Preserving Spin States upon Reflection: Linear and Nonlinear

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Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror Lei Kang, Sean Rodrigues, Mohammad Taghinejad, Shoufeng Lan, KyuTae Lee, Yongmin Liu, Douglas H Werner, Augustine Urbas, and Wenshan Cai Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03882 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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Preserving Spin States upon Reflection: Linear and Nonlinear Responses of a Chiral Meta-Mirror Lei Kang,†,‡,§ Sean P. Rodrigues,†, ‡ Mohammad Taghinejad,† Shoufeng Lan,† Kyu-Tae Lee,† Yongmin Liu,│ Douglas H. Werner,§ Augustine Urbas,║ and Wenshan Cai*,†,‡ †

School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive NW, Atlanta, Georgia 30332, United States ‡ School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332, United States § Department of Electrical Engineering and Center for Nanoscale Science, Pennsylvania State University, University Park, PA 16802, United States │ Department of Mechanical & Industrial Engineering and Department of Electrical & Computer Engineering, Northeastern University, Boston, MA 02115, United States ║ Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States

KEYWORDS: Metamaterials, chirality, circular dichroism, nonlinear optics, optical activity ABSTRACT: Conventional metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin state of light waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as ~0.5 is experimentally observed in a near infrared wavelength band. By imaging a fabricated pattern composed of the enantiomeric unit cells, we directly visualize the two key features of our engineered meta-mirrors, namely the chiral-selective absorption and the polarization preservation upon reflection. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime. Chiral metamirrors, exhibiting giant chiroptical responses and spin-selective near field enhancement, hold great promise for applications in polarization sensitive electro-optical information processing and biosensing. 1

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In terms of symmetry, chirality is identified as an omnipresent property in both the inorganic and organic world. The determination of matter’s chiral characteristics provides information about microscopic symmetry, such as the secondary structure of large biological molecules, and is crucial in molecular identification and analysis.1 When two objects have identical scalar physical properties, but opposite chiroptical responses, the structures are known as enantiomers. These types of materials have electric susceptibilities that respond differently to the handedness of the incident light, as the helical form of a circular polarized beam is in itself asymmetric. The twist in the polarization of the light is caused by the dynamic rotation of the electric and magnetic fields also known as the spin angular momentum of light. The direction of the optical torque carried by the light is denoted by the spin state as a positive or negative value for left and right circular polarized waves respectively. When the helical wave interacts with the chiral material, the polarization of the light can be controlled, based on chiroptical effects known as optical activity and circular dichroism (CD). However, conventional chiroptical devices are hindered by the extremely weak chiral response of natural materials. The desire to more efficiently use and control the interaction between circular polarizations and chiral media has inspired great interest in exploring artificial chiral structures. The modern study of artificial chirality can trace its roots back to the 1890s, when Bose’s twisted jute rotated the plane of polarization for millimeter waves.2 Harnessing plasmonic resonances that give rise to dramatically enhanced light-matter interaction, the recently emerging field of chiral metamaterials opens new opportunities for creating enormous chiroptical responses. By applying an intrinsic design principle, to form resonant architectures non-superposable on their own mirror image, researchers have realized a variety of chiral metamaterials consisting of two- and three-dimensional meta-atoms, which have brought large chiral responses to the nanoscale.3-18 In particular, the remarkably strong 2


