Enhanced Upconversion Luminescence by Two-Dimensional

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Enhanced upconversion luminescence by two-dimensional photonic crystal structure Chenchen Mao, Kyungtaek Min, Kyuyoung Bae, Suehyun Cho, Tian Xu, Heonsu Jeon, and Wounjhang Park ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00756 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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Enhanced upconversion luminescence by two-dimensional photonic crystal structure

Chenchen Mao,1 Kyongtaek Min,2 Kyuyoung Bae,1 Suehyun Cho,1 Tian Xu,3 Heonsu Jeon4 and Wounjhang Park1,*

1Department

of Electrical Computer & Energy Engineering, University of Colorado, Boulder, CO 80309-0425, U.S.A. 2Department

of Nano-Optical Engineering, Korea Polytechnic University, Siheung 15073, Republic of Korea 3Department

of Physics, Nantong University, Nantong, Jiangsu 226007, China

4Department

of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of

Korea *Corresponding

author: [email protected]

Abstract Upconversion nanoparticles (UCNPs) convert near-infrared excitation into visible emission with efficiencies far greater than those of two-photon absorption or second harmonic generation, enabling upconversion with low intensity, incoherent light. For widespread applications, however, further enhancement of upconversion efficiency is desired. Photonic crystal (PhC) structure embedded with UCNPs provides a new way to engineer the photonic environment and enhance upconversion luminescence. We incorporate silica-coated UCNPs into a two-dimensional (2D) thin film PhC structure, which exhibits enhanced local electric field at the near-infrared (NIR) excitation wavelength of UCNPs. Thanks to the nonlinearity of the upconversion process, the local field enhancement is amplified and results in a significantly enhanced luminescence intensity. We observed approximately 130 and 350-fold enhancements for green and red luminescence, respectively, and present a detailed analysis of the enhancement mechanism. Unlike the plasmonic nanostructure, which tends to cause severe luminescence quenching, the purely dielectric photonic crystal structure generally shows little quenching and provides a good alternative for many applications. Keyword: Luminescence upconversion, Upconversion nanoparticle, Photonic crystal, Selfassembly, Photoluminescence

Rare-earth activated upconversion nanoparticles (UCNPs) have begun to attract much research interest in the past decades. Their unique property of converting near-infrared (NIR) photons into visible (VIS) light with high efficiency makes them attractive for various applications including solar energy conversion,1 bioimaging2 and therapeutics.3 UCNPs are particularly appealing for biomedical imaging and therapeutic applications because they don’t blink, don’t bleach and are

