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Carbon nanotubes are a relatively simple material but they possess some incredibly unique characteristics; their optical properties, for example, are highly interesting to those working in the materials science sector.
Carbon nanotubes (CNTs) comprise of a one-dimensional hexagonal lattice of carbon atoms rolled to form a cylinder and fused together; these hollow tubes have unique and highly ordered atomic and electronic structure.
The electronic, optical, electrochemical and mechanical properties of carbon nanotubes are directionally dependent, i.e. they are anisotropic and tunable. CNTs have a high aspect ratio, meaning their length is quite long compared to their width. Many of the unique properties of CNTs are only applicable in one direction along the tube; for this reason, CNTs can be described as both metallic and as a semiconductor.
The way CNTs interact with electromagnetic radiation is unique, and they exhibit peculiar absorption, photoluminescence and Raman spectra. The optical properties of CNTs are largely determined by their unique electronic structure.
Light scatters elastically through the CNT, but only in one direction due to the tube’s non-linear refractive properties. Optical absorption occurs as light travels towards the CNTs and happens very quickly due to electronic transitions between energy levels. The non-linear polarization of electrons in the hexagonal lattice is thought to contribute to a large non-linear refractive index change, also known as the non-linear Kerr effect. Saturable absorption is a widespread phenomenon in any material that exhibits optical absorption due to the electronic transition between two energy levels. Once the CNTs reaches and exceeds its saturation point, the CNT becomes transparent.
Optical absorption in CNTs differs from the optical absorption properties of those found in bulk, three-dimensional materials. The absorption originates from electronic transitions between energy levels, which can be relatively sharp and can be employed to help identify CNT types.
The absorption of light by electrons in the CNT produces excitions, a charge separation of electrons and holes, observed in semiconducting nanotubes. As the electron-hole pair rapidly relaxes, there is the transition between excitation states which allows a CNT to exhibit fluorescence, photoluminescence, and electroluminescence. CNTs emit in the near-infrared and possess high photostability.
Quantum Computing and Cryptography
Carbon nanotube optics could pave the way towards optical-based quantum cryptography and quantum computing. Light is the major worldwide carrier of information, with single-photon sources being the key building block in a number of technologies in secure quantum communications metrology and quantum computing schemes.
Researchers from Los Alamos in the US, together with colleagues in Germany and France, have been exploring the enhanced potential of CNTs as single-photon emitters for quantum information processing. They are interested in nanotube integration into photonic cavities for manipulating and optimizing light-emission properties; such integration into electroluminescent devices could provide greater control the over timing of light-emission and could be integrated into photonic structures.
The researchers chemically modify the CNTs to purposefully introduce defects to create room-temperature single-photon emitters with telecom wavelengths. Their focus is on analysis of the defects for pushing quantum emissions to room temperature and telecom wavelengths.
Optical Probes in Biomedicine
Single-walled CNTs offer potential in a wide range of medical application including sensing, imaging, and drug delivery; they are also promising as optical probes in biomedicine.
CNTs have the ability to emit in the near-infrared range and exhibit photophysical properties; they also have excellent photostability and fluorescence, which is highly sensitive to the local environment, making them ideal optical probes.
They might also find use as an optical biosensor where they would be utilized as part of a sensor to detect various biomolecules. They can be functionalized with organic molecules or form part of a biocompatible matrix from where they could sense changes in the electro-optical properties, electrochemical fluorescence, and quenching mechanisms. The idea is that they will be able to provide a measurable response and allow monitoring of small molecules.
Sources and Further Reading
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