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Abstract
Chiral photonics concerns enantio-selective polarization control of linear and nonlinear optical functions. However, natural chiral media do not exhibit strong chiro-optic response. In our laboratory, guided by multiscale modeling, we have developed new chiral polymers and their nanocomposites. Their optical and magneto-optic activity can be amplified by manipulating supramolecular organization and by introducing strong coupling with plasmonic/excitonic, as well as paramagnetic nanoinclusions. We present here an account of our recent progress in the design, development and characterization of these materials that can have a potentially huge technological impact by allowing ultrathin, flexible, low-cost devices such as polarizers, optical isolators, optical magnetometers.
© 2017 Optical Society of America
1. Introduction
1.1 Linear regime
In isotropic and naturally gyrotropic media placed in a quasi-static magnetic field B, the refractive index, n, for each circular polarization state (R or L), is [1]
where κΝΟΑ is the chirality parameter (gyration constant) due to natural optical activity, κΜΟ is the magnetic gyration constant or “chirality” parameter due to the magneto-optic (MO) effect, ε' is the real part of the permittivity, and μ' is the real part of the permeability. A difference in index between polarization states produces a phase difference and, hence, rotation of the polarization plane of linearly polarized light. The rotation, θ, due to the MO effect depends upon the magnetic field and the magneto-electric constant (Verdet constant) of the medium: , where V is the Verdet constant, and d is the thickness. In terms of the gyration constant, this is,where λ is wavelength. Both κΝΟΑ and κΜΟ are microscopically related to induced magnetic dipole moments.Linear chiral photonics holds great promise for a wide range of applications including optical isolation, brain wave detection, near-zero and negative refractive index metamaterials, and linear spin (i.e. sense of circular polarization) controlled photonics in general [2–6]. In particular, an ability to develop tools for noninvasive measurements of the spatially and temporally resolved activity of complex neuronal circuits is of crucial significance for studying their anatomy and function. This type of brain measurement and its connection to behavior is one of science’s grand challenges. Real-time brain activity imaging can identify patterns indicating stress, fatigue, and other mental states or emotions. In addition, it can monitor pathologies ranging from Alzheimer’s disease, to autism, to epilepsy. The practical detection of the ultra-weak magnetic fields produced by brain activity is not possible with existing MO materials. Producing an interferometrically-measurable shift in the optical rotation (~1 µdeg) in response to sub-pT magnetic fields generated by brain activity, requires materials with a Verdet constant of at least 109 deg/T•m for a device of practical size. This value is roughly five orders of magnitude higher than that provided by current inorganic materials. For example, terbium gallium garnet (TGG), which currently dominates existing MO applications, has V = 104 deg/T•m in the visible. New inorganic MO materials that that improve on the performance of TGG continue to be discovered. Recent examples include Tb2Sn2O7 [7], Ho2Ti2O7 [8], and Sr2Tb8(SiO4)6O2 [9]. While these materials have higher Verdet constants than TGG, the improvement is by less than a factor of two. Thus, this approach does not appear to provide a pathway to dramatic increases in V that would enable game-changing new devices. Therefore, the core materials-related challenge is clearly the development of MO materials with dramatically increased Verdet constant. Optical magnetometry based on organic MO materials would be a game-changing alternative (see Section 2.3 for the most recent results). This approach can enable the massively parallel, high-resolution sensing of magnetic field patterns that reflect specific cognitive states. The anticipated low cost and compact size of the corresponding instruments will allow them to be used routinely and will not require exposure of subjects to ionizing radiation or large magnetic fields. With portable and affordable magnetoencephalography neurologists could conduct experiments in roaming individuals and directly connect behavior to specific neuronal activity, thus charting new ways to understand the brain. In addition to its medical application, this technology could revolutionize human-machine interaction by finally realizing a practical brain-computer interface, i.e., to directly control electronic and mechanical devices using brain activity. Possible applications range from user interfaces for personal computer applications to prosthetics and robotics.
