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Abstract
Semiconductor quantum dots (QDs) have recently caused a stir as a promising and powerful lighting material applied in real-time fluorescence detection, display, and imaging. Photonic nanostructures are well suited for enhancing photoluminescence (PL) due to their ability to tailor the electromagnetic field, which raises both radiative and nonradiative decay rate of QDs nearby. However, several proposed structures with a complicated manufacturing process or low PL enhancement hinder their application and commercialization. Here, we present two kinds of dual-resonance gratings to effectively improve PL enhancement and propose a facile fabrication method based on holographic lithography. A maximum of 220-fold PL enhancement from CdSe/CdS/ZnS QDs are realized on 1D Al-coated photoresist (PR) gratings, where dual resonance bands are excited to simultaneously overlap the absorption and emission bands of QDs, much larger than those of some reported structures. Giant PL enhancement realized by cost-effective method further suggests the potential of better developing the nanostructure to QD-based optical and optoelectronic devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The Nobel Prize in Chemistry 2023 was awarded to Moungi G. Bawendi, Louis E. Brus, and Alexei I. Ekimov for the discovery and synthesis of quantum dots. Semiconductor quantum dots are zero-dimensional semiconductor nanomaterials with size of less than 10 nm in three dimensions, considered as particularly superior novel materials mainly due to their irreplaceable advantages over traditional organic fluorescent dye molecules, such as high PL efficiency, broad excitation ranges but narrow emission peaks, widely tunable spectral band, high chemical stability, easy plasticity, and long lifetime. Thus, QD-based optical and optoelectronic devices have been extensively exploited in bio detection, displays, solar cells, nanoemitters, etc [1–3]. Fluorescence-based detection is the most widely used method for detecting weak signals from probe analytes at low concentrations to quantify and visualizing analytes, which is essential for life sciences and precision diagnosis [4–6]. Enhancing PL of QDs can reduce measurement time, increase detection sensitivity, and promote detection accuracy. However, the bottleneck of achieving both high efficiency emission and great radiation directivity still remains constraints.
Nanostructures tailoring optical resonance effects as novel platforms has aroused extensive attention in enhancing the fluorescence intensity [7–9]. Countless structures have been proposed in recent decades [7,10–14]. However, previously proposed nanostructures for PL enhancement of QDs are so complex that they are limited by intricate structural processing [15–17]. Obviously, 1D metallic gratings are the most direct and effective for uniform and reproducible fluorescence substrates. But most previous relative studies have failed to realize remarkable enhancement [18–20], due to enhancing PL intensity by overlapping resonance dips of nano gratings to either the absorption [11,21] or emission band of the QDs [17,22], only one or two orders of magnitude enhancement can be detected.
Here, we propose two kinds of metal gratings to excite dual resonance absorption for greatly improving PL intensity of CdSe/CdS/ZnS core/shell/shell QDs by simultaneously overlapping with the emission and excitation band of the QDs. The higher degree spectral overlap between resonance and the spectra of QDs renders the more electric field coupling to QDs enhanced to realize the higher local density of optical states, which can reduce the spontaneous lifetime and then increase in the emission intensity of the QDs. Large fluorescence signals can be obtained with dual-resonance gratings by using a low excitation light energy, which prevents fluorescent molecules from losing activity because of a high irradiation energy. Mode analysis on dual resonance absorptions is also displayed to explain physical mechanism. In addition, we exploit a two-step fabrication method based on holographic lithography (HL), which requires no additional etching and transfer steps, significantly reduces both production time and cost. A maximum 220-fold PL enhancement has been observed from a thickness of 35 nm QDs on the prepared Al-coated gratings with dual resonance absorptions compared to a silicon wafer taken as a control group. Such fluorescence substrates with high performance and low-cost processing can better match practical applications and industrial demand and are desirable for future growth.
