difference timedomain calculations. The investigation of optical losses in our system shows that at wavelengths shorter than λ = 390 nm Qexp is limited by self-absorption, indicating a good nanowire to cavity coupling. These results validate our hybrid approach as an efficient way to circumvent the processing issues that were so far preventing the insertion of ZnO emitters in photonic crystal nanocavities. Furthermore, we demonstrate that the degree of freedom along the groove can be used to move nanowire-induced nanocavities in space, position them deterministically, and tune their optical properties in the near-ultraviolet range. This striking feature opens the path toward the realization of versatile nanophotonic devices including movable and tunable all-dielectric NW nanolasers operating at high temperature. KEYWORDS: nanowire, nanocavity, nanomanipulation, photonic crystal, zinc oxide, silicon nitride, ultraviolet

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CdSe NWs embedded in SiN PhC waveguides with quality factors of up to 5 × 103.12 Alternatively, our group has proposed a NW-induced nanocavity design based on the integration of subwavelengths NWs in grooved silicon photonic crystals.13 It is a very versatile platform, as it can virtually be realized with various NW materials, it allows moving the nanocavity in space along the groove, and thus it enables its deterministic positioning. This approach has been implemented with InAsP/InP NWs, resulting in experimental quality factors as high as Qc = 9.5 × 103 at telecommunication wavelengths and leading to the demonstration of both the Purcell effect14 and continuous-wave lasing.15,16 The main limitation of the platform stems from the absorption cutoff of silicon that prevents NWs emitting at wavelengths shorter than 1 μm from being used. To circumvent such a limitation, we have proposed using silicon nitride instead of silicon, and we have theoretically shown that appropriate structural designs enable NW-induced nanocavity formation in SiN PhCs and lead to theoretical quality factors in the 104 to 105 range despite the much smaller refractive index of SiN.17

emiconductor nanowires (NWs) have been at the center of extensive research for the realization of advanced photonic devices such as efficient single-photon sources1,2 and nanolasers.3−9 Relatively thick NWs presenting intrinsic optical Fabry−Pérot modes have for example been used to realize nanolasers operating at up to room temperature.3−6 However, the realization of lower threshold nanolasers and the demonstration of cavity quantum electrodynamics effects require the hybrid integration of subwavelength NWs into nanocavities presenting a high quality factor on mode volume ratio. A large number of works have already reported the coupling of subwavelength NWs to plasmonic nanocavities and demonstrated lasing,7−9 but plasmonic nanocavities intrinsically suffer from significant absorption losses related to the presence of metal and to a poor overlap between the cavity mode and the gain medium. Such impediments can be alleviated by working instead with subwavelength NWs coupled to all-dielectric photonic crystal (PhC) nanocavities. Hybrid integration of subwavelengths NWs in PhC cavities has been reported in the green range with long CdS NWs embedded in modulated onedimensional PMMA PhCs, which did not lead to any measurable quality factor,10 in the ultraviolet range with a GaN NW vertically embedded in a thick TiO2 PhC cavity with a quality factors around 102,11 and in the red range with short © 2017 American Chemical Society

Received: February 6, 2017 Published: May 1, 2017 1040

DOI: 10.1021/acsphotonics.7b00116 ACS Photonics 2017, 4, 1040−1047

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Figure 1. (a) Schematic representation of the NW-induced nanocavity highlighting the main design parameters. (b) Confinement principle of the NW-induced cavity featuring the band diagrams of both the grooved waveguide and the nanocavity calculated by 3D-FDTD for t = 0.7a, r = 0.3a, wWG = 0.85a√3, d = 0.45a, w = 0.45a, δ = 0.45a, and L = 20a. The normalized frequency of the fundamental cavity mode is a/λ = 0.436. The white dot-dashed lines correspond to the light line. The horizontal dashed lines correspond to the edges of the photonic band gap. Vertical arrows highlight the gap between modes guided in the NW and in the groove. (c) Electric field of the fundamental nanocavity mode for the x and z polarization, as calculated by 3D-FDTD with identical parameters. The quality factor is Qth = 2.6 × 104 for a mode volume Vm = 5.9(λ/nrNW)3 = 3.4(λ/nrSiN)3.

