Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Weakly Tapered Silicon Nanopillar Resonators with Spatially WellSeparated Whispering Gallery Modes for Si-Based Lasers Myunghae Seo,† Kihyun Kim,† Hyeonsu Cho,† Sol Yoon,‡ Byoung Don Kong,*,‡ M. Meyyappan,§ and Chang-Ki Baek*,†,‡ Department of Creative IT Engineering and Future IT Innovation Lab. and ‡Department of Electrical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea § NASA Ames Research Center, Moffett Field, California 94035, United States Downloaded via 46.148.115.147 on August 5, 2019 at 15:04:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: A silicon-based laser is a critical component in the realization of full silicon (Si) photonic integrated circuits due to advantages in cost-competitive integration with the mature digital technology. The poor optical gain stemming from the indirect energy band structure of Si, however, has been a hurdle to realize this goal. An efficient Si nanocavity can become a key enabler to overcome the hurdle by earning the interaction time for ephemeral photons. Here, we propose a weakly tapered Si nanopillar whispering-gallery-mode resonator as a wavelength-scale Si cavity platform, which is suitable for lasing. We observe spatially well-separated and nondegenerate modes in the wide near-infrared spectral regime while maintaining a high quality factor. Mode analysis via experiments and simulations confirms spectrally and spatially well-separated resonant modes, suggesting that the diameter and slope of the Si nanopillar are crucial determinants for the formation and control of whispering gallery modes. Considering the versatile spectral tunability and selectivity with the high reproducibility, the proposed nanopillars can be a pathway that leads to compact Si-based lasers. KEYWORDS: silicon nanopillar resonators, silicon nanocavity, silicon-based lasers, whispering gallery mode, wavelength tunability, mode selectivity
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Er-doped Si lasers,9,10 and Ge-on-Si lasers.11,12 Recently, various hybrid structures of Si and two-dimensional materials such as graphene, MoTe2, and WSe2 have also been explored.13−16 All these attempts confirm and advocate the demand for a Si laser. Nevertheless, most of them still suffer from the poor emission efficiency that makes a compact Sicompatible high quality resonator2 a critical ingredient for the development of on-chip photonic devices as well as good resonators. Photonic crystals control the flow of light within a small periodic three-dimensional structure,17,18 but their complicated fabrication makes the approach less attractive.19 Si nanostructures with a size comparable to the optical wavelength have great potential to be an effective resonator
INTRODUCTION
Silicon (Si) photonics has been emerging as an important technology over the past decade thanks to the development of cost-effective, high performance, and compact photonic integrated circuits.1 However, efficient light emission in Si is difficult due to the ultralow radiative emission efficiency originating from the indirect nature of the material accompanied by unfavorable nonradiative effects such as free-carrier absorption and Auger recombination.2 Thus, realizing a Si laser has been a very challenging task but highly rewarding if successful. Many efforts have been reported on the fabrication of an all-Si laser. For instance, the stimulated Raman laser utilizes the effect of nonlinearity of light−matter interactions,3,4 and a quantum confinement effect in sub-10 nm Si nanowires has been shown to improve light emission.5,6 Combining Si with photonic materials with superior gain characteristics has resulted in III−V compounds-on-Si lasers,7,8 © XXXX American Chemical Society
Received: April 29, 2019 Accepted: July 10, 2019
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DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
Figure 1. SEM images of the fabricated asymmetric SiNPs: (a) negative SiNP; (b) positive SiNP. Each SiNP has a spacing of 4 μm to avoid optical interference from adjacent pillars. The scale bars represent 1 μm.
the same DT (∼1260 nm) and H (∼3200 nm). In addition, a sufficient spacing of 4 μm was assured between neighboring pillars to avoid spurious optical interferences by the adjacent pillars, which is presented in the top view SEM images of the SiNPs in Figure S1. The design parameters for this study were set to satisfy the Si gain frequencies and the PL measurement environment such as spot size (∼700 nm) and focal depth (∼2540 nm) of the pump laser. Detailed specifications of each SiNP are given in Table 1.
