Physics 437: molecules, optics, and spectroscopy

Qing-Bin Lu
PHY 376 ext. 3503

Applications of high-sensitivity femtosecond time resolved laser spectroscopic techniques to biological, environmental and medical studies:

pump-probe femtosecond laser transient absorption spectroscopy and
pump-probe femtosecond laser fluorescence up-conversion spectroscopy

The above powerful techniques provide us with unique capacity of obtaining real-time observation of reactions in various molecular systems of biological, environmental and medical significance, such as light-activated drugs, sensitizers in radiotherapy and anticancer drugs, and halogenated ozone-depleting molecules.

James. D.D. Martin
PHY 357 ext. 3201

One of our group's research areas involves cold, dense plasmas. These plasmas are formed by laser cooling gas-phase neutral atoms to temperatures of roughly 1 milliKelvin, then photoionizing them with additional laser light. These experiments probe a completely new regime of plasma physics, where little is known either quantitatively or qualitatively. For example, we are currently interested in the fundamental limits on the lifetime of such a plasma. This will determine its suitability for the "sympathetic cooling" of other species which cannot be directly laser cooled (molecules, for example).

This research involves extensive use of lasers and electronics. The laser light used for cooling is generated using home-built external cavity diode lasers (these are frequency-stabilized to better than 1 MHz using feedback). As such there are a variety of projects for students interested in laser technology and electronics. These will change depending on the current research interests of the group, but an illustrative example would be:

Construction and Characterization of a "Modeless" Dye Laser

Ultra-cold atoms have very little Doppler broadening of their resonance lines. As a result their excitation probability is highly susceptible to the mode structure of the exciting laser - an unsatisfactory condition. A modeless dye laser would avoid these problems. However, very little work has been done to optimize the performance of modeless dye lasers. The student would construct a modeless dye laser, then evaluate and optimize its performance for exciting ultra-cold atoms.

Please consult my website for an up-to-date listing of potential projects and further information.

Joseph Sanderson
PHY 361 ext. 6109

Coulomb imaging with ultrashort laser pulses

In the focus of a laser pulse which has a duration of a few femtosecond (10–15s) molecules are inertially confined (frozen), during this short time we can study the properties of the molecule such as geometry, alignment and its charge state. Coulomb imaging is possible because our laser pulse removes many electrons (typically 3–9) from a molecule causing an electrostatic explosion (by Coulomb repulsion) to occur. By detecting the atomic ions in coincidence and measuring their momenta we can get a complete description of the initial molecular geometry. By using a pump probe technique we can take snapshots of a molecule as it undergoes a chemical reaction and make a molecular movie. We are interested in improving our images, and allowing us to see ever bigger molecules, and more complex molecular dynamics. The progress of this program means quite diverse projects are available, such as developing data reduction algorithms to improve our molecular images, building molecular sources (not all molecules come in a bottle), conducting experiments using various combinations of laser pulse energy and width.

Ultrashort laser pulse generation and laser design

In order to make the clearest images of molecules, we need the shortest laser pulse possible. This is like having the shortest shutter speed when photographing an object in motion. The laser spectrum and its pulse length are related by a Fourier transform, so that the wider the spectrum the narrower the pulse length. In order to make the shortest pulses we need to understand and control the dispersion of light in our laser and search for the configuration, which can generate and sustain the broadest spectrum. Currently hollow fiber short pulse generation can generate attain 4fs but the best achieved for Coulomb imaging is 7fs, Attaining the world record is a challenging assignment.

Modeling of molecular ionization

In order to correctly image a molecule using Coulomb explosion, it is necessary to understand ever ionization step and predict the molecular deformation that results, In order to do this we have two computational methods currently under investigation. The first is a simple field ionisation model which is purely classical the second incorporates quantum mechanical tunneling, using the so called ADK tunneling theory. In particular we have been able to calculate the ionization rate as a function of vibrational level, something which will be a useful diagnostic, in experiments which aim to control vibrational level in a molecule using laser light.

Coherent control

Coherent control simply means being able to control the quantum states of matter by adjusting the parameters of a laser pulse, The aim of experiments taking place in Waterloo is to control the vibrational levels of a molecule using a Raman effect RCAP (Raman chirped adiabatic passage). The energy difference between two laser pulses is tuned continuously to make the molecule jump up and up through its vibrational levels. One of the reasons to do this is that by selecting specific vibraitonal modes it will be possible to selectively break the bonds of a molecule, making laser chemistry a reality.

James J. Sloan
C2 279B ext. 4401

• Stratospheric Studies and Satellite Measurements
• Tropospheric Aerosols and Urban Particulate Matter
• Regional Atmospheric Modelling

For more information contact Dr. Sloan directly or visit Waterloo Centre for Atmospheric Sciences (WCAS)

Donna Strickland
PHY 257 ext.2724

Laser development

Presently, a dual wavelength Ti:sapphire oscillator is being constructed in the lab. Two synchronously mode-locked pulse trains, having different wavelengths are output in two separate beams. Each beam comes from a separate optical cavity built around a single Ti:sapphire gain medium. The two pulse trains have been shown to lock together without an external synchronizer. One research project is the study of the cross mode-locking mechanism.

Mid-infrared generation

Tunable mid-infrared radiation from 8 to 12.5 m m has been generated by difference frequency mixing the two pulse trains from a dual wavelength Ti:sapphire oscillator. The efficiency of the system was low, with 10 m W of average power produced in the mid-infrared with 60 mW pump power in each beam. We are planning to do the difference frequency mixing intracavity. The advantage is that the power inside the cavity is an order of magnitude higher yielding a possible 100 fold increase in output power.

A high energy, low repetition rate mid-infrared laser system is also planned. The two Ti:sapphire wavelengths will be amplified to ~mJ energies in a single amplifier. The two amplified pulses will be difference frequency mixed together to produce high energy mid-infrared pulses.

Self-focusing of short pulses

The propagation of intense beams through nonlinear media is receiving renewed interest, as attainable laser powers exceed the Petawatt level. Ionizing and self-focusing in air has been observed with collimated beams of pulses having peak powers greater than one Terawatt. These propagation effects will be investigated in the laboratory. The ultimate goal of this research is to provide long channels of intense radiation, by controlling diffraction and self-focusing, for use in such applications as X-ray lasers and laser accelerators.