High-Performance Plasmonic Nanolasers with a Nanotrench Defect

Mar 19, 2018 - Recent developments in small footprint plasmonic nanolasers show promise for active optical sensing with potential applications in vari...
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High-Performance Plasmonic Nanolasers with a Nanotrench Defect Cavity for Sensing Applications Pi-Ju Cheng,† Zhen-Ting Huang,‡ Jhu-Hong Li,§ Bo-Tsun Chou,∥ Yu-Hsun Chou,§ Wei-Cheng Lo,‡ Kuo-Ping Chen,⊥ Tien-Chang Lu,*,§ and Tzy-Rong Lin*,‡,# †

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan Department of Mechanical and Mechatronic Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan § Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan ∥ ATD Device, United Microelectronics Corporation, Hsinchu 30075, Taiwan ⊥ Institute of Imaging and Biomedical Photonics, National Chiao Tung University, Tainan 71150, Taiwan # Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan ‡

S Supporting Information *

ABSTRACT: Recent developments in small footprint plasmonic nanolasers show promise for active optical sensing with potential applications in various fields, including real-time and label-free biochemical sensing, and gas detection. In this study, we demonstrate a novel hybrid plasmonic crystal nanolaser that features a ZnO nanowire placed on Al grating surfaces with a nanotrench defect nanocavity. The lasing action of gain-assisted defect nanocavity overcomes the ohmic loss parasitically in the plasmonic nanostructures. Therefore, the plasmonic nanolaser exhibits an extremely small mode volume, a narrow linewidth Δλ, and a high Purcell factor that can facilitate the strong interaction between light and matter. This can be used as a refractive index sensor and is highly sensitive to local changes in the refractive indices of ambient materials. By careful design, the near-ultraviolet nanolaser sensors have significant sensing performances of glucose solutions, revealing a high sensitivity of 249 nm/RIU and high resolution, with a figure of merit of 1132, at the resonant wavelength of 373 nm. KEYWORDS: surface plasmon resonance, nanolasers, optical sensors, sensitivity, Purcell factor

H

(FOM) compared to the passive scheme. Additionally, biochemical sensing applications that are hybridized with plasmonic modes are not limited to the visible and infrared regions.11 For example, surface plasmon resonant modes at the UV spectra (λ = 100−400 nm) can be essential in techniques used for various purposes, from UV astronomy to toxic gas sensing.12 In this wavelength regime, Al, an earth-abundant metal, has a strong plasmonic response.13 With regard to intrinsic material loss that often hinders lasing action or dispersively deteriorates the sensing performance, the suitable analyte candidate for the Al surface plasmonic laser sensor should have a low absorption window in the UV spectrum,14 for example, glucose in an aqueous solution. While conventional optical glucose sensors mainly rely on reaction-based probes, such as boronic acid receptors, glucose-binding proteins, or the usage of enzyme glucose oxidase (GOx),15 our proposed active plasmonic refractive index sensor may be a

ybridization of plasmonic effects in metallic structures has become one of the most reliable and promising techniques for sensing applications, including biochemical sensing and environmental detection. Various architectures of surface plasmon sensors have been proposed to achieve high sensing performances for real-time and label-free optical sensing, such as those based on surface plasmon (SP) resonance,1−3 plasmonic-enhanced fluorescence,4 or surfaceenhanced Raman scattering.5 It was recently proposed, both theoretically and experimentally, that surface plasmon sensors with an active (gain-assisted) resonator scheme can achieve a much higher sensitivity (S) due to the narrow linewidth (full width at half-maximum, fwhm) and high quality factor.6−8 Conversely, the sensing performance of surface plasmon sensors in conventional passive schemes9 is fundamentally limited by the strong nonradiative damping that substantially weakens the resonance signal and broadens the response linewidth.5,10 This damping also reduces the quality factor and degrades the sensitivity and signal resolution of the plasmonic resonant cavities. Therefore, the narrow linewidth of the SP lasing mode in the gain-assisted resonator allows the potential of active refractive index sensing with a considerably increased figure of merit © XXXX American Chemical Society

Special Issue: Recent Developments and Applications of Plasmonics Received: March 14, 2018 Published: March 19, 2018 A

