ARTICLE pubs.acs.org/JPCC
Spectrally Resolved Nonlinear Optical Response of Upconversion Lanthanide-Doped NaYF4 Nanoparticles Marcin Nyk,* Dominika Wawrzynczyk, Krzysztof Parjaszewski, and Marek Samoc Institute of Physical and Theoretical Chemistry, Department of Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
bS Supporting Information ABSTRACT: Fluoride (NaYF4) nanocrystals (ca. 30 nm size) codoped with the rare earth ions Yb3+ and Er3+/Tm3+ were synthesized with a wet chemical technique and characterized by TEM and XRD. The materials were well dispersible and stable in organic solvents. Nonlinear optical properties of these upconverting nanoparticles (UCNPs) were investigated as a function of wavelength in a wide spectral range with the Z-scan technique using a tunable femtosecond laser system. It was found that the nonlinear response exhibited strong anomalies close to the wavelengths of one-photon ff optical transitions of lanthanide ions. The strong nonlinearities around 980 nm were attributed to the multiphoton absorption and energy transfer processes from the Yb3+ ions (donor) to Er3+/Tm3+ ions (acceptor). These phenomena may find numerous applications including 3D biological imaging or optical switching mechanisms requiring strong near-resonant responses, not unlike those in e.g. metal vapors.
’ INTRODUCTION Upconverting nanoparticles containing lanthanide ions are promising materials for various optical applications such as imaging in biology and medicine,13 solar cell devices,4 sensing,5,6 and security.7 In particular, the NaYF4 matrix is considered to be among the most efficient host materials for formulating UCNPs; such systems doped with Yb3+ and Er3+/Tm3+ ions exhibit strong visible emission upon near-infrared (NIR) excitation. Recently, many technological and scientific activities have been focused on the synthesis of fluoride nanocrystals with different sizes and shapes.8,9 A great number of studies concerning linear optical properties were carried out in order to get a better insight into features of lanthanides luminescence in the NaYF4 matrix.10 However, to the best of our knowledge there have been no reports regarding wide wavelength range studies of nonlinear optical (NLO) properties of such systems. There is a huge interest in developing new organic materials exhibiting nonlinear absorption and determining their two-photon absorption cross sections. This interest stems from the importance of the nonlinear absorption process for new applications in photonics, nanophotonics, and biophotonics.11 While there has been considerable success in the past decade at preparing organic chromophores with large intrinsic nonlinearities, attention has recently focused also on understanding and exploiting nonlinearities of inorganic nanomaterials like CdS12 and ZnO.13 In this spirit, we have performed, what is the first to our knowledge, spectrally resolved study of the cubic NLO properties of UCNPs doped with lanthanide ions by using wide wavelength range low repetition rate femtosecond Z-scan measurements. This study was partly prompted by for the appearance of a single wavelength r 2011 American Chemical Society
study performed with an unamplified mode-locked laser.14 The aim of the present work is to point out the presence of strong nonlinearity anomalies that appear close to the ff electronic transitions of lanthanide ions and to initiate studies targeting the understanding of these phenomena and, ultimately, their practical applications. At the same time we want to indicate that care must be taken in interpreting the results of studies appearing in the literature where the employed laser parameters may be not well suited to distinguish between various mechanisms of the nonlinear response.