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highly robust. Among the various rare-earth doped nanomaterials, NaYF4 is known to be one of the most suitable host materials due to its small phonon energy,4 which results in inefficient multiphonon relaxation process and thereby high radiative transition efficiency. By doping with Yb3+ and Er3+ ions, visible emission is generated from Er3+ ions via NIR absorption and energy transfer by Yb3+ ions. This energy transfer upconversion (ETU) process is much more efficient than other frequency conversion processes, such as two-photon absorption and second harmonic generation which involve virtual states and therefore exhibit low efficiencies. As a result, the ETU process does not require phase matching and can be excited by low-intensity, incoherent light. For widespread applications, however, it is desirable to further enhance the luminescence efficiency and nanophotonics offer attractive solutions for this purpose. There is a large body of work on plasmonic nanostructures for upconversion enhancement.4-5 Surface plasmon is highly effective in localizing light and creating strong local electric field, which can be used to enhance upconversion. However, the mode volume where electric field is enhanced tends to be small and thus enhancement over large areas is generally difficult to achieve. Furthermore, metal inevitably quenches luminescence and thus the plasmonic nanostructure must be designed carefully to best utilize plasmonic field while minimizing the effect of quenching. In contrast, photonic crystals (PhCs) are dielectric nanostructures and thus exhibit less luminescence quenching than metallic nanostructures. Furthermore, by using band edge states, it is possible to excite optical resonances extended over the entire photonic structure, thereby achieving field enhancement over a large area.6 Incorporating UCNPs into PhCs has been attempted with some success. For example, upconversion intensity enhancement factors of 5 ~ 35 have been reported by using artificial opal structures.7 UCNPs deposited on top of an anodized alumina (AAO) exhibited the greatest enhancement reported so far with an enhancement factor of 65.8 Increased lifetime of upconverted luminescence has also been reported.9 While these results are promising, higher enhancement is desirable. A key factor that limits the enhancement in the previous works is the poor overlap between UCNPs and the regions of high electric field, as well as low index contrast. A vast majority of the previous reports used artificial opal structures which offer very small void space to embed UCNPs in. Furthermore, the void spaces are often not the regions of high electric field, leading to poor enhancement. In the AAO work, the UCNPs are deposited on top of the nanostructure and thus the interaction between UCNPs and AAO is weak due to poor overlap.8 In this study, we utilized a self-assembly technique that forces UNCPs into the nanoholes of PhC structure where electric field is strongly enhanced. The excellent overlap between UCNPs and the PhC’s hot spots results in a very high enhancement in upconversion luminescence: 130x for green and 350x for red emission. To our knowledge, this is the highest enhancement factors achieved by a purely dielectric PhC structure and is comparable to what was recently observed in an alldielectric metasurface.10

Results and discussion The NaYF4:Yb3+,Er3+ UCNPs were synthesized by modified coprecipitation method11 and were coated with thin silica shell.3c, 12 As-synthesized UCNPs were highly monodispersed, as shown in transmission electron micrograph (TEM) in Figure 1(a), indicating the formation of β-phase NaYF4 nanocrystals. The average diameter was 36.9 nm. The silica-coated UCNPs were examined


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by field emission scanning electron microscope (FE-SEM). As shown in Figure 1(b), the average thickness of SiO2 coating was 12.3 nm, resulting in an average diameter of 61.5 nm for silicacoated UCNPs. We then fabricated Si3N4 two-dimensional PhC structures by laser interference lithography. Scanning electron microscopy (SEM) showed the average periodicity was 596 nm and there is a slight anisotropy due probably to fabrication errors. The holes have an average diameter of 300 nm and depth of 300 nm. The schematic of the device is shown in Figure 1(c). Finally, the silica-coated UCNPs were filled into the holes by a self-assembly method.13 The key step in the self-assembly process is the O2 plasma treatment of the PhC surface which produces electrostatic repulsion between the PhC surface and UCNPs, resulting in UCNPs filling the holes only and not deposited on the top surface. The top-down and cross-sectional SEM images (Fig. 1(d) ~ (f)) show the UCNPs are selectively filled inside the holes. High-magnification SEM image is shown in Figure 1(e) and also in the inset of Figure 1(d) for better visualization. We also prepared a reference sample, where the silicacoated UCNPs are densely packed onto a microscope slide by dropcasting them in a confined cell (Fig. 1(g)) without the presence of PhC structure. The details of all fabrication processes are presented in the Methods section.

Figure 1. (a) TEM image of as-synthesized UCNPs, the average diameter is 36.9 nm. Scale bar: 100 nm. (b) SEM image of silica-coated UCNP, the average diameter is 61.5 nm. Scale bar: 100 nm. (c) Photonic crystal structure schematics. (d) Top-down SEM image of PhC sample. Scale bar: 1 µm. The inset shows magnified view of a single filled hole. Scale bar: 100 nm. (e) High-magnification SEM image of UCNPfilled holes. Scale bar: 1 µm. (f) Cross-sectional SEM image of PhC sample. Scale bar: 100 nm. (g) Crosssectional SEM image of reference sample. Scale bar: 1 µm.