Figure 2. (a) Schematic representation of the fabrication process flow: 1. Chemical vapor deposition of SiN on a Si(100) substrate; 2. Electron-beam lithography of alignment marks; 3. Transfer of alignment marks in SiN and Si by successive CHF3/CH4/O2 and SF6 reactive ion etching; 4. Electronbeam lithography of PhC waveguides; 5. C2F6/SF6 dry etching of SiN PhC waveguides; 6. Electron-beam lithography of grooves; 7. C2F6/SF6 dry etching of grooves; 8. Undercut of the SiN PhC by KOH etching of the Si substrate; 9. Transfer of ZnO NWs onto the SiN surface; 10. AFM nanomanipulation of NWs. (b) 5 × 5 μm2 AFM scan of a NW after ZnO NW dispersion. The PhC lattice constant is a = 166 nm, the NW length is L = 1.8 μm, and its diameter is δ = 63 nm. (c) 5 × 5 μm2 AFM scan of the NW shown in panel b after nanomanipulation and insertion in the groove. Panels d to k are scanning electron microscope images of fabricated nanocavities with respective lattice constants of 181, 178, 172, 169, 166, 163, and 160 nm. Panel i corresponds to the nanocavity shown in panel c. The white scale bars correspond to 1 μm. The geometric characteristics of this nanocavity subset are summed in Table 1.

In this Letter, we implement the NW-induced nanocavity design proposed in ref 17 by positioning single ZnO NWs in grooved SiN two-dimensional PhC waveguides. ZnO is chosen for three reasons: (i) the large oscillator strength and exciton binding energy that makes the semiconductor attractive for high-temperature cavity quantum electrodynamics, (ii) the absence of an established process for the fabrication of ZnO

photonic crystal nanocavities, which induces the necessity for the demonstration of a compatible hybrid approach, and (iii) the challenging ultraviolet emission range that will help us probe the quality of our PhC fabrication process. In the following, we demonstrate that subwavelength ZnO NWs exhibit optical confinement in the violet and near-ultraviolet range only when placed in grooved SiN PhC waveguides, thus 1041


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Table 1. Characteristics of Fabricated Nanocavities Investigated in Figure 2d−k: Lattice Constant, Hole Radius, NW Length, NW Diameter, Fundamental Resonance, and Experimental Quality Factor a [nm] r/a L [μm] δ [nm] λ0 [nm] Qexp × 10−3

181

178

175

172

169

166

163

160

0.27 2.2 64 420.1 1.8

0.28 2.1 58 419.9 1.9

0.32 2.4 65 415.3 1.8

0.3 3.5 78 403.1 2.1

0.29 3.1 55 406.1 1.7

0.31 1.8 63 393.5 1.4

0.28 2.4 77 387.5 1.0

0.29 2.7 58 385.4 1.0

Figure 3. (a) μPL spectra of the NW-induced nanocavities displayed in Figure 2d−k. The color scheme indicates the lattice constant of investigated nanocavities and applies for the whole figure. The black curve corresponds to the spectrum of the NW displayed in Figure 2b, before introduction in the groove. The vertical axis has a logarithmic scale. The spectra are vertically offset for clarity. The integration time varies from spectrum to spectrum. (b) Polarization properties of the NW emission (open circles) and the fundamental mode (closed circles) for the nanocavity investigated in Figure 2c,i andFigure 3a with a = 166 nm. The vertical bar indicates the orientation of the NW. The dashed lines are fitting curves of the form I(θ) = Imax cos2(θ − θ0) + Imin sin2(θ − θ0), with I the integrated intensity, θ the transmission angle of the polarization analyzer, θ0 the polarization angle of the considered signal, and Imax and Imin the highest and lowest values of I. The degree of linear polarization and the polarization orientation are respectively (Imax − Imin)/(Imax + Imin) = 0.07 and θ0 = 85° for the NW emission and (Imax − Imin)/(Imax + Imin) = 0.63 and θ0 = −2° for the fundamental mode. (c) μPL mapping of the fundamental and first-order mode for the nanocavity investigated in Figure 2c,i and Figure 3a with a = 166 nm. (d) Fundamental mode wavelength versus lattice constant for all investigated nanocavities. The dashed line is a linear fitting to the data. (e) Experimental quality factor of all investigated nanocavities as a function of the resonant wavelength. Closed and open circles correspond respectively to fundamental and higher order modes. The gray curve is the μPL spectrum of a NW outside the groove. The black dashed line and the red dotted line correspond respectively to the resolution limit and to absorption losses. The purple dot-dashed line corresponds to the maximal measurable quality factor taking into account theoretical losses, absorption losses, and the resolution limit. Panels f and g are μPL spectra of fundamental modes arising from two different nanocavities with a = 172 nm. Panel f corresponds to the light green spectrum given in panel a and emerging from the nanocavity displayed in Figure 2g. Spectra of panels f and g respectively exhibit resolution-limited quality factors Qexp = (2.1 ± 0.1) × 103 and Qexp = (2.0 ± 0.1) × 103. The green dots correspond to experimental data, and the black line is a Voigt fitting curve. The Gaussian component of the fitting curve is fixed at the resolution limit of the setup, 154 pm. The Lorentzian component of the fitting gives an estimation of the actual quality factors, respectively QLorentzian = (6.6 ± 2) × 103 and QLorentzian = (5.1 ± 0.5) × 103.

