Optical Chirality Enhancement Research Articles

Intrinsic Superchirality in Planar Plasmonic Metasurfaces.

Plasmonic metasurfaces with spatial symmetry breaking are crucial materials with significant applications in fields such as polarization-controlled photonic devices and nanophotonic platforms for chiral sensing. In this paper, we introduce planar plasmonic metasurfaces, less than one-tenth of a wavelength thick, featuring nanocavities formed by three equilateral triangles. This configuration creates uniform, thin metasurfaces. Through a combination of experimental measurements and numerical modeling, we demonstrate the inherent superchirality of these plasmonic metasurfaces. We address the challenge of achieving a strong enhancement of optical chirality in the visible spectrum, reaching levels comparable to those of 3D chiral metasurfaces.

Enhanced Circular Dichroism for Achiral Sensing Based on a DNA-Origami-Empowered Anapole Metasurface.

Circular dichroism (CD) spectroscopy has been extensively utilized for detecting and distinguishing the chirality of diverse substances and structures. However, CD spectroscopy is inherently weak and conventionally associated with chiral sensing, thus constraining its range of applications. Here, we report a DNA-origami-empowered metasurface sensing platform through the collaborative effect of metasurfaces and DNA origami, enabling achiral/slightly chiral sensing with high sensitivity via the enhanced ΔCD. An anapole metasurface, boasting over 60 times the average optical chirality enhancement, was elaborately designed to synergize with reconfigurable DNA origami. We experimentally demonstrated the detection of achiral/slightly chiral DNA linker strands via the enhanced ΔCD of the proposed platform, whose sensitivity was a 10-fold enhancement compared with the platform without metasurfaces. Our work presents a high-sensitivity platform for achiral/slightly chiral sensing through chiral spectroscopy, expanding the capabilities of chiral spectroscopy and inspiring the integration of multifunctional artificial nanostructures across diverse domains.

Lithography‐Free Fabrication of Terahertz Chiral Metamaterials and Their Chirality Enhancement for Enantiomer Sensing

AbstractChiral metamaterials comprise a promising platform for advanced optoelectronic and biomedical applications. However, conventional fabrication via lithography is limited by its complexity and high cost. Herein, the lithography‐free fabrication of terahertz chiral metamaterials and their enhancement for sensing the chirality of biocrystal enantiomers is presented. Chiral Au microstrip patterns (CHAMs) in a saw‐tooth shape are fabricated by combining two‐step buckling processes and glancing‐angle deposition. Non‐superimposable geometric chirality is achieved by controlling the tilt angle between the asymmetric and biaxial strain axes and the selective area deposition of the Au layers by using the shadow effect. The manufactured chiral metamaterials show mirror‐shaped terahertz circular dichroism (TCD) signals in the range of 0.2–2.5 THz. Coupling of the induced electric and magnetic dipoles to the chiral‐shaped Au surfaces results in effective optical chirality enhancement. Finite‐difference time‐domain computational simulations reveal the homogeneous distribution of optical chirality with an absolute maximum of 2.24 in the near field. Summing the TCD signals for enantiomeric cystine biocrystals onto the chiral metamaterials shows an ≈7‐fold amplification in magnitude. This enhancement can be attributed to the synergistic effects of superchiral field enhancement and the electromagnetic resonance between the CHAMs and biocrystals.

Local versus bulk circular dichroism enhancement by achiral all-dielectric nanoresonators

Abstract Large optical chirality in the vicinity of achiral high-index dielectric nanostructures has been recently demonstrated as useful means of enhancing molecular circular dichroism. We theoretically study the spatial dependence of optical chirality enhancement in the vicinity of high-index dielectric nanodisks and highlight its importance for the design of nanophotonic platforms for circular dichroism enhancement. Using a T-matrix framework, we demonstrate that, depending on the disk aspect ratio, chirality is enhanced preferentially along different directions. We employ various statistical procedures, including surface, volume and orientation averaging, to predict enhancement of chiroptical effects and show that optimal properties of a nanostructure depend substantially on whether spatial maximum or average chirality enhancement is sought after. The results indicate that at times it is beneficial to sacrifice helicity preservation for a larger field enhancement. Similarly, the optimal choice of the nanostructure is influenced by presence of a substrate, which limits the space available to be occupied by analyte molecules and impacts the optical chirality in the vicinity of the nanostructure.

