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Heong Sub Oh, Guang S. He, Wing-Cheung Law, Alexander Baev, Hongsub Jee, Xin Liu, Augustine Urbas, Chang-Won Lee, Byoung Lyong Choi, Mark T. Swihart, and Paras N. Prasad* Chiral photonics is an emerging direction that offers the potential to control both linear and nonlinear optical functions for applications ranging from optical switching, to negative- and near-zero refractive index metamaterials, to chiral bioimaging. However, realization of such applications requires materials with optical chirality at visible wavelengths that is orders of magnitude larger than that of naturally-occurring materials. We report the design, synthesis and supramolecular organization of a nanocomposite of quantum dots in a chiral polymer, in which resonant coupling between the excitonic manifolds of the polymer and quantum dots significantly amplify the chiral response. Specifically, we report gigantic enhancement of optical activity of chiral poly(fluorene-alt-benzodithiazole) via doping with CdTe/ZnS core-shell quantum dots. Excitonic coupling between the helical polymeric strands and single quantum dots as well as the coupling between helically arranged quantum dots was responsible for the observed gigantic enhancement. These nanocomposites also have an exceptionally large value of the effective nonlinear refractive index for right-circularly polarized light, 2.4 × 10−3 cm2 GW−1. This value is more than one order of magnitude larger than
Dr. H. S. Oh, Dr. G. S. He, Dr. W.-C. Law, Dr. A. Baev, H. Jee, Prof. M. T. Swihart, Prof. P. N. Prasad Institute for Lasers, Photonics and Biophotonics The University at Buffalo The State University of New York Buffalo, New York, 14260 E-mail: [email protected] X. Liu, Prof. M. T. Swihart Department of Chemical and Biological Engineering The University at Buffalo The State University of New York Buffalo, New York, 14260 Dr. A. Urbas Materials and Manufacturing Directorate Air Force Research Laboratory WPAFB Ohio, 45433, USA Dr. C.-W. Lee, Dr. B. Choi Frontier Research Laboratory Samsung Advanced Institute of Technology Yongin-Si, Gyeonggi-Do, 446–712, Korea Prof. P. N. Prasad Department of Chemistry Korea University Seoul, 136–701, Korea
DOI: 10.1002/adma.201304071
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Manipulating Nanoscale Interactions in a Polymer Nanocomposite for Chiral Control of Linear and Nonlinear Optical Functions
that of twisted π-system chromophores that have recently shown great promise for all-optical switching. The value of the nonlinear chirality parameter was 0.9 × 10−3 cm2 GW−1 which is comparable to the nonlinear refractive index itself. This provides strong evidence of enhanced magnetic coupling in the nanocomposite and opens the door to chiral control of linear and nonlinear optical functions. Chiral photonics employs materials with different refractive indices for right- and left-circularly polarized light to achieve new photonic functionality. Metaphotonics, the manipulation of electro-magnetic fields in nano-engineered (meta)materials to control propagation of light[1,2] has potential applications ranging from optical communications to biophotonics. Nonlinear metaphotonics, while still in its infancy, can provide further means of manipulating light. Chiral media, with their inherent coupling of electric and magnetic dipoles, can play a central role not only in metaphotonics but in a variety of related fields including chiral control of optical functions,[3] bio-sensing,[4] and chiral imaging.[5] The vast potential of chiral media in photonics has not yet been developed, simply because the optical activity of naturally-occurring materials at visible wavelengths is far too small for practical use in such applications.[6] However, chiral photonics is being redefined by a new class of flexible chiral materials with optical activity vastly exceeding that of natural materials. Here we provide the first experimental demonstration of resonant coupling of a chiral organic polymer with semiconductor quantum dots. Doping of chiral poly(fluorene-alt-benzodithiazole) (PFBT) with achiral CdTe/ZnS core-shell quantum dots produces a 70-fold increase of linear optical activity, along with unprecedented nonlinear chiral response. Linear enhancement exceeds predictions based on dipole-dipole interaction between a chiral molecule and a quantum dot,[7] suggesting that other mechanisms involving interactions at nanometer length scales, such as Coulombic coupling in helical arrangements of quantum dots,[8] and quasi-resonant coupling between molecular and quantum dot states, must be important. The chirality parameter reached 0.03 in thin films at visible wavelengths, large enough to have practical impact in devices such as polarization-sensitive phase-shifters. Nonlinear refractive index was near zero for one polarization, but for the other reached 2.4 × 10−3 cm2 GW−1, an order of magnitude larger than that of a promising chromophore recently studied by our group[9] thus showing great potential for chiral control of all-optical switching.