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chiroptical effects observed in chiral metamaterials have been reported to achieve ultracompact photonic devices, such as circular polarizers,7-13 for far-field polarization control of light. Owing to their circular-polarization-dependent field concentration, chiral metamaterials have also been employed to elicit increased chiral-selective nonlinearity.14-18 Furthermore, by leveraging plasmonic enhanced optical chirality in the near-field, chiral nano-architectures have been used as ultrasensitive biosensing platforms for CD spectroscopic analysis.19-24 Although chiroptical properties are conventionally defined and characterized via transmission measurements, reflective chiroptical responses are also found in nature. For instance, the Chrysina gloriosa, a type of jeweled beetle, reflects circularly polarized light of the opposite handedness in a different fashion due to their structure of extracellular chitin in their exoskeleton.25 Reflective chiroptical responses also can be found in artificial chiral media. For example, Zhang, et al, have studied the reflection behavior of chiral sculptured-thin-film mirrors consisting of an array of helical nanowires and observed the circularly polarized emission from colloidal nanocrystal quantum dots confined in the corresponding microcavities.26 More recently, by providing a proof-of-concept demonstration at microwave frequencies, Plum and Zheludev have elucidated the intriguing behaviors of metamaterial based chiral mirrors, abbreviated hereafter as meta-mirrors.27 In stark contrast to conventional mirrors that flip the spin state of incident light upon reflection at normal incidence, chiral meta-mirrors reflect light of one spin state while strongly absorbing the other. By presenting a theoretical analysis, Wang, et al., has revealed that the necessary condition to achieve such a meta-mirror is to simultaneously break the n-fold rotational (n > 2) and mirror symmetry at the unit cell level.28 The handednesspreserved reflection and chiral-selective absorption of meta-mirrors allow for the achievement of huge polarization contrast offering potential for applications in polarization manipulation and

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plasmonic enhanced chiroptical analysis. By leveraging chiral metamaterials in conjunction with hot-electron physics, unique devices can be created like that of the recently reported photodetector selectively sensitive to circularly polarized light.29 In this paper, we demonstrate a chiral meta-mirror consisting of a sandwich system with an asymmetric-hole array perforated film, a thin dielectric layer and an optically thick metallic back plane. The linear reflection measurement of the meta-mirror sample of both enantiomers with circularly polarized waves at normal incidence indicates a reflection circular dichroism of ~0.5 at a resonance wavelength around 880 nm. Moreover, the two distinct features of the meta-mirror, i.e., the chiral-selective absorption and the circular polarization preservation, are unambiguously identified through a comprehensive visualization of a fabricated hand pattern using the metamirror unit cells. Beyond the linear regime, the resonance behavior further gives rise to the handedness-dependent nonlinear generation from the meta-mirrors under circularly polarized excitations. More interestingly, we have demonstrated high-contrast second-harmonic optical imaging enabled by the resonance-enhanced and chiral-selective second harmonic signals. Given the structural simplicity of our design, we envision that the intriguing interplay between the meta-mirrors and other functional material systems that can be easily integrated with this metadevice would enable interesting functionalities, such as electrooptic modulation. In both naturally occurring chiral molecules and man-made chiral meta-structures, chiroptical responses stem from the magnetic dipoles excited by the electric field component of light, and vice versa (i.e., magnetic-field-excited electric dipoles).4, 30-32 In addition, the coupling of the two types of dipoles results in the breaking of degeneracy between the circularly polarized lights of opposite handedness, and thereby, the resultant chiroptical responses. These general principles of chirality lead to the effective design presented in this work. The dual-layer structure

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facilitates the generation of magnetic dipoles directed perpendicular to the light propagation direction, while the broken mirror symmetry of the in-plane pattern makes the coupling between the electric and the magnetic dipoles (and hence the charge-oscillation eigenmodes) sensitive to the spin state of the incoming photons. Schematics of the unit cells for both enantiomers of the metamaterial used in this study are illustrated in Figure 1a,b, respectively. Each enantiomer of the meta-mirror consists of a patterned gold (Au) film, separated from a silver (Ag) back plane by a thin dielectric spacer. The unit cell of the patterned layer has a hole formed from two intersecting rectangles and their mutual arrangement determines the chirality of the nanostructure. The optically thick silver back plane not only ensures zero transmissivity (such that T = 0 and accordingly, absorption satisfies A = 1 − R) as that required by a reflecting mirror, but also largely enhances the chiroptical response of the structure by providing a spin-state selective resonant cavity. Figure 1c,d show scanning electron microscope (SEM) images of the fabricated samples for the two enantiomers. We note that by satisfying the symmetry breaking conditions of the unit cell, our design allows the achievement of an extraordinary chiral optical response from planar photonic structures with just a single patterned layer, thusly resulting in a dramatic decrease of device fabrication complexity. To elucidate the chiral responses of the two enantiomers in the linear optical regime, fullwave simulations were performed using the commercial finite integration package CST Microwave Studio. As illustrated in Figure 2a, a polarization sensitive reflection band at around 850 nm can be clearly identified, when enantiomer A is illuminated by left- and right-handed circularly polarized (LCP and RCP) waves. Within this band, one of the incoming circularly polarized waves will be reflected, while the wave of the opposite circular polarization is absorbed by the meta-mirror. To unambiguously evaluate the corresponding reflecting behavior,

