Enhanced Two-Photon Photochromism in Metasurface Perfect

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Enhanced Two-photon Photochromism in Metasurface Perfect Absorbers Xiaojun Liu, Xiaomeng Jia, Martin C Fischer, Zhiqin Huang, and David R. Smith Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02042 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Enhanced Two-photon Photochromism in Metasurface Perfect Absorbers Xiaojun Liu1,*, Xiaomeng Jia2, Martin Fischer2,3, Zhiqin Huang1, and David R. Smith1,2 1. Center for Metamaterial and Integrated Plasmonics, Department of Electrical and Computer Engineering, Pratt School of Engineering, Duke University, Box 90291, Durham, NC 27708 2. Department of Physics, Duke University, Box 90305, Durham, NC 27708 3. Department of Chemistry, Duke University, Box 90354, Durham, NC 27708 *[email protected]

Light switchable materials are essential to optoelectronic applications in photovoltaics, memories, sensors, and communications. Natural switchable materials suffer from weak absorption and slow response times, preventing their use in low-power, ultra-fast applications. Integrating light switchable materials with metasurface perfect absorbers offers an innovative route to achieving desirable features for nanophotonic devices, such as directional emission, low-power and broadband operation, high radiative quantum efficiency and large spontaneous emission rates. Here we show an enhanced two-photon photochromism based on a metasurface perfect absorber: film-coupled colloidal silver nanocubes. The photochromic molecules – spiropyrans – are sandwiched between the silver nanocubes and the gold substrate. With nearly 100% absorption and an accompanying large field enhancement in the molecular junction, the transformation of spiropyrans to merocyanines is observed under excitation with 792 nm laser light. Fluorescence lifetime measurements on the merocyanine form reveal that large Purcell enhancement in the film-coupled nanocubes leads to large enhancements of the

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spontaneous emission rate and a high quantum efficiency. An averaged incident power as low as 10 µW is enough to initiate the two-photon isomerization of spiropyran in the film-coupled nanocubes, and a power of 100nW is able to excite the merocyanines to emit fluorescence. The power consumption is orders of magnitude lower than bare spiropyran thin films on silicon and gold, which is highly desirable for the writing and reading processes relevant to optical data storage. By sweeping the plasmonic resonance of the film-coupled nanocubes, wavelength specificity is demonstrated, which opens up new possibilities for minimizing the cross talk between adjacent bits in nanophotonic devices.

Keywords Metasurface; Photochromism; Two photon absorption; Plasmonics


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Introduction With increased data rates in information technology there is an increasing demand for high-performance switches and memory devices—especially data storage devices that have immense storage capacity, fast reading and writing rate, small volume, minimal cross talk between adjacent bits, and high reading sensitivity1, 2. Photochromic materials, which undergo reversible isomerization with absorption of electromagnetic radiation, have been shown to be a promising class of light sensitive materials for optical switches and memory devices. Spiropyrans have been widely studied for their reversible transformation to merocyanines under the exposure of UV light, and back to spiropyrans under visible light or heat. The UV-induced conversion process corresponds to a change in absorption spectra that enables strong absorption of visible light in the 550-600 nm band, corresponding to change of the color from transparent (spiropyrans) to deep blue (merocyanines). This process also results in a distinct change in emission behavior: the spiropyrans do not exhibit strong fluorescence emission, while the merocyanines show strong fluorescence emission centered at 650 nm3. These one-photon conversion processes have been demonstrated for nano-writing, memory devices and switches3-6. Spiropyrans can also be converted via a two-photon process, which enables the photochromic switching with near infrared light7-9. The two-photon process has the potential to be applied in many nanophotonic devices, especially optical storage devices, where the two-photon absorption is used during the writing process and one-photon absorption is used during reading/erasing process1, 2. One advantage of two-photon absorption in memory devices is decreased cross talk between adjacent data units. Since the probability of switching is proportional to the squared intensity of the incident field,


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the area of excitation is limited to a small area of the focus spot. However, high peak intensities are required for the two-photon process, and the spot size of focusing is limited

by the

diffraction

limit.