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Chirality can also provide an alternative route to metamaterials, nano-engineered structures with properties that cannot be achieved in naturally occurring materials.[10–13] These have been intensely studied since the first demonstrations of negative refractive index at microwave[14] and optical frequencies.[11,12] The mainstream approach in this field employs double negative metamaterials, i.e., materials with simultaneously negative dielectric permittivity, ε, and magnetic permeability, μ.[11–13] This requires subwavelength resonators, which are extremely challenging to fabricate, especially at large scale or in three dimensions. A radically different approach[2b],[15] is based on the difference in refractive index for right- and leftcircularly polarized light in chiral media, in which the refractive index is:[16] n R, L =
g : ± 6
(1)
where κ is the chirality parameter. In chiral media, the refractive index can be negative, for one circular polarization, without achieving negative values of μ and ε. This Figure 1. A schematic illustration of our overall approach to producing chiral metamaterials would require suppressing ε to near zero, or for metaphotonic and the interactions between chiral polymer chains and quantum dots. Panel increasing κ to of order one, and we do not (a) shows (1) chiral molecular building blocks undergoing (i) polymerization and helical selfattempt to achieve that here. Nonetheless, assembly to produce (2) chiral aggregates. Then (ii) incorporation of quantum dots produces (3) chiral nanocomposites that exhibit gigantic enhancement of chiral optical activity. Panel this work helps to establish design principles (b) illustrates excitonic coupling between a helical polymer and a quantum dot. The incident for enhancing chiral response that move the field with wave vector, k, excites a molecular dipole, μ, which induces dielectric response of the field closer to practice. quantum dot, affecting mutual orientation of electric and magnetic dipoles of the polymer7. We employ a bottom-up chemical Helical currents in assemblies of quantum dots generate magnetic dipole, M, and hence, approach[6,17] suitable for high throughput optical activity of the assembly. low-cost fabrication. Guided by multiscale modeling, we rationally designed a polymer with both local with nonlinear moieties such as nonlinear dyes or by utilizing chirality of pendant groups and conformational chirality of the intermediate layers of nonlinear dielectric.[21] In contrast, our backbone, and combined it with inorganic nanocrystals. Mulchiral approach introduces both chirality and nonlinearity at the tiscale modeling, from ab initio calculations of the gyration molecular and supramolecular scales. Here, we introduce mestensor of helical oligomers[6] to finite element analysis of optical oscopic resonant coupling in supramolecular assemblies to furrotation by planar chiral meta-atoms[18] guides this effort. Our ther amplify the chiral contribution to the intensity dependent general strategy (Figure 1a) leverages chirality across length refractive index: scales: (1) theory guided synthesis of chiral molecular building n R ,L (I) = g : ± 6 + (n2 ± 62 )I, (2) blocks,[6] (2) supramolecular organization into larger chiral [ 17,19 ] aggregates, and (3) excitonic enhancement of chirality. in which n2 is the nonlinear refractive index related microThe molecular building blocks are π-conjugated donor-acceptor scopically to the fully electric second hyperpolarizability, γeeee, chromophores with chiral substituents. Polymerization (i) and κ2 is the nonlinear chirality parameter. κ2 arises from produces helical conjugated macromolecules. Annealing of nonlinear coupling of electric and magnetic dipoles, and can thin films of these macromolecules induces substantial strucbe expressed, to first order in the magnetic interaction, via the tural enhancement of chirality due to supramolecular ordering mixed electric-electric-magnetic second hyperpolarizability, [ 17,20 ] and formation of helical fibrils. Further enhancement (ii) γeeem. The induced electric dipole moment of a nonlinear moloccurs when the polymer is intimately mixed with quantum ecule is expressed by an expansion over the powers of the elecdots (QDs).[7] Figure 1b schematically illustrates the coupling tric and magnetic fields, accounting for non-locality (spatial between the polymer and QDs. These interactions at the dispersion):[16] nanoscale produce chiral optical activity vastly exceeding that of c the chiral host polymer alone. Dipole-dipole coupling between : ind = " ieej E j + e i j l g lem i k ∂ E j /∂ xk achiral QDs and chiral molecules has been treated theoretiT cally,[7] but has not previously been studied experimentally. eem eeee + $ ieee j k E j E k + $ i j k E j Bk + ( i j kl E j E k E l Nonlinear metamaterials are traditionally made by doping eeem (3) + ( i j kl E j E k Bl + . . . , lithographically manufactured arrays of metallic resonators 1608
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g lem k (−T; T) = −
T 2 m : lg n mkng 2 −T 2 n⁄= 0 T ng
( eeem i jkl (T4 ; T1 , T2 , T3 ) j :ig p : pn :knm mlmg = ℘ (T pg − T1 − T2 − T3 − i pg ) mnp ×
(4)
(T ng 1 − T 2 − T 3 − i ng )(T mg − T 3 − i mg )
(5)
