Slant-gap plasmonic nanoantennas for optical chirality engineering and circular dichroism enhancement

Dielectric metasurface-assisted cavity ring-down spectroscopy for thin-film circular dichroism analysis

Chiral molecules show differences in their chemical and optical properties due to the different spatial arrangements of the atoms in the two enantiomers. A common way to optically differentiate them is to detect the disparity in the absorption of light by the two enantiomers, i.e. absorption circular dichroism (CD). However, the CD of typical molecules is very small, limiting the sensitivity of chiroptical analysis based on CD. Cavity ring-down spectroscopy (CRDS) is a well-known ultrasensitive absorption spectroscopic method for low-absorbing gas-phase samples because the multiple reflections of light in the cavity greatly increase the absorption path. By inserting a prism into the cavity, the optical mode undergoes total internal reflection (TIR) at the prism surface and the evanescent wave (EW) enables the absorption detection of condensed-phase samples within a very thin layer near the prism surface, called EW-CRDS. Here, we propose an ultrasensitive chiral absorption spectroscopy platform using dielectric metasurface-assisted EW-CRDS. We theoretically show that, upon linearly polarized and oblique incidence, the metasurface exhibits minimum scattering and absorption loss, introduces negligible polarization change, and locally converts the linearly polarized light into near fields with finite optical chirality, enabling CD detection with EW-CRDS that typically works with linearly polarized light. We evaluate the ring-down time in the presence of chiral molecules and determine the sensitivity of the cavity as a function of total absorption from the molecules. The findings open the avenue for the ultrasensitive thin film detection of chiral molecules using CRDS techniques.

Structured illumination microscopy for simultaneous imaging of achiral and chiral domains

We propose double structured illumination microscopy (SIM) method, which enables simultaneous imaging of achiral and chiral domains at sub-wavelength resolution. In double SIM, the illumination field is spatially structured both in the intensity and optical chirality so that moiré effects can be concurrently generated on the achiral and chiral fluorescent domains of a sample. This allows for down-modulating the high spatial frequency of both domains at the same time and thus provides sub-wavelength details after image reconstruction. We introduce the working principle of double SIM and theoretically demonstrate the feasibility of this method using different kinds of synthetic samples.

Circular dichroism spectroscopy and chiral sensing in optical fibers

Chirality is a property of broken mirror symmetry and detecting the handedness of chiral material in small quantities is an important problem in biology and biochemistry. Here, we present a waveguide-based method to measure chirality and distinguish the enantiomers of molecules. A bi-isotropic core in an optical waveguide lifts the degeneracy of modes in a cylindrically symmetric structure. This modal degeneracy lifting is exploited to measure the chirality of the core. The proposed sensor can determine the value of the chirality parameter of the material under test and it can be utilized for various materials with nonzero chirality parameter in different frequency bands. This approach improves the circular dichroism (CD) response and outperforms conventional CD spectroscopy methods by increasing their differential output signal. To compare the results with conventional CD spectroscopy, the CD parameter is adapted to optical waveguides.

Plasmonic elliptical nanoholes for chiroptical analysis and enantioselective optical trapping

A simple yet effective achiral platform using elliptical nanoholes for chiroptical analysis is demonstrated. Under linearly polarized excitation, an elliptical nanohole in a thin gold film can generate a localized chiral optical field for chiroptical analysis and simultaneously serve as a near-field optical trap to capture dielectric and plasmonic nanospheres. In particular, the trapping potential is enantioselective for dielectric nanospheres, i.e., the hole traps or repels the dielectric nanoparticles depending on the sample chirality. For plasmonic nanospheres, the trapping potential well is much deeper than that for dielectric particles, rendering the enantioselectivity less pronounced. This platform is suitable for chiral analysis with nanoparticle-based solid-state extraction and pre-concentration. Compared to plasmonic chiroptical sensing using chiral structures or circularly polarized light, elliptical nanoholes are a simple and effective platform, which is expected to have a relatively low background because chiroptical noise from the structure or chiral species outside the nanohole is minimized. The use of linearly polarized excitation also makes the platform easily compatible with a commercial optical microscope.

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.