platform for this purpose. Recently, an inverted Si nanocone has been shown to harness abundant whispering gallery modes (WGMs) at near-infrared wavelengths, providing an enhanced Purcell factor.20 The WGM traps photons for a long time by topologically confining their propagation, resulting in very high quality factor (Q) even in a very small mode volume.21 Single mode excitation and wavelength-selective characteristics are crucial factors to obtain desirable WGMs. However, Si nanocones have shown disordered WGMs with high degeneracy among modes; this not only causes difficulties in mode analysis but also makes it impractical for use in lasing. In this work, we report weakly tapered Si nanopillars (SiNPs) that are capable of exciting controlled numbers of well-ordered WGMs and show these structures to be remarkably suitable for light-emitting when combined with gain media. Two types of asymmetric SiNPs were fabricated to investigate the impact of the structural factors of SiNP on the formation of WGMs. Spatially well-separated optical resonances were observed by measuring photoluminescence (PL). The detailed analysis of WGMs utilizing optical simulations revealed that the simple mode controllability resides in the relations between the diameter and slope of SiNP.
Table 1. Specifications of the Fabricated Asymmetric SiNPs type
DT (nm)
DB (nm)
H (nm)
θB (deg)
negative SiNP positive SiNP
1263 1257
1166 1403
3197 3205
89.13 91.31
Whispering Gallery Modes of Weakly Tapered SiNP. Optical resonances of each SiNP and the resulting WGMs were characterized by confocal photoluminescence (PL) measurements at room temperature (LabRAM HR-800, Horiba). The schematic of the experimental setup is provided in the Supporting Information (Figure S2). Figure 2a shows the measured normalized PL emission spectra featuring broad background spectra that originate from Si band edge emissions between 950 and 1250 nm25 in both positive and negative SiNPs. Strong resonant peaks, with intensities as high as 3.38 times (at 1102.20 nm) compared to that of the background spectrum, can be seen in the PL spectrum of the negative SiNP. This feature, absent in the positive SiNP, indicates strong optical confinement in the negative SiNP. Figure 2b shows schematic diagrams of mode formations in the SiNPs that illustrate this difference. Without tapering, there can be Fabry−Perot (FP) waveguide mode constructed by both ends (top and bottom) along the vertical direction of the SiNP if the nanopillars have a heterogeneous interface.21 In our case of homogeneous SiNP on Si substrate, this mechanism is not valid. The positive SiNP without any reflection layer cannot sustain the FP modes, and it is difficult to trap photons effectively along the vertical direction, resulting in no pronounced resonant peak. When negative tapering is introduced, photons encounter tapered walls and are reflected back into the SiNP, forming FP waveguide modes. These photons, confined by the FP modes, travel within the SiNP along the vertical direction for an extended duration,
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RESULTS Device Fabrication. SiNPs were fabricated on a 700 μm thick lightly doped Si wafer (⟨100⟩, p-type/Boron, 8−12 Ω· cm), with the optical bandgap22 and refractive index23 almost identical to those in intrinsic Si. A thermally grown 500 nm silicon dioxide layer, patterned by e-beam lithography, served as the hard mask for inductively coupled plasma reactive ion etching (ICP-RIE). Carefully selected HBr/Cl2/O2 mixtures were used in the ICP-RIE to obtain accurately controlled angles of the asymmetric SiNPs. More specifically, the top diameter DT of the SiNPs was defined by the lithographic pattern size, whereas the bottom diameter DB and the bottom angle θB were determined by managing the amount of O2 during the etch step, which is responsible for anisotropic sidewall etching of SiNP.24 The height H of the SiNPs was controlled by the etch duration (600 s). Two types of weakly tapered (θB ≅ 90 ∓ 1°) asymmetric SiNPs were fabricated: a negatively tapered (negative) with a smaller diameter at the bottom than at the top (Figure 1a) and a positively tapered (positive) with a smaller diameter at the top than at the bottom (Figure 1b). Both types were designed to have almost B
DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 2. Effect of weak tapering on the mode formation in the SiNPs. (a) Measured photoluminescence emission spectra. Red: negative SiNP. Blue: positive SiNP. Both spectra were normalized to the maximum intensity in the negative SiNP. The free spectral range of the negative SiNP measured at the resonant wavelength of 1102.20 nm is 85.14 nm. (b) Schematic diagrams of mode formations in the negative and positive SiNPs.