’ METHODS AND INSTRUMENTS The syntheses of UCNPs codoped with Er3+/Tm3+ and Yb3+ were performed using procedures described in our previous papers.7,15 The morphology of the NaYF4 nanocrystals was examined with a FEI Tecnai G2 20 X-TWIN transmission electron microscope. Overall phase composition was determined by X-ray powder diffraction (XRD) with a Siemens D5000 diffractometer and Cu KR1 radiation, λ = 0.154 06 nm. Optical absorption and steady-state photoluminescence studies were used to characterize the spectral properties of the synthesized nanoparticles. UV vis absorption spectra were acquired using a Shimadzu UV-3600 spectrophotometer. A fiber-coupled continuous wave (CW) laser diode (Qphotonics) emitting at 976 nm was used as the excitation source for measurements of the UCNPs luminescence. The NLO measurements of UCNPs were carried out as Received: May 18, 2011 Revised: July 12, 2011 Published: July 19, 2011 16849
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The Journal of Physical Chemistry C described previously (e.g. ref 16), with some modifications. The present experiments were carried out using a laser system consisting of a Quantronix Integra-C regenerative amplifier operating as an 800 nm pump and a Quantronix-Palitra-FS BIBO crystal-based optical parametric amplifier. This system delivers wavelength tunable pulses of ∼130 fs length and was operated at the repetition rate of 1 kHz. The nanocrystals were dissolved in chloroform (g99.9%, Sigma-Aldrich) at the concentration ∼4% w/w. For the NLO experiment the solutions were placed in 1 mm path length Starna glass cuvettes, stoppered, and sealed with Teflon tape. Results obtained on the cells with solutions were calibrated against Z-scan measurements on a fused silica plate (4.66 mm thick) and compared with the measurements on an identical glass cell filled with the solvent alone. The output from the femtosecond pulsed laser in the range from 500 to 1500 nm was appropriately filtered using wavelength separators and color glass filters to remove unwanted wavelength components, attenuated to μJ/pulse range, and used as excitation source for simultaneous recording of standard open-aperture (OA) and closed-aperture (CA) Z-scan traces. The beam was focused so as to provide a focal spot in the range w0 ≈ 2560 μm (giving the Rayleigh range which was always taken well in excess of the total thicknesses of the cell and the reference silica plate), and the cuvette was made to travel in the Z-direction, typically from 20 to 20 mm. The data were collected using three InGaAs photodiodes (Thor Laboratories Inc.) that monitored the laser input, the OA signal, and the CA signal, respectively. The outputs were fed into three channels of a digital oscilloscope, and the data were collected by a computer using a custom LabVIEW software. The traces of the CA and OA scans obtained by dividing each of them by the laser input reference trace were analyzed with the help of a custom-fitting program that used equations derived by SheikBahae et al.17 The real and imaginary parts of the second hyperpolarizability γ of the solutes were computed assuming additivity of the nonlinear contributions of the solvent and the solute and the applicability of the Lorentz local field approximation.18 It needs to be stressed that there is no generally accepted way of presenting the results of the measurements of nonlinear optical properties of nanoparticles, especially in the case when the nanoparticles are doped with species that may be considered active chromophores. A large number of studies performed on solutions or suspensions of materials in a solvent quoted the macroscopic nonlinearity of the solute + solvent system, usually as the nonlinear refractive index n2 and the nonlinear absorption coefficient β. This way of presenting the results does not allow one to make any comparisons among different systems since the results obviously depend on the concentration of the active material in the solution. A more useful way of dealing with the data is to extrapolate the result to a pure material. However, for doped nanoparticles this will again scale with the concentration of the active species inside the nanoparticle. For codoped nanoparticles this issue becomes even more complex. Therefore, for this paper we decided to present the data, i.e., the hyperpolarizability or the two-photon absorption cross section, as per a single nanoparticle. In these calculations the molar mass of an individual spherical nanoparticle was estimated to be M = 91.9 106 g/mol. It should be noted that the total two-photon cross section per nanoparticle is an important parameter as it can be used to determine the brightness of two-photon excited emission from such a particle. A useful comparison of the merits of various nonlinear absorbers can also be obtained by scaling the twophoton absorption cross section using the molecular weight M,
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Figure 1. (a) X-ray diffraction patterns of NaYF4:2% Er3+, 20% Yb3+ UCNPs and theoretical (JCPDS no. 77-2042); the inset shows cell facecentered cubic structure: Na/RE atoms marked as purple, F atom marked as blue. (b) TEM, HRTEM, and SEAD representative micrographs of NaYF4:2% Er3+, 20% Yb3+ UCNPs showing the size uniformity of the particles.
i.e., by comparing σ2/M values. Other ways of comparing disparate nonlinear absorbers have been discussed recently.19
’ RESULTS AND DISCUSSION The XRD patterns shown in Figure 1a reveal that R-NaYF4: RE3+ (RE3+ = Er/Tm, Yb) nanocrystals were formed. All the diffraction peaks for R-NaREF4 (JCPDS no. 77-2042) shown in Figure 1a are characteristic of a pure cubic phase (space group: Fm3m). The chloroform dispersed UCNPs were uniform in size with a narrow size distribution, as shown by the TEM image (Figure 1b). The average size of the crystallite grains was determined to be ca. 30 nm. The structure of the obtained materials was also proved by the SAED. All reflexes observed in electronograms (Figure 1b) correspond to the face-centered cubic structure of NaYF4 matrix (see inset in Figure 1a). The results of representative OA and CA Z-scan normalized traces for UCNPs are shown in Figure 2. It was found that the 16850
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Figure 3. Dispersion of the imaginary part of the complex hyperpolarizability of NaYF4 UCNPs doped with 2% Er3+, 20% Yb3+ (a) and doped with 2% Tm3+, 20% Yb3+ (b); determined from Z-scan measurements on chloroform solutions containing 4.0% w/w of the nanoparticles.