(3D-FDTD). We finally show that such NW-induced cavities can be manipulated and deterministically positioned, and, going beyond the seminal work by Birowosuto et al.,14 we take advantage of these characteristics to control both the resonant wavelength and the optical losses of the nanocavity. The design we consider here is represented in Figure 1 and is constituted of a single ZnO NW placed in a grooved two-

proving that this platform effectively circumvents the limitations of silicon. We assess the origin of losses in such a system, and despite the PhC fabrication difficulties that we face at short wavelength, we successfully realize resolution-limited quality factors higher than Qexp = 2.1 × 103 at λ = 403 nm for a 3 SiN 3 mode volume Vm = 5.9(λ/nNW r ) = 3.4(λ/nr ) , as calculated by three-dimensional finite-difference time-domain simulations 1042


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the overall ZnO NW μPL polarization is nearly isotropic, the fundamental mode turns out to present a high degree of linear polarization oriented perpendicularly to the NW axis (Figure 3b). This is consistent with the 3D-FDTD calculations of Figure 1 showing that more than 80% of the electric field overlapping with the NW is x-polarized. In addition, the μPL mapping of the photonic crystal confirms the origin of the nanocavity modes that both arise from the NW (Figure 3c). Resonant modes are also found for the nanocavity subset displayed in Figure 2e−k (see μPL spectra in Figure 3a) and for all 40 nanocavities investigated here with resonances ranging from λ = 379 nm to λ = 430 nm (Figure 3d and e). An important result further proving the NW-induced nanocavity formation is shown in Figure 3d. Here, we plot the resonant wavelength of the fundamental mode as a function of the lattice constant. Although the data are somewhat scattered, the overall tendency clearly shows that the resonant wavelengths are mostly proportional to the lattice constant, as expected for PhC-based nanocavities. The data variations in Figure 3d can be attributed to fluctuations in the geometric parameters of fabricated nanocavities (Table 1). In particular, fluctuation of the SiN slab thickness estimated as ±3% and variation in the hole radius tend to blur the trend. Furthermore, the fundamental resonance is largely influenced by the NW diameter,17 which suffers here from a broad dispersion. The experimental quality factors of the fundamental and higher order modes are reported in Figure 3e as a function of the resonant wavelength. The cavity mode line widths are estimated using a Voigt fitting with the Gaussian component broadening set as the estimated resolution limit γ = 154 pm. First, we observe a few fundamental cavity modes with line widths close to the resolution limit, which lead to quality factors as high as Qexp = (2.1 ± 0.1) × 103 at λ0 = 403.1 nm (Figure 3f) or Qexp = (2.0 ± 0.1) × 103 at λ0 = 409.3 nm (Figure 3g). After deconvolution of the Voigt fitting, the Lorentzian component of the fitting gives substantially higher estimates of the intrinsic quality factor QLorentzian = (6.6 ± 2) × 103 and QLorentzian = (5.1 ± 0.5) × 103, showing that the measured line widths are here resolution limited. Second, the dispersion of experimental quality factors at a given wavelength suggests that fluctuations in fabrication imperfections are significant, as can be inferred from scanning electron microscope images of fabricated NWinduced nanocavities (Figure 2e−k). As reported in a previous paper17 and further investigated for the specific design considered here (Figure S2 in the Supporting Information), such imperfections can be attributed to various sources of asymmetry. Along the x axis, asymmetry arises from misalignment between the center of the waveguide and the center of the groove and misalignment between the center of the groove and the center of the NW and can individually bring the quality factor down to the low 103 range. Along the y axis, the nanocavity is intrinsically asymmetric, but mismatch between the groove depth and the NW diameter also tends to increase losses, leading to quality factors in the 103 range. The considered NWs may also present small defects on their sidewalls and sometimes a limited taper along the NW axis, which are sources of scattering losses and can bring the quality factor down to the 103 range. Finally, although induced losses are more difficult to evaluate, membrane and NW bending observed in Figure 2b−k does break the symmetry of the system and is therefore a likely source of mode propagation losses. Let us note that horizontal and vertical misalignments between the center of the NW and the center of the groove as