Furthermore,

the

fluorescence

lifetime

of

spiropyran/PMMA blends was found to be 5 ns, which limits the speed of response. Recently, integrating photochromic molecules with surfaces has drawn considerable attention for its potential for developing molecule-based optical devices10-13. Plasmonic metasurfaces, due to their ability to localize and enhance light at the subwavelength scale, are excellent candidates for ultra-fast switches and memory applications. By incorporating light sensitive materials with metasurfaces, the photochemical transformation can be induced within a single, a few, or a cluster of nanoantennas, scaling down the switching or memory unit to the subwavelength scale14-20. Associated with plasmonic field enhancement, Purcell enhancement introduced by nanoantennas leads to significant enhancement of the spontaneous emission rate, high fluorescence enhancement factors, and high quantum yield, which are desirable for fast data access21-25. Furthermore, owing to the high one- and two- photon absorption cross sections, nanoantennas facilitate ultra-low power consumption—an important feature in terms of minimal impact on the environment14, 16. Coupled nanoantennas are particularly interesting, since the hybridization of the plasmonic resonances associated with each individual antenna can lead to extremely large localized fields between the nanostructures. Coupled nanoantennas, such as bowties, gap-antennas, and film-coupled nanoparticles have been widely investigated for their scattering and field enhancement characteristics2629

, as well as nonlinear processes30-33 and photoluminescent properties22, 23, 25, 34. The film-

coupled colloidal silver nanocube platform has shown particular advantages for


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controlling radiative processes, since large-area “perfect” absorbing metasurfaces can be created by simple dip-coating methods26, 35, with additional desirable features such as directional emission, low-power and broadband operation,

high radiative quantum

efficiency, and large spontaneous emission rates21-23. Here, we integrate spiropyran/PMMA blends into a perfect absorbing metasurface—film-coupled colloidal silver nanocubes—which potentially enables ultralow power consumption for two-photon writing processes with ultra-fast response time for one-photon readout processes. The metasurface consists of 110 nm colloidally synthesized silver nanocubes densely spread over a gold substrate coated with ~4 nm spiropyran/PMMA blends and ~3 nm polyethylene layers, as shown in Fig. 1 (a). The resulting geometry corresponds to an optical patch antenna, with a region of field enhancement just below the nanocube. When the system is excited at resonance, most of the incident light is absorbed, leading to a strong dip in the reflectance. As has been shown in previous work, at resonance strong local electric fields are produced in the junction region, most strongly around the cube edges. With approximately 100% absorption on resonance and a field enhancement of over 200 in the molecular junction, the spiropyran molecules undergo isomerization to merocyanines by the absorption of two 792 nm photons. A 583 nm laser is used to probe the state of spiropyrans/mecrocyanines, giving rise to fluorescent emission centered at 650 nm. Fig. 1 (d) illustrates the energy level diagram of the two- and one-photon processes of one molecule in the junction. The large field enhancement and accompanying increased photonic density-ofstates result in modification of the spontaneous emission rate of the merocyanines in the


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gap region. The emission from a merocyanine molecule can be modeled as a monochromatic point-dipole emitting at 650 nm. The spontaneous decay rate of the dipole is given by 22

γ sp ( r ) =

πω 2 0 p ρ ( r , ω ) + γ int , 3hε 0

0 where ω is the emission frequency, p is the transition dipole moment, and γ int is the

internal non-radiative decay rate of the dipole. The first term corresponds to the radiative spontaneous emission rate, γr. Since the dipole is in the gap region of the film-coupled nanocubes, the photonic density of states ρ ( r, ω ) is modified from its free space value by the local electromagnetic environment. Through numerical simulations, we calculate the spontaneous emission rate of a dipole in the gap region of the film-coupled nanocube. Fig. 1(e) shows the relative spontaneous emission rate γ sp γ sp0

, where γ sp0 is the

emission rate in free space. The rate enhancement reaches 350 at the center of the nanocubes when assuming the dipole is oriented along the electric field, resulting in a shortened spontaneous lifetime. The radiative quantum efficiency, which is given by

QE = γ r γ sp , is spatially uniform and is larger than 0.3 over the gap region (Fig. 1 (f)). The color map represents the values of QE, where we see that QE reaches its maximum (>0.35) at the center of the cube. Fig. 1(f)). Therefore, the film-coupled nanocubes exhibit multiple features desirable for enhanced photochromism (1) Perfect absorption for the two-photon writing; (2) high radiative rate and high quantum efficiency for the one-photon readout. Fig. 2 shows the time-resolved emission of merocyanines from (a) film-coupled nanocubes, (b) on a gold film, and (c) on a silicon substrate. The samples are excited at