Plasmonic chirality of one-dimensional arrays of twisted nanorod dimers: the cooperation of local structure and collective effect

We study the chiral optical properties of one-dimensional arrays of plasmonic twisted nanorod dimers. By using finite-difference time-domain (FDTD) simulation and analytical approach based on the coupled dipole model, we have revealed unusual chiral optical responses due to the cooperation of local structure and collective effect. It is found that one-dimensional arrays of achiral unit may show chiral optical responses. Moreover, besides the classical bisignate lineshape of circular dichroism (CD) induced by localized surface plasmon resonance, a new CD peak/dip appears, originating from Wood anomaly. Near the Wood anomaly frequency, the optimal twist angle to achieve the highest CD has been shifted compared with that of single twisted nanorod dimer. The universal geometric configurations of the strongest chiral optical responses have been found.

Surface-enhanced circular dichroism by multipolar radiative coupling

We present numerical investigation of the mechanism of surface-enhanced circular dichroism of a chiral medium near nanoantennas. Strong circular dichroism was observed from the chiral medium surrounding nanoantennas with multipolar resonant modes, and the strong circular dichroism was more correlated to the multipolar resonances than to nearfield enhancement or optical helicity enhancement. According to this observation, we suggest multipolar radiative coupling between the nanoantennas and chiral medium as a possible mechanism of the strong chiral response. This work clarifies a mechanism of surface-enhanced chiral responses and would be useful for designing an enantiomeric-sensing platform and realizing devices relying on strong chirality, such as topological metamaterials for scattering-immune propagation of light and negative index metamaterials.

Design and characterization of a plasmonic Doppler grating for azimuthal angle-resolved surface plasmon resonances

We present a two-dimensional plasmonic Doppler grating (PDG) for broadband and azimuthal angle-resolved nanophotonic applications. The PDG consists of a set of non-concentric circular rings mimicking the wavefronts of a moving point source that exhibits the Doppler effect and thereby offers a continuous azimuthal angle-dependent lattice momentum for photon-plasmon coupling. The center and span of the working frequency window are fully designable for optimal performance in specific applications. We detail the design, fabrication and optical characterization of the PDG. The design of the Doppler grating provides a general platform for in-plane angle-resolved nanophotonic applications.

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.

Single-Crystalline Aluminum Nanostructures on a Semiconducting GaAs Substrate for Ultraviolet to Near-Infrared Plasmonics

Aluminum, as a metallic material for plasmonics, is of great interest because it extends the applications of surface plasmon resonance into the ultraviolet (UV) region and is superior to noble metals in natural abundance, cost, and compatibility with modern semiconductor fabrication processes. Ultrasmooth single-crystalline metallic films are beneficial for the fabrication of high-definition plasmonic nanostructures, especially complex integrated nanocircuits. The absence of surface corrugation and crystal boundaries also guarantees superior optical properties and applications in nanolasers. Here, we present UV to near-infrared plasmonic resonance of single-crystalline aluminum nanoslits and nanoholes. The high-definition nanostructures are fabricated with focused ion-beam milling into an ultrasmooth single-crystalline aluminum film grown on a semiconducting GaAs substrate with a molecular beam epitaxy method. The single-crystalline aluminum film shows improved reflectivity and reduced two-photon photoluminescence (TPPL) due to the ultrasmooth surface. Both linear scattering and nonlinear TPPL are studied in detail. The nanoslit arrays show clear Fano-like resonance, and the nanoholes are found to support both photonic modes and localized surface plasmon resonance. We also found that TPPL generation is more efficient when the excitation polarization is parallel rather than perpendicular to the edge of the aluminum film. Such a counterintuitive phenomenon is attributed to the high refractive index of the GaAs substrate. We show that the polarization of TPPL from aluminum preserves the excitation polarization and is independent of the crystal orientation of the film or substrate. Our study gains insight into the optical property of aluminum nanostructures on a high-index semiconducting GaAs substrate and illustrates a practical route to implement plasmonic devices onto semiconductors for future hybrid nanodevices.

HNO₃-assisted polyol synthesis of ultralarge single-crystalline Ag microplates and their far propagation length of surface plasmon polariton

We developed a HNO3-assisted polyol reduction method to synthesize ultralarge single-crystalline Ag microplates routinely. The edge length of the synthesized Ag microplates reaches 50 μm, and their top facets are (111). The mechanism for dramatically enlarging single-crystalline Ag structure stems from a series of competitive anisotropic growths, primarily governed by carefully tuning the adsorption of Ag(0) by ethylene glycol and the desorption of Ag(0) by a cyanide ion on Ag(100). Finally, we measured the propagation length of surface plasmon polaritons along the air/Ag interface under 534 nm laser excitation. Our single-crystalline Ag microplate exhibited a propagation length (11.22 μm) considerably greater than that of the conventional E-gun deposited Ag thin film (5.27 μm).