dimensional SiN PhC line-defect waveguide with air holes. It is a so-called mode-gap modulation cavity, but contrary to most designs,18−20 the light confinement does not arise from a structural modulation of the PhC. Instead, the refractive index modulation creates a gap between the mode guided in the NW and the mode guided in the grooved PhC (Figure 1b), which allows the light to be confined in the NW, the grooved waveguide acting as a barrier.13,17 The design is similar to those described in ref 17, but we consider here a notably smaller PhC slab thickness t = 0.7a for three reasons: (i) reducing the nanocavity mode volume; (ii) facilitating the PhC processing; and (iii) increasing the normalized frequency a/λ of the cavity mode, thus the PhC lattice constant needed to reach the nearultraviolet range where ZnO NWs emit. The hole radius is r = 0.3a, and the center of the innermost holes on opposite sides of the waveguide is separated by wWG = 0.85a√3. The groove is placed at the center of the waveguide with depth d = 0.45a and width w = 0.45a. A ZnO NW with a circular section, a diameter = 2.4 is δ = 0.45a, a length L = 20a, and a refractive index nZnO r positioned at the center of the groove and induces a fundamental cavity mode at the normalized frequency a/λ = 0.436 (Figure 1b), as calculated by 3D-FDTD. The fundamental cavity mode is represented in Figure 1c and exhibits an intrinsic quality factor as high as Qth = 2.6 × 104 for 3 SiN 3 a mode volume Vm = 5.9(λ/nNW r ) = 3.4(λ/nr ) . We implement the design in a 108-nm-thick SiN slab grown by chemical vapor deposition on a silicon(100) substrate.21 The 10 steps of the fabrication process flow are represented in Figure 2a. The photonic structures are first defined in the SiN slab through three electron-beam lithography and dry etching steps for the successive fabrication of alignment marks, PhCs, and grooves. The photonic membranes are then released by wet KOH underetching of the silicon substrate. Commercially available ZnO NWs provided by Novarials and presenting circular sections are then dispersed on the SiN surface. The NWs are located on the surface by atomic force microscopy (AFM), as shown in Figure 2b, manipulated on the SiN surface by the AFM cantilever, and placed inside the groove (Figure 2c). When needed, the AFM cantilever is used to clean the extra ZnO NW and dust lying in the groove vicinity. The process is completed for 40 different nanocavities with lattice constants varying from a = 160 nm to a = 181 nm, NW diameters ranging from δ = 50 nm to δ = 90 nm, and NW lengths varying from L = 1.7 μm to L = 4.9 μm. A subset of such nanocavities is exhibited in Figure 2d−k with their characteristics summed up in Table 1. The optical properties of NW-induced nanocavities are investigated by microphotoluminescence (μPL) at room temperature. The μPL spectrum of the NW of Figure 2b positioned on top of a SiN PhC with lattice constant a = 166 nm presents an emission peak at 375 nm with a long lowenergy tail corresponding to the room-temperature thermal broadening and longitudinal-optical phonon replica22 (black curve in Figure 3a). Because of its small diameter, the investigated NW does not intrinsically support optical modes (Figure S1 in the Supporting Information), and as a result, the spectrum exhibits no modulation arising from Fabry−Pérot resonances. As the NW does not couple efficiently to the underlying SiN PhC, no feature from the photonic band edges emerges from the spectrum. Once introduced in the groove, however, the μPL spectrum presents sharp peaks at λ0 = 394 nm and λ1 = 386.7 nm (light blue curve in Figure 3a), in good agreement with 3D-FDTD calculations of Figure 1. Whereas 1043