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polarization analysis of the reflection spectrum of enantiomer A for the two circular polarizations is performed and depicted in Figure 2b. The power reflection coefficient RLL (RLR) is defined as the LCP (RCP) portion of the reflected power from the metamaterial to that of the LCP incidence, while RLR (RRL) represents the cross conversion of circular polarization. In terms of the abbreviation, RLL implies an input LCP wave that is analyzed as the component of LCP light reflected back, whereas RLR implies an LCP wave input and the component of RCP light reflected back. As expected, RLR and RRL are identical due to the symmetry of the unit cell.33 In addition, a remarkable contrast between the co-polarized reflection spectra, i.e., RLL and RRR, is obvious at the wavelength around 850 nm. A comparison between Figure 2a,b and Figure 2c,d indicates the complementary spectral behavior in the reflection of enantiomers A and B. As depicted in Figure 2e, within the resonance band, enantiomer A is a mirror for RCP and an absorber for LCP waves, while enantiomer B strongly reflects LCP light and absorbs that with right circular polarization. The reflectance contrast can reach ~0.8 at the wavelength of 850 nm. Furthermore, besides the resonance around 850 nm, an additional reflection band exhibiting flipped chiral selectivity can be seen around 750 nm for both enantiomers, where, i.e., enantiomer A (B) is found to be more reflective for LCP (RCP) and more absorptive for RCP (LCP), respectively. We emphasize that, in addition to the chiral-selective absorption, there is a fundamental difference between our meta-mirror and a regular metallic mirror. It is known that, for ordinary reflective surfaces, including metallic mirrors, the circularly polarized light will flip its spin state upon reflection (Figure 2e). In this case, the reflected light from a regular mirror contains no component of the input polarization and only contains light of the opposite spin, i.e., RLR = RRL = 100% and RRR = RLL = 0%. In sharp contrast, the reflected light from the meta-mirror will

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preserve its initial state of circular polarization. This reflection behavior can be attributed to the tailored light-matter interaction between the circular polarization states and the meta-mirror. This is made evident from the distribution of the electric field cut in a plane 50 nm beneath the patterned Au layer of both enantiomers, as shown in Figure 2f-i. Corresponding to the resonance dip identified in Figure 2a,c, pronounced circular-polarization-dependent field enhancement is seen at 852 nm in enantiomer A and B for incidence of LCP and RCP waves, respectively, which foreshadows the resonance enhanced chiral-selective optical nonlinearity as will be presented and discussed later. To obtain reflection coefficients from the meta-mirror, we illuminated the sample with circularly polarized waves and analyzed the reflected light from the fabricated samples of both enantiomers. The measured results are shown in Figure 3 and all spectra were normalized to that of an unpatterned substrate. As illustrated in Figure 3a, a sharp reflection dip is observed at a wavelength around 880 nm for a LCP wave impinging upon enantiomer A, in comparison to the high reflectivity seen at the same band for RCP illumination. An additional pronounced but flipped circular dichroic contrast is seen in the wavelength range between 750 nm and 830 nm. When a circular polarized wave is incident on the sample, it is of interest to know whether the reflected beam maintains the input spin state or is flipped. The analysis of this is shown in Figure 3b. Analogously to the simulation, the metamaterial selectively absorbs one circular polarization and preserves spin state of the other. Furthermore, enantiomer B exhibits complementary chiral reflective behaviors, as indicated in Figure 3c and Figure 3d. The measured data are in good agreement with the simulations shown in Figure 2, and the slight discrepancies most likely arise from fabrication imperfections.