DOI: 10.1021/acsphotonics.8b00337 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics

Figure 1. | Schematic representation and characteristics of surface plasmon waveguide and periodic structures. (a) ZnO NWR on flat Al substrate. (b) ZnO NWR on Al plasmonic crystals. The geometrical factors are indicated: grating depth h, period length d, and period filling factor f. (c) We solved the hybrid plasmonic band structure of the two lowest-order bands. Modal profiles of the unit cell (indicated by the red box in the schematic) at the Brillouin zone edges are also shown. A wide band gap for the TM-like modes is shown and centered at the target wavelength λ0 = 373 nm (3.32 eV). The scheme and geometric parameters of the hybrid plasmonic waveguide are shown on the right.

Furthermore, the plasmonic resonant modes, which are localized in the hollow region and directly exposed to the surrounding environment, can benefit a refractive index sensor that is highly sensitive to detection. The proposed platform with a high quality factor and small mode volume is highly sensitive to variations of the refractive index of the surrounding media. Two key parameters, the sensitivity and resonance linewidth, are applied to quantitatively characterize the sensing performance of the nanolasers. The estimated FOM is 1 order of magnitude larger than those of plasmonic nanolaser sensors,7 suggesting the suitability for applications to real-time and label-free biochemical gas or glucose solution detection in the near-UV regime. Proposed nanocavity sensors built on metal nanostructures afford pointof-care use and can be feasibly integrated into modern nanoarray architectures.22

prospective scheme toward probe-free, real-time sensing for a glucose solution. In this study, we demonstrate a novel hybrid plasmonic nanolaser16,17 that features a ZnO nanowire (NWR) orthogonally placed on Al grating surfaces with a defect nanocavity at an emission wavelength of 373 nm. For the periodic hybrid plasmonic structure with Al gratings, we first solve the dispersion relation as a function of various structural parameters (grating depth h, period length d, and period filling factor f) to find the optimal geometrical design. The structure shows a wide hybrid plasmonic band gap that functions as an efficient transmission stopband.18,19 In practice, instead of considering perfectly extended gratings, we focus on reflectors with a finite period number, N, and the design rules regarding the structural parameters are investigated to realize high reflection. This periodic grating structure therefore features a highly reflective Bragg reflector. The corresponding reflectivity (R) is further presented as a function of the structural parameters and period number. Near the resonant wavelength, λ0 = 373 nm, the optimal plasmonic Bragg reflector can reduce the mirror loss by 10-fold compared with that of an NWR end facet without gratings. We next introduce a defect between the Bragg reflectors as a nanocavity by changing the length and depth of one grating. To optimize the structural parameters that affect the performance of the nanosensor, we analyze the radiative and nonradiative loss mechanisms around the plasmonic reflectors in detail. The proposed plasmonic crystal nanolasers20,21 are found to exhibit a small mode volume and low threshold gain (gth). In addition, the Purcell factor of the resonant defect modes is hence expected to be high, which would facilitate a strong interaction between light and matter. Owing to these advantages, a hollow nanotrench defect is further introduced between the high-reflection plasmonic reflectors in comparison with the unetched defect. The proposed nanolaser was then experimentally demonstrated, revealing an extremely narrow linewidth above the threshold.

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RESULTS AND DISCUSSION The hybrid plasmonic waveguides are illustrated in Figure 1. The structure consists of a ZnO NWR on Al surfaces without (Figure 1a) or with periodic gratings (Figure 1b). The NWR-Al structure can host hybrid plasmonic modes without gratings, where the optical field intensity of these modes is strongly squeezed near the metal-dielectric interface.23 However, the clear facets on the ends of the NWR exhibit a lower reflectivity as end mirrors. One common choice to enhance reflectivity is to use a distributed Bragg reflector (DBR) with an alternating dielectric structure. Furthermore, to achieve a high Bragg reflectivity,18,19 the refractive indices of the alternating dielectric materials can be selected so that higher-order diffractions are evanescent. This can be done by producing gratings on the Al surfaces, analogous to DBRs (gratings with a period length, d, and period filling factor, f). In a quasi-two-dimensional waveguide, the refractive index of the bulk material in the DBR is replaced by the corresponding effective refractive index, neff, of the guided modes. To determine the optimal effective refractive index, we vary the grating depth, h, to achieve a wide B