Figure 2. Representative open (a, c) and closed aperture (b, d) Z-scans and the theoretical fits for NaYF4 UCNPs doped with 2% Er3+, 20% Yb3+ at 990 nm (a, b) and doped with 2% Tm3+, 20% Yb3+ at 970 nm (c, d). Black symbols and red lines show raw Z-scan data/traces and fits, respectively.
shapes of the traces change drastically for both samples around ca. 980 nm, proving occurrence of the nonlinear absorption. Figure 3 shows the dispersion of the imaginary parts of the complex hyperpolarizability of NaYF4 UCNPs. Because of a large error associated with the determination of the real (refractive) part of γ (which is determined from the differences between the closed-aperture signals for the solvent and the solution), it was not possible to find clear trends for the presently available Re(γ) data, which was scattered in values from 3.0 to 1.5 1029 esu and from 1.5 to 1.5 1029 esu for the samples doped with erbium(III) and thulium(III), respectively. However, the imaginary part of γ does show definite peaks in the range ca. 10 00011 000 cm1, corresponding to laser wavelengths of ca. 9001000 nm; thus, there seems to appear an absorption process in that wavelength range, having a multiphoton character. Assessing the shapes of the OA Z-scan traces similarly as in ref 20, we may conclude that the process taking place here is quadratic in intensity and thus formally of two-photon absorption character; therefore, we may present the results as effective two-photon absorption cross sections σ2.
The observed power-dependent variation in the transmittance, interpreted as multiphoton absorption, appears at the wavelength range, which coincides with that of absorption to 2F5/2 metastable electronic level of Yb3+ ions. Therefore, any interpretation of the mechanisms of that process must take the presence of this absorption into account. Before discussing the behavior of our samples in the 900 1000 nm region, we must remark that, recently, Komarala et al.14 performed a single wavelength (800 nm which corresponds to 12 500 cm1) NLO study of a similar system codoped with erbium/ytterbium. Those authors reported the nonlinear absorption coefficient β and the nonlinear refractive index n2 (called γ in their paper) values for the sample NaYF4 codoped with 2% Er3+ and 20% Yb3+ obtained from the fits of the Z-scans as 2.73 1013 m/W and 2.76 1019 m2/W, respectively. It is not clearly stated in their paper if the values correspond to the 5 mg/mL concentration in toluene (after subtracting the nonlinearity contributions of toluene solvent and the cell silica walls) or if they are extrapolated to the solvent-free nanoparticles. Examination of the Z-scan traces and the relevant parameters quoted in the paper indicates that the former is true. Therefore, values of β and n2 for solvent-free nanoparticles should be ∼200 times larger (5 mg/mL corresponds to about 0.5 wt %), neglecting the differences in densities of the solution components, i.e., β ≈ 5 1011 m/W and n2 ≈ 5 1017 m2/W. Those values can then be compared with ones obtained for our 16851
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The Journal of Physical Chemistry C samples: NaYF4:2% Er3+, 20% Yb3+ and NaYF4:2% Tm3+, 20% Yb3+ at 800 nm. The nonlinear parameters at that wavelength appear to be much smaller than those in the vicinity of 980 nm, but they are measurable and we calculate the values (extrapolated to pure, solvent-free nanoparticles) as β ≈ 0.22 1013 m/W, n2 ≈ 2.2 1019 m2/W and β ≈ 0.25 1013 m/W, n2 ≈ 0.71 1019 m2/W, for the two kinds of nanoparticles, respectively. Therefore, in our case, for both erbium and thulium doped samples the β values at 800 nm are 103 times lower compared to the values obtained by Komarala et al.14 We note here that, in addition to the strong 980 nm absorption which is apparently due to Yb3+ ions, indeed we detect a weaker nonlinear absorption around 800 nm (12 500 cm1) which may have its origin in one-photon absorption by Er3+ ions (involving excitation of the 4I9/2 level). One might speculate that the smaller amplitude of the nonlinear effects at 800 nm compared to that at 980 nm is associated with the relatively low concentrations of Er3+ (2% compared to the 20% of