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To better visualize the chiral selective absorption and the preserved circular polarization in our meta-mirrors, we created a pattern of a human hand using the enantiomeric unit cells and observe the images under select polarizations. Figure 4a depicts the configuration of the fabricated sample, in which enantiomer A and B were employed to form the exterior and interior regions of the hand. On the exterior of the pattern, a gold layer was left unpatterned to serve as a regular mirror in the background. According to the measured linear spectra (Figure 3), the wavelengths of incident light were selected to be in the range from 850 to 910 nm to optimize the image contrast. The hand pattern was then illuminated by LCP, linearly polarized, and RCP light, then collected with an objective and illustrated in Figure 4b-d, respectively. As evidenced by the figure, the hand pattern is only visible under circularly polarized illumination. In particular, a bright (dark) hand stands out from a dark (bright) background under LCP (RCP) incidence, which is due to the complementary reflection characteristics of enantiomer A and B under circularly polarized illumination as shown in Figure 3 from 850 to 910 nm. Essentially, the images presented in Figure 4b-d embody the reflection behavior discussed in Figure 3a,c, i.e., the chiral-selective reflection of both enantiomers. Additionally, it should be noted that switching the spin state of the incident light reverses the contrast of the images. On the contrary, as shown in Figure 4c, linearly or randomly polarized incidence is unable to distinguish the enantiomeric characteristics of the pattern and cannot produce recognizable images of the sample, because such input contains equal amounts of the two circular polarization components. As we have stressed earlier, a regular mirror will flip the incident circular polarization to the opposite handedness, while a chiral meta-mirror preserves the circular state of polarization upon reflection. This drastic contrast can be visualized when we collect the images of the entire metapattern, including the unpatterned metallic surface in the surrounding area, after a circular

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polarization analyzer (Figure 4e-g). The images serve as a visual presentation of the circular polarization preservation characteristic of the meta-mirror upon reflection that is observed in Figure 3b,d, after circular polarization analysis. As demonstrated in Figure 4e,f, for LCP incident light, a bright hand against a dark surrounding area can be seen with a LCP analyzer, while, the entire metamaterial pattern becomes dark when a RCP analyzer is applied. The contrast between the two enantiomers is even stronger when analyzed by circular polarization. Furthermore, the imaging behavior completely inverts when a RCP wave impinges upon the sample, i.e., a dark hand stands out with a bright background with a RCP analyzer implemented (Figure 4g), while an image identical to Figure 4f will be produced with a LCP analyzer inserted (not shown). These results comprehensively visualize the chiral-selective absorption and the circular polarization preservation properties of our meta-mirrors. It is even more intriguing to compare these behaviors with the imaging property of a regular mirror, here seen as the unpatterned area around the meta-mirror pattern. It can be seen that, the regular mirror will be dark (bright) when the incident light has the same (opposite) spin state as the circular analyzer (Figure 4e-g). In comparison, in the absence of the analyzers, the regular mirror is always bright (Figure 4b-d). This is consistent with the discussion above: an ordinary mirror flips the incoming circular polarization into the opposite one, while chiral meta-mirrors reflect and preserve the incoming polarization for a given spin state. Beyond the chiroptical response in the linear regime, the strong resonances including the handedness-sensitive field concentration observed in the meta-mirror is expected to significantly increase chiral-selective optical nonlinearity. Therefore, we have conducted nonlinear chiroptical measurements. In the experiment, we excite the meta-mirror samples of both enantiomers with a circularly polarized ultrafast laser and collect the generated nonlinear optical signals in the