DOI: 10.1021/acsphotonics.8b00337 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 2. | Reflectivity and mirror quality factor versus grating reflector period number and corresponding resonant mode profiles. (a) Reflectivity, R, and corresponding mirror quality factor, Qmir, for various period numbers, N, on each side of the defect cavity. (b) Side view of resonant mode profiles in the y−z plane at period numbers N = 2, 4, 6, and 8. When N is small, the mirror loss is still large. The field of the resonant defect mode inevitably leaks into the surroundings and exhibits obvious scattering loss at the end facets. For a larger period number (N = 6), R and Qmir are approximately maximized (R ∼ 89% and Qmir = 99). We therefore chose N = 6 for the calculation throughout this study.

hybrid plasmonic band gap. Because Re[neff] decreases with h and we need a relatively high contrast in the effective indices in alternating regions, we conclude that the depth should be larger to achieve the wide band gap. The dependence of the effective refractive index of the guided modes on the grating depth can be understood as follows: the photonic TM01-like guided mode of the small hexagonal ZnO NWR (with a side length of 35 nm below the cutoff dimension of the TM01 guided mode) leaks slightly to the ambient environment and is strongly coupled to the SP waves. The fundamental hybrid plasmonic guided mode with the highest index value, Re[neff] = 1.6, corresponds to a grating depth of zero. As the grating depth increases, so does the air gap. Consequently, a larger part of the field occupies the air gap region underneath the NWR, which has a refractive index of 1. This results in a lower Re[neff] value for the hybrid plasmonic gap mode. At h larger than 10 nm, the coupling strength between the leaky photonic mode of the NWR and the SP mode on the metal substrate becomes weak, as the effective refractive index minimally changes with grating depth. As an example, the field confinement at h = 20 nm is weak and the mode spreads into the ambient environment, as shown in Supporting Information, Part 1. As a result, we chose h = 10 nm for the gratings in the following calculated results, and the corresponding real part of the index neff is 1.16. This moderate index contrast still ensures a large band gap for TM-like guided modes. Additionally, we numerically solved the Bloch modes characterized by the period length and period filling factor to obtain a suitable set of structural parameters that maximize the band gap of TM-like guided modes and to shift the gap center toward the target wavelength, λ0, of 373 nm, which is the peak emission wavelength of the ZnO NWR. We start with a unit cell with a period length of 134 nm, obtained from the Bragg condition for plane waves propagating along the z direction, and f is set to 50%. The estimated value of the period length deviates slightly from the standard Bragg condition (d = 0.36λ0) because the

plasmonic guided modes have different mode profiles in etched and unetched regions. This mode mismatch may introduce unwanted scattering and higher-order diffraction; therefore, fine-tuning is required to realize near-complete destructive interference. At a period length of 119 nm and grating depth of 10 nm, the band edge modes are also solved. To meet the target wavelength, a period filling factor of 50% is chosen. With properly chosen values of the structural parameters (d, h, and f), this hybrid periodic plasmonic structure forms a wide band gap for TM-like modes that is centered at the target wavelength. Instead of considering an infinite array of gratings, we consider the plasmonic Bragg reflector with a finite period number, N, to meet the experimental requirements. We first consider the limiting case where the NWR is placed on a flat Al surface (h = 0). When the incident wave is coupled to the propagating surface plasmonic wave, most of the energy of the wave mode is transmitted through the end facets, with a low reflectivity of 8.1%, as discussed in Supporting Information, Part 2. This leaky mirror loss can be significantly suppressed by plasmonic reflectors, as we consider here. Conversely, when the grating depth is larger than 5 nm, the hybrid plasmonic gap modes also become leaky and prominent unwanted scattering occurs. Therefore, part of the incoming energy that encounters abrupt facets would be scattered into higher-order diffracted waves or higher-order guided modes and then rescattered into the ambient environment. As a result, to achieve a low radiative loss and high reflectivity, we choose a period filling factor of 50% and grating depth of 5 nm. We conclude that, in addition to a sufficiently small scattering ratio ( h). As Figure 5a shows, the sensitivity rapidly increases from 117 to 249 nm/RIU, while the defect depth increases from 10 to 90 nm, and the sensitivity reaches a maximum value of 270 nm/RIU corresponding to a depth of 140 nm. For a shallower trench, the confined resonant field could interact with the gain medium NWR with more efficiency, as Figure 5b shows, and hence has a smaller transparency threshold gain. While the defect depth increases, the overlap of the confined resonant field and gain medium is drastically reduced to boost the transparency threshold gain (see Figure 5c). The enlarged sensing surface due to an