1.2 Nonlinear regime
For an optically nonlinear medium the index of refraction, n, is given by
for each of the circular polarization states (R and L), where κΝΟΑ is the chirality parameter due to natural optical activity, n2 is the nonlinear refractive index, κ2 is the nonlinear chirality parameter, and I is light intensity. The difference in the indices for the two circular polarizations causes a phase shift and, hence, rotation of the polarization plane of linearly polarized light. The total amount of rotation, caused by natural optical activity and intensity-dependent optical activity together can be controlled internally, by judicious design of molecular structure, and externally, by applying intense laser light. Even though the nonlinear chiral term, κ2 , is not expected to provide notable enhancement in overall n2 value, it can lead to notable differences in the intensity dependent term of Eq. (2) for opposite senses of the circular polarization, which might ultimately be used in new designs of all-optical switching devices. Moreover, combining high refractive nonlinearity with a negative sign with nonlinear chirality could be exploited in near-zero-refractive-index metamaterials [2]. In this case, the refractive index can be tuned towards negative values through zero for one of the circular polarizations of the incoming light.1.3 Microscopic origin
On the molecular scale κΝΟΑ, κΜΟ, and κ2 are microscopically related to the induced magnetic dipole moment, m, coupled to electronic transitions, induced by the electric component of the incident optical field [10]. Whereas κΝΟΑ and κ2 can only exist in chiral molecules - those which are not superimposable with their mirror images, κΜΟ exists in most organic materials when a static magnetic field is applied. In small molecules the optical and magneto-optical activity can be enhanced through an increased density of the excited states upon coupling with small metal clusters [11] or while using a zwitterionic motif in the structure [12].
1.4 Mesoscopic origin
On the mesoscopic scale, magnetic interactions are effectively enhanced when helicity is incorporated into the structure. This can be realized in π-π stacked supramolecular assemblies [13], in 3D patterns made by means of direct laser writing in a photocurable polymer [14], or in polymers equipped with chiral pendants, resulting in helical twisting of the rigid backbone [15]. In particular, by using π-conjugated chiral polymers with a high degree of electronic delocalization, extra amplification can be achieved due to the enhancement of the induced electric dipole moment and due to the induced displacement of delocalized electron density along a helical path, therefore generating helical microcurrents. Moreover, dipole-dipole coupling with plasmonic and/or excitonic nanoparticles can additionally result in re-normalization of the electric transition dipole moments, hybridization of molecular states with nanoparticles states and, in general, an increase in the density of states, leading to gigantic enhancement of all of the above mentioned properties [15–17].
2. Chiral polymers
2.1 Organic synthesis
The molecular building blocks of chiral polymers being developed for chiral photonics purposes are usually fluorenes [15,16,18] or thiophenes [19,20]. Helicity with an enantiomeric excess can be sterically induced by the incorporation of enantiopure chiral pendant groups (e.g. dimethyloctyl, methylbutyl, ethylhexyl) into the monomers with subsequent polymerization. Attachment of appropriate chiral pendants not only generates helicity and determines optical activity, but also improves solubility and processability of the rigid backbone. Repeating units of helical polymers can be further functionalized with one or more substituents (e.g., phenyl groups with or without π-electron donor/acceptor) at the opposite side from the chiral pendant, which allows one to study the effect of electron density on the optical activity of polymers. Formation of bound excitons with greater mobility through the conjugated helical backbone can further enhance magnetic coupling in these systems. Parameters of helicity of polymeric strands, such as the pitch and the coil radius can be manipulated via manipulating their chemical structures, thereby optimizing their optical response. Usually the polymers are designed to have low glass transition temperature, to facilitate thermal re-arrangement of cast or coated films without degradation. Molar mass is also optimized to make longer strands with better optical response.
Fused polyfluorene systems like the poly(fluorene-quinoxaline) systems shown in Fig. 1 have been already synthesized to show an enhanced optical activity [18]. The monomers required to produce these polymers can be prepared from commercially available starting materials in just a few steps [18]. To access the effect of systematic insertion of phenyl groups the monomer structures can be varied: one phenyl, two phenyls, two condensed benzenes (naphthalene) and three condensed benzenes (anthracene) (Fig. 2). Also, position-fixing structures (e.g. tert-butyl) in pre-determined positions can be introduced in the polymer chains to force the rotation of the main chain, inducing high helicity conformations. Representative combinations are presented in Fig. 2.