Figure 3. Optical simulations using three-dimensional FDTD simulation. (a) Normalized electric field intensity. Red: negative SiNP. Blue: positive SiNP. (b) Mode analysis of the resonances observed in the negative SiNP. WGMs are classified into four wavelength regions according to the mode number. (c) Field distributions in the xy-plane of the WGM (l = 1) formed in each region. TM modes were observed from the magnitude of electric field component EZ and TE modes from the magnitude of magnetic field component HZ. Note that a direct visual comparison of TM and TE modes field intensity is unsuitable. (d) The magnitude of electric field components EX and EZ of the TMl91 in the xz-plane indicated by an asterisk in (a). The resonant wavelengths correspond to 1080.19 nm (l = 5), 1090.25 nm (l = 4), 1101.60 nm (l = 3), 1114.32 nm (l = 2), and 1130.46 nm (l = 1). The black dotted lines indicate the contour of the negative SiNPs, and the white dotted lines indicate the z-position where the field intensity is maximum.
experiencing multiple reflections at the surface. In the meantime, each perimeter, which can be defined on any horizontal cross section of the SiNP with a different height, establishes another type of cavity mode, called WGMs,26 and the trapped light resonates with these modes. Our negative SiNP effectively combines these two cavity modes. Each WGM has a specific resonant frequency that is defined by the total internal reflection around the perimeter, and these resonance frequencies appear as the peaks in Figure 2a. More analysis about the coupling of FP modes and WGMs will be provided later with the aid of optical simulations. The diameter varies with the height in the tapered SiNPs. As such, many WGMs for different wavelengths can exist, which appear as multiple resonant peaks. If the diameter variation is
wide, i.e., a pillar has more radii per unit length, it can support more WGMs, resulting in many spectrally adjacent WGMs. This is nonideal for a resonator, and previously reported tapered Si nanocones display nearly indistinguishable and overlapping multiple peaks, making them impractical.20 The distinguishable and controllable WGMs in our negative SiNP here are accomplished by adopting the near minimal-tapered structures (−1°) and developing the process and design techniques to achieve this challenging task. The adopted weakly tapered structure limits the variation of the diameter along the height, while maintaining the coupling possibility between the FP modes and WGMs, resulting in spectrally wellseparated resonant peaks with wide frequency spacing over the gain wavelength range of Si. The free spectral range (FSR), i.e., C
DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 4. Diameter dependence of WGMs in negative SiNPs. WGMs observed by (a) PL measurements and (b) optical simulations for negative SiNPs with increasing DT and DB at 20 nm intervals from 1263 and 1166 nm, respectively, while keeping θB = 89.13°. In PL measurements, the baseline was subtracted and the magnitude was normalized to maximum peak intensity. The gray shades indicate wavelength regions II and III in Figure 3b. Each mode is red-shifted as the diameter increases.
of the FSR is provided in the Supporting Information. There are no resonant peaks observed in the positive SiNP as expected. Tracing the light propagation in time scale reveals that the negative SiNP sustains resonant modes for a longer period of time, enabling effective confinement of light within the structure, and the absence of such a feature for the positive SiNP. The electric field intensity variations over time, which present additional information about this resonance, are provided in Figure S3. Figure 3b shows the mode analysis of the resonant peaks observed in Figure 3a. Clearly distinguishable WGMs are distributed over a wide wavelength range as a function of diameter and height of the negative SiNP. They can be classified into four wavelength regions denoted as I, II, III, and IV, essentially corresponding to an integer number of waves across the circumference. For instance, if the number is N for the region I, then N − 1, N − 2, and N − 3 are for region II, III, and IV, respectively. Within each region, there can be different resonant wavelengths with small changes in diameter. Multiple polarization modes such as transverse-magnetic (TM) and transverse-electric (TE) can exist for each resonant wavelength. We defined three mode numbers to describe the TM(TE)lnm: the azimuthal number n represents the number of wavelengths along the perimeter of SiNP in the xy-plane, the radial number m represents the number field maxima along the radius, and the axial number l represents the order of the axial standing wave as for FP modes. In the optical simulations, TM modes at m = 1, 2 and TE modes at m = 1 are observed in the wavelength between 950 and 1250 nm with varying n, and five available resonant wavelengths exist in each region of the studied negative SiNP with varying l. The analysis shows that the dominant peaks in Figure 3a are TM (m = 1). The reason for this turns out to be the relatively higher Q of TMn1 compared to those of the other modes. More analysis on Q for each mode is provided in Figure S5. The field distributions on the horizontal and vertical cross section of the SiNP reveal great details of the WGMs formed in each region. Figure 3c shows the field distributions in the xyplane of the WGM (l = 1) formed in each region. TM modes are shown from the magnitude of the electric field component
