Optical vortices and chiral matter
Light has a spatial "shape," not just wavelength and polarization.
Among these, light where the phase of light changes helically around the propagation axisOptical vortexIt is called a light vortex. Light vortices can carry orbital angular momentum, and as light with a spatial structure different from ordinary plane waves, they have been attracting attention in recent years.
On the other hand, some molecules and crystals cannot be superimposed on their mirror images. This type of chiralityChiralityand substances with chirality are called chiral substances.
We are theoretically studying how the orbital angular momentum and spatial structure of optical vortices affect the optical response of chiral matter.
What is an optical vortex?
In optical vortices, the phase of light is subject to the azimuthal angle φ
exp(iℓϕ)
changes as follows. The integer ℓTopological chargeand is called, it represents how many times the phase winds around the optical axis.
In an ideal paraxial optical vortex, per photon
Lz = lħ
and carry orbital angular momentum. Since the phase cannot be uniquely determined at the center of the optical axis, a phase singularity appears where the intensity becomes zero, typically forming a ring-shaped light intensity distribution.
The orbital angular momentum of optical vortices imparts a new degree of freedom to various light-matter interactions, such as particle rotation, atom/molecule excitation, optical communication, and quantum information.
What is a chiral substance
Chiral substances are substances whose structures cannot be completely superimposed on their mirror image.
Similar to how your right and left hands are mirror images of each other but do not overlap, chiral molecules exist as two enantiomers, corresponding to right-handed and left-handed forms. However, many of their physical and chemical properties are identical, and to distinguish them optically, one must utilize the chirality inherent in light itself.
A typical method isCircular dichroismWe read information about the chirality of a substance by measuring the difference in absorptivity for right- and left-circularly polarized light. The asymmetry between the local chirality of the light field and the excitation rate of chiral molecules can be described using optical chirality density.
Is it easy to distinguish chiral substances?
Right-handed and left-handed chiral substances, while having different shapes, share many almost identical physical and chemical properties, such as mass, energy levels, and normal absorption spectra. Therefore, it is not easy to distinguish between the two by simply applying ordinary light.
Representative identification methodsCircular dichroismThis is done by irradiating a substance with right-handed circularly polarized light and left-handed circularly polarized light and measuring the slight difference in their absorption amounts. By investigating this difference, information about whether the substance is right-handed or left-handed can be obtained.
However, this absorption difference is generally small, requiring highly sensitive measurements. Therefore, we are researching ways to detect the handedness of chiral substances using a different approach by leveraging the spatial structure and angular momentum of optical vortices.