These sum-over-states expressions show that the high density of states in hybrid organic/inorganic complexes can directly influence the magnitude of these quantities. Furthermore, coupling of states via magnetic interaction in Equation (4) and (5) implies further enhancement, because the magnetic dipole of helically arranged and excitonically-coupled QDs, generated by induced helical microscopic currents, and the intrinsic magnetic moment of chiral molecules interact, and the total magnetic moment can add up. We suggest that this is the physical origin of the dramatic enhancement reported here. This mechanism of enhancement is quite different from plasmonic enhancement reported earlier.[17,19] This mechanism of enhancement, being resonant in nature, offers the advantage of frequency selectivity by judicious choice of energy level matching between the chiral polymer and the QDs. With or without the QDs, the optical activity of the polymer depends strongly on its time-temperature history; annealing leads to formation of helical fibrils and dramatic structural enhancement of chirality with little effect on optical absorbance.[17] Pure PFBT absorbance at 467 nm decreased slightly and red shifted after annealing at 150 °C (Figure S3a). The corresponding circular dichroism (CD) band also showed very little shift, while the maximum CD increased by two orders of magnitude, from −0.82 to −89 mdeg (Figure S3a, Supporting Information). Optical activity of the PFBT/CdTe/ZnS nanocomposite thin film, after annealing, was dramatically enhanced (Figure 2) compared to the pure polymer. Figure 2 presents this as the absorbance dissymmetry ratio, -gabs. The corresponding maximum CD value was approximately −2800 mdeg (Figure S3c). Before annealing, the absorbance spectrum of the PFBT/CdTe/ZnS film (Figure S3c) was similar to that of the pure PFBT film (Figure S3a). However, after annealing, the absorbance increased and the absorbance maximum was
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where "ieej is linear electric polarizability, and g lem k is the gyration tensor, the microscopic measure of natural optical activity directly related to the macroscopic chirality parameter, κ. $ ieee j k is electric hyperpolarizability, $ ieem is electric-magnetic hyperpolarjk izability, ( ieeee j kl is electric second hyperpolarizability, and finally, is mixed electric-magnetic second hyperpolarizability. ( ieeem j kl The last coefficient ( ( ieeem j kl ) is the microscopic measure of thirdorder nonlinear chirality and is directly related to the nonlinear chirality parameter κ2 of Equation (2). We focus here on bulk random media, in which second-order nonlinear optical processes are dipole-forbidden. Both the gyration tensor and the electric-magnetic second hyperpolarizability couple electric, μ, and magnetic, m, dipoles, thereby influencing the propagation of light beams with different senses of circular polarization:
Figure 2. Absorbance dissymmetry ratio and chirality parameter are enhanced dramatically in PFBT/CdTe/ZnS nanocomposites, but not in PFBT/ZnO nanocomposites. (a) Dissymmetry ratio spectra of PFBT, PFBT/ZnO and PFBT/CdTe/ZnS nanocomposites after annealing. Note that the curve for the CdTe/ZnS/PFBT film has been scaled by a factor of 0.01 to make all three curves visible. (b) Ellipticity of CdTe/ZnS/PFBT film and corresponding chirality parameter computed via Kramers-Kronig transformation as described in Supporting Information.
red-shifted to ∼500 nm, near the maximum of the QD absorbance (Figure S3c), reflecting strong coupling between PFBT and QDs. PFBT/(∼4 nm)ZnO nanocomposites, in contrast, showed a slight decrease in both absorbance and CD (Figure 2, Figure S3b). If CD enhancement in PFBT/CdTe/ZnS resulted from changes in polymer conformation or other structural changes, then it should be observed for both the PFBT/(∼4 nm) ZnO and PFBT/CdTe/ZnS nanocomposites, because the constituent QDs are of similar size and surface termination. The fact that (∼4 nm) ZnO QDs do not produce CD enhancement suggests that electronic coupling between the QDs and polymer is necessary for dramatic CD enhancement. The degree of enhancement of CD and absorbance dissymetry ratio in the PFBT/CdTe/ZnS nanocomposites depends upon the QD/PFBT ratio, the annealing temperature, and the film thickness. However, in all cases, the addition of QDs led to a dramatic increase in the intensity of the CD band near 500 nm (Figure S2, Figure S3, Figure S6, Figure S8, Table S1). The maximum CD was generally obtained by annealing at 150 °C (Figure S2b, Figure S6, Table S2).
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Figure 3. Nonlinear optical characterization of polymer and nanocomposite films. Closed aperture Z-scan curves for (a) and (b) pure PFBT thin film sample (280 nm thick), (c) and (d) PFBT/QD nanocomposite thin film sample (420 nm thick). (a) and (c) are taken with right-circularly polarized light whereas (b) and (d) with left-circularly polarized light. The solid black line in (c) represents a reference Z-scan curve for 1 mm thick quartz glass plate, which has a slightly smaller effect on nonlinear transmission compared to the nanocomposite, despite being 2400 times thicker. The laser pulses parameters for Z-scan measurements: ∼775 wavelength, ∼160 nm pulse width, 0.5 μJ energy for film and 1.6 μJ for quartz glass plate, focusing length of 10 cm. The films are coated on glass slides.