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583nm, and the emission is measured at (650 ± 5) nm. The film-coupled nanocubes have a gap thickness of 7 nm, which consists of a (4.12 ± 0.25) nm thick spiropyran/PMMA blend and a (2.83 ± 0.21) nm thick polyethylene layer, as confirmed by ellipsometry (see Methods). The nanocubes are uniformly and densely deposited (~20 µm-2). The gold film and the silicon samples are coated with the same spiropyran/PMMA blends and polyethylene layers with the same thicknesses. Comparing to the gold film sample and the silicon sample, a significant shortening of the merocyanines lifetime is seen in the film-coupled nanocubes sample. The lifetime can be determined by fits to the data deconvolved with the instrument response function (IRF)36. The lifetime for the silicon sample is ~574 ps. The lifetime for the gold film sample is ~209 ps, which exhibits an increased decay rate due to the coupling of the emission to surface-plasmons37. For the film-coupled nanocubes sample, the spontaneous decay is so fast that the measured lifetime is limited by the IRF of the system. As the film-coupled nanocube system has been demonstrated to present large spontaneous emission rate enhancement, the actual lifetime of the nanocube sample is estimated to be much smaller than the temporal resolution limit of the single photon counting photomultiplier tube (~180 ps)22. A significant reduction in the fluorescence lifetime is seen in the nanocube sample when comparing to the gold and silicon samples. Fig. 2 also compares the time-resolved emission of merocyanines before and after the two-photon absorption of the 792 nm light. A 68-fold enhancement in the fluorescent emission intensity is seen in the nanocube sample, after a 3.6 ms average exposure to 40 µW, 792nm laser light focused by a 40X, NA = 0.7 objective (see Methods). This enhancement indicates massive isomerization of spiropyrans to merocyanines induced by


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two-photon absorption of 792nm light, due to the large field enhancement in the gap region of the nanocube sample. By contrast, even as the power of the 792 nm laser is increased to 1.2 mW, there is no obvious change in the fluorescence emission intensity for the gold film and silicon samples, indicating extremely weak two-photon absorption. As a reference, we verify the isomerization process in the gold film and silicon samples under UV radiation. After exposing the gold film and silicon samples to a 670 µW continuous laser centered at 405 nm for the same amount of time, the silicon sample shows an 18-fold enhancement in the fluorescence emission intensity, while the gold film sample has a 6-fold enhancement due to non-radiative quenching close to the gold surface. To confirm that the nonlinear absorption is indeed a two-photon process, we performed a power dependence study on the film-coupled nanocubes sample. The fluorescence intensity at 650 nm is measured with the excitation wavelength fixed at 583 nm and the pump wavelength at 792 nm. As the spiropyran and merocyanine forms are in equilibrium in spiropyran/PMMA blends, the merocyanine form always presents and emits fluorescence centered at 650 nm. We first measured the power dependence at 583 nm before two-photon absorption by varying the power from 100 nW to 15 µW using a neutral density filter. The sample was then exposed to 40 µW light at 792 nm for ~3.6 ms. Due to the two-photon photochromism, spiropyran molecules undergo isomerization, resulting in a large amount of merocyanines and an associated strong fluorescence emission. To avoid the saturation of the detector, we vary the power of 583 nm light from 2 nW – 1 µW. The results are detailed in Fig. 3 (a). Blue circles show the power dependence of 583 nm light before exposing to 792 nm light, while the red circles