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reflection direction. The corresponding nonlinear spectra when the metamaterial is pumped at a series of excitation wavelengths with a constant intensity are shown in Figure 5a,b. For most excitation wavelengths, a sharp peak can be identified in the generated nonlinear spectrum at the frequency-doubled wavelength. More importantly, this second harmonic generation (SHG) signal is closely dependent on the spin state of the pump light and the enantiomeric properties of the meta-mirror. Furthermore, for excitation at shorter wavelengths (e.g. 740 nm), the SHG peak is accompanied by a broadband nonlinear spectrum that can be attributed to multi-photon luminescence.34 The spin-dependent harmonic generation of light observed in our chiral metamirror originates from its chiroptical responses in the linear regime. In particular, a chiralselective resonance in the reflection spectrum corresponds to an eigenmode of the system. Such an eigenmode is associated with condensed light intensity, enhanced optical absorption, and hence the reflection dip in the linear spectrum. The enantiomeric properties of the structures in the linear regime are further transduced to their second-harmonic generation circular dichroism, as nonlinear wave mixing processes are positively related to the local intensity of the light wave. By integrating the signal in the frequency-doubled peaks of Figure 5a,b, we obtain the SHG excitation spectrum of both enantiomers pumped by circular polarized waves, as shown in Figure 5c,d. The resonance behavior of the SHG efficiency in the meta-mirror is evident, recalling the linear chiroptical response shown in Figure 3. For instance, the SHG excitation spectrum of enantiomer A under LCP and RCP excitation peaks at a fundamental wavelength (λω) around 770 nm and 850 nm, respectively, which is correlated to the linear reflection behavior shown in Figure 3a. Moreover, a SHG efficiency contrast larger than 3× (4×) is observed at λω ~760 nm between RCP (LCP) and LCP (RCP) beams incident on enantiomer A (B). To better elucidate the chiral-selective nonlinearity of the meta-mirror, second harmonic generation circular 10


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dichroism (SHG-CD) is utilized to quantify the harmonic optical activity. Here SHG-CD is defined as 2( − )/( + ), is plotted in Figure 5e for both enantiomers. A

remarkably large harmonic optical activity is observed at the two resonances. In particular, a near unity SHG-CD value of enantiomer A is found at λω around 770 nm, at where enantiomer B experiences a SHG-CD value of approximately −1, while, at λω around 850 nm, enantiomer A and B have a SHG-CD value of roughly −0.5 and 1, respectively. These results reveal that second harmonic excitation can boost the contrast of nonlinear images,14 as seen in later experiments. To investigate the potential of the meta-mirror for nonlinear imaging, we pump the patterned hand with a circularly polarized ultrafast wave at two fundamental wavelengths, i.e., 770 nm and 850 nm, where the SHG-CD signal peaks and acquire the nonlinear images formed by the generated nonlinear signals. As illustrated in Figure 6a, a bright hand in a dark background is clearly observed in the second-harmonic image of the sample under a LCP excitation at λω = 770 nm, while Figure 6b indicates that the image brightness of the interior and exterior area of the hand reverses when the handedness of the excitation is switched to RCP. This observation is in accord with the chiral-selective SHG efficiency of the two enantiomers illustrated in Figure 5c,d. Importantly, compared with the linear images shown in Figure 4, a dramatically improved contrast is identified from these SHG images. Moreover, the unpatterned section surrounding the metamaterial area is invariably dark due to the much lower SHG efficiency of the flat surface. By implementing a point-by-point calculation of the SHG-CD with photon counts recovered from both Figure 6a and Figure 6b, we obtain the corresponding SHG-CD image of the metamaterial pattern, as shown in Figure 6d. It can be seen that, owing to the enantioselectivity of the SHG signal generated from the metamaterial pattern, the SHG-CD image provides 11