Incorporation of QDs into the nanocomposite was confirmed by HRTEM of drop-cast samples and SEM imaging of the spin-coated films that were characterized optically. Pure PFBT was slightly aggregated at room temperature. After annealing, nanotubes were observed in TEM (Figure S11a) but these did not appear helical. In contrast, many straight and looped lefthanded helical nanotubes were formed upon annealing of PFBT/QDs nanocomposites (Figure S11d). QDs within nanotubes are uniformly distributed after annealing at 150 °C. Lattice fringes of the QDs are visible in HRTEM images before annealing (Figure S11b). Such images also show that the QDs are not aggregated. Selected-area electron diffraction (Figure S11c) shows evidence of orientational ordering of the QDs, even prior to annealing. After annealing, lattice fringes attributed to the polymer are visible (Figure S11e, Figure S10). The lattice spacing observed (∼0.8 nm) is too large to arise from the QDs. Similar bands of lattice fringes have previously been reported in twisted fibers of crystalline polymers.[22] Corresponding selected area electron diffraction patterns are clearly different showing some degree of ordering of QDs before annealing, but a single dominant orientation after annealing. Strong coupling between semiconductors and photonic modes has been reported previously in J-aggregated dyes,[23] organic dyes,[24] and layers of semiconductor nanocrystals.[25] We attribute enhanced optical activity of PFBT/QD nanocomposites to simultaneous (i) Coulomb interaction between molecular dipoles and QDs, leading to induced dielectric response of the particles,[7] (ii) dipole-dipole coupling of QDs, helically ordered by interactions with the polymer fibrils, which leads to a build-up
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of magnetic moment, and (iii) resonant coupling (requiring the overlap of absorption bands) between the rich manifold of states in QDs and discrete states of macromolecules, resulting in amplified magnetic moment as well as extra resonances in the resonant denominators of Equation (4) and (5). The nonlinear behavior of PFBT/QD nanocomposites was characterized by the closed-aperture Z-scan method.[26] Z-scan curves for a pure annealed PFBT thin film (280 nm thick) and PFBT/QD nanocomposite (420 nm thick) are presented in Figure 3 for both right- and left-circularly polarized incident ef f light. The nonlinear refractive index, n2 = n2 ± 62 , was determined by comparison to a 1 mm thick quartz plate with known n2.[26] For right circularly polarized light, the PFBT/QD nanoef f composite had n2 = 2.4 × 10−3 cm2 GW−1 (Figure 3c), more than one order of magnitude larger than that of very promising twisted π-system chromophore (TMC-2) that we have recently ef f characterized.[6] For the left-circularly polarized wave, n2 was −3 2 −1 less than 0.6 × 10 cm GW (Figure 3d), implying a value of κ2 near 0.9 × 10−3 cm2 GW−1, which is comparable to the value of the nonlinear refractive index itself. This provides strong evidence of enhanced magnetic coupling in the nanocomposite, considering magnetic interaction is generally at least 100 times weaker than the corresponding electrical one. To conclude, the nanocomposite of chiral polymers with QDs demonstrated here increases the linear and nonlinear chirality parameters to a level where they can have practical impact on the propagation of light. It represents an important step along the path to a new realm of applications in metaphotonics, including negative refraction,[2] non-linear optics,[27] and
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[2]
[3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]
Experimental Section Synthesis of Nanocomposites: We introduced chirality in PFBT by using a fluorene-based monomer modified with (S)-3,7-dimethyloctyl substituents at the 9-positions.[17] PFBT was synthesized using palladium-catalyzed Suzuki polycondensation as the final step and purified with Soxhlet extraction. We confirmed the structure by 1H NMR, GPC, and DSC. The dodecanethiol-capped CdTe/ZnS core shell QDs were prepared following established methods.[34,35] CdTe cores of ∼2.8 nm diameter were coated with a ∼1 nm ZnS shell. The absorbance spectrum of the QDs overlaps with that of the PFBT (Figure 2b). The sample films were prepared by spin-coating (PFBT/QDs, 4 mg/4 mg, in 1 mL of chloroform/chlorobenzene, 9/1, v/v), followed by annealing at 150 °C for 15 minutes.
[16] [17] [18]
[19] [20] [21] [22] [23]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[24] [25] [26]
Acknowledgements This work was supported in part by a grant from the Air Force Office of Scientific Research (grant no. FA95500610398). The authors also thank Frontier Research Laboratory of Samsung Advanced Institute of Technology for support, and thank Prof. Natalia Litchinitser for valuable discussions.