Yb3+). Since the nonlinear absorption bands in the studied nanoparticles appear to coincide with the one-photon absorption bands of the ions present in them, a question arises whether the processes observed are due to true two-photon absorption (a basically instantaneous process involving only virtual intermediate states) or to sequential absorption of two photons (involving excitation to a real intermediate level). It has to be stated that Z-scans measurements alone do not provide any indication concerning the mechanism of the nonlinear absorption process. In fact, the total nonlinear absorption can be approximated as the sum of both contributions: dΦ ¼ N0 σ2 Φ2 N0 σ01 Φ N1 σ12 Φ dz dN1 N1 ¼ N0 σ 01 Φ N1 σ 12 Φ dt τ1 The first equation here describes the absorption losses encountered by the photon flux Φ due to one-photon absorption and the two kinds of the nonlinear absorption. The second equation describes the dynamics of the evolution of the population N1 of the excited state. The absorption cross sections σ01 and σ12 refer to one-photon absorption by the ground and excited state, respectively, and σ2 is the two-photon absorption cross section. It should be noted that: • For a single laser pulse of duration tp where tp,τ1, far from saturation, one has N1 , N0 so the second equation may be integrated to give N1 ≈ (1/2)N0σ01Φtp, and thus the effective two-photon cross section due to sequential absorption is σ2,eff = σ2 + (1/2)σ01σ12tp. Obviously, for long-lived intermediate states the effective two-photon cross section should scale with the laser pulse duration. • For bursts of several laser pulses like in the case of chopped mode-locked excitation (as used in ref 14) or Q-switched modelocked lasers the effective cross section contribution by sequential absorption will scale with the laser pulse duration multiplied by the number of pulses falling within the lifetime of the intermediate state. We believe this scaling may be the reason for much bigger values of the nonlinear absorption coefficients obtained using chopped mode-locked excitation compared to our lowrepetition-rate laser pulses. The discrimination between the instantaneous and sequential nonlinear absorption contribution should be performed using
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standard time-resolved techniques like the pumpprobe transient absorption technique. These experiments are planned. Considering the details of the sequential absorption that may be involved in the nonlinear processes seen in our nanoparticles, we may visualize them as in Figure 6. Although our measurements involve the nonlinear absorption, it is the light emission processes that accompany them that are of the most interest. The upconversion excitation of the upper level 4F7/2 in Er3+ ion may require sequential two-photon absorption at either 810 or 970 nm.21 With the pump close to 810 nm, the ground-state absorption 4I15/2 f 4I9/2 is followed by nonradiative decay to the 4 I11/2 level (see Figure 6). From there, the population is taken to the 4F7/2 level by excited-state absorption. With a pump wavelength around 970 nm, the upconversion excitation scheme of the upper level 4F7/2 is similar; however, it requires two processes: the ground-state absorption 4I15/2 f 4I11/2 and excited-state absorption process from 4I11/2 to 4F7/2 level. At 970 nm there are two ions involved in energy transfer upconversion process. Energy absorbed by Yb3+ ion and populating 2F5/2 level is transferred to a neighbor Er3+ ion; thus, the already excited ion is excited to a higher energy state by the transferred energy (cooperative sensitization), which finally results in the emission of a higher energy photon, i.e., upconverted emission.22 Azuel23 proved that the energy transfer upconversion process is the most efficient in ErYb and TmYb systems. The NaYF4 matrix has a small phonon energy (∼400 cm1) and is wellknown as the most efficient host for upconverting lanthanides (Er3+/Yb3+ and Tm3+/Yb3+).24 Therefore, the upconversion processes should also be seen as a strong nonlinear absorption (see Figures 3 and 4). It should be noted that the maximum NLO values for NaYF4:2% Er3+, 20% Yb3+ and NaYF4:2% Tm3+, 20% Yb3+ were found