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correspond to measurements after two-photon absorption. The two lines are parallel and are fitted to a line with a slope close to 1, indicating the linear dependence of the fluorescence emission against the excitation power. At least a 40-fold enhancement is seen between the two lines over a large range of excitation power, which can essentially serve as the ON(1) and OFF(0) states. For the power dependence study of the pump beam, we fixed the 583 nm light at an optimized power, 100 nW, and varied the power of the 792 nm light from 1 - 64 µW. The results are detailed in Fig. 3 (b), where the data is fitted to a line with slope = 2.15, where A = 5.06*103, B = 1.30*105. B represents the fluorescent emission before two-photon absorption, which is close to the measured value. This measurement verifies the nonlinear absorption of the 792nm light is indeed a twophoton process. When the pump power is larger than 20 µW, the fluorescence emission starts to deviate from the power rule. This deviation may result from two competing effects: (1) as the pump power increases, more spiropyrans are isomerized to merocyanines; and (2) the large localized fields in the gap region causes local heating, converting the merocyanines back to spiropyrans. As a result, the isomerization reaction is close to equilibrium. In our experiment, we choose 40 µW as the optimized pump power, under which the merocyanines in the gap region are close to maximum. To test information storage capacity, patterned “smiley face” samples were fabricated using electron-beam lithography. As shown in Fig. 4, the dimensions of each of the patterns are 60 µm, 50 µm, 40 µm, 30 µm, and 20 µm, while widths of the metal lines are 6 µm, 5 µm, 4 µm, 3 µm, and 2 µm, respectively. Similar to the uniform nanocube samples, a layer of (4.12 ± 0.25) nm spiropyran/PMMA blend was spin-coated on top of the patterned sample, followed by layer-by-layer deposition of (2.83 ± 0.21) nm


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polyethylene layers. 110 nm silver colloidal nanocubes were spread densely and uniformly on top by a simple dip-coating method. Figure 4 (a) shows the SEM images of the patterned sample and the distribution of the nanocubes. The patterned samples consist of two parts: (1) spiropyrans in the nanocavities defined by the nanocubes and gold film; (2) spiropyrans between silicon wafer and the nanocubes (no gold film). Note that only the regions defined by film-coupled nanocubes allow for the two-photochromism. The samples were measured using a 0.5 µW, 583 nm laser with and without the exposure to 40 µW, 792 nm light. Fig. 4 (b) and (c) show the fluorescence intensity images before and

after

two-photon

photochromism,

respectively.

Before

the

two-photon

photochromism, the fluorescence intensity is low across the entire sample, and the filmcoupled nanocubes provide little enhancement of the fluorescence. After the twophotochromism, the spiropyrans within the nanocavities are isomerized to merocyanine, leading to an over 40-fold enhancement in fluorescence emission in the patterned region. For the regions with only cubes and silicon substrate, there is no significant local field enhancement at 792nm due to the absence the nanocavities. Yet, the silver nanocubes provide some local field enhancement at 792nm that leads to weak two-photon absorption, giving rise to an 10-fold enhancement in fluorescence emission in the non-patterened region. Spiropyrans have shown a broad wavelength range for one-, two-, and threephoton absorption. However, a narrow absorption band is desirable to minimize the cross talk between adjacent units. The film-coupled nanocubes facilitate a narrow, tunable absorption band over the photochromic molecular spectrum. To demonstrate this effect, a set of film-coupled nanocube samples with different gap sizes were fabricated to resonate


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at different wavelengths. To make the samples comparable, we fixed the thickness of the spiropyran/PMMA blend to be ~4 nm, but varied the thickness of polyethylene using layer-by-layer deposition38. The resulting film-coupled nanocubes samples are resonant at 702.9 nm, 722.5 nm, 756.5 nm, 787.4 nm, 811.8 nm, and 842.5 nm, as shown in Fig. 5(a). Fig. 5(b) shows the absorption characteristics of these samples at 792 nm–the pump wavelength, and 650 nm–the fluorescence emission wavelength. The absorption at 792 nm reaches a maximum for the sample resonant at 792 nm, while the absorption at 650 nm decreases as the resonance red-shifts. The samples were measured using a 0.1 µW, 583 nm laser with and without exposure to 40 µW, 792 nm light. The fluorescence enhancement was calculated by comparing the fluorescence intensity before and after the two-photon absorption. The sample resonance exactly at 792 nm exhibits the highest fluorescence enhancement, due to the near 100% coupling of the 792 nm light into the nanocavity. As the resonance is blue-shifted or red-shifted, less energy from the 792 nm light is coupled to the nanocavity, and consequently fewer spiropyrans within the nanogap regions are isomerized to merocyanines. As a result, the fluorescence enhancement decreases when the resonance of the sample is tuned red or blue. This trend is due to two factors: (1) the absorption at the pump wavelength—792 nm; (2) the absorption at the fluorescence wavelength—650 nm. The most blue shifted sample shows the lowest fluorescence enhancement, since it couples the least energy at the pump wavelength, but absorbs most energy at the fluorescence wavelength. The most red shifted sample couples only 70% of the energy at the pump wavelength; however, due to the low absorption at 650 nm, the fluorescence intensity drops by only 20% when compared to the sample resonant at 792 nm.