In order to quantitatively evaluate the anticipated field enhancement at the UCNP absorption wavelength of 980 nm, we performed finite-difference time-domain (FDTD) simulations for UCNP-filled PhC structure using commercial software Lumerical. As shown in Figure 2(a), the band structure shows a -point (k = 0) resonance at 980 nm, the absorption wavelength of UCNP. This mode provides both strong electric field enhancement inside the holes and good coupling


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with the normally incident light. Figure 2(b) and (c) shows the transmission spectra of as-fabricated and UCNP-filled PhC structure, showing the general agreement between the simulation and experiment and thereby confirming the accurate fabrication of photonic crystal as designed and the selective filling of holes with UCNPs. The simulations showed two dips corresponding to the two split modes at the -point, as shown in the band structure. But the experimental spectra show a single broad dip in both as-fabricated and UCNP-filled PhCs. There are two possible reasons for the discrepancy. First, the PhC structure has some inhomogeneity just like any lithographically fabricated nanostructures. This leads to broadening of the spectral features. Also, in the experiments, the incident light is not fully collimated and may have some divergence, producing a range of incident angles and consequently broadening of transmission dips. Despite the broadening, the agreement between the simulated and experimentally observed central dip wavelengths for as-fabricated and UCNP-filled PhCs is a strong indication that the fabricated PhC samples did have the prescribed optical properties.

Figure 2. (a) Photonic band structure of UCNP-filled PhC structure in normalized units of a/ for frequency and 2/a for wave number, where a is the period. (b) Measured and simulated transmission spectra of asfabricated PhC structure. (c) Measured and simulated transmission spectra of PhC sample with UCNPs filled inside the holes.

Next, the electric field profile at excitation wavelength (980 nm) in a unit cell was simulated to show a highly localized electric field profile concentrated inside the holes, as shown in Figure 3(a). It is noted that the electric field is intense in the vicinity of the side wall, which is due to the discontinuity of permittivity. This mechanism has been used to create an air-guided waveguide in


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silicon.14 In this work, we utilize the same mechanism to create strongly enhanced field inside the holes where UCNPs are filled. By monitoring the absorption enhancement factor, which is defined as 𝐹𝑎𝑏𝑠 =

∫|𝐸|2𝑑𝑉 ∫|𝐸0|2𝑑𝑉

where 𝐸 is the electric field with the photonic crystal structure and 𝐸0 is the incident electric field, we were able to design a device with a resonance peak at 980 nm (Figure 3(b)). This structure is expected to provide absorption enhancement of 12.3 at the absorption wavelength.

Figure 3. (a) Electrical field distribution at 980 nm in the cross-sectional plane of a single unit cell. The black dashed lines indicate the interfaces between air, Si3N4 and substrate. (b) Absorption enhancement factor, as defined in the text, as a function of wavelength.

We now proceed to characterize the upconversion properties of the PhC and reference samples. The emission spectra of NaYF4:Yb3+,Er3+ nanoparticles arise from the green 2H11/2–4I15/2 (522 nm), 4S –4I 4 4 3/2 15/2 (540 nm) and red F9/2– I15/2 (660 nm) transitions. The green emission originates entirely from two-photon upconversion process while the red emission may have contributions from both two- and three-photon upconversion processes.4, 11a The typical photoluminescence (PL) spectra of both PhC and reference samples are shown in Figure 4(a), where the PL intensity of reference UCNP sample was magnified by 50-fold for better visualization. To further investigate the performance of PhC structure, we measured the visible emission intensity under various excitation power densities. In order to accurately compare the PL intensities of PhC and reference samples, one must estimate the average number of UCNPs in each unit cell area of both samples. By analyzing multiple top-down and cross-sectional SEM images of PhC sample, we obtain 46.2 as the average number of UCNPs within a nanohole. This number is further confirmed by self-assembly process simulation using modified Jodrey-Tory algorithm.15 Figure 1(g), the cross-sectional image of reference sample, reveals the average number of UCNPs within the film in identical unit cell area is 4.1 × 103. These parameters allow us to calibrate the PL intensities to account for the difference in number of UCNPs in the PhC and reference samples.