at 990 nm (10 101 cm1) with (β ≈ 116 1013 m/W, n2 ≈ 4.9 1019 m2/W) and at 970 nm (10 309 cm1) with (β ≈ 15.44 1013 m/W, n2 ≈ 0.01 1019 m2/W), respectively; however, the accuracy of wavelength tuning of the femtosecond OPA (partly due to the natural line width of a femtosecond laser which is on the order of 10 nm) is not sufficient to decide whether these wavelengths are indeed identical to or slightly shifted from the Yb3+ absorption band. The presence of sequential absorption, whether involving the same ion or two different ions, and the resulting upconversion process does not preclude the existence of simultaneous twophoton absorption through a virtual intermediate energy level.15 In fact, it can be argued that in examined system both processes occur and the efficiency of the instantaneous two-photon absorption process may indeed be enhanced if the virtual electronic levels invoked in that process are close to the metastable 2F5/2 energy level of Yb3+ (see Figure 6). It should be reiterated that both the up-conversion processes and the two-photon absorption process, depend quadratically on the incident excitation intensity,15 therefore they can be easily confused in an experiment with no temporal resolution. It should be stressed that, unlike organic two-photon absorbers, thanks to long lifetimes of the intermediate states, the upconverting nanoparticles show efficient upconverted luminescence also with CW excitation. The experiment shown in Figure 5 illustrates upconversion luminescence obtained under CW and pulsed conditions. The vials and cuvettes contain the same concentrations of the erbium- and thulium-doped NaYF4 UCNPs. The pictures on the left show traces of the upconverted emission (the yellow emission is the sum of red 4F9/2 f 4I15/2 and the green 2H11/2/4S3/2 f 4I15/2 luminescence corresponding to the 16852
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Figure 4. Dispersion of the effective two-photon absorption cross section (taken per an individual NP) and comparison to one-photon absorption spectrum (right-hand blue scale) of NaYF4 UCNPs doped with 2% Er3+, 20% Yb3+ (a) and doped with 2% Tm3+, 20% Yb3+ (b), respectively. The insets in (a, b) show the magnified spectral region of 9001000 nm (the strongest peak is clipped in both insets).
intra 4f4f electronic transitions of the Er3+ ions, and the visible blue luminescence is a radiative deactivation of the 1G4 to the 3H6 energy levels of Tm3+ ions) under CW laser diode excitation at 976 nm (see Supporting Information, Figures S1 and S2). The microscope objective (photographs in the right column of Figure 5) focuses near-infrared (976 nm) light from the Quantronix femtosecond pulsed laser system. In this case there is evidence easily visible to the naked eye that the emission observed here is proportional to the exciting light intensity in a power higher than one: the visible emission is confined to a small point at the focus of the objective lens (see the arrows). These photographs mimic ones usually presented for the case of two-photon excited
fluorescence of dyes and taken as proof of the intrinsic capability of that process to be used for 3D imaging. Indeed, the upconverted emission seen in Figure 5 with femtosecond excitation possesses the same capability, although issues may arise concerning the long luminescence lifetimes which may limit the available scan speeds. Currently, the upconversion microscopy is at the early stage of investigations. A study by Yu et al.25 showed that rare-earth nanophosphors exhibit a unique upconversion luminescence mechanism and imaging modality. Those authors developed a new threedimensional visualization method of laser scanning upconversion luminescence microscopy with little photobleaching26 and no 16853
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Figure 6. Energy level diagram of Er3+, Yb3+, Tm3+ ions and possible mechanisms for the upconversion luminescence in NaYF4 NPs.
Figure 5. An experiment illustrating efficient visible upconverted PL under excitation with a CW laser diode at 976 nm (left column) and the two-photon PL spot (indicated by red arrows) under femtosecond pulsed laser operating also at 976 nm (right column).
