Second harmonic generation (SHG) is a nonlinear optical process, 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.

Today, SHG is typically produced in nonlinear media, like certain optical crystals, and the effect is widely used by the laser industry, for example, to make green 532 nm lasers from a 1064 nm source.

Now, Naomi Halas and colleagues have designed a new nonlinear optical material for this frequency doubling process, known as a nanocup (or half-shell), which consists of a dielectric nanoparticle upon which a semicircle layer of metal (gold in this case) 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. Halas' team showed that the resonances of this structure respond to both the electric and magnetic field components of light, and possess unique light-refractive properties.

The Rice researchers succeeded in generating second harmonic UV light from individual nanocups by tuning the magnetic plasmon resonance to the incoming laser light beam with a wavelength of 800 nm. They also found that they could increase the intensity of the SHG by tilting the nanoparticle with respect to the incoming laser light. The scattered SHG signal at 400 nm was collected and analysed by a CCD camera. The team observed that the intensity of the SHG increases as the angle between the incident beam and the symmetry axis of the nanocup is increased (see figure).

Equivalent to conventional nonlinear optical crystals
"Our technique generates frequency-doubled light at efficiencies equivalent to those produced by conventional nonlinear optical crystals with the same thickness," Halas told "Our work on 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 team, photonic devices, such as optical parametric oscillators or amplifiers and electro-optic or acousto-optic modulators, could be made using these types of structures. The nanocups could also be integrated into silicon photonics for on-chip optical sources or for measurements in future work.

The research is detailed in Nano Letters.