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significantly improved contrast compared to those seen in Figure 6a,b. In addition, the SHG images of the sample pumped by a circularly polarized wave at λω = 850 nm are depicted in Figure 6d,e. A dark (bright) hand can be recognized from the bright (dark) background when excited by LCP (RCP) laser light, although the overall moderate contrast of the two images is constrained by the SHG efficiency at λω = 850 nm. Nevertheless, the corresponding SHG-CD image shown in Figure 6f presents well-resolved details of the metamaterial pattern, which again clearly demonstrates the ability provided by the SHG-CD equation in visualizing the chirality of the metamaterial nanostructures. We notice that a round-shaped dark area is observed in Figure 6c. This is attributed to the signal of the fundamental beam reflected by the regular mirror area that was not completely eliminated by the short-pass filter implemented. Nevertheless, possessing a value of SHG-CD close to zero, it has less influence on the visibility and contrast of the pattern’s SHG-CD image. No such round dark area is found in Figure 6f, because of the much lower transmission rate of the filter at λω = 850 nm. We have designed and fabricated a metamaterial-based chiral mirror with giant chiroptical responses in both the linear and nonlinear regimes. Compared with regular (e.g. metallic) mirrors, there are two prominent features of the meta-mirror that have been identified, namely the strong chiral-selective absorption and the handedness-preserved reflection from the surface. In addition, chiral resonance tailored nonlinear generation and second harmonic signal based imaging are observed in the meta-mirror under circularly polarized ultrafast excitations. Only requiring a single patterned layer, our design is more fabrication-friendly as compared with many of the most efficient chiral metamaterials that require genuinely three-dimensional building blocks. Moreover, the unpatterned back plane may allow for easier integration of various constituent materials as the core of the three-layer metamaterial, which, along with the 12


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topologically continuous structures of the design, may enable chiroptical responses that can be purposely tuned by external stimuli, such as an electric field.

METHODS Sample Fabrication. The chiral metamaterial is created via a multi-step lithography process. Photolithography was used to design the sample layout of the chiral mirror. Silver was electron beam evaporated onto the sample and lift-off was performed. A coating of Al2O3 was subsequently deposited with a thickness of 145 nm. Electron beam lithography (JEOL JBX9300FS EBL) was carried out to define the upper, intersecting-rectangle pattern. A layer 50 nm in thickness of gold was deposited and another lift off process was performed. In order to achieve results similar to that of the simulation and protect the sample from degradation, PMMA was spin coated to form a transparent top cladding of 1 µm in thickness on the sample. Linear Optical Characterization. Light provided by a tungsten light source was polarized using a linear polarizer and a waveplate to create the different incident spin states. The analysis was performed in reflection mode using strict placement of beam splitters and optics. A homemade spectroscopy system dedicated to the characterization of small sample areas was used for the characterization of the sample. A spectrometer and a silicon CCD detector collected the light passed through the spectroscopy system. The analysis of the circular polarization that was reflected from the sample is achieved by the placement of a circular analyzer which is composed of a quarter wave plate followed by a linear polarizer. Images were captured using a long pass filter at 850 nm.

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Nonlinear Optical Characterization. A Ti:sapphire ultrafast oscillator (Spectra-Physics, Mai Tai HP) with a pulse duration of 100 fs, a repetition rate of 80 MHz was applied as the excitation source for the measurements herein. This laser was tuned from 730 to 1000 nm every 10 nm to acquire the data for the nonlinear measurements above. The incident beam had a diameter of roughly 40 µm and had an average power of 80 mW. To create the two spin states, a quarter wave plate was adjusted to interact with the linearly polarized wave at specified angles. The nonlinear images were achieved by using a series of short pass filters.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions L.K., S. P. R and M. T. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). This material is based upon work partially supported by the National Science Foundation under Grant No. ECCS1609567 and by a collaborative support from the Air Force Research Laboratory through Azimuth subcontracts 238-5404-GIT. W.C. and Y.L. acknowledge support from Samsung 14


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Global Research Outreach (GRO) Program. Y.L. also acknowledges the partial financial support from the Office of Naval Research under Grant No. N00014-16-1-2409. S.P.R. acknowledges the support of the National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DGE-1650044.

