Fig. 1. Schematic drawing for nanophotonic chiral sensing. Chiral molecules (top) and nanophotonic sensors (bottom; depicted by gold nanorods as an example) interact with each other, resulting in various phenomena listed in the figure. Nanophotonic sensors can be achieved by various optical systems such as nanoparticles, plasmonic structures, photonic crystals, and metasurfaces.

Fig. 2. Schematic drawing for the electromagnetic description of molecular chirality. Microscopic view provides the multipole moments of each molecule (E1, electric dipole moment; E2, electric quadrupole moment; M1, magnetic dipole moment). Macroscopic view provides the medium parameters of the molecule ensemble. In the macroscopic medium level, left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) lights experience different refractive indices, n˜±κ˜, respectively.

Fig. 3. Transmission-type CD measurement. LCP and RCP lights are transmitted separately through the chiral sample (i.e., chiral molecules and/or a nanophotonic sensor), and their difference in transmission is recorded. The difference in transmission is equivalent to that in absorption, CD.

Fig. 4. Enhancement of single-molecule CD by the superchiral fields. (a) The first experimental realization of the dissymmetry factor enhancement by the mirror geometry^[84]. (b) The limit of localized surface plasmon resonance to enhance the optical chirality density (left) with the uniform sign (c.f. the dipolar field enhancement)^[154]. (c) An example of the uniform optical chirality density in plasmonic structures^[155]. (d) The uniform optical chirality density enhancement in dielectric nanoparticles^[63].

Fig. 5. Decomposition of CD in a molecule-nanosensor system^[112]. (a) Schematic drawing for the system consisting of a molecule film-coated (green) gold nanodisk array (gold). (b) Inherent CD of molecules is enhanced by the strong near-field of the nanostructure. (c) CD is induced by the presence of chiral molecules in the vicinity of the nanostructure. Decomposed CD of the system coupled to (d) ORD-only molecules (κ=0.001) and (e) CD-only molecules (κ=0.001i).

Fig. 6. (a) Energy diagram of the achiral nanosensor resonance ω0 before coupling to molecule analytes (black). The optical responses of the achiral nanosensor to LCP and RCP incidences are identical. After coupling, two effects are involved; the dielectric effect due to the refractive index n decreases the nanosensor resonance frequency ω0−δω0(n). The chiral effect due to the chirality parameter κ increases or decreases the resonance frequency ω0−δω0(n)±δω0(κ) according to the nanosensor characteristic, while it makes the nanosensor resonance circular dichroic (red and blue). (b) Spectral lineshapes of the absorption before (black) and after (red and blue) coupling. The resulting CD spectrum shows an asymmetric lineshape.

Fig. 7. Energy diagram of the chiral nanosensor resonance ω0 before coupling to molecule analytes.

Fig. 8. (a)–(d) Plasmon-induced CD by the Coulomb interaction between chiral molecules and plasmonic nanoparticles^[162]. (a) Normalized extinction spectra of a chiral molecule (black: the E1 moment μ→ and the M1 moment m→), a silver nanoparticle (blue), and a gold nanoparticle (red). (b) Field enhancement near each particle. (c) Normalized CD and ORD spectra of the chiral molecule. (d) Normalized CD spectra of chiral-molecule-coupled silver (blue) and gold (red) nanoparticles. (e) Nanophotonics-induced CD by the electromagnetic interaction between chiral media and nanostructures. Differential scattering cross sections of a gold nanoparticle embedded in the ORD-alone chiral medium^[120].

Fig. 9. Chiral Purcell-enhancement of fluorescent CD in a hypothetical helicity-preserving Fabry-Perot cavity^[101]. Cavity resonances provide the CD enhancement, i.e., ΔΓ/ΔΓ0>1.

Fig. 10. Absorption-based nanophotonic chiral sensing. (a) Plasmon-induced CD of riboflavin bilayer-coated gold nanoislands^[114]. (b) Optical chirality enhancement of gold gammadion arrays and their chiral sensing^[82]. (c) Zeptomole-level chiral sensing by twisted optical metamaterials^[168].

Fig. 11. Nanophotonic chiral sensing using collective circular dichroism of a gold helicoid array^[107]. (a) Schematic drawing for the sensing mechanism. (b) Electron (left) and optical (right) microscope images of the helicoid array. (c) Near-field profiles of the uniform optical helicity density h within the array plane upon LCP and RCP incidences. (d) Sensitivity of the helicoid array sensor for various chiral molecule species.

Fig. 12. Light emission by chiral-molecule-nanostructure complexes. (a) Luminescence-based chiral sensing by chiral quantum metamaterials^[103]. By the chiral Purcell effect, quantum dots composed of metamaterials show different luminescence by the presence of chiral molecules, allowing the sub-zeptomole level sensitivity. (b) Circularly polarized organic light-emitting-diodes (CP-OLEDs)^[176] and their spectra of the CP photoluminescence (CPPL) and the PL dissymmetry factor gPL. Circularly polarized light (CPL) is emitted by chiral Frenkel excitons and/or chiral plasmons. (c) Chiral laser by the Fabry-Perot cavity with gain media, fluorescein (FITC) binding with l-tryptophan and green fluorescent proteins (GFPs)^[177].

Fig. 13. CD measurement for the bulk sample using conventional CD spectrophotometers. Intensities of two circularly polarized lights (I±) are plotted. Lights experience different attenuation coefficients μ± within the chiral molecule sample according to their handedness.

Fig. 14. CD instrumentations. (a) Direct subtraction, (b) self-interference, and (c) polarization modulation methods (S, source; LP, linear polarizer; QWP, quarter-wave plate; D, detector; M, monochromator; R, retarder; LI, lock-in amplifier). Polarization states of light are depicted by arrows in the optical path. In (b), the second linear polarizer is tilted by a small angle θ=δ with respect to the optic axis of the first linear polarizer. In (c), the polarization state of the light after the retarder continuously varying at a frequency ω.

Fig. 15. (a) FDCD spectroscopy and (b) CPL spectroscopy. S, source; LP, linear polarizer; QWP, quarter wave plate; D, detector. Polarization states of light are depicted by arrows in the optical path. A set of the linear polarizer and the quarter wave plate is used in this figure for the sake of simplicity, but the polarization modulation with a retarder and a detector coupled to a lock-in amplifier can be used^[181].

Fig. 16. (a) Circular differential scatterometry^[186] and (b) its application to single-particle sensing of plasmonic nanoparticle-protein complexes^[109].

Fig. 17. Chiral (a) optical force on a chiral object^[189] and (b) refraction of light in a chiral medium^[122].