[27] [28] [29]
Received: August 13, 2013 Revised: September 26, 2013 Published online: December 5, 2013
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[32] [33] [34]
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surface-enhanced Raman optical activity.[28] Unprecedented levels of anisotropy factor in the visible region were achieved in a nanocomposite of a chiral polymer with QDs that can resonantly couple to the polymer. The obtained maximum dissymmetry ratio of 0.27 is one to two orders of magnitude larger than values typically reported for chiral polymers[29] and supramolecular assemblies.[30] It is also an order of magnitude higher than was recently achieved through precise chiral arrangement of gold nanoparticles using DNA.[19] The nanocomposite is also highly nonlinear, with the nonlinear refractive index for one sense of the circular polarization at least one order of magnitude higher than that of a very promising nonlinear dye, and nonlinear chirality of the same order as the nonlinear refractive index itself. This opens the door to chiral control of optical nonlinearities. We expect that further increases in bulk chirality parameter will be achieved by optimizing the polymer/QD combination, by simultaneous coupling to plasmonic nanoparticles, and by patterning the nanocomposite using two-photon lithography to produce stacked chiral nano-and micro-structures.[31,32] To this end, we have already demonstrated direct two-photon laser writing of submicron features that can provide chiral optical activity in the mid-IR.[33]
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Heong Sub Oh, Guang S. He, Wing-Cheung Law, Alexander Baev, Hongsub Jee, Xin Liu, Augustine Urbas, Chang-Won Lee, Byoung Lyong Choi, Mark T. Swihart, and Paras N. Prasad* Chiral photonics is an emerging direction that offers the potential to control both linear and nonlinear optical functions for applications ranging from optical switching, to negative- and near-zero refractive index metamaterials, to chiral bioimaging. However, realization of such applications requires materials with optical chirality at visible wavelengths that is orders of magnitude larger than that of naturally-occurring materials. We report the design, synthesis and supramolecular organization of a nanocomposite of quantum dots in a chiral polymer, in which resonant coupling between the excitonic manifolds of the polymer and quantum dots significantly amplify the chiral response. Specifically, we report gigantic enhancement of optical activity of chiral poly(fluorene-alt-benzodithiazole) via doping with CdTe/ZnS core-shell quantum dots. Excitonic coupling between the helical polymeric strands and single quantum dots as well as the coupling between helically arranged quantum dots was responsible for the observed gigantic enhancement. These nanocomposites also have an exceptionally large value of the effective nonlinear refractive index for right-circularly polarized light, 2.4 × 10−3 cm2 GW−1. This value is more than one order of magnitude larger than
Dr. H. S. Oh, Dr. G. S. He, Dr. W.-C. Law, Dr. A. Baev, H. Jee, Prof. M. T. Swihart, Prof. P. N. Prasad Institute for Lasers, Photonics and Biophotonics The University at Buffalo The State University of New York Buffalo, New York, 14260 E-mail: [email protected] X. Liu, Prof. M. T. Swihart Department of Chemical and Biological Engineering The University at Buffalo The State University of New York Buffalo, New York, 14260 Dr. A. Urbas Materials and Manufacturing Directorate Air Force Research Laboratory WPAFB Ohio, 45433, USA Dr. C.-W. Lee, Dr. B. Choi Frontier Research Laboratory Samsung Advanced Institute of Technology Yongin-Si, Gyeonggi-Do, 446–712, Korea Prof. P. N. Prasad Department of Chemistry Korea University Seoul, 136–701, Korea
DOI: 10.1002/adma.201304071
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Manipulating Nanoscale Interactions in a Polymer Nanocomposite for Chiral Control of Linear and Nonlinear Optical Functions
that of twisted π-system chromophores that have recently shown great promise for all-optical switching. The value of the nonlinear chirality parameter was 0.9 × 10−3 cm2 GW−1 which is comparable to the nonlinear refractive index itself. This provides strong evidence of enhanced magnetic coupling in the nanocomposite and opens the door to chiral control of linear and nonlinear optical functions. Chiral photonics employs materials with different refractive indices for right- and left-circularly polarized light to achieve new photonic functionality. Metaphotonics, the manipulation of electro-magnetic fields in nano-engineered (meta)materials to control propagation of light[1,2] has potential applications ranging from optical communications to biophotonics. Nonlinear metaphotonics, while still in its infancy, can provide further means of manipulating light. Chiral media, with their inherent coupling of electric and magnetic dipoles, can play a central role not only in metaphotonics but in a variety of related fields including chiral control of optical functions,[3] bio-sensing,[4] and chiral imaging.[5] The vast potential of chiral media in photonics has not yet been developed, simply because the optical activity of naturally-occurring materials at visible wavelengths is far too small for practical use in such applications.[6] However, chiral photonics is being redefined by a new class of flexible chiral materials with optical activity vastly exceeding that of natural materials. Here we provide the first experimental demonstration of resonant coupling of a chiral organic polymer with semiconductor quantum dots. Doping of chiral poly(fluorene-alt-benzodithiazole) (PFBT) with achiral CdTe/ZnS core-shell quantum dots produces a 70-fold increase of linear optical activity, along with unprecedented nonlinear chiral response. Linear enhancement exceeds predictions based on dipole-dipole interaction between a chiral molecule and a quantum dot,[7] suggesting that other mechanisms involving interactions at nanometer length scales, such as Coulombic coupling in helical arrangements of quantum dots,[8] and quasi-resonant coupling between molecular and quantum dot states, must be important. The chirality parameter reached 0.03 in thin films at visible wavelengths, large enough to have practical impact in devices such as polarization-sensitive phase-shifters. Nonlinear refractive index was near zero for one polarization, but for the other reached 2.4 × 10−3 cm2 GW−1, an order of magnitude larger than that of a promising chromophore recently studied by our group[9] thus showing great potential for chiral control of all-optical switching.