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All the data in Figure 4 were obtained by this calibration process and represent the PL intensity from the same number of UNCPs with and without PhC structure. The power dependent green and red luminescence of PhC and reference samples are shown in Figure 4(b) and (c), respectively. Both green and red emission intensities exhibit quadratic dependence on the excitation power in the weak excitation regime and a linear dependence in the strong excitation regime. The quadratic dependence on incident intensities is a consequence of the sequential absorption of two photons required for upconversion. The slightly higher slope for the red emission indicates a small contribution by the three-photon process, which, as explained later, leads to larger enhancement than green. In the strong excitation regime, the intermediate energy level (4I11/2 of Er3+) is saturated and therefore upconversion is accomplished by single photon absorption, resulting in a linear power dependence.4, 11a, 16 The PL enhancement factors were calculated by comparing the integrated PL intensity over the green and red emission band. The enhancement factor for green and red is power dependent, as shown in Figure 4(d) and (e). In the strong excitation regime, the enhancement factors are small, 11 for green and 49 for red, respectively. The enhancement factor increases sharply as the incident power is decreased. It reaches a plateau in the weak excitation regime, reaching their maximum values of around 130 for green and 350 for red, respectively.

Figure 4. (a) PL spectra of reference UCNP sample (blue) and PhC-UCNP sample (green) excited by a 980 nm laser. The PL intensity of reference UCNP sample is magnified by 50-fold for better visualization. (b) Power dependent PL intensity of green emission from the PhC-UCNP and reference samples. The PhCUCNP sample is denoted by green open squares, while the reference UCNP sample is represented by black open squares. The green and black lines are obtained by the least-square fitting from which yield the slopes as denoted. (c) Power dependent PL intensity of red emission from the PhC-UCNP and reference UCNP samples. The PhC-UCNP sample is denoted by red open squares, while the reference sample is represented by black open squares. The red and black lines are obtained by the least-square fitting from which yield the slopes as denoted. (d) PL enhancement factor of the green upconverted luminescence as a function of the


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excitation power density. (e) PL enhancement factor of the red upconverted luminescence as a function of the excitation power density.

The measured enhancement factors are compared with the predictions by the rate equation analysis.11a Since the red emission possibly involve three-photon upconversion process, which will add significant difficulties to the rate equation analysis, we will closely examine only the green PL enhancement in this work. From the rate equation analysis we reported previously,4, 11a, 17 the green luminescence photon flux can be expressed as, 𝛷𝑆 =

𝛷𝑊 =

𝑊𝐴4 2

𝜎Φ#(1)

𝑐𝑑4(𝑐𝐵𝑑2𝑁𝐷 + 𝑊𝐴2)𝑁2𝐷

𝑊𝐴40 𝑊𝐴4

𝑊𝐴40 𝑁𝐷0

[

𝑐𝐹𝑑2𝑁𝐴 𝑊𝐴2 +

𝑊𝐷10(𝑐𝐵𝑑2𝑁𝐷 + 𝑊𝐴2) 𝑐𝐹𝑑2𝑁𝐴

]

(𝜎𝛷)2#(2)