2. Dual surface plasmon resonance excitation
The CdSe/CdS/ZnS QDs consisting of a semiconductor core (CdSe) and double semiconductor shells (CdS and ZnS) was tailored as a prober to evaluate the amplification of PL from dual-resonance gratings at visible wavelengths in our case. The details of preparing of red-emitting CdSe/CdS/ZnS QDs is provided in Supplement 1. CdSe QDs, group II-VI semiconductor QDs, has broad PL peak between 450 to 800 nm and unique optical properties. The growth of ZnS and CdS shells around CdSe QDs is a method for enhancement of CdSe PL quantum yield and photostability [23–25]. Figure 1(a) shows transmission electron microscopy (TEM) images of the synthesized QD solution depicting the morphology and particle size. The QDs were spin-coated on a silicon wafer as a uniform layer as shown in Fig. 1(b), and its thickness and refractive indices (see Fig. S1(b) in Supplement 1) are measured by spectroscopic ellipsometry. The QD properties are related to QD size. The UV–VIS absorption and PL spectra of the synthesized QDs solution at 405 nm excitation wavelength are displayed in Fig. 1(c). The PL wavelength at room temperature is 624 nm.
Fig. 1. (a) TEM image of the synthesized CdSe/CdS/ZnS QDs solution (b) The image of QDs spin-coated on a silicon wafer under UV light excitation. (c) The UV-VIS absorption and PL spectra of the synthesized QDs solution.
Surface plasmon wave is a binding electromagnetic wave originating from the collective electron oscillation propagating along the metal-dielectric interface, which is often categorized into two classes: propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). The momentum of the surface plasmon polariton (SPP) mode, kSP, is always larger than that of a free-space photon of the same frequency, k0, so the SPP mode cannot be resonantly excited unless the phase match condition is fulfilled. The commonly used optical phase-matching configurations for exciting surface plasmon resonance are the prism-coupled [26,27] and the grating-coupled configuration [28,29]. The grating-coupled approach can support low cost, high throughput, miniaturization, and ease of integration compared to the prism-coupled configuration. The diffraction at a metallic diffraction grating provides additional in-plane momentum ($m{{2\pi } / P}$) to compensate for the wave vector mismatch between the incident light and the surface plasmon wave,
2.1 Grating on metal substrate
The two kinds of dual-resonance gratings are provided in Fig. 2. The first kind is a PR grating on metal substrate and the second kind is a PR grating coated by metal. Initially, we explore the PL enhancement based on the first model (Fig. 2(a)) with dual SPR resonant absorptions. Our numerical simulations were performed by RSoft CAD software based on rigorous coupled wave analysis (RCWA). The experimental spectrometer measurements use natural light, also called unpolarized light, are defined as a set of linearly polarized light with different vibration directions and the same irradiance. It has no special preference for the direction in which it vibrates. Thus, the averaged simulation results of two vertical polarizations are used to ideally mimic the measured polarization conditions. The slanted groove wall resulting from the HL technology should be considered in the calculation. All tilt angles $\alpha $ were set to 80° in our simulations and the small slope of the groove walls has a minor impact on the resonance dips. The refractive indices of PR are shown in Fig. S1(a) of Supplement 1. Regarding metal selection, the optical properties of different metals are closely related to the real and imaginary parts of their dielectric constants. Materials with a large negative real and small positive imaginary dielectric values have the capability of supporting plasmon excitation. The double resonance dips appear at different wavelengths with different metal substrates shown in Fig. 3(a) and their geometric parameters are displayed in Table S1 of Supplement 1. However, the developer will corrode some metal substrates during the processing, the addition of an appropriate anti-corrosion layer is required to protect metal. A dual-resonance grating (Pattern #1) with good performance was fabricated on a copper substrate without anticorrosion layer, and the simulated and experimental reflectance spectra are displayed in Fig. 3(b). Table 1 provides the simulated and experimental structural parameters of this grating pattern. But Cu can only show plasmon excitations at wavelengths longer than 580 nm, there is no way to design the double dips to simultaneously cover the absorption and emission bands of the QDs used in our experiment, only the emission band at 624 nm can be resonantly enhanced. The position and number of resonance dips are not only related to the material, but also closely depend on the structural parameters. The resonance wavelengths of dual resonance gratings can be adjusted in accordance with the absorption and PL spectra of different QDs. The calculated electromagnetic field distributions are used to provide intuitive explanations for the formation of resonance absorption at 624 nm. According to the electric field $|{{E_x}} |$ distribution (Fig. 3(c)), we can infer that the guided mode resonance (GMR) is excited in the grating layer. The magnetic field H in the y direction (Fig. 3(d)) indicates the light coupling to SPP waves is trapped at the dielectric-metal (air or PR) through the excitation of SPP modes and enhanced field occupies above the metal surface. The full width at half maximum (FWHM) of the spectrum is narrower than the absorption dips caused by SPP mode alone. This suggests this resonance dip1 results from the coupling effect of GMR and SPR, dip2 is realized due to totally plasmonic resonance.