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In summary, we have shown that film-coupled colloidal silver nanocubes can dramatically enhance the two-photon photochromism of spiropyrans embedded in the gap region of a plasmonic nanostructure. The nanocavities defined by the silver nanocubes and the gold film are capable of coupling nearly 100% of the pump light, thus inducing massive isomerization of spiropyrans to merocyanines. The film-coupled nanocubes produce large Purcell enhancements, with directional emission and high quantum efficiency. For merocyanines embedded within the nanocavities, this enhancement translates to a lower excitation power and significantly shorter emission lifetime. These effects facilitate low power reading and writing if applied to optical data storage. We show that a power as low as 10 µW is sufficient to induce the two-photon isomerization of spiropyran in the film-coupled nanocubes, while a power of 10 nW is able to excite the merocyanines to emit fluorescence. By sweeping the plasmonic resonance of the filmcoupled nanocubes, we demonstrate the two-photon absorption is maximized at resonance. This spectral filtering creates a selectable wavelength region within the spectrum where the photochromic molecules can absorb, and can consequently minimize the cross talk between adjacent bits when applied to optical data storage devices. Note that spiropyrans, though showing desirable properties, are not optimized for practical optical devices. Merocyanines are sensitive to heat and are thus not stable enough to act as a storage material. The spiropyran-PMMA blends show significant fatigue after only several isomerization cycles. However, spiropyrans are not the only molecules that exhibit two-photon photochromism, and are not the only molecules that can be integrated with the metasurface perfect absorbers. The metasurface platform has the flexibility to enhance the two-photon absorption of other photochromic materials, many of which may


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have better characteristics for storage application. For example, thermo-stable photochromic polymers, such as fulgides and anthracenes, can be integrated with metasurface platform for future applications39, 40. To obtain storage materials with high fatigue resistance, copolymerization of the photochromic molecule and the corresponding monomer (PMMA in this paper) is desirable, which has been shown to sustain hundreds of on-off cycles41-43. Furthermore, metasurface perfect absorbers may also be extended into 3D platforms by distributing coupled colloidal clusters within the photochromic materials for functional photonic devices. Methods Simulations COMSOL MultiPhysics was used to calculate the field distribution and scattering properties of the film-coupled nanocubes. Since the nanocubes are randomly distributed over a substrate with a density of ~20 µm-2, we assume the collective properties of the surface correspond to the properties of a single 110nm silver nanocube separated from a gold substrate by a 7 nm spacer layer. The refractive index of the spacer layer was chosen to be 1.45, which is a typical value for organic layers. The spiropyran molecule was modeled as a monochromatic dipole with an emission wavelength of 650 nm. To evaluate the emission properties of the dipole coupled to the nanocavity defined by the filmcoupled nanocube, the Green’s function of the system was calculated by varying the spatial position of the dipole on a 15-by-15 grid under the nanocube. The total decay was then calculated from the Green’s function. The radiative quantum efficiency was estimated by comparing the radiative decay rate and the total decay rate. The radiative decay rate was found by subtracting the nonradiative decay rate from the total decay rate,


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where the nonradiative decay rate was evaluated by integrating all metal losses in the system23. Sample fabrication The fabrication procedure is summarized in Fig. S2. 4% PMMA in anisole (MicroChem 495PMMA-A4) was diluted with pure anisole to obtain 0.2% PMMA. A spiropyran colorant, 1′,3′-Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] (Sigma-Aldrich), was dissolved in 0.2% PMMA solution to obtain a 3mM spiropyran solution. The solution was spin-coated on 100-nm-thick gold substrates on silicon wafers (Platypus technologies), at a speed of 3000 rpm for 2 minutes. The thin layer was hardened by baking the sample in a vacuum oven at 70oC for 30 mins. Four polyelectrolyte (PE) multilayers were grown on top of the spiropyran/PMMA thin layer. The sample is merged in a 3mM poly(strenesulphonate) (PSS) and 1M NaCl solution for 5 mins, and rinsed with a 1M NaCl solution for 1 min, followed by immersion in a 3mM poly(allylamine) hydrochloride (PAH) and 1M NaCl solution for 5 mins, and rinsing in 1M NaCl for 1 min. After the desired number of polymer layers was reached, the sample was thoroughly rinsed in water. 110 nm silver nanocubes with a 3 nm polyvinylpyrrolidone (PVP) coating (Nanocomposix) were deposited on top of the substrate. A 5 µL aqueous nanocube solution was dropped on the surface of the substrate, and incubated for 50 mins at 4oC. The negatively charged nanocubes facilitate electrostatic adhesion to the positively charged top polymer layer (PAH) of the substrate, forming a uniformly, densely distributed layer of nanocubes. The nanocubes not adhered to the substrate were removed by rinsing the substrate with water. The final density of the nanocubes on the substrate