Table 1. Optical Characteristics of Plasmonic Resonant Modes in Etched and Unetched Defect Nanocavities for Sensing Glucose Solutions at 373 nma unetched etched

L (nm)

R

Γwg

gth (cm−1)

S(nm/RIU)

FOM

105 96

83% 85%

1.12 0.418

4.66 × 104 8.02 × 104

90 109

409 495

a

The structural parameters are as follows: hc = 6 nm, h = 6 nm, N = 6, d = 105 nm, and f = 60%. E

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Figure 5. | Sensitivity and transparency threshold gain versus depth of etched defect nanocavity. (a) Sensitivity of proposed nanocavities in a glucose solution. The sensitivity, S, increases from 117 to 249 nm/RIU while the defect depth increases from 10 to 90 nm; the rapid increase of S ceases for defect depths over 90 nm, implying that the FOM becomes saturated (∼1132). For a shallower trench, the confined resonant field could interact with the gain medium NWR more efficiently and therefore has a smaller transparency threshold gain, gtr. While the defect depth increases, the overlap of the confined resonant field and gain medium is drastically reduced to boost the transparency threshold gain. (b), (c) Side view of resonant mode profiles |E| in the y−z plane at defect depths, hc, of 10 and 90 nm. At hc = 90 nm, S = 249 nm/RIU, and the FOM is 1132. When hc increases, a strong concentration of the plasmonic resonant mode remains in the low-index defect region and the increased sensing area therefore provides a better overlap with the environment.

increasing hc allows the confined resonant field in the defect to interact more strongly with the analytes, whereas the corresponding plasmonic mode remains robust against changes in the defect depth hc (see Figure 5b, c). As Figure 5a shows, the sensitivity no longer rapidly increases with defect depth over 90 nm, implying that the FOM is becoming saturated (∼1132). However, the stronger confinement in the deeper defect will reduce the overlap between the plasmonic mode and gain medium, i.e., NWR (see Figure 5b). Moreover, the plasmonic mode starts to become coupled to higher-order guided modes, leading to possible field leakage and causing the transparency threshold gain to rapidly increase. As a result, considering the sensitivity and laser performance, we estimate the best working defect depth as hc = 90 nm, with S = 249 nm/RIU, FOM = 1132, and gtr = 2.32 × 105 cm−1. This design reveals that the hybrid plasmonic crystal nanolaser sensor has the high sensitivity and high resolution required for glucose solution sensing.

In addition, when a cavity is etched into a nanotrench, the resonant modes are strongly confined inside the cavity; conversely, an etched cavity provides an increased surface for functionalized molecules to bind to for sensing or for further chemical surface treatment. The simulated near-UV nanolaser sensing of the glucose solution demonstrated that the novel designed optical sensors have a noteworthy sensing performance, with a high sensitivity of 249 nm/RIU and high resolution, with an FOM of 1132. In particular, the FOM is 1 order of magnitude higher than those of state-of-the-art SPR sensors. The outstanding performance, attributed to ultrastrong mode confinement, an ultranarrow linewidth, and strong lightmatter interaction, provides opportunities for various applications of plasmonic nanolasers in sensing and detection.

