Joseph Sanderson

Associate Professor

Office: PHY 361
Phone: (519) 888-4567 ext. 36109
Email: j3sander@uwaterloo.ca
Website: Femtosecond Laser Lab

Research interests

My research and that of my students focuses on the study of how matter interacts with intense Femtosecond laser pulses.

One of the ways which the interaction of matter with femtosecond laser pulses can be utilised is as a means of imaging some of the smallest fastest moving and most complex units of matter, molecules.

Coulomb imaging

In my lab and at the ALLS (Advanced Laser Light Source) laboratory we can use the two most important properties of a femtosecond laser pulse, its tiny duration (1fs=one thousand million millionth of a second, which is inconceivably short to a human being but is quite normal for molecules which are used to vibrating on this kind of timescale) and its high intensity (this is partly a consequence of the shortness of the pulse because intensity is power divided by area and power is energy divided by the pulse duration).

The pulse length acts like the shutter speed of a camera allowing us take a snapshot of a molecule in motion and the intensity gives us a means to make the image. We use the high intensity to rip many electrons from the molecule (the laser light develops a momentary electric field stronger than the one which binds the electrons to the atoms) which then explodes because there are not enough electrons left to bind the positively charged atomic ions together. This is called a Coulomb explosion after the physicist Charles de Coulomb who gave his name to the electrostatic force between two charged objects. To use this explosion as a way of imaging the molecule we need to detect all of the fragment ions created and measure their momentum then we can work backwards to see what the original geometry of the molecule was.

Here are a couple of recent images showing how we can make a “molecular movie” of a molecule stretching in the laser field, in the first movie a CO2 molecule expands from 7fs to 200fs and in the second movie over the same time period we measure how the bend angle of the molecule relates to how close in time the two molecular bonds break (ninety degrees means simultaneous)

We are currently studying how small molecules like CO2 are affected by the laser and what the best conditions for imaging are, we are also interested looking at how the electrons in the molecule are controlled by the laser (recollision) and initiate molecular processes such as ionisation and dissociation.

Another way in which matter responds to femtosecond laser pulses is to reorganise itself and generate surprising new kinds of material, such as carbon chains

Femtosecond laser induced Nano particle production

'Nano particle production and charactorisation is now one of the hot topics in science, engineering and industry. Perhaps surprisingly (given its ability to disrupt matter by rapidly removing many electrons from anything in its way) femtosecond laser pulses have recently been shown to be a highly promising tool in generating nano particles. In fact it is precisely this disruptive capability which is the quality which promotes the nano particle production. When femtosecond laser pulses interact either with a liquid or a solid they cause a the material to be rapidly reduced to ionised atomic and molecular fragments which can then self organise to form either nano scale surface features which dramatically modify the properties of a material or nano particles of varying sizes and properties. One of the nicest particles is a polyyne chain which consists of carbons bound to each other and capped at each end by a hydrogen atom. These chains have been found to have extraordinary properties such as strength beyond that of diamond (also made of carbon).

Usually polyynes are produced from graphite powder (soot) in a liquid heated by an electric discharge or high energy laser pulse. Recently it has been found that instead of this messy method it is possible to take a clean solvent solution and irradiate it with pulses from a femtosecond laser. The result is a solution containing polyynes and other fragments of the initial solvent. One of the ways that the femtosecond laser is able to process the liquid effectively is by generating a bright filament, in which nonlinear interactions between the laser field and the liquid increase the length of the focus by orders of magnitude.

Recent publications

• Time of flight mass spectrometry of polyyne formation in the irradiation of liquid alkanes with femtosecond laser pulses. A.A. Zaidi, A. Hu, M.J. Wesolowski, X. Fu, J.H. Sanderson, Y. Zhou, W.W. Duley. Carbon 2010 (in press)
• Synthesis of polyyne molecules from hexane by irradiation of intense femtosecond laser pulses. Y. Sato, T. Kodama, H. Shiromaru, J.H. Sanderson, T. Fujino, Y. Wada, T. Wakabayashi, Y. Achiba. Carbon, Volume 48, Issue 5, April 2010, Pages 1673-1676
• Inelastic rescattering processes in molecules measured with few-cycle laser pulses. François Légaré, Irina A Bocharova, Phillippe Lassonde, Reza Karimi, Joseph H Sanderson, Tudor Johnston, Jean-Claude Kieffer and Igor V Litvinyuk. 2009 J. Phys. B: At. Mol. Opt. Phys. 42 235601
• Ultrafast imaging of polyatomic molecules with simplex algorithm. Jean-Paul Brichta, Aden N. Seaman, Joseph H. Sanderson. Computer Physics Communications, 180, (2) 197-200 (2009)
• Direct synthesis of polyyne molecules in acetone by dissociation using femtosecond laser irradiation. A. Hu, J. Sanderson, A.A. Zaidi, C. Wang, T. Zhang, Y. Zhou, W.W. Duley Carbon, Volume 46, Issue 13, November 2008, Pages 1823-1825