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was ~20 µm-2, which is confirmed by scanning electron microscopy. The samples were stored in the dark to avoid fatigue of the spiropyran, and measured within two days of fabrication to avoid silver oxidation. The patterned samples in Fig.4 were fabricated using electron beam lithography. 75 nm gold was evaporated on Si wafer with a 5 nm titanium adhesion layer, followed by the same fabrication procedures of spiropyran/PMMA and PE layers, and the same procedure of depositing nanocubes. Optical measurements The reflectance of the samples was measured using Fourier transform infrared spectroscopy (FTIR) with a 36X, NA = 0.5 objective.To confirm the uniform distribution of the nanocubes, each sample was measured at three different locations, and the resonances were found to be around (792 ± 5) nm. The time-resolved fluorescence emission was measured using a fluorescence lifetime imaging system (FLIM), as shown in Fig. S3. The pump pulses for TPA were from a Coherent Chameleon Ultra II (792nm, 80MHz, ~100 fs duration). A portion of the beam was sent to a Coherent Miro OPO and was converted to 583 nm to serve as excitation source for fluorescence. The beams were focused on the sample by a 40X, NA = 0.7 Nikon objective. The focal spot size was nearly diffraction limited. The laser beams were steered by an electrically controlled scanning mirror to scan over a field of view (FOV) of 180 µm, with 128 pixels per line. The pixel scanning rate is 31250 s-1. To induce the transformation of spiropyran to merocyanine form by TPA, the pump light at 792 nm was focused on the sample for 1min. The average dwell time on each pixel was ~3.6 ms. The pump beam was then blocked, and the excitation source for fluorescence at 583 nm was


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unblocked and focused onto the sample. Fluorescence emission was collected through the objective, reflected by a dichroic mirror (Semrock FF625), passed through a 600nm longpass filter (Edmund 62985) and a 650 ± 5 nm band-pass filter (Edmund 33336) to remove the excitation light, and imaged onto a single-photon counting photomultiplier tube (PMC-100-4). The PMT was connect to a time-correlated single photon counting module (Becker-Hickl, SPC-150), which enables the recording of the fluorescence intensity as well as the lifetime at each pixel. The temporal resolution was around 180 ps. Each image was acquired at a resolution of 128x128 with 5s integral time, repeated 5 times. For the uniform film-coupled nanocube samples, all the pixels from the same image were summed to get the total fluorescence intensity or the lifetime curve. Acknowledgement This work was supported by the Air Force Office of Scientific Research (AFOSR, Grant No. FA9550-12-1-0491, Grant No. FA9550-18-1-0187). We thank Warren S. Warren for providing the optical measurement apparatus.

Supporting Information - Numerical simulation of excitation field enhancements - Fluorescence lifetime imaging system setup and IRF - Fatigue measurements