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Figures and Captions

Figure 1. Schematics and SEM images of the metamaterial based chiral mirror for both enantiomers. (a−b) Schematic of the enantiomeric unit cells that consist of a patterned gold film, which is separated from an optically thick silver back plane by a thin Al2O3 spacer. The unit cell of the patterned layer is formed from the union of two intersecting rectangles. Geometrical parameters: P = 400 nm, l = 260 nm, w = 210 nm, v = 25 nm, u = 75 nm, tAu = 50 nm and ts = 145 nm. (c−d) SEM images of enantiomers A and B of the chiral meta-mirrors. Scale bar represents 500 nm.

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Figure 2. Simulated linear chiroptical responses of both enantiomers. (a) Total reflection spectra of enantiomer A under LCP and RCP illumination, respectively. (b) The corresponding reflection spectra of the co- and cross-polarization components. (c−d) Graphics of the counterparts for enantiomer B. A chiral selective reflection band around 850 nm can be identified. (e) Schematics of the reflection behavior of the meta-mirror and a regular mirror. A meta-mirror composed of enantiomer A (B) reflects RCP (LCP) incident light and absorbs the entire LCP (RCP) component. The reflected light from the meta-mirror will preserve its initial state of circular polarization. For a regular mirror, the circularly polarized light will flip its circular polarization upon reflection at normal incidence. (f−g) The electric field distributions cut from a plane 50 nm beneath the Au layer of enantiomer A at the resonance wavelength of 852 nm. (h−i) The electric field distributions of enantiomer B at 852 nm.

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Figure 3. Measured linear chiroptical responses of both enantiomers. (a) Reflection spectra of enantiomer A under LCP and RCP illumination. (b) The corresponding reflection spectra of the co- and cross-polarized components. (c) Reflection spectra of enantiomer B under LCP and RCP illumination, respectively. (d) The corresponding reflection spectra of the co- and crosspolarization components for enantiomer B. Representative images of the corresponding samples are shown in the insets.

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Figure 4. Visualization of the chiral selective absorption and the circular polarization preservation of the meta-mirrors. (a) Schematic of the created hand pattern. The exterior and interior regions of the hand are formed by enantiomer A and B, respectively. The metamaterial pattern is surrounded by unpatented metal, serving as a regular mirror in the background. (b−d) Microscopic images of the fabricated metamaterials illuminated by LCP, linear/random polarized and RCP waves, respectively. (e−g) Corresponding images taken with a circular polarization analyzer, when the pattern is illuminated by both circular polarizations. In the case of RCP illumination with the LCP analyzer, an image identical to Figure 4f is produced.

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Figure 5. Chiral-selective nonlinear responses from both enantiomers. (a) Nonlinear spectra of enantiomer A under LCP and RCP excitations, respectively, at a series of excitation wavelengths of constant intensity. (b) Depictions of the counterparts for enantiomer B. (c−d) SHG excitation spectra for enantiomer A and B. The empty circles represent the measured results, and the solid curves serve to guide the eye. (e) Second-harmonic circular dichroism SHG − CD 2( − )/( + ) of both enantiomers.

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Figure 6. Nonlinear imaging of the chiral meta-mirror. (a−b) Nonlinear images of the hand pattern under LCP and RCP excitation, respectively, at a fundamental wavelength of 770 nm. (c) SHG-CD image obtained from Figure 6a,b. (d−e) Nonlinear images of the pattern under circularly polarized excitation at 850 nm. (f) The corresponding SHG-CD image is obtained from Figure 6d,e. The scale bars represent 20 µm.

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Table of Contents Graphic

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TOC Graphic 60x35mm (300 x 300 DPI)


Preserving Spin States upon Reflection: Linear and Nonlinear

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