Fig. 1 Representative fused polyfluorene-based polymers. The R groups are typically homochiral dimethyloctyl, methylbutyl, or ethylhexyl moieties.
Fig. 2 Representative fused polyfluorene-based polymers. The top row shows systematic addition of phenyl rings, while the bottom row illustrates the addition of t-butyl groups to enforce the rotation of the backbone.
2.2 Coupling with inorganic moieties
The polymers are designed to incorporate moieties that can couple to inorganic nanoparticles such as noble metal plasmonic ones (Au, Ag) as well as semiconductor quantum dots. Both electron-rich anionic groups and Lewis bases strongly bind to noble metals. For example, binding can occur either through anionic phenoxide or dicyanomethylene groups or via the pyridine groups, which are also chromophore building blocks. The polymers can also be functionalized with thiol groups to immobilize gold nanoparticles and obtain uniform chiral distribution of gold NPs throughout the helical conformation of a chiral polymer. A variety of plasmonic particles, including metallic nano-spheres, nano-rods, and core-shell particles, can be used to tune the overlap between the intrinsic molecular response and the plasmon band. Plasmonic interaction between Au NPs and a chiral polymer was demonstrated and resulted in more than two orders of magnitude enhancement of optical activity in the visible as compared to the un-doped polymer [15]. An in situ photochemical decomposition of noble metals (Au and Ag) precursors can also be used to produce nanocomposite materials with desired optical activity. The advantage of this method is that a high loading of metallic nano-particles can be produced without aggregation.
The excitonic enhancement of chiro-optical response by a variety of semiconductor nanocrystals (quantum dots) is a promising path to getting polymer based nanocomposite materials with ultra-high optical activity. For example, nanocomposites of poly(fluorenebenzothiadiazole) (PFBT) doped with CdTe/ZnS core-shell quantum dots (QDs) have shown a great enhancement of PFBT circular dichroism: almost two orders of magnitude compared to the pristine polymer [16]. Excitonic coupling between the helical polymer molecule and single QDs as well as the coupling between QDs can result in enhancement that can be even superior to plasmonic enhancement obtained with gold NPs. Surface modification is essential to ensure that the nanoparticles can be stably dispersed in solvents that dissolve the conjugated polymer host material and that are suitable for use in spin-coating of thin films, and to ensure that the nanoparticles remain well dispersed in the polymer host after solvent evaporation. The mechanism of chirality transfer from a chiral molecule to an achiral nanoparticle was suggested based on dipole-dipole coupling, resulting in the induced chiral re-distribution of surface charges, and on the local field enhancement [17].
Table 1 summarizes the chiro-optical properties of a few fluorene-based polymers and nanocomposites. One has to bear in mind that the dispersion of the real part of the measured quantities, that actually defines the amount of the phase shift between different polarization states, does not exactly follow that of the imaginary (absorptive) part. The two parts are linked through Kramers-Kronig transformation [10] and as such are spectrally shifted with respect to each other. It implies that the spectral position of (nonlinear) chirality parameter peak value does not coincide with that of the absorbance peak value. It is of course understood, that the very nature of light-matter interaction assumes a relatively fast fade-out of the real part far from any electronic resonance. A figure-of-merit, defined as the ratio of ellipticity to absorbance, becomes important for assessing the actual practicality of a material. The immediate technological importance of the achieved enhancement of linear and nonlinear optical activity is a potential ability to achieve a negative or near-zero refractive index (meta)material [2]. The mainstream approach in this field employs double negative metamaterials, i.e. materials with simultaneously negative dielectric permittivity, ε, and magnetic permeability, μ, in efforts to produce near-zero or negative refractive index [2]. This requires sub-wavelength resonators - “meta-atoms”, which are extremely challenging to fabricate with conventional “top down” techniques (e-beam lithography), especially at large scale or in three dimensions.
Table 1. Optical properties of representative chiral polymers.