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References 1. Dvornikov, A. S.; Walker, E. P.; Rentzepis, P. M. The Journal of Physical Chemistry A 2009, 113, (49), 13633-13644. 2. Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, (4920), 843-845. 3. Klajn, R. Chemical Society Reviews 2014, 43, (1), 148-184. 4. Seefeldt, B.; Kasper, R.; Beining, M.; Mattay, J.; Arden-Jacob, J.; Kemnitzer, N.; Drexhage, K. H.; Heilemann, M.; Sauer, M. Photochemical & Photobiological Sciences 2010, 9, (2), 213-220. 5. Triolo, C.; Patanè, S.; Mazzeo, M.; Gambino, S.; Gigli, G.; Allegrini, M. Opt. Express 2014, 22, (1), 283-288. 6. Berkovic, G.; Krongauz, V.; Weiss, V. Chemical Reviews 2000, 100, (5), 17411754. 7. Matczyszyn, K.; Olesiak-Banska, J.; Nakatani, K.; Yu, P.; Murugan, N. A.; Zaleśny, R.; Roztoczyńska, A.; Bednarska, J.; Bartkowiak, W.; Kongsted, J.; Ågren, H.; Samoć, M. The Journal of Physical Chemistry B 2015, 119, (4), 1515-1522. 8. Lin, L.; Zhang, Z.; Lu, Z.; Guo, Y.; Liu, M. The Journal of Physical Chemistry A 2016, 120, (40), 7859-7864. 9. Dvornikov, A. S.; Malkin, J.; Rentzepis, P. M. The Journal of Physical Chemistry 1994, 98, (27), 6746-6752. 10. Baron, R.; Onopriyenko, A.; Katz, E.; Lioubashevski, O.; Willner, I.; Wang, S.; Tian, H. Chemical Communications 2006, (20), 2147-2149. 11. Browne, W. R.; Feringa, B. L. Annual Review of Physical Chemistry 2009, 60, (1), 407-428. 12. Wesenhagen, P.; Areephong, J.; Fernandez Landaluce, T.; Heureux, N.; Katsonis, N.; Hjelm, J.; Rudolf, P.; Browne, W. R.; Feringa, B. L. Langmuir 2008, 24, (12), 63346342. 13. Areephong, J.; Browne, W. R.; Katsonis, N.; Feringa, B. L. Chemical Communications 2006, (37), 3930-3932. 14. Kunwar, P.; Hassinen, J.; Bautista, G.; Ras, R. H. A.; Toivonen, J. 2016, 6, 23998. 15. Fu, S.; Zhang, X.; Han, Q.; Liu, S.; Han, X.; Liu, Y. 2016, 6, 36701. 16. Nozaki, K.; Shinya, A.; Matsuo, S.; Suzaki, Y.; Segawa, T.; Sato, T.; Kawaguchi, Y.; Takahashi, R.; Notomi, M. Nat Photon 2012, 6, (4), 248-252. 17. Ditlbacher, H.; Krenn, J. R.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Opt. Lett. 2000, 25, (8), 563-565. 18. Bouhelier, A.; Bachelot, R.; Lerondel, G.; Kostcheev, S.; Royer, P.; Wiederrecht, G. P. Physical Review Letters 2005, 95, (26), 267405. 19. Zijlstra, P.; Chon, J. W. M.; Gu, M. Nature 2009, 459, (7245), 410-413. 20. Ren, H.; Li, X.; Zhang, Q.; Gu, M. Science 2016, 352, (6287), 805-809. 21. Ciracì, C.; Rose, A.; Argyropoulos, C.; Smith, D. R. Journal of the Optical Society of America B 2014, 31, (11), 2601-2607. 22. Akselrod, G. M.; Argyropoulos, C.; Hoang, T. B.; Ciracì, C.; Fang, C.; Huang, J.; Smith, D. R.; Mikkelsen, M. H. Nat Photon 2014, 8, (11), 835-840. 23. Rose, A.; Hoang, T. B.; McGuire, F.; Mock, J. J.; Ciracì, C.; Smith, D. R.; Mikkelsen, M. H. Nano Letters 2014, 14, (8), 4797-4802.


