Donna Strickland

Table of contents:

Helpful Webpages

Link to inclusion in public webpage:

Speaker notes:

Introduction slide / General speaker notes:

Synopsis of work:

In 1985, Gérard Mourou (supervisor)and Donna Strickland (PhD student) succeeded in creating ultrashort high-intensity laser pulses without destroying the amplifying material. First they stretched the laser pulses in time to reduce their peak power, then amplified them, and finally compressed them. The intensity of the pulse then increases dramatically. "Chirped pulse amplification" has revolutionized laser physics, and has abundant applications including corrective eye surgeries.

Researcher's background:

Donna Strickland was born in Guelph, Ontario, Canada. She became interested in laser and electrooptics early and studied at McMaster University in Hamilton, Ontario. She pursued her doctoral studies in the U.S. at the University of Rochester, where she did her Nobel Prize awarded work. She obtained her PhD in 1989. She subsequently has worked at Princeton University and since 1997 at the University of Waterloo in Canada.

Societal relevance:

CPA is the current state-of-the-art technique used by most of the highest-power lasers in the world.

Before the introduction of CPA in the mid-1980s, the peak power of laser pulses was limited. In order to keep the intensity of laser pulses below the threshold of the nonlinear effects, the laser systems had to be large and expensive. CPA can achieve orders-of-magnitude higher peak power than laser systems could generate before it's invention.

In addition to the higher peak power, CPA makes it possible to miniaturize laser systems. A compact high-power laser, known as a tabletop terawatt laser, can be created based on the CPA technique.

CPA is used in all of the highest-power lasers (greater than about 100 terawatts) in the world, with the exception of one 500TW facility.

  • CPA can accelerate protons for proton therapies that are used to treat deep-tissue tumors, like those that develop in the brain.
  • CPA has been used in Lasik eye surgery to quickly slice open the lens of the eye without damaging the surrounding tissue, and nearly 10 million patients in the United States have had their vision corrected with the procedure.
  • CPA lasers opened up an industry for more precise machining of a wide range of materials. This process uses thermal energy to remove material from metallic or nonmetallic surfaces, including machining of brittle materials like the cover glass used in smartphones..
  • CPA has allowed scientists to generate huge quantities of charged particles and light for heating matter to stellar interior conditions, as well as generate copious quantities of electron-positron pairs emulating electron-positron pair plasma powered by supermassive black holes.
  • CPA techniques have been used to take ultrafast images of split-second processes at the molecular level in order to study how atoms behave. Ultrahigh intensity lasers based on CPA generate high-energy photon sources that can probe dense matter and even nuclear structures. Better understanding and visualizing the fundamental nature of atoms can lead to advances in fusion and creating new materials.

General citations and resources:

https://en.wikipedia.org/wiki/Donna_Strickland

https://uwaterloo.ca/physics-astronomy/people-profiles/donna-strickland

https://www.nobelprize.org/uploads/2018/10/strickland-lecture.pdf

https://www.nobelprize.org/womenwhochangedscience/stories/donna-strickland

https://en.wikipedia.org/wiki/Chirped_pulse_amplification

https://www.rochester.edu/newscenter/what-is-chirped-pulse-amplification-nobel-prize-341072/

Slide 1: Lasers introduction

Science details:

Lasers produce a very narrow beam of light, and they are useful in many different technologies. LASER = Light Amplification by Stimulated Emission of Radiation.

Light is a wave, and different colours are different wavelengths. Wavelength changes continuously across the rainbow, where blue has the shortest wavelength and red has the longest. Most light we see is incoherent, meaning it's made up of lots of different wavelengths. Lasers are made from coherent light, which is light that is all the same wavelength and in phase (lined up peak-to-peak), allowing it to be focused to a point. Lasers can concentrate a lot of energy onto a single point, and can therefore do things like cut through materials, and travel long distances.

Citations and resources:

https://spaceplace.nasa.gov/laser/en/

Figures:

Top left: Example animation of incoherent light (like from a lightbulb)

Bottom left: Example animation of coherent light (like from a laser)

Top right: Simplified diagram of a light wave, showing the wavelength

Bottom right: Animation of a laser turning on

All animations from https://spaceplace.nasa.gov/laser/en/

Slide 2: Introduction to two-photon absorption

Science details:

Light acts both like a wave and a particle, and it can interact with matter in interesting ways. Atoms can interact with photons (light packets) by absorbing them or emitting them, and if a photon with enough energy is absorbed by an atom it will free an electron from that atom. The higher the frequency of a photon, the more energy it carries.