Fig. 2. Schematic diagram of the proposed metallic gratings a) PR grating on metal film with QDs and b) Al-coated PR grating with QDs.
Fig. 3. (a) Simulated reflectance spectra of the designed gratings on different metal substrates (b) Simulated and measured reflectance spectra of the prepared grating using HL. Distribution of (c) the electric-field amplitude $|{{E_x}} |$ and (d) magnetic-field amplitude $|{{H_y}} |$ at the resonance wavelength of 624 nm. (e) Sample images of gratings (f) Preparation of PL enhancement from QDs upon Patterns #1 and #2 under UV light irradiation captured by a camera
Table 1. Structural parameters of optimized and experimentally prepared grating samples on Cu film.
Strong local electric field hotspots raise both radiative and nonradiative decay rate of QDs nearby. It is obvious that a part of the near-field hotspot region is distributed inside the PR grating. Therefore, the field-enhanced area with that the QDs can contact is so insufficient that there is no more notable PL enhancement. Pattern #2 enables a single resonance dip to deviate from the QD emission bands. The reflectance spectra are optimized calculatedly and recorded experimentally as shown in Fig. 3(b) (dotted lines). When a 325 nm laser is incident on the well-prepared fluorescence substrates to excite QDs, grating pattern #1 (Fig. 3(e)) clearly outperforms pattern #2 ((Fig. 3(e)) (20×) in PL enhancement as shown in Fig. 3(f). The previous theoretical analysis that PL intensity can be boosted by matching the resonance dips with the emission band of QDs is verified in this case.
2.2 Metal-coated grating
Inspired by the above model, the success of nanograting-based SPP-assisted PL enhancement techniques relies on concentrating field enhancement in the QD film, increasing resonance strength and forming two resonance dips to simultaneously overlap the absorption and emission bands of the QDs. Our proposed Al-coated gratings (Fig. 2(b)) satisfy these requirements. We plot the real and imaginary parts of the dielectric functions of aluminum (Al) and copper (Cu) as obtained from Palik [30], as plotted in Fig. S2 of Supplement 1. Al exhibits the plasmonic activity from 200 nm to 800 nm, which makes Al more attractive at short wavelengths than other metals. Although Al (Cu) is unstable, surface oxidation occurs inevitably on the surface of Al (Cu) and a thin Al2O3 (CuO) layer will affect its plasmonic properties (see Fig. S3(e) in Supplement 1), in our case, the oxide layer keeps the QDs away from the metal, which contributes to suppress fluorescence quenching.