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Figure 4. (a) Scanning electron microscope image of grooved SiN PhCs after NW transfer with lattice constants a = 169 nm, a = 166 nm, and a = 163 nm. Dashed squares indicate the limits of the AFM scan area shown in panels b to f before and after NW nanomanipulation. The white scale bar corresponds to 5 μm. (b) AFM scan of a single ZnO NW positioned on the edge of a PhC prior to nanomanipulation. Its length is L = 3.2 μm and its diameter is δ = 70 nm. The dimensions of this scan and the following are 5 × 5 μm2. (c) AFM scan of the same NW positioned by AFM nanomanipulation next to the groove of the PhC with a = 169 nm. Panels d, e, and f are AFM scans of the same NW placed inside the groove and translated over PhC waveguides with respective lattice constants a = 169 nm, a = 166 nm, and a = 163 nm. (g) Normalized μPL spectra of the ZnO NW at the positions shown in panels c, d, e, and f. The vertical axis has a logarithmic scale, and spectra are vertically offset for clarity. (h) Resonant wavelength of the fundamental and higher order modes versus lattice constant for the μPL spectra shown in panel g. The dashed lines are linear fitting to the data. (i) Experimental quality factor versus lattice constant for the μPL spectra shown in panel g. The mode order is indicated above each data point. (j) Intensity enhancement of the fundamental mode as a function of the experimental quality factor for the three nanocavity spectra presented in panel g. The dashed line is a linear fitting to the data.

the absorption losses using the absorption coefficient values measured by Srikant et al. for a bulk ZnO grown on c-plane sapphire24 (red dotted line in Figure 3e). We can finally calculate the maximum measurable quality factor Qmax (purple dot-dashed line in Figure 3e), taking into account the theoretical losses Qth−1, absorption losses Qabs−1, and the resolution limit of the experimental setup. In other words, Q−1 max −1 −1 corresponds to the calculated optical losses Qabs + Qth convolved with the optical setup resolution. Qmax fairly fits the sharp quality factor decrease at shorter wavelengths as well as the highest quality factors obtained at longer wavelengths. One can nevertheless see that the actual absorption cutoff of our NW-induced nanocavities is red-shifted by a few nanometers as compared to the cutoff deduced from the bulk ZnO absorption data. This can be accounted for by the tensile strain measured in bulk ZnO grown on sapphire,24 which is expected to be higher than in NWs and induces a small band gap blue shift. It is worth highlighting that self-absorption limiting the quality factor for λ < 390 nm confirms that the coupling between the nanocavity mode and the NW inferred from 3DFDTD calculations is not impaired by side issues such as NW polarization or fabrication disorder. Such a significant coupling is exactly what is desired for the development of nanophotonic devices. Now that we have successfully implemented the NWinduced nanocavity design in grooved SiN PhC waveguides, let us demonstrate its main benefits: the possibility to move the nanocavity along the groove, reposition it at will, and tune its optical properties. To that end, we consider a single groove

well as the impact of NW bending, defects, and tapers can be overcome by choosing defectless and taperless NWs with diameters matching the groove width and height. Next, we investigate Figure 3e in more detail at short wavelength, where one can observe a drastic decrease of the maximum quality factor as the resonant wavelength gets closer to the ZnO NW emission peak. Such a sharp increase in losses cannot be accounted for by scattering losses due to fabrication imperfections, as the latter tend to follow a 1/λ2 behavior.23 Besides, as the lattice constant decreases and the relative size of the groove and NW increases, one would expect a much smoother degradation of the quality factor as long as the modes guided in the grooved and NW structures remain within the PhC band gap.17 We rather attribute this quality factor degradation to the NW self-absorption. Absorption losses can be estimated by the following equation:

Q abs−1 =

αλ Γ 2πnrNW

with α the wavelength-dependent absorption coefficient and Γ the confinement factor of the electric field energy in the absorbing NW, i.e., Γ=

∑NW ∈r |E|2 ∑tot ∈r |E|2

with ϵr the position-dependent dielectric constant and E the position-dependent electric field. For the mode calculated in Figure 1, the confinement factor reaches Γ = 19%. We calculate 1044