background fluorescence, by introducing a reverse excitation dichroic mirror and the confocal pinhole technique. They also emphasized that laser scanning upconversion luminescence microscopy imaging of UCNPs may be readily conducted in conjunction with conventional confocal imaging to provide more details about complex biological samples. On the other hand, the system described in ref 27 involves a multiphoton process similar to twophoton fluorescence microscopy but does not provide highly localized emission of upconverted luminescence from the focal volume and thus does not have natural depth-discriminating capability. The nonlinear optical effects seen in the nanoparticles investigated in the present work hold much promise for further development of high-quality multicolor upconversion phosphors or versatile photosensitizers for wide application in biological imaging at NIR excitation. We need to comment here on the merit of these nanoparticles compared to the typical two-photon chromophores. The biggest effective two-photon absorption cross section measured here is 8 1044 cm4 s/nanoparticle, or 8 106 GM (GoeppertMayer units) for the erbiumcodoped nanoparticles. This corresponds to σ2/M ≈ 0.1, a value ∼1 order of magnitude lower than those for typical good organic two-photon chromophores.2830 However, it must be kept in mind that in imaging applications it is the total luminescence available from a marker that determines the brightness of the imaged spot, so the relevant parameters are the total nonlinear absorption cross section, the luminescence quantum yield, and also the resistance of the marker to bleaching (determining the
number of luminescence photons available from the marker before it is bleached). The combination of these parameters offered by the upconverting nanoparticles may indeed be advantageous. In addition, since the effective nonlinear absorption cross section is expected to depend on the pulse duration as described above, the use of longer (e.g. picosecond) pulses or pulse bursts may lead to improvement of imaging parameters. Although we have not been able to obtain a consistent picture of the dispersion of the nonlinear refraction parameters of the nanoparticles in the present study, the further investigation of this aspect of the cubic NLO properties of these systems is needed. Because of the presence of sharp peaks of the effective nonlinear absorption, the nonlinear refraction should also show interesting anomalies in its dispersion and relatively high values of n2 may be available with fine-tuning of the wavelength and laser pulse duration. The nonlinear refraction phenomena in upconverting nanoparticles may potentially find numerous applications in switching mechanism requiring strong near-resonant responses. We conclude that nonlinear optical studies of the rare-earth ions codoped NaYF4 UCNPs should be carried out in more detail. In particular, it is necessary to clarify the details of the relative contributions of the sequential absorption related to the upconversion process and that of the instantaneous two-photon absorption process under different conditions: with various detuning from the one-photon transitions in the lanthanide ions and with different laser pulse parameters.
’ SUMMARY Systematic studies of the nonlinear optical properties of lanthanide doped fluoride nanocrystals have been commenced. Until now, most of the NLO research carried out with the determination of the spectral dependences of the parameters concentrated on organic and organometallic compounds. Similarly to those cases, attempts to draw any conclusions from singlewavelength NLO studies are not likely to be very useful in recognizing the potential of lanthanide-doped nanoparticles, especially because of the importance of the sharp absorption transitions in these systems. Our wide-wavelength range measurements of femtosecond Z-scan show that OA and CA Z-scan traces reveal strong and rapidly changing anomalies close to the 16854
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The Journal of Physical Chemistry C lanthanide ions absorption bands. We believe that in this system there is a possibility of the simultaneous contributions of two processes. One of them is the sequential absorption leading to the well described in the literature upconversion light emission which is observed even with CW sources. With the use of a highenergy femtosecond pulse laser, there is, however, a probability of a substantial contribution of instantaneous two-photon absorption which may be resonantly enhanced in the vicinity of one-photon transition wavelengths. Further studies are needed to evaluate the relative contributions of these two processes.
’ ASSOCIATED CONTENT
bS
Supporting Information. Room temperature photoluminescence spectra of NaYF4:2% Er3+, 20% Yb3+ and NaYF4:2% Tm3+, 20% Yb3+ UCNPs in chloroform dispersion under excitation with a CW laser diode at 976 nm. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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(18) Samoc, M.; Samoc, A.; Luther-Davies, B.; Humphrey, M. G.; Wong, M. S. Opt. Mater. 2003, 21, 485. (19) Schwich, T.; Cifuentes, M. P.; Gugger, P. A.; Samoc, M.; Humphrey, M. G. Adv. Mater. 2011, 23, 1433. (20) Samoc, M.; Morrall, J. P.; Dalton, G. T.; Cifuentes, M. P.; Humphrey, M. G. Angew. Chem., Int. Ed. 2007, 46, 731. (21) Springer Handbook of Lasers and Optics; Tr€ager, F., Ed.; Springer: New York, 2007. (22) Nyk, M.; Kuzmin, A.; Prasad, P. N.; Strek, W.; de Araujo, C. B. Opt. Mater. 2009, 31, 800. (23) Auzel, F. Chem. Rev. 2004, 104, 139. (24) Aarts, L.; van der Ende, B. M.; Meijerink, A. J. Appl. Phys. 2009, 106, xxxx. (25) Yu, M. X.; Li, F. Y.; Chen, Z. G.; Hu, H.; Zhan, C.; Yang, H.; Huang, C. H. Anal. Chem. 2009, 81, 930. (26) Ungun, B.; Prud’homme, R. K.; Budijono, S. J.; Shan, J. N.; Lim, S. F.; Ju, Y. G.; Austin, R. Opt. Express 2009, 17, 80. (27) Kim, D. H.; Kang, J. U.; Waynant, R. W.; Ilev, I. K. Upconversion Fiber-Optic Confocal Microscopy Using a Near-Infrared Light Source; IEEE: New York, 2007. (28) He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008. (29) Lin, T. C.; Chung, S. J.; Kim, K. S.; Wang, X. P.; He, G. S.; Swiatkiewicz, J.; Pudavar, H. E.; Prasad, P. N. In Polymers for Photonics Applications Ii; Springer-Verlag: Berlin, 2003; Vol. 161, p 157. (30) Christodoulides, D. N.; Khoo, I. C.; Salamo, G. J.; Stegeman, G. I.; Van Stryland, E. W. Adv. Opt. Photon. 2010, 2, 60.