The mechanism of optical chirality enhancement in vertical-cut slant-gap antenna

The mechanism of optical chirality enhancements (OCE) in slant-gap antenna with vertical-cut configuration by normal and oblique incident light with linear polarization is demonstrated. The main contribution of OCE is originated from the inner product of the incident H field and the localized surface plasmon resonant (LSPR) evanescent E field. The pure LSPR near-field term E·H gives a net zero contribution. Based on these findings, we can accurately count for the asymmetric OCE distribution due to the substrate effect. The detailed analysis of OCE contribution might provide a route for designing a more efficient chiral molecule sensor.

Optical Chirality Enhancement in Hollow Silicon Disk by Dipolar Interference

AbstractOptical chirality enhancement is highly demanded for enantioselective interaction of circularly polarized light with chiral molecules. The chirality enhancement in the coaxial air hole of a hollow silicon disk depends on three aspects, namely, the enhancements of electric and magnetic fields and a factor determined by the phases of their field components. In the spectral regime of dipole resonances, maximum chirality enhancement with sign consistency and uniform spatial distribution in the air hole can be obtained in association with both magnetic dipole resonance and anapole. Due to dipolar interference, the chirality is nulled at their coincidence, around which the sign of chirality is reversed. Maximum chirality with both positive and negative signs can be found between magnetic dipole resonance and anapole in the vicinity of their coincidence. This situation is maintained under size scaling so that the operation wavelength can be broadly tuned. The optical chirality can be further improved by merely adjusting the hole radius, by which the optimal spatially averaged optical chirality enhancement factor can reach 39 and −23. The simple strategy for optimizing Mie resonators presented in this work may benefit the design of Mie resonator‐based achiral metasurfaces for chirality detection application.

Generation of optical chirality patterns with plane waves, evanescent waves and surface plasmon waves.

We systematically investigate the generation of optical chirality patterns by applying the superposition of two waves in three scenarios, namely free-space plane waves, evanescent waves of totally reflected light at dielectric interface and propagating surface plasmon waves on a metallic surface. In each scenario, the general analytical solution of the optical chirality pattern is derived for different polarization states and propagating directions of the two waves. The analytical solutions are verified by numerical simulations. Spatially structured optical chirality patterns can be generated in all scenarios if the incident polarization states and propagation directions are correctly chosen. Optical chirality enhancement can be obtained from the constructive interference of free-space circularly polarized light or enhanced evanescent waves of totally reflected light. Surface plasmon waves do not provide enhanced optical chirality unless the near-field intensity enhancement is sufficiently high. The structured optical chirality patterns may find applications in chirality sorting, chiral imaging and circular dichroism spectroscopy.

Dynamic manipulation of optical chirality for gammadion nanostructures

We present a method to achieve the dynamic manipulation of optical chirality for a gammadion nanostructure. The gammadion nanostructure is illuminated by two counter-propagating linearly polarized lights (LPLs) with orthogonal polarization directions. By adjusting the phase difference between the two LPLs, the optical chirality enhancement can be adjusted periodically as a function of the phase difference and the optical chirality manipulation range lies between those obtained by single left- and right-handed circularly polarized light excitation. This work provides a convenient and feasible method to achieve the dynamic manipulation of optical chirality for chiral plasmonic nanostructures.

Importance of higher-order multipole transitions on chiral nearfield interactions

Abstract Surface-enhanced circular dichroism (SECD) of chiral molecules adsorbed on plasmonic nanostructures can substantially enhance chiroptical molecular signals by several orders, which is otherwise very weak to be directly measured. Several mechanisms were proposed to explain this extreme enhancement, but the exact mechanism is still controversial. We investigate strong higher-order multipole contribution to SECD near plasmonic nanostructures using the superposition T-matrix method and discuss how 3-dimensional full-field simulations implementing a homogeneous chiral medium have succeeded in the reconstruction of the extreme enhancement. We also discuss how theoretical studies modeling chiral molecules based on dipole approximation have failed to reconstruct the extreme enhancement and show that SECD enhancement of such chiral dipoles is directly governed by optical chirality enhancement. In addition, strong multipolar transitions in subwavelength chiral plasmonic nanoparticles are discussed based on the T-matrix. This work reviews theoretical frameworks describing chiral molecules, demonstrates significant contribution of a multipolar transition on the extreme SECD enhancement near plasmonic nanostructures, and emphasizes the importance of a multipolar transition in chiral nearfield interaction.

