Spotlight on Chemists

Scott Cushing: Ultrafast Lasers and Quantum Entanglement

Ultrafast lasers

Ultrafast Lasers

A laser is a device that stimulates atoms or molecules to emit light at particular wavelengths and amplifies that light, resulting in a very narrow beam of radiation. In fact, what many may not know is that laser is actually an acronym for “light amplification by the stimulated emission of radiation.” The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths [1]. A specific type of laser that is designed particularly to be able to have very short light pulses, of the order of picoseconds or femtoseconds*, is the ultrafast laser, that Dr. Scott Cushing and his team work with [4]. In the past 30 years, the development of ultrafast lasers has gained continuous attention and application. Over the years, several techniques have been devised to produce ultrashort pulses.

The accessibility of ultrafast lasers has led to the analysis of a wide range of chemical, physical and biological phenomena. 

Applications of Ultrafast Lasers

Ultrafast lasers can be used for high quality micromachining (a manufacturing technology that allows engineers to create small, intricate parts that can then be used in experiments) of brittle materials like glass. They are thus often used for cutting glass with flexible geometries and high quality edges. These properties have made them ideal for the large-scale manufacture of displays for portable devices, like phones and tablets [2].

A very strongly increasing use of ultrashort laser pulses is also to be recognized in the field of neuroscience. The idea is to use light to activate the signalling of neurons to study the neural network of the brain. Through the excitation of multiple photons, a volume of neurons can be mapped [4]. Deep laser engraving using pulsed fibre lasers –  a type of ultrafast lasers – is used to incise 3D forms such as logos, drawings, text, serial numbers and barcodes into metal parts and workpieces [3]. Ultrafast laser technology is also put to use in various other systems, such as synchrotrons and X-ray free-electron lasers. 

It’s safe to say that lasers have more uses than just as a unique toy for one’s cat – they can actually be applied across many different fields. Dr. Scott Cushing of The California Institute of Technology specialises in research in this domain, with a focus on ultrafast lasers, and also works to apply concepts such as entanglement (see below) to improve conventional microscopy methods. 

* A picosecond is one trillionth of a second; a femtosecond is one quadrillionth, or one millionth of one billionth, of a second.

Entanglement

Quantum entanglement is thought to be one of the trickiest concepts in science, but the core issues are simple. Dr. Cushing encourages students to understand the idea through a simple thought experiment: suppose we have the ability to split a photon in half, resulting in two individual photons. If done correctly, we can imagine a system wherein we don’t know which photon is which. This means that unless we measure one of the two photons, we can’t know the properties of the other.

In fact, in principle, even if one of these photons is sent all the way across the galaxy, only as soon as the earthly photon is measured, the properties of the other one are revealed. This is perhaps what prompted Einstein to refer to the theory as, “action at a spooky distance.” [4]. Essentially, we can say that our two photons are entangled, since information about one improves our knowledge of the other.

Ultrafast Laser Spectroscopy

While much of the effort into quantum-related phenomena is still in foundational physics, there are numerous new avenues as to how quantum entanglement can be applied to imaging and sensing. It also has great application in the field of spectroscopy, something Dr. Cushing and his team investigates. 

A particularly interesting application of ultrafast lasers is in ultrafast laser spectroscopy – it ​​uses ultrashort laser pulses to study the structure of atoms and molecules in a medium, and their dynamics, on extremely short time scales. It usually involves exciting the medium with one (or more) ultrashort laser pulses. This can vaporise and ionise materials, and the light flash emerging from the vaporised material can be analysed through spectroscopic methods. This helps in identification of atoms and molecules. Using entanglement, the technique can be enhanced and refined even further – to the equivalent of a laser pointer – and can also make itself more affordable. The trick lies, according to Dr. Cushing, in making good entangled photons. 

If rendered functional, entanglement spectroscopy can be used in something called “ghost imaging.” Ghost imaging is often understood as imaging using light that has never physically interacted with the object to be imaged. Instead, if we apply our photon entanglement example mentioned above and consider the light as consisting of just two halves of one photon, one half interacts with the object and the other falls onto the imaging detector. Entangled photons, in this manner, can also be used to obtain 3D images of the brain. 

Learn More

If you’d like to hear more about the fascinating world of lasers and ongoing research into its uses, visit us on Spotify to listen to our ChemTalk podcast with Dr. Scott Cushing, assistant professor at the California Institute of Technology, to learn more about various spectroscopic instruments, discuss how he prepares his students for success, and why he thinks he has ‘scientific ADHD’. 

Find the ChemTalk podcast here: https://open.spotify.com/episode/6Pup7zBcHDMURIj4HWR0Ay

Works Cited 

[1] Hecht, J. (2022, October 3). laser. Encyclopedia Britannica. https://www.britannica.com/technology/laser

[2] “What are ultrafast lasers?”Azo Materials. 30 April 2021. https://www.azom.com/article.aspx?ArticleID=20372

[3] “Deep Laser Engraving: How It Works and What You Need.” Alex Fraser. LaserAX. 27 January 2021. 

[4] Cushing, Scott. Personal Interview. Conducted by Roxanne Salkeld. 1 November 2022.