Ideally, we have assumed that Al coating can be fully covered around rectangle grating (unattainable actually). We exploit this ideal model to clearly reveal the dual resonance absorption mechanism. Figure 4(a) illustrates the resulting spectral response to normally incident light of the dual-resonance Al-coated grating with periodicity P = 625 nm, grating thickness hg = 260 nm, fill factor F = 0.62, Al thickness hm = 50 nm and QDs thickness hQD = 35 nm. As seen from magnetic field strength in the x–z plane at resonance dips in insets, it is apparent that field enhancement distributes along metal surface and in grating slits, which implies different resonant modes are excited. Furthermore, the modal analysis method is conducted to clarify accurately the resonant mode types supported in grating by calculating the coupling strength of incident light to a certain mode. Under normal incidence, the refractive indices of the metal (Al) used in the calculation are ${n_m} = 0.498 + 3.8853i$ at $\lambda = 405 nm$ and ${n_m} = 1.3984 + 7.4572i$ at $\lambda = 624 nm$. When the transverse magnetic (TM) wave impinges on a grating, the magnetic field $|{{H_y}} |$ can be described in the form of a modal expansion [31],
Fig. 4. (a) Simulated reflectance spectra of the ideal Al-coated rectangular grating. The insets are magnetic-field amplitude at the resonance wavelength of 405 nm and 624 nm. (b-c) The complex refractive indices $n^{\prime\prime}$ of the first 20 modes within the groove corresponding to the resonance wavelengths of 405 nm and 624 nm. (e-f) The amplitudes |Am| and |Bm| of the first 20 modes within the groove corresponding to the resonance wavelengths of 405 nm and 624 nm.
We write the effective refractive indices ${n_{eff}}$ in the form of ${n_{eff}} = {n^\prime } + n^{\prime\prime}i$ and the ${n_{eff}}$ of each mode can be obtained by the characteristic equation of the interaction between the incident light and the grating for TM polarization [32],
Depending on the actual situation, the slanted wall of the trapezoidal type formed by the HL method leads the metal coating to cover continuously around the grating. Then the impact of QD coating considered, we further optimize structural parameters. All grating parameters are shown in Table 2. The simulated and measured reflectance spectra of Al-coated grating pattern #1 with only one absorption generated at 405 nm within the excitation band of QDs are recorded under normal illumination shown in Fig. 5(a). Figure 5(b) presents corresponding scanning electron microscope (SEM) images of Pattern #1. Only the diffraction order m = 1 satisfies Eq. (1) to excite SPR and creates a resonance dip at a wavelength of 405 nm. And there is a stronger FPR brought by the QD layer whose refractive index is higher than that of air and trapped inside the QD layer in the groove, as shown in red dotted boxes of Fig. 6(a). Therefore, the FWHM of this resonance dip is relatively narrower on account of coexistence of SPR and FPR. In terms of experimental data, the broader FWHM is related to the optical loss caused by scattering from the surface roughness of Al.
Fig. 5. (a) Measured and simulated reflectance spectra and (b) top view and side view SEM images of the Al-coated grating with single resonance dip (Pattern #1). (c) Measured and simulated reflectance spectra and (d) top view and side view SEM images of the Al-coated grating with dual resonance dips (Pattern #2).
Fig. 6. (a) Distribution of magnetic-field $|{{H_y}} |$ amplitude of the proposed single-resonance grating without QD layer and with QD layer. Distribution of (b) magnetic-field $|{{H_y}} |$ amplitude and (c) electric-field $|{{E_y}} |$ of the proposed dual-resonance grating at the resonance wavelengths of 405 nm (excitation). (d) Distribution of magnetic-field amplitude of dual-resonance grating at 624 nm (emission).
Table 2. Structural parameters of optimized and experimentally prepared Al-coated grating samples.
Regarding the dual-resonance grating, we discuss the influence of structural parameter of Al-coated grating on dual resonance dips in more detailed in Supplement 1. The simulated and measured reflectance spectra are provided in Fig. 5(c), and the top view and side view of SEM images are showed in Fig. 5(d). There are two resonance absorptions corresponding to excitation and emission bands of QDs and the physical mechanism to form dual resonances has been described above. As seen from the field distributions $|{{H_\textrm{y}}} |$ at dual resonance presented in Fig. 6(b) and (d), SPP modes propagating at the upper surface of the grating and cavity modes trapped inside groove are excited simultaneously within this grating. SPP modes at dual wavelength are excited by different diffraction orders, SPR formations at 405 nm and 624 nm are attributed to the diffraction orders m = 2 and m = 1 in Eq. (1), respectively. The analysis described above only considers the TM polarization (polarization direction perpendicular to grating lines), because there is no resonance effect under TE polarization (polarization direction parallel to grating lines). But when the QD layer is introduced, there is one more resonance absorption which is excited under TE polarization within the excitation band of QDs. As seen from the $|{{E_\textrm{y}}} |$ field distribution (Fig. 6(c)), cavity mode is excited due to interference between incident light and the light which is reflected by the bottom of the slits.