the spacing between successive resonant modes of the negative SiNP, is as large as 85.14 nm with respect to the 1102.20 nm resonant peak. The largely expanded FSR stemming from the small mode volume of our negative SiNP pushes the differentorder modes to the edge of the Si gain range, avoiding mode degeneracies and, more importantly, opening the possibility of single mode operation.27 The strength of a resonant mode can be evaluated by using the quality factor (Q). The resonant peaks were normalized by subtracting the baseline between wavelengths of 950 and 1250 nm, followed by fitting with a Lorentzian function28 to estimate the Q from the PL spectrum. Specifically, Q was calculated via the expression, Q = λ0/Δλ, where λ0 is the wavelength at a peak maximum and Δλ is the full width at halfmaximum (fwhm) of a peak. The calculated Q of the negative SiNP was placed between 871 and 1804, which represents the minimum and maximum values of Q of all resonances obtained within the gain range of Si (950−1250 nm). Our SiNP could be thought of as a multicavity structure in which several circular nanocavities are stacked vertically, leading to a wide variation of Q with inevitable overlaps. The values are higher than the Q reported in the typical widely tapered Si nanocones20 or at least similar. This is because the slope of the negative SiNP only affects the number of modes but has little effect on the Q of each mode. This point will be further elaborated in the following sections. Optimal Angle by Optical Simulations. Optical simulations were performed using the three-dimensional finite-difference time-domain (FDTD) method (Lumerical Solutions, Inc.) to design and analyze the WGMs of the negative SiNP. The simulation results for the same size negative and positive SiNPs in the PL measurements above are summarized in Figure 3. The electric field spectral intensity is presented in Figure 3a. As with the PL measurement results, the occurrence of optical resonance is evident, and the trains of peak arrays are observed from the negative SiNP. Each mode is spatially well separated with wide spacing, and the FSR at the resonant wavelength of 1101.60 nm is 84.08 nm; considering the unavoidable surface roughness in all real samples, this is in excellent agreement with the experimental results. Verification D
DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials EZ, while TE modes are shown from the magnitude of the magnetic field component HZ. In each mode, n decreases with increasing wavelength. TM111, TM82, and TE101 were formed in region I, TM101, TM72, and TE91 in region II, TM91, TM62, and TE81 in region III, and TM81, TM52, and TE71 in region IV. As for the vertical distribution, the magnitudes of the electric field components EX and EZ of the TMl91 in the xz-plane are shown in Figure 3d. The modes shown in Figure 3d are indicated by an asterisk in Figure 3a, whose resonant wavelengths correspond to 1080.19 nm (l = 5), 1090.25 nm (l = 4), 1101.60 nm (l = 3), 1114.32 nm (l = 2), and 1130.46 nm (l = 1). As analyzed in the previous section, WGMs are formed at different positions as a function of the diameter and height in the negative SiNP. Excitation at higher z-position corresponds to a longer wavelength since a longer wavelength requires a longer path which can be accommodated by a larger diameter. This also indicates that the FP modes were well coupled to WGMs with resonant frequencies matched. For instance, the five field maxima in TM591 mode analysis (the leftmost column 5 of Figure 3d) illustrate the constructed 2 × λ FP modes along the vertical direction that can be coupled with a WGM along the wavelength-matched perimeter of the SiNPs on the xyplane. Controllability of WGMs. The potential of engineering the WGMs in the negative SiNP for wide frequency range is further elaborated below. Figure 4 summarizes the diameter dependence of WGMs. The left column (Figure 4a) shows the spectra from PL measurements, while the right column (Figure 4b) shows FDTD simulation results for the same structures. The top and bottom diameters (DT and DB, respectively) of the negative SiNPs increase from DT = 1263 nm and DB = 1166 nm to DT = 1323 nm and DB = 1226 nm, respectively, with a 20 nm interval. The tapering angle was maintained as constant at θB = 89.13°. To compare the peak resonance properties, the baseline was subtracted from the PL spectra, and the spectra were normalized by the maximum peak intensity. Besides the close resemblance between the experimental and simulation results, this systematical study highlights essential engineering parameters of the SiNP WGMs. The resonant wavelength of WGMs shifts