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crossing three PhCs of lattice constants a = 169 nm, a = 166 nm, and a = 163 nm (Figure 4a) and a single NW with a length L = 3.2 μm and a diameter δ = 70 nm (Figure 4a and b). The NW is manipulated by an AFM cantilever on the SiN surface and brought on top of the PhC of lattice constant a = 169 nm (Figure 4c). It is then moved along the groove (Figure 4d) and translated over 36 μm through the PhCs with lattice constants a = 169 nm, a = 166 nm (Figure 4e), and a = 163 nm (Figure 4f). Details of the NW nanomanipulation from its original position to the PhC with lattice constant a = 166 nm can be found in the Supporting Information. As the NW is moved in space, we probe its optical properties by μPL (Figure 4g−j). In agreement with the results of Figure 3, the spectrum does not exhibit any optical confinement when the NW is placed outside the groove. Once the NW is introduced in the grooved waveguide with a = 169 nm, nanocavity resonances appear with a fundamental mode at λ0 = 402.7 nm. As the NW is translated to the PhC waveguides with shorter lattice constants, the nanocavity resonances are blue-shifted with the fundamental mode appearing at λ0 = 396.8 nm for a = 166 nm and at λ0 = 390.9 nm for a = 163 nm. As represented in Figure 4h, this relationship between the lattice constant and the mode wavelength is observed not only for the fundamental mode but also for higher order modes. Such a result is essentially similar to Figure 3d, but we here confirm the linear trend for an identical NW, in a configuration bypassing most of the data noise that emerges from geometric fluctuations in Figure 3d. In addition to the fundamental and higher order modes identified in Figure 4h, let us mention that one can also observe broader resonances (Figure 4g) that may be attributed to unintended localization due to PhC disorder. Furthermore, through NW nanomanipulation, not only are we able to deterministically position the nanocavity and tune its resonant wavelength, we can also in turn control the cavity to NW coupling and adjust absorption losses (Figure 4i). As a result, as we shorten the lattice constant surrounding the NWinduced nanocavity, the spectral overlap between the cavity mode and the NW absorption increases and the experimental quality factor of the fundamental mode decreases from Qexp = (1.8 ± 0.1) × 103 at λ0 = 402.7 nm down to Qexp = (1.4 ± 0.1) × 103 at λ0 = 396.8 nm and Qexp = (1.2 ± 0.1) × 103 at λ0 = 390.9 nm, in good agreement with the absorption cutoff curve inferred in Figure 3e. Simultaneously, not only does the line width of higher order cavity modes tend to broaden as the lattice constant shortens, but the third-order mode even disappears for a = 163 nm, demonstrating maximal overlap with the NW absorption and confirming the absorption cutoff evidenced in Figure 3e. Note that for a = 169 nm the first- and second-order modes show some deviation from that trend due to accidental mixing with the broader resonances that we mentioned earlier. Finally, it is worth mentioning that the change of quality factor of these single-NW-induced nanocavities gives us some insight into the cavity quantum electrodynamics effect at play in our system. The small change in lattice constant is expected to have a negligible impact on the nanocavity mode volume or the extraction efficiency. As a result, variations in the intensity enhancement observed in our system are essentially defined by the Purcell effect Fp ∝ Qexp/Vm, which in turn should mainly depend on the quality factor. This is in good agreement with the observation of an intensity enhancement proportional to the experimental quality factor, as shown in Figure 4j. One should however be cautious with the latter result because

beyond the limited effect of the lattice constant, extraction efficiency of the cavity mode may be affected by the photonic membrane bending that can be observed in Figure 4d−f. As a conclusion, we have been able to realize ZnO-NWinduced nanocavities in grooved SiN PhCs exhibiting resolution-limited quality factors and small mode volumes. With quality factors higher than Qexp = (2.1 ± 0.1) × 103 at λ0 = 403.1 nm, such NW-induced nanocavities outperform previous PhC nanocavities embedding NWs as reported in the green10 and the ultraviolet12 ranges. Our result also extends the use of SiN for hybrid PhC nanocavities from the previously reported green25 down to the near-ultraviolet range: it proves that SiN absorption is not a limiting factor in that configuration and that the processability of SiN is well adapted to the near-ultraviolet range despite the PhC fabrication challenges that arise at such short wavelengths. Furthermore, besides an early work on Anderson localization in disordered two-dimensional ZnO PhCs,26 embedding ZnO emitters in photonic crystal nanocavities has not been reported due to the difficulty in processing ZnO: the present results incidentally show that the hybrid approach followed here is an efficient way to circumvent such processing issues. More importantly, we would like to emphasize that controlling both the wavelength and the optical losses of the NW-induced nanocavity through nanomanipulation goes beyond the original work by Birowosuto et al.14 and constitutes the most striking feature of this system: it opens the path to the demonstration of cavity quantum electrodynamics effects with deterministically positioned emitters and the realization of various versatile nanophotonic devices including movable and tunable all-dielectric NW nanolasers. It is worth noting however that the ZnO NWs used in this work are not optimal for such applications because of the nonradiative recombinations of carriers at the NW surfaces: a ZnMgO shell structure would be needed to minimize such a detrimental effect. Finally, because of its hybrid nature, it is worth noting that our nanocavity design can be implemented beyond ZnO NWs, making it a unique multimaterial nanophotonic platform. In particular, since PhC fabrication issues have already been overcome for the challenging near-ultraviolet range, it should easily allow for the realization of NW-induced nanocavities with materials emitting at longer wavelengths, across the visible and near-infrared ranges that were not accessible with the silicon PhC platform realized by Birowosuto et al.,14 e.g., InGaN/GaN NWs,27 diamond NWs,28 GaP NWs,29 or lead halide perovskite NWs.30