In addition, localized surface plasmon resonance (LSPR) can be excited by the surface rough structure of the metal layer coated on the grating, and the grating skeleton adds the region where LSPR hotspots can occur on the surface of Al coating. Our patterned gratings support the coexistence of both local and propagating components of SPP modes at the upper interface of the metal layer. The hotspot region extends to the grating ridges, grooves, and inclined sidewalls inside the QD layer. This suggests that the area that QDs can fully contact with the whole hotspots is larger, and the field strength is higher than that of a typical metal grating, as described in our previous work [33].
3. PL performance enhancement
3.1 Holographic lithography for fabrication of plasmonic gratings
A cost-effective method for the fabrication of metal gratings is developed, which can be summarized as a two-step process including photoresist patterning via holographic lithography and metal or QDs deposition. To fabricate the first grating, a Cu film with thickness of 100 nm was first deposited on the silicon wafer and then a photoresist grating was prepared on the metal layer, as depicted in Fig. 7(a). The processes for the second grating are performed in the reverse order of the first structure. The grating pattern was recorded in the photoresist layer spin-coated on the silicon substrate, and then a thin Al layer of about 50 nm was deposited on the PR grating, as displayed in Fig. 7(b). All the PR gratings are realized using holographic lithography framed by blue-dotted lines, and an overview of this process is presented in Fig. 7(c), detailed description and characterizations are provided in Supplement 1.
Fig. 7. Process flow of the metallic grating involving (a) PR grating on Cu film with QDs and (b) Al-coated PR grating with QDs. (c) Flow diagram and experimental setup of HL. All processes related to HL are framed by blue dashed lines
3.2 PL performance enhancement
The resulting PL enhancement from QDs on unpatterned substrate (Fig. 8(a)), the well-prepared gratings with single resonance dip (Fig. 8(b)) and double resonance dips (Fig. 8(c)) was observed using a laser scanning inverted confocal microscope. A 405 nm laser is utilized to excite the QDs and the emission light from 550 nm to 700 nm is collected. The parameter settings, including laser power, camera gain settings, as well as the average PL intensities, are presented in the bottom right corner of the images. There is a significant distinction between the patterned samples with SPR assistance and the unpatterned samples. Since the enhanced fluorescence intensity of the patterned samples reached the saturation state of the test instrument at the same excitation energy, the laser energy of the patterned samples is 20 times less than that of the control group. The fluorescence signal of the Si substrate as a reference is almost imperceptible. The single-resonance grating pattern #1 shows approximately 50 times higher PL intensity than the bare Si substrate. The most striking conclusion to emerge from the intensity value comparison indicates a 220-fold increase in PL intensity exhibited by dual-resonance grating pattern #2 compared to the bare Si substrate.
Fig. 8. Fluorescence images of samples coated with 35 nm CdSe/CdS/ZnS QDs layer on different fluorescent substrates using laser scanning inverted confocal microscope: (a) Bare Si substrate, (b) single-resonance Al-coated grating (Pattern #1), (c) dual-resonance Al-coated grating (Pattern #2). Fluorescence images of samples coated with 30 nm QDs layer on different substrates using a laser scanning inverted confocal microscope: (d) bare Si substrate, (e) unpatterned Al film deposited on Si substrate, (f) dual-resonance Al-coated grating (Pattern #3).