2.3 Coupling with stable organic biradicals
MO activity in our all-organic materials can be enhanced: (i) structurally, by optimizing the helicity and conjugation of the polymer chains [4,19,20]; and (ii) magnetically, through the introduction of new spin states upon inclusion of stable organic biradicals [4] or superparamagnetic nanoparticles. A synergistic combination of these two effects can potentially generates ultra-high MO activity: reported Verdet constant values of up to 106 deg/T•m in the visible [4,19,20]. An organic biradical bTbK (bis-TEMPO-bisketal, Fig. 3) couples two TEMPO radicals via a rigid bisketal linkage to provide an exceptionally stable biradical [21]. Most biradicals are quite reactive and are decomposed or polymerized at room temperature; however, the bTbK molecule is known to be stable in its biradical form at room temperature. Stability of the biradical units of bTbK can be further improved by replacing the peripheral methyl groups with larger entities (R1) as illustrated in Fig. 3 [22]. This also improves the solubility of the biradicals together with the chiral polymers in organic solvents that are suitable for use in spin-coating of thin films. The length and electron density of the biradical molecule can also be extended as displayed in Fig. 3 (lower right) by adding a π-building block (R2) at the center of the bTbK unit. Magnetically doped quantum dots such as InP:Mn [23,24] are another potential development of this strategy. They can potentially contribute both to excitonic enhancement of natural optical activity and to increased magnetic coupling.
The above mentioned strategies of doping with biradicals and superparamagnetic nanoparticles can of course be combined with one another. Beyond simply doping with multiple types of nanoparticles, it has become possible to produce hybrid, multicomponent nanoparticles that combine plasmonic, semiconductor, and/or superparamagnetic domains in single multi-domain nanoparticles. Previously published examples include Au-Fe3O4, Au-PbSe, Au-Fe3O4-PbSe, Ag-CdSe, Au-CdSe, Pd-CdSe, Pt-CdSe, PbSe-Fe3O4, FePt-CdS, and Au-Cu2-xSe [25–28]. Well-developed methods of linking organic molecules to nanoparticles can be used to tether the stable biradicals to the nanoparticles, providing control of the interactions of different components of nanocomposite materials with one another and with the host polymer.
3. Characterization methods
Overall, the correlation of the experimental data with synthetic modifications provides rigorous structure–property relationships that are needed to guide the development of nanocomposites with improved properties. Characterization of chiral nanocompsoite materials requires the whole arsenal of methods and techniques. This includes linear and nonlinear circular dichroism spectroscopy (CD) as well as magnetic circular dichroism spectroscopy (MCD). Direct measurements of the Faraday rotation at ambient temperature and moderate magnetic fields can be made using magnetic field modulation technique [29]. It is also important that the MCD includes the ability to measure the magnetic field response on CD signals across wide ranges of both magnetic field and temperature. The dispersion of the chirality parameter and/or Verdet constant is obtained with Kramers-Kronig transform of CD/MCD spectra (ellipticity) [10,4]. For nonlinear characterization a modification of the Z-scan method, employing circularly polarized light, can be used to quantify both polarization-sense-dependent two-photon absorption and nonlinear refractive indices for the left and right circular polarizations [30].
4. Outlook
New classes of low-cost, flexible, and versatile materials with exceptional optical/magneto-optical activity promise to be of considerable technological importance. They will enable breakthrough developments in sensing and imaging and shape technological innovation in numerous related areas. Such materials can potentially be produced with custom-tailored photonic functionality in large-format and at low-cost, which would have a broad and transformative impact in all above mentioned technological areas by enabling on-demand roll-to-roll fabrication of flexible chiral photonic materials with unprecedented throughput and manufacturing efficiency. This requires the synthesis of well-defined polymers with precisely controlled repeating units. These materials must maintain chemical, thermal, and photochemical stability, which may be compromised by extending the conjugation length. Uniform dispersion of stable organic biradical dopants is required in the polymer host to provide optimal coupling between their partially-filled orbitals and those of the polymer backbone, while preserving the desired helical conformation of the polymer chains.
Funding
Air Force Office of Scientific Research Grant numbers FA9550-11-1-0121 and FA9550-09-1-0258.
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