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METHODS 3D-FDTD Calculations. Three-dimensional finite-difference time-domain calculations are carried out using our home-built algorithm. The spatial resolution is set to σ = a/ 20, a being the lattice constant, and the calculation domains along the x, y, and z axes are respectively 450σ, 300σ, and 1200σ. The magnetic field component |Hy| is first calculated using a punctual broadband TE-like light source. Band diagrams are then extracted by a Fourier transform of |Hy|. Resonant mode electric field profiles are finally calculated by exciting the resonant wavelength using a narrow-band light source. Resulting quality factors are deduced from the energy decay of the resonant mode. Mode volumes are obtained from the electric field profile, following Vm =

∑ ∈r |E|2 ∈r |Emax |2

with ϵr the

position-dependent dielectric constant and E the position1045


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dependent electric field. The refractive indices used for SiN and ZnO are respectively nSiN = 2.0 and nZnO = 2.4. r r Fabrication. The SiN layer is deposited on a Si(100) substrate at approximately 200 °C in an electron-cyclotronresonance plasma-enhanced chemical vapor deposition reactor. Si atoms are provided by a hydrogen-free precursor gas introduced directly in the deposition chamber, whereas N2 is introduced in the plasma chamber as a source of N atoms. After deposition, a 200-nm-thick ZEP-520 positive-tone resist is spincoated on the SiN layer, and the PhC patterns are defined by electron-beam lithography, using a 100 kV acceleration voltage and a 0.6 nA current. The base dose level is 240 μC/cm2, no systematic proximity effect correction is applied, and the dose level is optimized as a function of the pattern size. The patterns are then developed in a ZED-N50 solution provided by Zeon Chemicals. The PhC patterns are subsequently transferred to the SiN layer by capacitively coupled plasma reactive ion etching using a C2F6/SF6 gas mixture. The SiN photonic membranes are released by wet etching of the silicon substrate in a wet KOH/H2O (1:5) solution at 60 °C for 15 min. Novarials NWs are provided as ∼1 cm2 sheets of bundled NWs. We put the NW sheet in an 2-propanol (IPA) solution and separated the NWs within the solution by ultrasonication. We then transferred the NWs by spin-coating the solution on the SiN PhCs and evaporated the residual IPA on a hot plate at 200 °C for 3 min. The density of ZnO NWs transferred on the SiN surface can be controlled via the relative amount of NWs in solution as well as the spin-coating speed. For the AFM imaging and AFM manipulation, we use silicon cantilevers respectively in tapping and contact modes. The cantilevers have an 8 nm tip radius and a 42 N/m spring constant. Optical Measurements. NWs are excited at Pexc = 10 μW with a frequency-tripled pulsed laser emitting at 266 nm and operating at an 80 MHz rate. The laser is focused with a Mitsutoyo microscope objective dedicated to the ultraviolet range with a 50-fold magnification and a 0.42 numerical aperture. The emitted signal is collected in a confocal configuration, coupled to a multimode fiber, dispersed on a 1200 grooves/mm grating and recorded with a charge-coupled device camera. Polarization properties are investigated by adding a polarization analyzer on the signal collection path, just before the fiber. The resolution setup in the considered wavelength range is 154 pm.

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Author Contributions

S.S. carried out FDTD calculations with the support of H.T., performed the nanomanipulation experiments with the assistance of A.Y., and conducted the optical experiments with the support of M.T. Fabrication of SiN photonic crystal membranes was performed by S.S. and T.T. with the assistance of M.T. and E.K. S.S. analyzed the data and wrote the manuscript through contributions of all authors. M.N. conceived and supervised the entire project. All authors have given approval to the final version of the manuscript. Funding

This work has been supported by the JSPS KAKENHI Grant Number 15H05735. Notes

The authors declare no competing financial interest.

