Chiral nanophotonics have been investigated to confine optical and electromagnetic waves to subwavelength modes, which are structures with dimensions smaller than the wavelength of the propagating waves. Consequently, optical nanostructures with both extrinsic and intrinsic chiralities have recently been developed and improved. Here, we provide a comprehensive summary of chiral nanophotonics.
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What is Nanophtonics?
Nanophotonics deals with interactions between light and matter in the nanoscale dimensional range. Dielectric nanophotonic devices, such as photonic crystal devices, enable the manipulation of light at wavelength scales. Light-guiding structures that permit subwavelength confinement of the optical mode are crucial to creating compact integrated photonic devices.
The confinement limit of a guided optical mode in dielectric waveguides is determined by the diffraction limit, which is usually approximately λ0/ɳ, where λ0 and ɳ stand for the wavelength in free space and refractive index, respectively. However, plasmonic devices have shown the ability to support subwavelength optical modes, known as surface plasmon polaritons, at metal-dielectric interfaces.
Chiral Nanophotonics
Chirality is a characteristic of an object that cannot coincide with its mirror image when translated or rotated. To create artificial structures that promote stronger chiral light-matter interactions than those found in natural materials for practical applications, researchers often turn to metasurfaces, metamaterials, and plasmonic nanostructures. These structures are essential for various applications in chiral nanophotonics, such as biochemical chiral sensing, chiral emission or lasing, and chiral nonlinear responses.
Advanced nanophotonics research focuses on the creative manipulation of chiral light emission and the improvement of chiral light-matter interactions. This cutting-edge field is exploring new avenues for chiral materials and chiral optical cavities to create the next generation of ultra-compact devices.
By using nanostructured optical resonators, it is possible to increase the local optical field by scaling the light below the diffraction limit. This method enables the creation of electric and magnetic field distributions that can enhance the chiral optical density and lead to the manipulation of light of molecular dimensions in the evanescent fields at optical frequencies.
Currently, chiral nanomaterials are being explored for various technological applications. These applications include chiral molecular sensing, synthesis and separation, nanorobotics, super-resolution imaging, and the development of ultrathin broadband optical components for chiral polarization layers (CPL), such as CPL absorbers, polarizers, and mirrors.
Chiral Optical Density
The greater the chiral optical density, the more discerning the light absorption becomes for particular molecular species, which is an essential feature for ultrasensitive sensing. This also entails the manipulation of polarization-based light-matter interactions in nonlinear frequency conversion, light emission, and lasing processes.
Novel dielectric and plasmonic systems exhibit a wide range of topological features, including spin-orbit coupling, chiral valley coupling, helical edge states, transverse photon spin, and momentum-space polarization singularities. These characteristics offer a multitude of potential applications with remarkable capabilities, such as directional emission and propagation, along with quantum information processing.
Chiral Nanostructures for Enhanced Optical Density
The field of chiral nano-optics has helped achieve subwavelength modal confinement within molecular volumes and substantially increased the chiral optical density. Significant effort has been invested in this area, resulting in the creation of numerous optical nanostructures that exhibit both extrinsic and intrinsic chirality.
However, creating three-dimensiona (3D)l helical structures is necessary to display significant chiral responses, which presents a challenge for the design and fabrication of nanostructures. Thus, integrating chiral platform materials, such as two-dimensional (2D) materials, transition metal dichalcogenides, and quantum emitters, can help address this challenge.
One of the main difficulties in chiral sensing is the need to fine-tune and/or switch the chiral response at either nanophotonic or magneto-plasmonic interfaces. This is because chiral interactions are weak and sophisticated strategies that are necessary to overcome the limitations of current technologies.
Recent Studies
Chiral nanostructures can exhibit optical properties that are specific to polarization and can be manipulated in micro- to nanoscale optical element engineering. A promising approach for creating these complex 3D structures in glass is the use of direct femtosecond laser writing.
Despite its potential, the underlying mechanism of femtosecond laser-induced chirality remains unclear because of the complicated chemical and physical processes that occur during brief interactions between light and matter.
An article published in Light discussed the development of a phenomenological model for two-layer phase shifters, which was used to study laser-induced optical chirality in silica glass.
The presence of nanograting anisotropic characteristics and stress-induced linear birefringence phenomenon in silica glass allowed for the quantitative interpretation of femtosecond laser-induced optical chirality in this work. Furthermore, these two contributions exhibit chiral optical characteristics and symmetry breaking due to misalignment of the eigenaxes.
Another study, published in the Journal of Physics D: Applied Physics, reported the construction of a fully connected neural network model to design chiral plasmonic metamaterial structures. The model was optimized, and its interpretability was increased using permutation.
The results revealed that the gold length of the resonator was the most important structural parameter affecting the chiral dichroism (CD) response. Furthermore, using the peak magnitude of the CD and the corresponding wavelength as the output in the forward prediction improved the accuracy of the peak magnitude of the CD prediction, avoiding the need for auxiliary networks and simplifying the network structure.
Conclusion
In conclusion, the exploration of chiral nanophotonics has significantly advanced our ability to confine optical and electromagnetic waves to subwavelength modes. The development and enhancement of optical nanostructures with both extrinsic and intrinsic chirality has opened new possibilities for various technological applications.
Nanophotonics enabled the manipulation of light at wavelength scales using dielectric nanophotonic devices. Confinement of the optical modes in these structures is crucial for creating compact integrated photonic devices.
Chiral nanophotonics, which focuses on the creation of artificial structures with unique chiral properties, have demonstrated their importance in applications such as biochemical chiral sensing, chiral emission or lasing, and chiral nonlinear responses. Researchers are actively exploring novel avenues in this cutting-edge field, aiming to manipulate chiral light emission and improve chiral light-matter interactions to develop ultra-compact devices.
Thus, chiral nanophotonics hold great promise for the advancement of optical technologies, offering unique opportunities for creating innovative devices and systems with enhanced functionalities. Ongoing research in this field is essential for unlocking the full potential of chiral nanostructures and their applications in diverse domains.
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References and Further Reading
Xie, S., Gu, L., Guo, J. (2023). Design of chiral plasmonic metamaterials based on interpretable deep learning. Journal of Physics D: Applied Physics, 57(4), 045103. https://iopscience.iop.org/article/10.1088/1361-6463/ad0567/meta
Lu, J., Tian, J., Poumellec, B., Garcia-Caurel, E., Ossikovski, R., Zeng, X., Lancry, M. (2023). Tailoring chiral optical properties by femtosecond laser direct writing in silica. Light: Science & Applications, 12(1), 46. https://doi.org/10.1038/s41377-023-01080-y
Chiral Nano-Optics: Challenges and Opportunities Accessed on 14 January 2023.
Chirality and Nanophotonics Accessed on 14 January 2023.
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