Frequency doubling with nanocups

Researchers at Rice University in Houston, Texas have discovered a new type of material for converting red light into blue light.
Harmonic generation
Second harmonic generation (SHG) also called frequency doubling is a nonlinear optical process, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons.
The principle is that in quantum mechanics, the recolliding electron is represented as a de Broglie wave. This wave shifts over the molecular or atomic orbital, from which it was originally ionized. During this shifts, interferences occur, that modulate the amplitude and phase of the harmonic radiation.
Nanocups are three-dimensional artificially designed plasmonic nanostructures. Second harmonic generation is an important nonlinear optical process that has been used since the 1960s for making new light sources, optical crystals and the effect is widely used by the laser industry and in metrology applications in which two photons are converted into a single photon with twice the energy, and therefore twice the frequency or half the wavelength of the initial photons. The process was first demonstrated in 1961 when researchers focused a ruby laser with a wavelength of 694 nm into a quartz sample and observed that the light subsequently emitted had a wavelength of 347 nm.
Nanocup (or half-shell) consists of a dielectric nanoparticle upon which a semicircle layer of metal has been deposited. The device possesses "plasmonic resonances" – collective oscillations of the metal's conduction electrons – that can strongly interact with light at certain resonance frequencies. The resonances of this structure respond to both the electric and magnetic field components of light, and possess unique light-refractive properties.
Second harmonic UV light can be generated from individual nanocups by tuning the magnetic plasmon resonance to the incoming laser light beam with a wavelength of 800 nm. The intensity of the SHG can be increased by tilting the nanoparticle with respect to the incoming laser light and as the angle between the incident beam and the symmetry axis of the nanocup is increased.
Nanocup morphologies
The synthesis of carbon nanostructures, with interesting morphologies, has created a revolution in nanotechnology; carbon nanotube is a case in point, but other nanoscale morphologies of graphitic carbon could provide compelling uses. In particular short structures, including very short nanotubes, have proven impossible to be grown by existing techniques due to the difficulty in controlling and terminating growth during initial stages.
Architectures engineered from graphitic carbon, having up to 105 times smaller length/diameter (L/D) ratios compared to conventional nanotubes, reveal unique morphologies of nanocups, nanorings, and large area connected nanocup arrays. Such highly engineered hollow nanostructures can be fabricated using precisely controlled short nanopores inside anodic aluminum oxide templates.
The nanocups can be effectively used to hold and contain other nanomaterials, for example, metal nanoparticles, leading to the formation of multi component hybrid nanostructures with unusual morphologies. They can open up possibilities to integrate new morphologies of graphitic carbon in nanotechnology applications.
Nanocups have the ability to bend light in a specific direction. The nanocups could also be integrated into silicon photonics for on-chip optical sources or for measurements in future work. Nanocups could lead to the development of other, similar types of nonlinear optical materials that are designed to work at specific wavelengths of light, say, in the infrared or ultraviolet, or at wavelengths that are currently inaccessible to existing nonlinear optical materials.
According to the researchers, photonic devices, such as optical parametric oscillators or amplifiers and electro-optic or acousto-optic modulators could be made using these types of structures.


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