to longer wavelength as the diameter increases.29 Meanwhile, in each wavelength region sharing the same mode, the number of modes and interval between the modes stay constant. Thus, the diameter of the SiNP affects only the shift of the resonant wavelength of WGMs. As an example, we analyzed this character from the diameter variation with the increase of DT and DB as much as 60 nm. Theoretically, however, this spectral shift can be achieved as long as the resonance exists within the gain range of Si (950−1250 nm), which allows us to engineer the resonant wavelength to tune it to the desired frequency. The slope dependence of WGMs was also studied, and the results are summarized in Figure 5. For a fixed top diameter (DT = 1263 nm), the slope θB was changed by ±0.5°. It turns out that the slope of the negative SiNP directly affects the number and interval between the modes. As the slope of the negative SiNP becomes gentler (θB decreasing), the number of modes increases and the interval between modes gets wider, resulting in the obscure distinction and overlaps between the wavelength regions sharing the same modes. In contrast, as the slope of the negative SiNP becomes steeper (θB increasing), the number of modes decreases significantly, and the interval between modes gets narrower, making the distinction between
Figure 5. Slope dependence of WGMs in negative SiNPs. WGMs were optically simulated for negative SiNPs with increasing and decreasing θB by 0.5°, while maintaining DT = 1263 nm. Each spectrum was normalized based on the maximum intensity. The gray shade indicates wavelength region III in Figure 3b.
the wavelength regions clearer. When θB = 89.63°, only three TM91 modes can be found in region III, and the dominant modes are clearly distinguishable, which contrasts with six TM91 modes distributed in the same region when θB = 88.63°. Further details on the mode analysis as a function of θB are provided in Figure S5a. It is worth noting that the resonant wavelength shift toward longer wavelength as θB increases is actually not the effect of the slope variation. It is rather the result of the increased overall diameter as DB also increases when θB increases. Interestingly, the slope of the negative SiNP does not significantly affect the Q of the resonant modes (Figure S5b). As such, we can conclude that WGMs become more spatially separated and distinguishable without loss of Q as θB approaches perpendicular orientation. The above traitsa compact Si-compatible optical resonator with the tunability and no loss of Qare important characteristics that may have many implications in integrated optics applications utilizing a cavity. Especially, the optical paths in our SiNPs are confined within the small footprint of the nanopillars unlike traditional cavities with double-reflection surfaces that require lateral dimensions greater than at least several times the wavelength. This is an advantage that enables enhancement by combining many pillars with the same diameter (for single frequency) or with multiple diameters (for multiple frequencies). For instance, our SiNPs can be applied to many chemical/biological sensor applications that utilize the resonance shift of an optical mode. In this case, the binding of molecules or particles to an optical cavity can cause mode shift or splitting, and the subtle change of modes can be detected with great sensitivity due to the high optical Q. In addition, the changes in the surrounding environment such as temperature, pressure, and electric and magnetic fields can also shift the resonance, and there are many applications taking advantage of these properties.30 More importantly, the tunability and wide FSR are crucial features in realization of a Si-based laser. The tunability allows to select modes within the SiNPs by controlling the number of modes within the gain wavelength range of Si. The wide FSR enables to place only the desired mode within the gain range of Si, while keeping all E
DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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extinction coefficient of Si were taken from the experimental data (ref 33) and then fitted appropriately for the simulations. The results are provided in Figure S6. To identify resonances in a single SiNP, all boundary conditions were set to perfectly matched layers (PMLs). The z-polarized electric (magnetic) dipole sources were set to be located close to the sidewall of the SiNP to excite the TM (TE) modes. The wavelength range was set between 950 and 1250 nm, which is near the band edge of Si. To find the resonant wavelengths, frequency domain point monitors were located near the top facet of the structure where the electric (magnetic) field intensity was expected to be the highest, and the field intensity variations over wavelength were recorded. The field distributions at the resonant wavelengths were also recorded by using frequency domain profile monitors at the xy- and xz-planes. TM (TE) modes were observed from the electric (magnetic) field components EZ (HZ) for propagation in the xy-plane.