To investigate the LSPR effect of the Al overlayer on the fluorescence enhancement, we performed another set of experiments to compare the PL enhancement of the 30 nm QD layer on the unpatterned Al film deposited on the Si substrate. Figure 8(d-f) shows the fluorescence images of bare Si substrate, Al film deposited on Si substrate and Al-coated 610 nm pitch grating with dual resonance dips (Pattern #3). The geometric parameters of grating Pattern #3 are identical with Pattern #2, and only the thickness of QD layer is different. As shown from the images, the slight LSPR effect is excited by the irregular nanostructure due to the surface roughness of the Al film, and about 5-fold PL enhancement is achieved from the QDs on a uniform Al coat when compared to the bare Si substrate as a reference. A maximum 105-fold PL intensity enhancement is achieved by Al-coated grating pattern #3 with dual resonance dips when compared to reference. Note that the thickness of QD layer has great influence on the performance of PL enhancement. Once the thickness is overly small, the QD film is uneven and air holes will appear on the surface (Fig. 8(b)). If the film is too thick, PL intensity will be greatly weakened. The optimal range of the thickness is between 30 nm and 50 nm, as displayed in Supplement 1 (Fig. S3(f)). In addition, the above tests are carried out under normal incidence. PL enhancement will gradually attenuate as the incidence angle increase due to resonance dip deviation and diminished dip strength, as shown in Fig. S4 in Supplement 1. When the PL intensity is tested at 45° incident angle, only a 10-fold PL enhancement is detected by Al-coated 390 nm pitch grating (Pattern#1). PL spectra are detected from QDs dispersed on different fluorescent substrates as displayed in Fig. 9(a).
Fig. 9. (a) PL spectra detected from QDs dispersed on Si substrate, single-resonance grating (Pattern #1), dual-resonance grating (Pattern #2) under normal incidence, and Pattern #1 under 45° incident angle (b) Time-resolved photoluminescence spectra for samples coated with the thickness of 35 nm CdSe/CdS/ZnS QDs layer on different fluorescent substrates.
From the perspective of quantum dots, the fluorescence enhancement is ascribed frequently to the increase in the spontaneous emission rate and the internal quantum yield. To further explain the inherent mechanisms, based on Fermi’s golden rule, the room temperature Time-resolved photoluminescence spectra (TRPL) measurements are performed by time-correlated single photon counting. The normalized TRPL decay curves of the QDs deposited on the Si substrate and Al-coated grating (Pattern #1 and #2) are presented in Fig. 9(b). The spontaneous emission decay lifetimes of QDs on different samples are obtained by fitting the experimental photoluminescence decay data using a stretched exponential relaxation model [34], $I(t) = {I_0}\textrm{exp} [{ - {{({t / \tau })}^\beta }} ]$, where I0 is the PL intensity at t = 0, τ is the PL decay time, and $\beta $ is the dispersion exponent. Although the measurement is conducted at a large incident angle due to the instrument configuration, the QDs on the Al-coated grating pattern #2 exhibit an obviously reduced PL decay time of 3.18 ns compared to 6.45 ns on the bare Si substrate. This result is essentially identical to the mechanism reported in the previous investigations [22,35], indicating that the the local field enhancement induced by dual-resonance gratings improve the radiative recombination rate in QDs for the significantly increased emission intensity due to the stronger QDs-resonance mode coupling.
4. Conclusion
We have proposed two kinds of gratings with dual resonance absorptions formed by SPP and other resonances coupling, and demonstrated the PL signal of the QDs can be tremendously enhanced by the Al-coated gratings with dual resonance absorptions owing to the coexistence of various resonance modes coupling with the QDs, the concurrence of the excitation and emission field enhancement of QDs, and all field enhancement concentrated in the QD film. We have experimentally fabricated the proposed grating using the facile method based on holographic lithography. By observing the resulting PL enhancement from fluorescence images, QDs film on the prepared dual-resonance grating can generate overall photoluminescence enhanced by up to 220 times, as compared to bare Si substrate. These results pave the way for fluorescence-enhanced photonic structures to further realize novel functionalities and expand their applications.