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Chirality can also provide an alternative route to metamaterials, nano-engineered structures with properties that cannot be achieved in naturally occurring materials.[10–13] These have been intensely studied since the first demonstrations of negative refractive index at microwave[14] and optical frequencies.[11,12] The mainstream approach in this field employs double negative metamaterials, i.e., materials with simultaneously negative dielectric permittivity, ε, and magnetic permeability, μ.[11–13] This requires subwavelength resonators, which are extremely challenging to fabricate, especially at large scale or in three dimensions. A radically different approach[2b],[15] is based on the difference in refractive index for right- and leftcircularly polarized light in chiral media, in which the refractive index is:[16] n R, L =
g : ± 6
(1)
where κ is the chirality parameter. In chiral media, the refractive index can be negative, for one circular polarization, without achieving negative values of μ and ε. This Figure 1. A schematic illustration of our overall approach to producing chiral metamaterials would require suppressing ε to near zero, or for metaphotonic and the interactions between chiral polymer chains and quantum dots. Panel increasing κ to of order one, and we do not (a) shows (1) chiral molecular building blocks undergoing (i) polymerization and helical selfattempt to achieve that here. Nonetheless, assembly to produce (2) chiral aggregates. Then (ii) incorporation of quantum dots produces (3) chiral nanocomposites that exhibit gigantic enhancement of chiral optical activity. Panel this work helps to establish design principles (b) illustrates excitonic coupling between a helical polymer and a quantum dot. The incident for enhancing chiral response that move the field with wave vector, k, excites a molecular dipole, μ, which induces dielectric response of the field closer to practice. quantum dot, affecting mutual orientation of electric and magnetic dipoles of the polymer7. We employ a bottom-up chemical Helical currents in assemblies of quantum dots generate magnetic dipole, M, and hence, approach[6,17] suitable for high throughput optical activity of the assembly. low-cost fabrication. Guided by multiscale modeling, we rationally designed a polymer with both local with nonlinear moieties such as nonlinear dyes or by utilizing chirality of pendant groups and conformational chirality of the intermediate layers of nonlinear dielectric.[21] In contrast, our backbone, and combined it with inorganic nanocrystals. Mulchiral approach introduces both chirality and nonlinearity at the tiscale modeling, from ab initio calculations of the gyration molecular and supramolecular scales. Here, we introduce mestensor of helical oligomers[6] to finite element analysis of optical oscopic resonant coupling in supramolecular assemblies to furrotation by planar chiral meta-atoms[18] guides this effort. Our ther amplify the chiral contribution to the intensity dependent general strategy (Figure 1a) leverages chirality across length refractive index: scales: (1) theory guided synthesis of chiral molecular building n R ,L (I) = g : ± 6 + (n2 ± 62 )I, (2) blocks,[6] (2) supramolecular organization into larger chiral [ 17,19 ] aggregates, and (3) excitonic enhancement of chirality. in which n2 is the nonlinear refractive index related microThe molecular building blocks are π-conjugated donor-acceptor scopically to the fully electric second hyperpolarizability, γeeee, chromophores with chiral substituents. Polymerization (i) and κ2 is the nonlinear chirality parameter. κ2 arises from produces helical conjugated macromolecules. Annealing of nonlinear coupling of electric and magnetic dipoles, and can thin films of these macromolecules induces substantial strucbe expressed, to first order in the magnetic interaction, via the tural enhancement of chirality due to supramolecular ordering mixed electric-electric-magnetic second hyperpolarizability, [ 17,20 ] and formation of helical fibrils. Further enhancement (ii) γeeem. The induced electric dipole moment of a nonlinear moloccurs when the polymer is intimately mixed with quantum ecule is expressed by an expansion over the powers of the elecdots (QDs).[7] Figure 1b schematically illustrates the coupling tric and magnetic fields, accounting for non-locality (spatial between the polymer and QDs. These interactions at the dispersion):[16] nanoscale produce chiral optical activity vastly exceeding that of c the chiral host polymer alone. Dipole-dipole coupling between : ind = " ieej E j + e i j l g lem i k ∂ E j /∂ xk achiral QDs and chiral molecules has been treated theoretiT cally,[7] but has not previously been studied experimentally. eem eeee + $ ieee j k E j E k + $ i j k E j Bk + ( i j kl E j E k E l Nonlinear metamaterials are traditionally made by doping eeem (3) + ( i j kl E j E k Bl + . . . , lithographically manufactured arrays of metallic resonators 1608
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g lem k (−T; T) = −
T 2 m : lg n mkng 2 −T 2 n⁄= 0 T ng
( eeem i jkl (T4 ; T1 , T2 , T3 ) j :ig p : pn :knm mlmg = ℘ (T pg − T1 − T2 − T3 − i pg ) mnp ×
(4)
(T ng 1 − T 2 − T 3 − i ng )(T mg − T 3 − i mg )
(5)
