Surface-enhanced chiroptical spectroscopy with superchiral surface waves

Plasmon-enhanced circular dichroism spectroscopy of chiral drug solutions

We investigate the potential of surface plasmon polaritons at noble metal interfaces for surface-enhanced chiroptical sensing of dilute chiral drug solutions with nl volume. The high quality factor of surface plasmon resonances in both Otto and Kretschmann configurations enables the enhancement of circular dichroism differenatial absorption thanks to the large near-field intensity of such plasmonic excitations. Furthermore, the subwavelength confinement of surface plasmon polaritons is key to attain chiroptical sensitivity to small amounts of drug volumes placed around ≃100 nm by the metal surface. Our calculations focus on reparixin, a pharmaceutical molecule currently used in clinical studies for patients with community-acquired pneumonia, including COVID-19 and acute respiratory distress syndrome. Considering realistic dilute solutions of reparixin dissolved in water with concentration ≤5 mg/ml and nl volume, we find a circular-dichroism differential absorption enhancement factor of the order ≃20 and chirality-induced polarization distortion upon surface plasmon polariton excitation. Our results are relevant for the development of innovative chiroptical sensors capable of measuring the enantiomeric imbalance of chiral drug solutions with nl volume.

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.

Signal and noise analysis for chiral structured illumination microscopy

Recently, chiral structured illumination microscopy has been proposed to image fluorescent chiral domains at sub-wavelength resolution. Chiral structured illumination microscopy is based on the combination of structured illumination microscopy, fluorescence-detected circular dichroism, and optical chirality engineering. Since circular dichroism of natural chiral molecules is typically weak, the differential fluorescence is also weak and can be easily buried by the noise, hampering the fidelity of the reconstructed images. In this work, we systematically study the impact of the noise on the quality and resolution of chiral domain images obtained by chiral SIM. We analytically describe the signal-to-noise ratio of the reconstructed chiral SIM image in the Fourier domain and verify our theoretical calculations with numerical demonstrations. Accordingly, we discuss the feasibility of chiral SIM in different experimental scenarios and propose possible strategies to enhance the signal-to-noise ratio for samples with weak circular dichroism.

Enhanced Chiral Sensing with Dielectric Nanoresonators

Chiro-sensitive molecular detection is highly relevant as many biochemical compounds, the building blocks of life, are chiral. Optical chirality is conventionally detected through circular dichroism (CD) in the UV range, where molecules naturally absorb. Recently, plasmonics has been proposed as a way to boost the otherwise very weak CD signal and translate it to the visible/NIR range, where technology is friendlier. Here, we explore how dielectric nanoresonators can contribute to efficiently differentiate molecular enantiomers. We study the influence of the detuning between electric (ED) and magnetic dipole (MD) resonances in silicon nanocylinders on the quality of the CD signal. While our experimental data, supported by numerical simulations, demonstrate that dielectric nanoresonators can perform even better than their plasmonic counterpart, exhibiting larger CD enhancements, we do not observe any significant influence of the optical chirality.

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.