other modes out of the gain range. Accordingly, the proposed SiNPs provide a well-suited cavity platform that can serve for the long-awaited all-Si laser. Although we obtained experimentally high Q of up to 1804, the lasing action is also a function of a combined gain material. Chen et al. observed lasing in the wavelength-scale InGaAs/ GaAs nanopillar on Si with a relatively low Q (∼206) WG-like mode.8 Our Q is similar or several times higher than this and higher than the Q reported from any wavelength-scale SiNP20 so far. Thus, we believe this is sufficient to consider Si-based lasers. As for better Q, the increase of the diameter of the WGM greatly increases Q as it reduces the bending loss.31 To increase Q without a significant increase in the volume of the SiNPs, we need fine control of the negative tapering (θB ≅ 90°) to avoid mode overlap that can cause mode broadening. Reducing contamination and surface roughness after the etch process via sacrificial oxidation can also be another strategy to increase Q.32
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* Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00810. Top view SEM images of SiNPs, schematic of the confocal PL measurement system, electric field intensity variations over time from the SiNPs, more analysis on Q in the negative SiNP, method for the high Q calculations using FDTD, mode analysis as a function of θB in the negative SiNPs, the complex refractive index of Si used for the simulations, and verification of FSR (PDF)
CONCLUSION We have demonstrated a wavelength-tunable and Si-integrated micro-optical cavity with high Q by managing the slope of silicon nanopillars with proper process control and judicious design aided by accurate optical simulation. The results from our experimental and theoretical study demonstrated that the structural changes of SiNP have a significant influence on the formation of WGMs. Specifically, the negative SiNP show enhanced resonance supported by combining FP modes and WGMs that confines a large amount of light for a long duration of time. The weakly tapered structure exhibits spatially wellseparated WGMs in the spectral pattern with wide spacing, while maintaining high Q. The dominant modes correspond to TM (m = 1), and they are well distributed across the structure as a function of the SiNP diameter and height. This result reveals wide mode tunability confirmed by a detailed mode analysis varying the SiNP structural parameters. The mode analysis shows that WGMs can be spatially more separated and more distinguishable without affecting high Q by adjusting the slope of SiNP closer to vertical, suggesting the mode selection possibility. Our results clearly indicate that the design of Si resonators at the wavelength range near the band edge of Si is available for a wide range of light-emitting applications. Moreover, this Si-friendly cost-effective, compact cavity platform is a crucial component for the realization of Sibased lasers. Because the Si-based laser is one of the missing links toward Si on-chip integrated photonic circuits, our study is useful to expedite the arrival of true Si integrated photonic circuits.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected]. *E-mail
[email protected]. ORCID
Myunghae Seo: 0000-0002-4784-9656 Kihyun Kim: 0000-0001-7578-7977 Chang-Ki Baek: 0000-0002-2852-6683 Author Contributions
M.S. conceived the idea, designed the experiments, fabricated devices, analyzed the data, and performed FDTD simulations. K.K., H.C., and S.Y. characterized the devices and analyzed the data. B.K., M.M., and C.B. supported the research, fabrication, and characterization of devices. B.K. and C.B. supervised the research equally. B.K. and C.B. contributed equally as corresponding authors to this work. All authors discussed the results and contributed to the writing of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the MSIT (Ministry of Science and ICT), Korea, under the “ICT Consilience Creative program” (IITP-2019-2011-1-00783) supervised by the IITP (Institute for Information & communications Technology Promotion), the “Development of highly sensitive Si photodetector and optimization technique of its characterization of Nd:YAG laser” supervised by IITP (No. 2018-0-01283), the “Nano·Material Technology Development Program” supervised by the NRF (National Research Foundation of Korea, (2009-0082580)), and “Development of mass productive mid-temperature thermoelectric module based on top-down process technology” supervised by MSS&TIPA (S2714114).
METHODS
Photoluminescence Measurements. A continuous wave laser with a wavelength of 514 nm was focused perpendicularly on the individual SiNPs through the objective (×100, NA 0.9). The spot size and focal depth of the incident pump laser were ∼700 nm and ∼2540 nm, respectively. The power of the excitation source was adjusted using a neutral density filter with transmittance of 50%. The backscattered light from the SiNPs passed through the same objective and was separated from the incident light by a dichroic mirror. The light was collected and analyzed through a near-infrared spectrometer combined with InGaAs detector arrays. Light shorter than λ = 570 nm was cut off by using a long-pass edge filter. The schematic of the confocal PL measurement system is presented in Figure S2. Optical Simulations. SiNPs were designed with the same parameters as the fabricated devices. The refractive index and F
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DOI: 10.1021/acsanm.9b00810 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