2

Here, 𝛷𝑆 and 𝛷𝑊 are luminescence photon flux in the strong and weak excitation regimes, respectively. 𝑁𝑖 is the population density in the energy level 𝑖. The subscripts D1 and D0 refer to the excited (2F5/2) and ground state (2F7/2) levels of donor (Yb3+), respectively, while A4 and A2 represent the 4S3/2 and 4I11/2 levels of acceptor (Er3+), respectively. 𝑊𝐴4 and 𝑊𝐴40 indicate the total and radiative decay rates, respectively, from the initial state 4S3/2 to the final state 4I15/2. 𝑊𝐴2 is the total decay rate from the initial state 4I11/2 to the final state 4I15/2. The energy transfer coefficient 𝑐𝑑4 represents the Förster energy transfer process between the donor and the acceptor 𝐴4 level. For the energy transfer between donor and acceptor 𝐴2 level, the additional subscripts 𝐹 and 𝐵 in 𝑐𝑑2 coefficient indicate the forward (donor to acceptor) and backward (acceptor to donor) energy transfers. 𝑁𝐷 and 𝑁𝐴 are the doping densities of donor and acceptor, respectively. 𝜎 is the absorption cross section of the donor ion and 𝛷 is the incident photon flux. Equations (1) and (2) clearly show how upconversion luminescence intensity may be affected by photonic nanostructures. Specifically, photonic nanostructures can modify three different aspects of upconversion: (a) the emission efficiency (𝑊𝐴40 𝑊𝐴4), (b) energy transfer rates characterized by the c coefficients in Equation (2), and (c) the absorption cross-section 𝜎. We have previously shown that the energy transfer rate between two closely spaced ions is already very high and is difficult to enhance much further.4 Emission efficiency is determined by the interplay between the enhancement by the Purcell effect and the decrease by quenching. In our work, we are using the band edge states which have large mode volume extending over the entire PhC structure. Therefore, the Purcell enhancement is expected to be small. On the other hand, some quenching is expected by the surface states produced by the fabrication process. However, compared to metallic structure, the purely dielectric PhC used in this work should exhibit much less quenching. Finally, we expect a substantial enhancement in absorption due to the field enhancement by PhC structure. Based on this discussion, we can write the anticipated enhancement factors as below.


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𝐹𝑆 =

𝐹𝑎𝑏𝑠

𝐹𝑊 =

𝑄

#(3)

𝐹2𝑎𝑏𝑠 𝑄

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#(4)

where 𝐹𝑎𝑏𝑠 is the absorption enhancement factor and 𝑄 is the quenching factor defined as,

[ ] [ ] 𝑊𝐴40

𝑄 = 𝑊𝐴4

𝑊𝐴40

𝑟𝑒𝑓

𝑊𝐴4

= [𝑊𝐴4]𝑃ℎ𝐶 [𝑊𝐴4]𝑟𝑒𝑓#(5) 𝑃ℎ𝐶

The second equality follows from the fact that we have negligible Purcell enhancement and thus the radiative transition rate is not affected by the PhC structure. The total decay rate 𝑊𝐴4 can be directly measured by the transient PL spectroscopy. As shown in Figure 5, the green luminescence decay rates were found to be 9.70×103 s-1 and 8.53×103 s-1, respectively, for the PhC and reference samples. The faster decay of PhC sample is attributed to the increased non-radiative transition rate, which is most likely due to energy transfer to surface states on the sidewalls of PhC. From these data, the quenching factor was determined to be 𝑄 = 1.1. It is noted that this quenching factor is lower than those observed in plasmonic nanostructures where the quenching factor varies from 1.4 to 8.5, depending on the geometry.11a, 17b In general, small metal volume leads to lower quenching but in general metals tend to cause more severe quenching than dielectrics due to their higher loss parameters.

Figure 5. Time-resolved photoluminescence decay at 550 nm of reference and PhC samples under 980 nm excitation.

The absorption enhancement factor is calculated from simulation and, as shown in Figure 3(b), 𝐹𝑎𝑏𝑠 = 12.3 at 980 nm for our PhC structure. Given the quenching factor of 1.1, Equation (3) predicts upconversion enhancement factor of 𝐹𝑆 = 11.2 in the strong excitation regime, which


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agrees well with the measured enhancement factor of 11, as shown in Figure 4(d). For the weak excitation regime, 𝐹𝑎𝑏𝑠 = 12.3 and 𝑄 = 1.1 gives 𝐹𝑊 = 137, which again is in good agreement with the measured value of 130. The consistencies between measurement and simulation in both excitation regimes is a strong validation of the experimental results and analysis presented in this paper. Finally, we note that the higher enhancement factor of 350 observed for the red emission is due to the contribution by the three-photon process, which scales cubically with the local intensity enhancement. Thus, even a small contribution by three-photon process can yield a large enhancement, as observed in this study.