These sum-over-states expressions show that the high density of states in hybrid organic/inorganic complexes can directly influence the magnitude of these quantities. Furthermore, coupling of states via magnetic interaction in Equation (4) and (5) implies further enhancement, because the magnetic dipole of helically arranged and excitonically-coupled QDs, generated by induced helical microscopic currents, and the intrinsic magnetic moment of chiral molecules interact, and the total magnetic moment can add up. We suggest that this is the physical origin of the dramatic enhancement reported here. This mechanism of enhancement is quite different from plasmonic enhancement reported earlier.[17,19] This mechanism of enhancement, being resonant in nature, offers the advantage of frequency selectivity by judicious choice of energy level matching between the chiral polymer and the QDs. With or without the QDs, the optical activity of the polymer depends strongly on its time-temperature history; annealing leads to formation of helical fibrils and dramatic structural enhancement of chirality with little effect on optical absorbance.[17] Pure PFBT absorbance at 467 nm decreased slightly and red shifted after annealing at 150 °C (Figure S3a). The corresponding circular dichroism (CD) band also showed very little shift, while the maximum CD increased by two orders of magnitude, from −0.82 to −89 mdeg (Figure S3a, Supporting Information). Optical activity of the PFBT/CdTe/ZnS nanocomposite thin film, after annealing, was dramatically enhanced (Figure 2) compared to the pure polymer. Figure 2 presents this as the absorbance dissymmetry ratio, -gabs. The corresponding maximum CD value was approximately −2800 mdeg (Figure S3c). Before annealing, the absorbance spectrum of the PFBT/CdTe/ZnS film (Figure S3c) was similar to that of the pure PFBT film (Figure S3a). However, after annealing, the absorbance increased and the absorbance maximum was
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where "ieej is linear electric polarizability, and g lem k is the gyration tensor, the microscopic measure of natural optical activity directly related to the macroscopic chirality parameter, κ. $ ieee j k is electric hyperpolarizability, $ ieem is electric-magnetic hyperpolarjk izability, ( ieeee j kl is electric second hyperpolarizability, and finally, is mixed electric-magnetic second hyperpolarizability. ( ieeem j kl The last coefficient ( ( ieeem j kl ) is the microscopic measure of thirdorder nonlinear chirality and is directly related to the nonlinear chirality parameter κ2 of Equation (2). We focus here on bulk random media, in which second-order nonlinear optical processes are dipole-forbidden. Both the gyration tensor and the electric-magnetic second hyperpolarizability couple electric, μ, and magnetic, m, dipoles, thereby influencing the propagation of light beams with different senses of circular polarization:
Figure 2. Absorbance dissymmetry ratio and chirality parameter are enhanced dramatically in PFBT/CdTe/ZnS nanocomposites, but not in PFBT/ZnO nanocomposites. (a) Dissymmetry ratio spectra of PFBT, PFBT/ZnO and PFBT/CdTe/ZnS nanocomposites after annealing. Note that the curve for the CdTe/ZnS/PFBT film has been scaled by a factor of 0.01 to make all three curves visible. (b) Ellipticity of CdTe/ZnS/PFBT film and corresponding chirality parameter computed via Kramers-Kronig transformation as described in Supporting Information.
red-shifted to ∼500 nm, near the maximum of the QD absorbance (Figure S3c), reflecting strong coupling between PFBT and QDs. PFBT/(∼4 nm)ZnO nanocomposites, in contrast, showed a slight decrease in both absorbance and CD (Figure 2, Figure S3b). If CD enhancement in PFBT/CdTe/ZnS resulted from changes in polymer conformation or other structural changes, then it should be observed for both the PFBT/(∼4 nm) ZnO and PFBT/CdTe/ZnS nanocomposites, because the constituent QDs are of similar size and surface termination. The fact that (∼4 nm) ZnO QDs do not produce CD enhancement suggests that electronic coupling between the QDs and polymer is necessary for dramatic CD enhancement. The degree of enhancement of CD and absorbance dissymetry ratio in the PFBT/CdTe/ZnS nanocomposites depends upon the QD/PFBT ratio, the annealing temperature, and the film thickness. However, in all cases, the addition of QDs led to a dramatic increase in the intensity of the CD band near 500 nm (Figure S2, Figure S3, Figure S6, Figure S8, Table S1). The maximum CD was generally obtained by annealing at 150 °C (Figure S2b, Figure S6, Table S2).
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Figure 3. Nonlinear optical characterization of polymer and nanocomposite films. Closed aperture Z-scan curves for (a) and (b) pure PFBT thin film sample (280 nm thick), (c) and (d) PFBT/QD nanocomposite thin film sample (420 nm thick). (a) and (c) are taken with right-circularly polarized light whereas (b) and (d) with left-circularly polarized light. The solid black line in (c) represents a reference Z-scan curve for 1 mm thick quartz glass plate, which has a slightly smaller effect on nonlinear transmission compared to the nanocomposite, despite being 2400 times thicker. The laser pulses parameters for Z-scan measurements: ∼775 wavelength, ∼160 nm pulse width, 0.5 μJ energy for film and 1.6 μJ for quartz glass plate, focusing length of 10 cm. The films are coated on glass slides.