Funding
National Natural Science Foundation of China (U2241226).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. G. S. Selopal, H. Zhao, Z. M. Wang, et al., “Core/shell quantum dots solar cells,” Adv. Funct. Mater. 30(13), 1908762 (2020). [CrossRef]
2. N. Sun, D. Malide, J. Liu, et al., “A fluorescence-based imaging method to measure in vitro and in vivo mitophagy using mt-Keima,” Nat. Protoc. 12(8), 1576–1587 (2017). [CrossRef]
3. H. Moon, C. Lee, W. Lee, et al., “Stability of quantum dots, quantum dot films, and quantum dot light-emitting diodes for display applications,” Adv. Mater. 31(34), 1804294 (2019). [CrossRef]
4. C. Liu, Y. Chen, Y. Wang, et al., “A highly efficient fluorescence-based switch-on detection method of 5-formyluracil in DNA,” Nano Res. 10(7), 2449–2458 (2017). [CrossRef]
5. R. Jackson, R. P. Oda, R. K. Bhandari, et al., “Development of a fluorescence-based sensor for rapid diagnosis of cyanide exposure,” Anal. Chem. 86(3), 1845–1852 (2014). [CrossRef]
6. Q. Di, L. Li, X. Miao, et al., “Fluorescence-based thermal sensing with elastic organic crystals,” Nat. Commun. 13(1), 5280 (2022). [CrossRef]
7. J. Sun, Z. Li, Y. Sun, et al., “Uniform and reproducible plasmon-enhanced fluorescence substrate based on PMMA-coated, large-area Au@ Ag nanorod arrays,” Nano Res. 11(2), 953–965 (2018). [CrossRef]
8. J. H. Lin, H. Y. Liou, C.-D. Wang, et al., “Giant enhancement of upconversion fluorescence of NaYF4: Yb3+, Tm3 + nanocrystals with resonant waveguide grating substrate,” ACS Photonics 2(4), 530–536 (2015). [CrossRef]
9. C. Becker, S. Burger, C. Barth, et al., “Nanophotonic-enhanced two-photon-excited photoluminescence of perovskite quantum dots,” ACS Photonics 5(11), 4668–4676 (2018). [CrossRef]
10. H. Naiki, A. Masuhara, S. Masuo, et al., “Highly controlled plasmonic emission enhancement from metal-semiconductor quantum dot complex nanostructures,” J. Phys. Chem. C 117(6), 2455–2459 (2013). [CrossRef]
11. Y. Yang, A. Dev, I. Sychugov, et al., “Plasmon-enhanced fluorescence of single quantum dots immobilized in optically coupled aluminum nanoholes,” J. Phys. Chem. Lett. 14(9), 2339–2346 (2023). [CrossRef]
12. Y. Wang, H. Gao, Y. Liu, et al., “Large-scale controllable fabrication of aluminum nanobowls for surface plasmon-enhanced fluorescence,” Nano Res. 16(7), 10131–10138 (2023). [CrossRef]
13. G. Chen, D. Wang, W. Hong, et al., “Fluorescence enhancement on large area self-assembled plasmonic-3D photonic crystals,” Small 13(9), 1602612 (2017). [CrossRef]
14. L. Trotsiuk, A. Muravitskaya, O. Kulakovich, et al., “Plasmon-enhanced fluorescence in gold nanorod-quantum dot coupled systems,” Nanotechnology 31(10), 105201 (2020). [CrossRef]
15. L. Li, W. Wang, T. S. Luk, et al., “Enhanced quantum dot spontaneous emission with multilayer metamaterial nanostructures,” ACS Photonics 4(3), 501–508 (2017). [CrossRef]
16. H. Kishida and M. H. Mikkelsen, “Ultrafast lifetime and bright emission from graphene quantum dots using plasmonic nanogap cavities,” Nano Lett. 22(3), 904–910 (2022). [CrossRef]
17. S. Liu, A. Vaskin, S. Addamane, et al., “Light-emitting metasurfaces: simultaneous control of spontaneous emission and far-field radiation,” Nano Lett. 18(11), 6906–6914 (2018). [CrossRef]
18. H. Takase, M. Yamauchi, H. Chida, et al., “Enhanced single-photon emission from single quantum dots interacting with a one-dimensional plasmonic chip,” J. Phys. Chem. C 126(11), 5189–5197 (2022). [CrossRef]