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METHODS Fabrication. A minimum-migration method is adopted to grow the high-quality single-crystalline Al film by MBE on the epi-ready Si substrate. The high-quality single-crystalline ZnO NWRs were synthesized by a hydrothermal method and were soaked in an isopropyl alcohol solution. Semiconductor gain nanostructures, such as nanorods or nanoparticles in the solutions, can typically experience severe photocorrosion during high power optical pumping. Through hybridization with an atomically thick surface protection layer, ZnO nanorods have been demonstrated to reach low photocorrosion.28,29 Focused ion beam (FIB) nanopatterning is adopted to define and mill the nanostructures-gratings and one open trench in the middle of gratings, onto the metallic surface (see Supporting Information, Part 5). We then sprayed the NWR solution on the prepatterned templates. The templates were kept in the nitrogen environment to prevent oxidation. Measurement. We mounted nanolaser samples in a highvacuum chamber under controlled temperature. The Al nanostructures were positioned using a SEM image from the measurement system, and the precise location of the single ZnO NWR was identified by recognizing the optical image

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CONCLUSIONS We proposed and demonstrated a novel hybrid plasmonic crystal nanolaser that features a ZnO NWR placed on Al grating surfaces with a nanotrench defect cavity at the emission wavelength of 373 nm. To reduce the mirror loss at the end facets of the NWR, Al gratings serving as hybrid plasmonic Bragg reflectors were investigated. We analyzed the dispersion relations of the Bloch modes in the hybrid plasmonic crystal and showed the relationships between the stopbands and structural parameters (grating depth, period length, period filling factor). The design rule for an efficient reflector was then applied, and a Bragg mirror was demonstrated with a reflectivity of ∼89%. The proposed plasmonic nanolasers exhibit a small mode volume of 5.75 × 10−4λ03 and low threshold gain of 3.40 × 104 cm−1. Therefore, the nanolaser structure can be expected to have a high Purcell factor, facilitating the strong interaction between light and matter. F

DOI: 10.1021/acsphotonics.8b00337 ACS Photonics XXXX, XXX, XXX−XXX

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ACS Photonics

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from a nitrogen-cooled charge coupled device (CCD) camera installed in the measurement system. A 355 nm Nd:YVO4 pulse laser with a 1-kHz repetition rate and 0.5 ns pulse duration was then used to pump the NWR. For the focusing of the incident beam, we used a long-working-distance 100× near-UV infinitycorrected objective lens with a 0.55 numerical aperture. The light spot with a size of ∼15 μm ensured that only the target NWR was illuminated. Emissions from the NWRs were collected by the same path into a 600-μm core UV optical fiber and analyzed using a 320 mm single monochromator attached to a CCD. The linewidths of the emission spectra were restricted by the instrument’s resolution of ∼0.2 nm.

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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.8b00337. Modal characteristics of hybrid plasmonic waveguides and plasmonic crystals, Mirror loss: reflectivity and scattering ratio of Brag grating reflectors, Reflectivity, transmissivity and scattering at grating reflectors, Fillets of grating sharp corners in FEM simulation and corresponding reflectivities, and FIB nano-patterning (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.C.L.). *E-mail: [email protected] (T.R.L.). ORCID

Yu-Hsun Chou: 0000-0002-3183-1398 Author Contributions

P.J.C., Z.T.H., K.P.C., T.C.L. and T.R.L. initiated the study. P.J.C., Z.T.H., B.T.C., and T.R.L. performed the numerical calculation and simulation. Y.H.C., T.C.L., and J.H.L. performed the optical experiments. P.J.C., Z.T.H., J.H.L., B.T.C., and T.R.L. wrote the manuscript. P.J.C. and Z.T.H. equally contributed to this work. All authors analyzed the calculated and experimental data. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the help of Prof. Nan-Nong Huang at National Taiwan Ocean University. Author Dr. Pi-Ju Cheng would like to thank Prof. Shu-Wei Chang and Dr. Shiue-Yuan Shiau for their insightful discussion. Author Prof. Tzy-Rong Lin expresses his deepest gratitude to his Father, Mr. Hsing-Chung Lin, for his cultivating parenting and frequent encouragement during his research, and he shows his endless love to his Father by this paper. This work is supported by Ministry of Science and Technology (MOST), Taiwan (Grant No.: MOST 1062221-E-009-112-MY3, MOST 105-2221-E-002-079, MOST 105-2221-E-019-049-MY3, MOST 103-2221-E-224-002-MY3 and MOST 103-2221-E-019-028-MY3).

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REFERENCES

(1) Homola, J. Surface plasmon resonance sensors for detection of chemical and biological species. Chem. Rev. 2008, 108, 462−493. G

DOI: 10.1021/acsphotonics.8b00337 ACS Photonics XXXX, XXX, XXX−XXX