Incorporation of QDs into the nanocomposite was confirmed by HRTEM of drop-cast samples and SEM imaging of the spin-coated films that were characterized optically. Pure PFBT was slightly aggregated at room temperature. After annealing, nanotubes were observed in TEM (Figure S11a) but these did not appear helical. In contrast, many straight and looped lefthanded helical nanotubes were formed upon annealing of PFBT/QDs nanocomposites (Figure S11d). QDs within nanotubes are uniformly distributed after annealing at 150 °C. Lattice fringes of the QDs are visible in HRTEM images before annealing (Figure S11b). Such images also show that the QDs are not aggregated. Selected-area electron diffraction (Figure S11c) shows evidence of orientational ordering of the QDs, even prior to annealing. After annealing, lattice fringes attributed to the polymer are visible (Figure S11e, Figure S10). The lattice spacing observed (∼0.8 nm) is too large to arise from the QDs. Similar bands of lattice fringes have previously been reported in twisted fibers of crystalline polymers.[22] Corresponding selected area electron diffraction patterns are clearly different showing some degree of ordering of QDs before annealing, but a single dominant orientation after annealing. Strong coupling between semiconductors and photonic modes has been reported previously in J-aggregated dyes,[23] organic dyes,[24] and layers of semiconductor nanocrystals.[25] We attribute enhanced optical activity of PFBT/QD nanocomposites to simultaneous (i) Coulomb interaction between molecular dipoles and QDs, leading to induced dielectric response of the particles,[7] (ii) dipole-dipole coupling of QDs, helically ordered by interactions with the polymer fibrils, which leads to a build-up
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of magnetic moment, and (iii) resonant coupling (requiring the overlap of absorption bands) between the rich manifold of states in QDs and discrete states of macromolecules, resulting in amplified magnetic moment as well as extra resonances in the resonant denominators of Equation (4) and (5). The nonlinear behavior of PFBT/QD nanocomposites was characterized by the closed-aperture Z-scan method.[26] Z-scan curves for a pure annealed PFBT thin film (280 nm thick) and PFBT/QD nanocomposite (420 nm thick) are presented in Figure 3 for both right- and left-circularly polarized incident ef f light. The nonlinear refractive index, n2 = n2 ± 62 , was determined by comparison to a 1 mm thick quartz plate with known n2.[26] For right circularly polarized light, the PFBT/QD nanoef f composite had n2 = 2.4 × 10−3 cm2 GW−1 (Figure 3c), more than one order of magnitude larger than that of very promising twisted π-system chromophore (TMC-2) that we have recently ef f characterized.[6] For the left-circularly polarized wave, n2 was −3 2 −1 less than 0.6 × 10 cm GW (Figure 3d), implying a value of κ2 near 0.9 × 10−3 cm2 GW−1, which is comparable to the value of the nonlinear refractive index itself. This provides strong evidence of enhanced magnetic coupling in the nanocomposite, considering magnetic interaction is generally at least 100 times weaker than the corresponding electrical one. To conclude, the nanocomposite of chiral polymers with QDs demonstrated here increases the linear and nonlinear chirality parameters to a level where they can have practical impact on the propagation of light. It represents an important step along the path to a new realm of applications in metaphotonics, including negative refraction,[2] non-linear optics,[27] and
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[2]
[3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]
Experimental Section Synthesis of Nanocomposites: We introduced chirality in PFBT by using a fluorene-based monomer modified with (S)-3,7-dimethyloctyl substituents at the 9-positions.[17] PFBT was synthesized using palladium-catalyzed Suzuki polycondensation as the final step and purified with Soxhlet extraction. We confirmed the structure by 1H NMR, GPC, and DSC. The dodecanethiol-capped CdTe/ZnS core shell QDs were prepared following established methods.[34,35] CdTe cores of ∼2.8 nm diameter were coated with a ∼1 nm ZnS shell. The absorbance spectrum of the QDs overlaps with that of the PFBT (Figure 2b). The sample films were prepared by spin-coating (PFBT/QDs, 4 mg/4 mg, in 1 mL of chloroform/chlorobenzene, 9/1, v/v), followed by annealing at 150 °C for 15 minutes.
[16] [17] [18]
[19] [20] [21] [22] [23]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[24] [25] [26]
Acknowledgements This work was supported in part by a grant from the Air Force Office of Scientific Research (grant no. FA95500610398). The authors also thank Frontier Research Laboratory of Samsung Advanced Institute of Technology for support, and thank Prof. Natalia Litchinitser for valuable discussions.
[27] [28] [29]
Received: August 13, 2013 Revised: September 26, 2013 Published online: December 5, 2013
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surface-enhanced Raman optical activity.[28] Unprecedented levels of anisotropy factor in the visible region were achieved in a nanocomposite of a chiral polymer with QDs that can resonantly couple to the polymer. The obtained maximum dissymmetry ratio of 0.27 is one to two orders of magnitude larger than values typically reported for chiral polymers[29] and supramolecular assemblies.[30] It is also an order of magnitude higher than was recently achieved through precise chiral arrangement of gold nanoparticles using DNA.[19] The nanocomposite is also highly nonlinear, with the nonlinear refractive index for one sense of the circular polarization at least one order of magnitude higher than that of a very promising nonlinear dye, and nonlinear chirality of the same order as the nonlinear refractive index itself. This opens the door to chiral control of optical nonlinearities. We expect that further increases in bulk chirality parameter will be achieved by optimizing the polymer/QD combination, by simultaneous coupling to plasmonic nanoparticles, and by patterning the nanocomposite using two-photon lithography to produce stacked chiral nano-and micro-structures.[31,32] To this end, we have already demonstrated direct two-photon laser writing of submicron features that can provide chiral optical activity in the mid-IR.[33]
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