Stress‐Induced 3D Chiral Fractal Metasurface for Enhanced and Stabilized Broadband Near‐Field Optical Chirality

AbstractMetasurfaces comprising 3D chiral structures have shown great potential in chiroptical applications such as chiral optical components and sensing. So far, the main challenges lie in the nanofabrication and the limited operational bandwidth. Homogeneous and localized broadband near‐field optical chirality enhancement has not been achieved. Here, an effective nanofabrication method to create a 3D chiral metasurface with far‐ and near‐field broadband chiroptical properties is demonstrated. A focused ion beam is used to cut and stretch nanowires into 3D Archimedean spirals from stacked films. The 3D Archimedean spiral is a self‐similar chiral fractal structure sensitive to the chirality of light. The spiral exhibits far‐ and near‐field broadband chiroptical responses from 2 to 8 µm. With circularly polarized light (CPL), the spiral shows superior far‐field transmission dissymmetry and handedness‐dependent near‐field localization. With linearly polarized excitation, homogeneous and highly enhanced broadband near‐field optical chirality is generated at a stably localized position inside the spiral. The effective yet straightforward fabrication strategy allows easy fabrication of 3D chiral structures with superior broadband far‐field chiroptical response as well as strongly enhanced and stably localized broadband near‐field optical chirality. The reported method and chiral metasurface may find applications in broadband chiral optics and chiral sensing.

Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced Circular Dichroism Spectroscopy to Polarization-Preserving Mirrors

Circular dichroism (CD) spectroscopy is an important technique to investigate the structural information about chiral molecules. The intrinsically weak chirality of molecules often requires long optical paths and high-concentrated molecules. Metasurfaces can generate localized strong chiral fields to enhance chiral light-molecule interactions near the surface. However, losses and structural chirality of metasurfaces can diminish the inherent CD of the chiral molecules and induce interfering spectral features that disturb the chiral analysis of the molecules. Herein, we realize achiral titanium dioxide metasurfaces to enhance the near-ultraviolet CD of chiral molecules. The device is completely lossless for wavelengths greater than 360 nm and produces intense chiral hotspots in the near-field. An 80-fold enhancement of optical chirality is numerically demonstrated, giving rise to a 50-fold enhancement of CD. In addition, we show that the metasurface can function as a polarization-preserving mirror and enhance optical chirality in the far-field. Our results provide a scheme to ultracompact CD spectrometers and chiral cavities.

Chiral plasmonic nanocrescents: large-area fabrication and optical properties.

Large-area arrays of substrate-supported plasmonic gold crescents are fabricated by using the new colloidal lithography technique, which is based on an in situ-deposited silica resistance layer. The method provides the means to control the particles' asymmetry just by changing the mutual deposition angle of gold and silica. Asymmetric crescent structures exhibit a pronounced circular dichroism in near-infrared region, with the chiral asymmetry factor reaching 0.2. According to the simulation, the optical chirality enhancement reaches between one and two orders of magnitude and is localized near the crescents' tips.

Analytic Optimization of Near-Field Optical Chirality Enhancement.

We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization.

Enhanced Optical Chirality through Locally Excited Surface Plasmon Polaritons

The possibility to enhance chiral light-matter interactions through plasmonic nanostructures provides entirely new opportunities for greatly improving the detection limits of chiroptical spectroscopies down to the single molecule level. The most pronounced of these chiral interactions occur in the ultraviolet (UV) range of the electromagnetic spectrum, which is difficult to access with conventional localized plasmon resonance based sensors. Although Surface Plasmon Polaritons (SPPs) on noble metal films can sustain resonances in the desired spectral range, their transverse magnetic nature has been an obstacle for enhancing chiroptical effects. Here we demonstrate, both analytically and numerically, that SPPs excited by near-field sources can exhibit rich and non-trivial chiral characteristics. In particular, we show that the excitation of SPPs by a chiral source not only results in a locally enhanced optical chirality but also achieves manifold enhancement of net optical chirality. Our finding that SPPs facilitate a plasmonic enhancement of optical chirality in the UV part of the spectrum is of great interest in chiral bio-sensing.

Globally enhanced chiral field generation by negative-index metamaterials

We show that negative-index metamaterials can generate chiral electromagnetic fields, which can be used to enhance molecular chiroptical signals by inducing strong local magnetic fields. Compared to circularly polarized light (CPL), the enhanced chiral field generated by a double-fishnet negative-index metamaterial shows a 3.5-fold enhancement of the volume-averaged optical chirality and possesses the same handedness as that of incident CPL, thereby forming a globally enhanced chiral field. By analyzing near-field configurations, the mechanism of the optical chirality enhancement by double-fishnet negative-index metamaterial is elucidated. We thus anticipate that these metamaterials have potential applications in chiroptical spectroscopy.