By Carolyn Mathas, Contributor
Solid-state lasers generate light in the visible range. Detecting molecules requires radiation in the mid-infrared range and these lasers are difficult to manufacture, especially if the form required is extremely short, intense pulses. Researchers have been searching for a simple method to produce these infrared laser pulses for a long time. A team at Vienna University of Technology in cooperation with Harvard University has achieved just that. Their results are presented in the journal “Nature Communications.”
Generating laser light in the mid-infrared range requires customized quantum cascade lasers manufactured in the Nano-Center of TU Wien. These quantum cascade lasers use tiny, nanometer-sized structures that precisely adjust the wavelength of the light. This range of frequencies are arranged regularly and always have the same distance in between them like the teeth of a comb, giving the spectrum its name: frequency comb.
For this solution, it’s the frequencies and the phase with which the respective light waves oscillate. Akin to two pendulums connected by a rubber band, they can either swing towards each other or away from each other. And the two vibration modes have slightly different frequencies. It’s the same with laser light comprised of different wavelengths. The individual light waves of the frequency comb oscillate exactly in sync and can generate short, intense laser pulses. Or there is a shift between their oscillations whereby no pulses are created but the laser lights with a near-continuous intensity.
The TU Wien team built a tiny modulator into its quantum cascade laser which the light waves pass by repeatedly. An alternating electrical voltage is applied to the modulator and depending on the frequency and strength of the voltage, different light oscillations are excited in the laser. When the frequencies of the frequency comb oscillate in sync, they can combine into short, intense laser pulses – more than 12 billion times per second.
The new technology is easily miniaturized to build compact measuring instruments or to perform measurements that require two photons simultaneously.