High-frequency light is useful. The higher the frequency of the light, the shorter the wavelength – and the shorter the wavelength, the smaller the objects and details the light can see.
Violet light can therefore show you smaller details than, for example, red light, because it has a shorter wavelength. But to see very, very small things – to the scale of a billionth of a meter, thousands of times smaller than the thickness of a human hair – to see those things, you have to extreme ultraviolet light (and a good microscope).
Extreme ultraviolet light, with wavelengths between 10 and 120 nanometers, has many applications in medical imaging, studying biological objects and deciphering the fine details of computer chips during their manufacture. However, producing small and affordable sources of this light has been a major challenge.
We have found a way to make nanoparticles of an ordinary semiconductor material emit light at a frequency up to seven times higher than the frequency of the light sent to them. We have generated blue-violet light from infrared light and it will be possible to generate extreme ultraviolet light from red light using the same principles. Our research, conducted with colleagues from the University of Brescia, the University of Arizona and Korea University, is published in Science Advances.
The power of harmonics
Our system starts with an ordinary laser that produces long-wavelength infrared light. This is called the pump laser and there is nothing special about it – such lasers are commercially available and can be compact and affordable.
Zalogina et al. / Scientific Advances, Author provided
But then we fire short pulses of light from this laser at a specially designed nanoparticle of a material called aluminum gallium arsenide, and that’s where it gets interesting.
The nanoparticle absorbs energy from the laser pulses and then emits its own burst of light. By carefully designing the size and shape of the nanoparticle, we can create powerful resonances to amplify certain harmonics of the emitted light.
What exactly does that mean? Well, we can make a useful analogy with sound.
Wikimedia/Y Landman
When you pluck a string on a guitar, it vibrates with what is called his fundamental frequency – which makes up the main note you hear – plus small amounts of higher frequencies called harmonics, which are multiples of the fundamental frequency. The body of the guitar is designed to produce resonances that boost some of these harmonics and dampen others, creating the overall sound you hear.
Both light and sound have similarities in their physics – they are both propagating waves (acoustic waves in the case of sound and electromagnetic waves in the case of light).
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In our light source, the pump laser is like the main note of the string, and the nanoparticles are like the guitar body. Except that the special thing about the nanoparticles is that they greatly amplify those higher harmonics of the pump laser, producing light with a higher frequency (up to seven times higher in our case, and a correspondingly seven times shorter wavelength).
What it’s good for
This technology allows us to create new light sources in parts of the electromagnetic spectrum, such as the extreme ultraviolet, where there are no natural light sources and where the current constructed sources are too large or too expensive.
Conventional microscopes using visible light can only study objects up to about one ten-millionth of a meter in size. The resolution is limited by the wavelength of the light: violet light has a wavelength of about 400 nanometers (a nanometer is one billionth of a meter).
But there are plenty of applications, such as biological imaging and electronics manufacturing, where being able to see down to one billionth of a meter would be a huge help.
Right now, to look at those scales you need “super-resolution” microscopy, which allows you to see details smaller than the wavelength of the light you’re using, or electron microscopes, which use no light at all and create an image using of a flux of electrons. However, such methods are quite slow and expensive.
Read more: A quantum hack for microscopes could reveal life’s undiscovered details
To understand the benefits of a light source like ours, we need to look at computer chips: they are made of very small components with dimensions of almost a billionth of a meter. During the manufacturing process, it would be helpful for manufacturers to use extreme ultraviolet light to monitor the process in real time.
This would save resources and time on bad batches of chips. The scale of the industry is such that even a 1% increase in chip revenue can save billions of dollars a year.
In the future, nanoparticles like ours could be used to produce small, low-cost sources of extreme ultraviolet light, illuminating the world of extremely small things.