An analogy used to understand this (used by Donna Strickland) is imagining photons as basketball players, and the nets as interactions with atoms. The energy of the photon is represented by the height of the basketball player, and the height of the basketball net is the energy required to free and electron from the atom. Some photons with low frequency, like red and green photons for example, won't be tall enough (have enough energy) to shoot the basketball through the hoop (free an electron) despite the number of red or green photons playing basketball. A violet photon, on the other hand, is tall enough to shoot a hoop and free an electron, and the number of freed electrons is equal to the number of violet photons playing basketball. If the photon is ever taller than the net, the outgoing electron will carry that extra energy by going extra fast. This is how light regularly interacts with matter.

Donna Strickland studies 'non-linear' optics, which is when light of one or more frequencies interacts with an atom, and the emitted light will be of different frequencies. These types of interactions usually require very high powered lasers, and tell us a lot about what things are like on the atomic level.

A common example of non-linear optics is two-photon absorption, when two photons are simultaneously absorbed instead of one, freeing an electron with the energy of the two input photons combined. In our analogy, the laser stacks basketball player photons on top of one another, giving them the combined height required to shoot a hoop and free an electron. Here, two red photons working together in a laser are equivalent to one violet photon when interacting with an atom.

Another Nobel Laureate, Maria Goeppert Mayer, predicted two-photon absorption in 1931.

Citations and resources:

https://www.youtube.com/watch?v=sI_e7c085LM

https://www.nobelprize.org/uploads/2018/10/strickland-lecture.pdf

https://en.wikipedia.org/wiki/Nonlinear_optics

https://spie.org/news/1204-nonlinear-optics-simplified?SSO=1

Figures:

Left: different energy/frequency photons interacting with an atom (basketball net). Only the violet photon is tall enough to score.

Right: Two-photon absorption depicted with two red photons stacked on top of one another, analogous to a coherent laser. The stacked photons are now tall enough to score (free an electron).

Both figures are from Donna Strickland's Nobel Prize lecture.

https://www.nobelprize.org/uploads/2018/10/strickland-lecture.pdf

https://www.youtube.com/watch?v=sI_e7c085LM

Slide 3: Chirped Pulse Amplification

Science details:

Chirped Pulse Amplification (CPA) was developed by Donna Strickland to create a very powerful short pulse of laser light, with high photon density in both time and space. Higher density of photons leads to the observation of multiphoton interactions, and in the search for higher order nonlinear harmonic generation, lasers with higher and higher density of photons were required. In photonics, energy density is discussed as intensity (energy per unit area per unit time), and maximizing intensity requires both high energy and short pulses. Prior to CPA, physicists were producing lasers either high energy or short pulse – a combination of the two caused damage to the gain medium due to self-focussing (a non-linear runaway process where the beam focusses onto itself until the center of the beam has high enough intensity to damage the gain material). CPA solved this problem.

CPA starts with a short pulse produced from an oscillator, and this pulse is stretched out long enough such that when it passes through the gain medium in the amplifier it won't allow for nonlinear interactions. After it's amplified, the stretched pulse is compressed back to a short, high power pulse at output.

To create the initial pulse, Strickland combined many waves of different wavelengths such that they all peak at some time t=, where they all add constructively, and interfere destructively at all other t. The more wavelengths added, the shorter the pulse.

As the pulse travels down optical fiber, spectral bandwidth is added and it is stretched in duration. The ordinary refractive index is wavelength dependent, causing the red wavelengths to lead and the blue to lag, dispersing the colours in the pulse and lengthening the pulse with propagation. This frequency sweep in time of the pulse is known as a chirp, resembling the chirp of a bird.

The stretched pulse could then safely be amplified without non-linear effects causing damage.

Once amplified, a pair of parallel gratings is used to compress the linearly chirped pulse back down to its minimal pulse duration. The gratings cause the colours to diffract at different angles, causing the different wavelength photons to travel different distances. The gratings are set up such that it reverses the chirp, compressing the pulse duration back to the original length.

Citations and resources:

https://www.youtube.com/watch?v=sI_e7c085LM

https://www.nobelprize.org/uploads/2018/10/strickland-lecture.pdf

Figures:

Bottom: Stages of CPA, starting at the oscillator producing the initial pulse and ending with a short high energy pulse.

Top right: Diagram of the diffraction gratings, demonstrating the compression of the chirp, achieved by creating wavelength dependent path lengths in the compressor.

Top left: Diagram of multiple wavelengths superimposed to create a short pulse.

All figures are from Donna Strickland's Nobel Prize lecture cited above.