Conclusions In this study, we incorporated upconverting nanoparticles in a 2D planar PhC structure for upconversion enhancement. We used electrostatically charged surface to self-assemble UNCPs only inside the nanoholes of PhC structure where the electric field is strongly enhanced due to the dielectric discontinuity. By placing UCNPs in the region of enhanced local electric field, all optical processes involved in upconversion can be enhanced. However, our previous study showed the dominant enhancement mechanism is the absorption enhancement.4,11a The positive effect of local field enhancement is partially offset by the inevitable quenching. The combined effects of local field enhancement and quenching resulted in an overall enhancement factors of 130 for green and 350 for red luminescence. Compared to plasmonic nanostructures, the local field enhancement is not as high in the dielectric PhC structure. This lower field enhancement is offset by the fact that the purely dielectric PhC structure causes much less quenching than the plasmonic structures made of metal. Another important advantage of PhC structure is the capability of achieving enhancement over large areas. This is in sharp contrast to plasmonic structures which tend to produce extremely intense optical hotspots with very small volume. This leads to higher peak enhancement but achieving high enhancement over a large volume is difficult. In this respect, PhC structure provide an excellent alternative suitable for applications like lighting, displays and sensors that may require a large active area.

Funding Sources This work was supported in part by the National Science Foundation through Grant DMR-1420736, MRSEC: Soft Materials Research Center, the Colorado Office of Economic Development & International Trade (CTGG1 2017-0609), and the National Research Foundation (NRF) grants funded by the Korea Government (MEST) (2014R1A2A1A11051576 and 2018R1C1B5042877).

Methods Synthesis of hexagonal phase (β-phase) NaYF4: Yb3+, Er3+ UCNPs UCNPs were synthesized using a modified coprecipitation method10 where 0.3 g of YCl3, 0.1 g of YbCl3, and 0.01 g of ErCl3 were dissolved into 36 mL of octadecene and 6 mL of oleic acid with vigorous stirring and the mixture was heated up to 160 ˚C. Simultaneously, 0.2 g of NaOH and 0.296 g of NH4F were dissolved in 20 mL of methanol. Subsequently, the methanol mixture was added to the precursor solution and stirred at 60 ˚C for 30 min. Afterwards, the mixture was heated