19. X. Liu, S. P. McBride, H. M. Jaeger, et al., “Hybrid nanostructures of well-organized arrays of colloidal quantum dots and a self-assembled monolayer of gold nanoparticles for enhanced fluorescence,” Nanotechnology 27(28), 285301 (2016). [CrossRef]
20. S.-Y. Cho, H.-J. Jeon, H.-W. Yoo, et al., “Highly enhanced fluorescence signals of quantum dot–polymer composite arrays formed by hybridization of ultrathin plasmonic Au nanowalls,” Nano Lett. 15(11), 7273–7280 (2015). [CrossRef]
21. P. Yaiwong, C. Lertvachirapaiboon, K. Shinbo, et al., “Tunable surface plasmon resonance enhanced fluorescence via the stretching of a gold quantum dot-coated aluminum-coated elastomeric grating substrate,” Anal. Methods 14(33), 3188–3195 (2022). [CrossRef]
22. J. H. Song, T. Atay, S. Shi, et al., “Large enhancement of fluorescence efficiency from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons,” Nano Lett. 5(8), 1557–1561 (2005). [CrossRef]
23. F. Heydaripour, M. Molaei, M. Karimipour, et al., “Conversion of the yellow to blue emission of CdSe quantum dots (QDs) via ZnSe shell growth,” J. Mater. Sci.: Mater. Electron. 30(12), 11378–11382 (2019). [CrossRef]
24. G. X. Wang, M. S. Park, H. K. Liu, et al., “Synthesis and characterization of one-dimensional CdSe nanostructures,” Appl. Phys. Lett. 88(19), 193115 (2006). [CrossRef]
25. S. Deka, A. Quarta, M. G. Lupo, et al., “CdSe/CdS/ZnS double shell nanorods with high photoluminescence efficiency and their exploitation as biolabeling probes,” J. Am. Chem. Soc. 131(8), 2948–2958 (2009). [CrossRef]
26. A. S. Lambert, S. N. Valiulis, A. S. Malinick, et al., “Plasmonic biosensing with aluminum thin films under the Kretschmann configuration,” Anal. Chem. 92(13), 8654–8659 (2020). [CrossRef]
27. H. R. Gwon and S. H. Lee, “Spectral and angular responses of surface plasmon resonance based on the Kretschmann prism configuration,” Mater. Trans. 51(6), 1150–1155 (2010). [CrossRef]
28. G. Ruffato, E. Pasqualotto, A. Sonato, et al., “Implementation and testing of a compact and high-resolution sensing device based on grating-coupled surface plasmon resonance with polarization modulation,” Sens. Actuators, B 185, 179–187 (2013). [CrossRef]
29. K. Tawa, H. Hori, K. Kintaka, et al., “Optical microscopic observation of fluorescence enhanced by grating-coupled surface plasmon resonance,” Opt. Express 16(13), 9781–9790 (2008). [CrossRef]
30. D. Lynch, “WR Hunter in Handbook of optical constants of solids,” Volume 1, 275 (1985).
31. G. Kang, J. Wang, P. Li, et al., “Modes manipulation within subwavelength metallic gratings,” Plasmonics 11(4), 1169–1174 (2016). [CrossRef]
32. Z. Lin, B. Wang, Z. Huang, et al., “Bi-directional & multi-functional terahertz beam splitter with stacked configuration,” Eur. Phys. J. Plus 137(9), 1094 (2022). [CrossRef]
33. Y. Chai, F. Li, J. Wang, et al., “Enhanced sensing performance from trapezoidal metallic gratings fabricated by laser interference lithography,” Opt. Lett. 47(4), 1009–1012 (2022). [CrossRef]
34. M. Berberan-Santos, E. N. Bodunov, B. Valeur, et al., “Mathematical functions for the analysis of luminescence decays with underlying distributions 1. Kohlrausch decay function (stretched exponential),” Chem. Phys. 315(1-2), 171–182 (2005). [CrossRef]
35. Y. Ito, K. Matsuda, Y. Kanemitsu, et al., “Mechanism of photoluminescence enhancement in single semiconductor nanocrystals on metal surfaces,” Phys. Rev. B 75(3), 033309 (2007). [CrossRef]
