Metamaterials enable photothermal circular dichroism spectroscopy for photophysics and photochemistry

In the field of biology and medical research various tools can be employed to separate different chiral species where light beams interact with chiral matter which produces different reactions and dissipation. There are several available methods used in chiral photochemistry, however, all the existing methods have difficulties in achieving strong chiral-matter interaction capable of providing strong chiro-optical effects for various photochemistry applications (for example, polarization-sensitive surface photochemistry which will allow for nanopatterning in photosensitive films).

A team of researchers at University of Electronic Science and Technology of China: Dr. Xiang-Tian Kong and Professor Zhiming Wang in collaboration with Dr. Larousse Khosravi Khorashad and Professor Alexander O. Govorov from Ohio University proposed a chiral spectroscopy-photothermal circular dichroism. It involved a combination of both the thermal and plasmonic physics. Their objective was to better describe the effects of the photothermal plasmonic circular dichroism (CD). The work is currently published in the research journal, Nano Letters.

Plasmonic nanocrystals exhibit excellent photoheating properties thus describing their vast use in photothermal applications. The desired temperature distributions in a plasmonic heater are achieved through its geometry while manipulation of the light can be realized by plasmonic metamaterials and nanostructures. Plasmonic planar materials are associated with high optical absorption in thin systems. Therefore, perfect absorbers consist of the resonator at the top layer and a back reflector and can achieve unity absorption coefficient while asymmetric chiral absorbers have strong circular dichroism signals. Although chiral elements can be of different shapes, in this study the authors used the geometry of the perfect absorber with -shaped nanocrystals in combination with the effects of photothermal plasmonic nanostructures.

The authors observed that plasmonic nanocrystals are good heaters and absorbers. Thus, they successfully generated significant circular dichroism effect with very large photothermal g-factors. For instance, the chiral absorber investigated here produced a large g-factor of about 0.6. Furthermore, significant strong photothermal circular dichroism was obtained from the various calculated temperatures thereby confirming the ability of the planar metastructures to generate large asymmetry when illuminated with chiral light. Generally, in plasmonic metamaterials, the chiro-optical effect is always larger than the equivalent effect in plasmonic bioassembly.

The effect of photothermal plasmonic circular dichroism in plasmonic metamaterials absorbers is dependent on the strong circular dichroism effects and efficient absorption of light resulting in heating. As a result, a strong photothermal circular dichroism effect can be generally obtained from the combination of the above two dependent factors. Even though the obtained magnitudes for maximum asymmetry parameters and absorption coefficients were relatively smaller, the g-factor counterparts were much higher. This was attributed to the choice of materials that focused on the applications in chiral photodetectors, surface photochemistry, and bolometers.

The difference in the chiro-optical effects in the plasmonic metamaterials and plasmonic bioassembly has several advantages. For example, it can be utilized in chiral bolometers (for measuring circular polarization degree of the incident light) and polarization-sensitive surface photochemistry. The authors are optimistic that their findings will advance further development of photochemistry and other associated applications.