The Light Fantastic
When White Light Lasers Meet Photonic Devices
When we think of lasers, we usually imagine a narrow beam of light of a very particular colour such as that from a laser pointer - which is red, green or perhaps yellow. This single colour characteristic comes from the underlying physics of lasers. Within a laser, each photon is created as an exact replica of the others, so they all have the same wavelength and direction. This is one of the properties that make lasers so useful in a range of applications. But in some ways, it’s also a hindrance.
Almost as soon as lasers were invented, scientists began investigating the possibility of broadening out the very narrow wavelength range. At first this amounted to little more than a slight smearing of the spectral peak. But during the last decade with the advent of advanced nonlinear optical materials and nano engineered optical fibres, lasers have been wavelength broadened to the extent that the light that comes out really does appear white. Scientists refer to such laser generated white light as a supercontinium.
One area in which supercontinium light promises great advances is in integrated photonic devices. These are essentially the optical equivalent of conventional electronic chips, such as a computer CPU. But optical chips have the potential to operate very much faster than their electronic equivalents in many applications.
If one were able to run photonic chips using the white light of a supercontinium, there would be the exciting possibility to revolutionize numerous fields which require use of many different wavelengths at the same time, such as optical sensing and characterization, spectroscopy, tomography, metrology, and telecommunications. But before it’s possible to unlock this potential, it’s necessary to overcome the fundamental tendency of different colours to propagate quite differently in physical media due to the effects of dispersion and diffraction.
Dispersion is the tendency of materials to have different refractive indices for different wavelengths – it’s what causes white light to be split into a rainbow by water droplets or a glass prism. Diffraction is a property experienced by all waves, including light, whereby the wave is scattered at an aperture or when it encounters a structure that has a regular series of lines or dots. Diffraction is dependent on wavelength so if a structure is created that has a series of ridges with the right spacing, the effect is to also split the light into a spectrum.
Although dispersion and diffraction are common terms in optics, these phenomena are not exclusive to light. Wave particle duality tells us that electrons moving in a silicon chip are also in effect, tiny waves. These electron waves experience diffraction as they pass the regular rows of atoms in the crystal leading to a range of useful effects. In electronics, engineers use external applied voltages to control this movement of electrons and to some extent modify the effects of diffraction. What optical engineers would dearly love to do is something similar in optical chips. Unfortunately there isn’t a simple equivalent of an applied external voltage for an optical chip, but a group of researchers have recently discovered a way to do something very similar.
Dr Ivan Garanovich works as a research scientist in the Nonlinear Physics Centre at ANU. He’s part of a team investigating ways to simulate electric fields in optical chips. Their work centres on what are known as photonic lattices. These are regular arrays of lines created in transparent materials such as fused silica. The light waves interact with the lattice in the silica in a very similar way to electrons in the natural lattice of atoms in a semiconductor crystal. And this gives them the potential to make optical devices such as transistors and switches.
What the team have found is that if the regular lattice of the photonic device is bent into a gradual curve, the effect on the photons is almost exactly the same as that electrons experience when a voltage is applied to a semiconductor. If the curve is just in one direction, it’s like a DC voltage. If the curves wave from side to side the effect is the same as AC.
“What we’re really excited about is being able to use this technology to overcome diffraction and dispersion and synchronise the passage of different colours of light through the device,” Dr Garanovich explains.
The microscopic photonic lattices are created by burning tiny grooves into a slab of polished quartz using a powerful laser on a computer controlled micromanipulation stage. Researchers then investigate the propagation of supercontinium light through these structures in various configurations and compare this with theory.
By calculating what the diffraction and dispersion effects will be within the photonic lattice and then introducing a curve to the structure, the researchers have been able to control the propagation of each wavelength. In the simplest case, this allows white light to propagate through the dispersive and diffractive lattice without splitting into component colours. But that’s just the beginning of what’s possible.
The important thing is that having an optical equivalent to voltage gives engineers an extra degree of freedom in designing components. This means that it’s possible to utilize nonlinear effects at multiple wavelengths in the same medium. Using this technology, optical structures can be created that guide and process light in ways that far more closely mirror electronic devices. And that opens up a world of exciting possibilities.