’ ACKNOWLEDGMENT The authors acknowledge support from the Foundation for Polish Science (under “Welcome” and “Homing” programs). ’ REFERENCES (1) Nyk, M.; Kumar, R.; Ohulchanskyy, T. Y.; Bergey, E. J.; Prasad, P. N. Nano Lett. 2008, 8, 3834. (2) Guo, H.; Idris, N. M.; Zhang, Y. Langmuir 2011, 27, 2854. (3) Zhan, Q.; Qian, J.; Liang, H.; Somesfalean, G.; Wang, D.; He, S.; Zhang, Z.; Andersson-Engels, S. ACS Nano 2011, 5, 3744. (4) Shalav, A.; Richards, B. S.; Trupke, T.; Kramer, K. W.; Gudel, H. U. Appl. Phys. Lett. 2005, 86, xxxx. (5) Vetrone, F.; Naccache, R.; Zamarron, A.; de la Fuente, A. J.; Sanz-Rodriguez, F.; Maestro, L. M.; Rodriguez, E. M.; Jaque, D.; Sole, J. G.; Capobianco, J. A. ACS Nano 2010, 4, 3254. (6) Vetrone, F.; Naccache, R.; Zamarron, A.; Juarranz de la Fuente, A.; Sanz-Rodriguez, F.; Martinez Maestro, L.; Martin Rodriguez, E.; Jaque, D.; Garcia Sole, J.; Capobianco, J. A. ACS Nano 2010, 4, 3254. (7) Kim, W. J.; Nyk, M.; Prasad, P. N. Nanotechnology 2009, 20, xxxx. (8) Mai, H. X.; Zhang, Y. W.; Sun, L. D.; Yan, C. R. J. Phys. Chem. C 2007, 111, 13730. (9) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (10) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (11) Samoc, M.; Samoc, A.; Humphrey, M. G.; Cifuentes, M. P.; Luther-Davies, B.; Fleitz, P. A. Mol. Cryst. Liq. Cryst. 2008, 485, 894. (12) Venkatram, N.; Rao, D. N.; Akundi, M. A. Opt. Express 2005, 13, 867. (13) Sreeja, R.; John, J.; Aneesh, P. M.; Jayaraj, M. K. Opt. Commun. 2010, 283, 2908. (14) Komarala, V. K.; Wang, Y. J.; Xiao, M. Chem. Phys. Lett. 2010, 490, 189. (15) Kumar, R.; Nyk, M.; Ohulchanskyy, T. Y.; Flask, C. A.; Prasad, P. N. Adv. Funct. Mater. 2009, 19, 853. (16) Babgi, B.; Rigamonti, L.; Cifuentes, M. P.; Corkery, T. C.; Randles, M. D.; Schwich, T.; Petrie, S.; Stranger, R.; Teshome, A.; Asselberghs, I.; Clays, K.; Samoc, M.; Humphrey, M. G. J. Am. Chem. Soc. 2009, 131, 10293. (17) Sheik-bahae, M.; Said, A. A.; Wei, T. H.; Hagan, D. J.; Van Stryland, E. W. IEEE J. Quantum Electron 1990, 26, 760. 16855
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