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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.7b00116. Additional calculation and optical measurement data (PDF) Nanomanipulation data (AVI)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Sylvain Sergent: 0000-0001-9604-9455 Masato Takiguchi: 0000-0001-6294-905X Atsushi Yokoo: 0000-0002-8296-2136 Eichi Kuramochi: 0000-0002-6284-823X 1046


ACS Photonics

Letter

crystal operating at telecom wavelengths. ACS Photonics 2017, 4, 355− 362. (16) Takiguchi, M.; Yokoo, A.; Nozaki, K.; Birowosuto, M. D.; Zhang, G.; Tateno, K.; Kuramochi, E.; Shinya, A.; Notomi, M. Continuous-wave operation and 10-Gb/s direct modulation of InAsP/ InP sub-wavelength nanowire laser on silicon photonic crystal. APL Photonics 2017, 2, 046106. (17) Sergent, S.; Tachiguchi, M.; Taniyama, H.; Shinya, A.; Kuramochi, E.; Notomi, M. Design of nanowire-induced nanocavities in grooved 1D and 2D SiN photonic crystals for the ultra-violet and visible ranges. Opt. Express 2016, 24, 26792−26808. (18) Lalanne, P.; Sauvan, C.; Hugonin, J. P. Photon confinement in photonic crystal nanocavities. Laser Photonics Rev. 2008, 2, 514−526. (19) Kuramochi, E.; Notomi, M.; Mitsugi, S.; Shinya, A.; Tanabe, T. Ultrahigh-Q PhC nanocavities realized by the local width modulation of a line defect. Appl. Phys. Lett. 2006, 88, 041112. (20) Song, B.-S.; Noda, S.; Asano, T.; Akahane, Y. Ultra-high-Q photonic double-heterostructure nanocavity. Nat. Mater. 2005, 4, 207−210. (21) Yamada, K.; Tsuchizawa, T.; Nishi, H.; Kou, R.; Hiraki, T.; Takeda, K.; Fukuda, H.; Ishikawa, Y.; Wada, K.; Yamamoto, T. High performance silicon photonics technology for telecommunications applications. Sci. Technol. Adv. Mater. 2014, 15, 024603. (22) Voss, T.; Bekeny, C.; Wischmeier, L.; Gafsi, H.; Börner, S.; Schade, W.; Mofor, A. C.; Bakin, A.; Waag, A. Influence of excitonphonon coupling on the energy position of the near-band-edge photoluminescence of ZnO nanowires. Appl. Phys. Lett. 2006, 89, 182107. (23) Minkov, M.; Dharanipathy, U. P.; Houdré, R.; Savona, V. Statistics of the disorder-induced losses of high-Q photonic crystal cavities. Opt. Express 2013, 21, 28233−28245. (24) Srikant, V.; Clarke, D. R. Optical absorption edge of ZnO thin films: the effect of substrate. J. Appl. Phys. 1997, 81, 6357. (25) Mukherjee, I.; Hajisalem, G.; Gordon, R. One-step integration of metal nanoparticle in photonic crystal nanobeam cavity Opt. Opt. Express 2011, 19, 22462−22469. (26) Wu, X.; Yamilov, A.; Liu, X.; Li, S.; Dravid, V. P.; Chang, R. P. H.; Cao, H. Ultraviolet photonic crystal laser. Appl. Phys. Lett. 2004, 85, 3657. (27) Bertness, K. A.; Sanford, N. A.; Davydov, A. V. GaN nanowires grown by molecular beam epitaxy. IEEE J. Sel. Top. Quantum Electron. 2011, 17, 847−858. (28) Babinec, T. M.; Hausmann, B. J. M.; Khan, M.; Zhang, Y.; Maze, J. R.; Hemmer, P. R.; Lončar, M. A diamond nanowire single photon source. Nat. Nanotechnol. 2010, 5, 195−199. (29) Assali, S.; Zardo, I.; Plissard, S.; Kriegner, D.; Verheijen, M. A.; Bauer, G.; Meijerink, A.; Belabbes, A.; Bechstedt, F.; Haverkort, J. E. M.; Bakkers, M. P. A. M. Direct band gap wurtzite gallium phosphide nanowires. Nano Lett. 2013, 13, 1559−1563. (30) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, V.; Trinh, M. T.; Jin, S.; Zhu, X.-Y. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nat. Mater. 2015, 14, 636−642.

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Nanomanipulating and Tuning Ultraviolet ZnO ... - ACS Publications

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