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24. Bakker, R. M.; Yuan, H.-K.; Liu, Z.; Drachev, V. P.; Kildishev, A. V.; Shalaev, V. M.; Pedersen, R. H.; Gresillon, S.; Boltasseva, A. Applied Physics Letters 2008, 92, (4), 043101. 25. Hoang, T. B.; Akselrod, G. M.; Argyropoulos, C.; Huang, J.; Smith, D. R.; Mikkelsen, M. H. 2015, 6, 7788. 26. Moreau, A.; Ciraci, C.; Mock, J. J.; Hill, R. T.; Wang, Q.; Wiley, B. J.; Chilkoti, A.; Smith, D. R. Nature 2012, 492, (7427), 86-89. 27. Lin, L.; Zheng, Y. 2015, 5, 14788. 28. Paolo, B.; Jer-Shing, H.; Bert, H. Reports on Progress in Physics 2012, 75, (2), 024402. 29. Hanke, T.; Cesar, J.; Knittel, V.; Trügler, A.; Hohenester, U.; Leitenstorfer, A.; Bratschitsch, R. Nano Letters 2012, 12, (2), 992-996. 30. Nevet, A.; Berkovitch, N.; Hayat, A.; Ginzburg, P.; Ginzach, S.; Sorias, O.; Orenstein, M. Nano Letters 2010, 10, (5), 1848-1852. 31. Liu, X.; Larouche, S.; Bowen, P.; Smith, D. R. Opt. Express 2015, 23, (15), 19565-19574. 32. Lassiter, J. B.; Chen, X.; Liu, X.; Ciracì, C.; Hoang, T. B.; Larouche, S.; Oh, S.H.; Mikkelsen, M. H.; Smith, D. R. ACS Photonics 2014, 1, (11), 1212-1217. 33. Harutyunyan, H.; Volpe, G.; Quidant, R.; Novotny, L. Physical Review Letters 2012, 108, (21), 217403. 34. Lee, B.; Park, J.; Han, G. H.; Ee, H.-S.; Naylor, C. H.; Liu, W.; Johnson, A. T. C.; Agarwal, R. Nano Letters 2015, 15, (5), 3646-3653. 35. Akselrod, G. M.; Huang, J.; Hoang, T. B.; Bowen, P. T.; Su, L.; Smith, D. R.; Mikkelsen, M. H. Advanced Materials 2015, 27, (48), 8028-8034. 36. Enderlein, J.; Erdmann, R. Optics Communications 1997, 134, (1), 371-378. 37. Chance, R. R.; Prock, A.; Silbey, R. The Journal of Chemical Physics 1975, 62, (6), 2245-2253. 38. Guo, Y.; Geng, W.; Sun, J. Langmuir 2009, 25, (2), 1004-1010. 39. Yokoyama, Y. Chemical Reviews 2000, 100, (5), 1717-1740. 40. Dvornikov, A. S.; Rentzepis, P. M. Research on Chemical Intermediates 1996, 22, (2), 115-128. 41. Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. Macromolecules 2002, 35, (25), 9377-9382. 42. Radu, A.; Byrne, R.; Alhashimy, N.; Fusaro, M.; Scarmagnani, S.; Diamond, D. Journal of Photochemistry and Photobiology A: Chemistry 2009, 206, (2), 109-115. 43. Zhu, M.-Q.; Zhu, L.; Han, J. J.; Wuwei, W.; Hurst, J. K.; Li, A. D. Q. Journal of the American Chemical Society 2006, 128, (13), 4303-4309.


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FIGURE CAPTIONS Figure 1. Film-coupled nanocubes (a) Schematic of silver nanocubes deposited on a gold film coated spiropyran/PMMA blend and PE layers. (b) Absorption spectrum of filmcoupled 110nm silver nanocubes resonance around 792nm (blue). The absorption is 97% at resonance. Emission spectrum of spiropyran/PMMA blend on silicon substrate (red). The fluorescence emission is centered at 650nm. (c) Magnetic field distribution in the gap region when at resonance. (d) Scheme of one- and two- photon process of spiropyrans. Enhancement of spontaneous emission rate (e) and quantum efficiency (f) relative to a dipole in free space as a function of position under the nanocube.

Figure 2. Time-resolved emission of merocyanines from (a) film-coupled nanocubes; (b) gold substrate; (c) silicon substrate. Blue lines: before two-photon absorption. Red lines: after two-photon absorption. Yellow lines: after UV exposure.

Figure 3. Power dependence study at (a) excitation wavelength (583nm), and (b) pump wavelength (792nm). A = 5.06*103, B = 1.30*105.

Figure 4. (a) SEM image of the pattern sample. (b) Fluorescent intensity image in log scale before two-photon absorption. (c) Fluorescent intensity image in log scale after two-photon absorption.

Figure 5. Specificity of wavelength of the system. (a) The reflectance spectra of the filmcoupled nanocubes with various thickness of the PE layers. (b) Absorptions at 792nm (circles) and 650nm (stars) of samples with different resonance wavelengths in (a). (c) Fluorescent enhancement of various samples with different resonance wavelengths in (a). The enhancement is calculated as the ratio between the fluorescent intensity after and before the two-photon absorption.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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TOC


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Enhanced Two-Photon Photochromism in Metasurface Perfect

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