Doctoral defense: linearity is totally boring

Wednesday 11 Apr 18
by Søren Ravn


Morten Bache
DTU Fotonik
+45 45 25 37 75
In his pursuit to control laser light, Morten Bache has acquired an in-depth understanding of nonlinear effects. This knowledge has now made him DTU’s latest Dr.techn.

On 23 March, Associate Professor Morten Bache from DTU Fotonik defended his doctoral dissertation.

The doctoral defence was the climax so far of ten years of passionate work to achieve optimal control of laser beams, so that the powerful and extremely short laser light impulses can be used for examining and manipulating atomic and molecular processes. Research that is constantly generating new and surprising knowledge.

“The laser was invented more or less in parallel with the discovery of chaos and fractals, and it was realized that the underlying mathematical models contained a universal description of how nonlinear effects behave. We use these models to study nonlinear effects when strong laser light travels through a material. It’s a rich and complex field, and each time we use the laser in a new context, we acquire new knowledge,” he says.

The dissertation gathers 10 years of research building on Morten Bache’s PhD studies at DTU and postdoc studies in Italy. The aim has been to compress powerful infrared laser impulses into such short duration that researchers can freeze atomic and molecular timescales which—for example in a chemical reaction—pass from one product to another. A timescale that is so fast that it cannot be measured by electronic means. In the field in which atoms and molecules live, ultrashort laser impulses are required to be able to see what is happening.

"It’s a rich and complex field, and each time we use the laser in a new context, we acquire new knowledge."
Morten Bache

“In my field of research in atomic and molecular optical physics, we are driven by having so much control over the laser light that we can measure, control, and manipulate the smallest things at molecular or atomic level, and constantly generate new ideas. There is no limit to how short we need laser impulses to be. The shorter the laser impulses, the more we can examine. We’ve sufficient time resolution to study molecules that vibrate, but it’s a challenge to make short laser impulses with the completely right ‘colour’ in the mid-infrared region, where molecular vibrations have their natural fluctuation frequencies,” he explains.

Must use nonlinear techniques
An example would be to use laser light to study the dynamics of a water molecule. Water is an extremely complex matter, and a field with so much still to explore. And the study of the time dynamics of molecular vibrations in water requires strong and ultrashort laser impulses with long wavelengths in the mid-infrared region. Researchers also wish to arrive right down to where they can time dissolve electron movements, which requires powerful laser impulses with ultrashort wavelengths in the ultraviolet region.

“A joint feature is that these laser impulses cannot be created directly by commercial infrared lasers, so we must use nonlinear techniques to convert the laser beam wavelength into shorter or longer wavelengths. This means that we control what ‘colours’ the laser light consists of, and—at the same time—we achieve impulses that are so short that we can study the timescales that belong to physics,” he says and elaborates:

“The point of departure is actually very simple. We take a commercial laser and basically smash the light into a piece of material. This may be glass, water, a gas, an optical fibre or—as in my case—a crystal. And we then study what happens. We have sufficient peak power in the laser light to induce nonlinear effects anywhere—even in air. The trick is to understand and control the nonlinear effects we see.”

Light through crystals
Generally speaking, Morten Bache’s research—and, in particular, his dissertation—is based on laser light which passes through a type of crystals used on a daily basis throughout the world to make laser light with new colours. The crystal has proved to give access to controlling nonlinear effects that no other systems provide. The crystal is the same as that used as far back as the first laser experiments in the 1960s, and a similar crystal is today used in all green laser pointers. It is thus surprising that it keeps providing new and interesting results.

“By manipulating the crystal, I achieve control of and access to nonlinear effects that you cannot achieve elsewhere. The application perspectives arise from having control over and understanding the laser light and the nonlinear effects by means of the ultrashort laser impulses. Linearity is totally boring. By embracing nonlinearity, we become able to create a laser light which can really make electrons jump.”

The title of Morten Bache’s doctoral dissertation is: ‘Cascaded Nonlinearities for Ultrafast Optical Science and Applications.’