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to 100 °C for degassing and then to 320 °C under argon atmosphere for 1 hr. Once the mixture was cooled, it was washed in water and ethanol via centrifugation and redispersion. The final UCNP precipitates were dried at 50 °C overnight and redispersed in toluene the following day. Synthesis of silica coating shell on UCNPs 2.5 mg UCNPs were dispersed in 260 μL cyclohexane and sonicated for 30 min. Then, 6.4 μL Igepal CO-520 and 390 μL cyclohexane were added to this solution and stirred for 10 min. 26 μL Igepal CO-520 and 5.2 μL 30 wt.% ammonia were added and the container was sealed and sonicated for another 20 min. After that, 6.0 μL TEOS was added into the solution and the solution was shaken for 48 hr. Silica-coated UCNPs were precipitated by adding acetone, and were washed with ethanol/water (1:1) three times. Device fabrication A 300 nm thick Si3N4 thin-film was deposited on a 4″ fused quartz substrate using plasmaenhanced chemical vapor deposition (310PC, Surface Technology Systems). After the substrate was diced into 1 cm × 1 cm square pieces, a 2D PhC grating pattern was obtained by laser interference lithography, followed by the pattern transfer down to the Si3N4 layer using CF4/O2/N2 (40 : 2 : 2 in sccm) reactive-ion etching (Plasmalab 80 Plus, Oxford Instruments).12 The silica-coated UCNPs were selectively filled in the PhC hole arrays through template-assisted self-assembly. First, the PhC structure and a piece of microscope slide went through an O2 plasma etching process (March Jupiter III) to make the photonic crystal surface hydrophilic and hydrolyze upon addition of silica-coated UCNPs solution. Then, the PhC structure was mounted onto another microscope slide with a spacer layer around the PhC sample. Afterwards, the microscope slide that went through O2 etching was placed face-down on top of the other microscope slide with the PhC structure and spacer layer. The assembly of two microscope slides with the PhC structure inside was fastened by two binder clips and mounted vertically. Finally, a 10 µL of 10 mg/mL of silicacoated UCNP solution was injected in the gap between the two microscope slides and left overnight for drying. Due to the ionized silanol groups present on the surface of the PhC structure and silica-coated UCNPs, the particles repel the surface of the PhC device and self-assemble within the holes only.13 No additional washing or surface cleaning steps were necessary. For comparison, a reference sample with silica-coated UCNPs deposited on a piece of microscope slide without the presence of PhC structure was also prepared. The particles were densely packed at the surface of the slide, which went through identical O2 plasma etching process as the PhC sample did. 1 cm × 1 cm square well was formed on the plasma-etched glass slide by sticking synthetic acrylic tape. 20 µL of silica-coated UCNP solution was dropcasted within the well boundary. Due to the hydrophilic nature of the glass surface after O2 plasma etching treatment, the solution spread uniformly within the well. The sample was then covered with a petri dish to protect it from ambient air flow and dried at room temperature. Optical Spectroscopy We measured the photoluminescence spectroscopy of both the PhC-UCNP and reference UCNP samples using a spectrometer (Acton SpectraPro 300i) equipped with a liquid nitrogen cooled charge-coupled device (CCD) (Roper Scientific) detectors. The excitation source was a 980 nm laser diode (OEM laser). The incident light was directed to the samples through a dichroic mirror at normal incidence while transmitting the visible photoluminescence signal from the UCNPs. A


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collimating lens and a focusing lens were used to collect luminescence from the samples and then refocused onto the spectrometer input slit. For time-resolved photoluminescence spectroscopy, a pulsed 980 nm diode laser was used to excite samples at normal incidence. The pulse shape and duration were adjusted by a laser diode current driver (Thorlabs LDC205C) that was modulated by a function generator (Wavetek). A clean up filter (Semrock LL01980) with bandwidth of 3.7 nm was used to remove laser tail outside central laser wavelength. The decay measurements were conducted on monochromator (Sciencetech 9057F) equipped with a visible PMT (Hamamatsu H11461P-11), which is connected to a multi-channel scaler (Stanford Research Systems SR430). FDTD simulation Numerical simulations were conducted using a commercial software package (Lumerical Solutions) based on Finite-Difference Time-Domain (FDTD) technique to extract the PhC band structures, absorption enhancement factor spectrum as well as the electric field profile in unit cell. The structure was set periodic in x and y directions, where periodic boundary conditions were applied. The perfectly matched layer (PML) boundary condition was applied along the z direction. The refractive index dispersion relation for the fused quartz was obtained from the data set by Palik.18 The band structure simulation was conducted with diode sources randomly distributed in the vicinity of the hole area and signal was collected at randomly located monitors. The bands that were contributed by air and glass substrate have been removed for better visualization. References (1) (2)

(3)

(4)

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For Table of Contents Use Only Enhanced upconversion luminescence by two-dimensional photonic crystal structure Chenchen Mao, Kyungtaek Min, Kyuyoung Bae, Suehyun Cho, Tian Xu, Heonsu Jeon and Wounjhang Park


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Enhanced Upconversion Luminescence by Two-Dimensional

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