A Trio of Terahertz Papers

A large portion of my work during my PhD, from 2014 to the end of 2017, was with the goal of producing high energy terahertz pulses. Terahertz radiation is named after the unique frequency band centered on 1 THz (1*10^12 Hz) that is between radio-frequency technology and optical technology. We were interested in making this terahertz radiation for a European project called AXSIS, but it has implications on basic science as well. The results of this work were obtained during my PhD, but as always the analysis and writing up for publication took some time (along with the peer review process…). With the publication of a third article my output from that time period is finally at a close, with the rest of the team of course continuing to have success after success.

The first paper was a short “proof-of-principle” paper that showed that we properly understood our new method for terahertz generation (i.e. it worked like we expected). And beyond showing that it worked, we already produced a record energy in such terahertz pulses. The paper was entitled “Narrowband terahertz generation with chirped-and-delayed laser pulses in periodically poled lithium niobate” and was published in the journal Optics Letters in June of 2017. Although at only 4 pages we managed to fit 5 figures and a lot of information.

Of course as it is with any scientific project, we immediately had the question of “what next”. For the AXSIS project we needed much higher energy pulses, and although we had an engineering solution to improve slightly (thanks to our collaborators in Japan for manufacturing the largest crystals in the world!) we needed to also better understand what was going on. This took the rest of 2017, but using some nuances of the data from the first experiments from 2016 and a simple model, we realized that a so-called “higher-order” property of our driving laser pulses was limiting the efficiency.

In early 2017 we designed and tested a method to compensate for this limitation. That method, along with the engineering solution, and pumping two crystals at once, resulted in another increase in the pulse energy by 15 times by the end of 2017! The model and method we used to increase the efficiency of the process were all important results for the community and the final result of high energy pulses was the cherry on top. We took the better part of 2018 to finish the analysis and to put the results together in to a manuscript. After a long peer-review process (and a rejection!) we were finally accepted to Nature Communications, and published in June of 2019. The paper is titled “Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation“, and was also written about in the news section of the DESY webpage.

After getting the milestone result in the Nature Communications paper we could reflect on past nuanced data from the original experiments in 2016, and also imagine the broader impact of this higher-order property of the driving laser pulses. With a detailed analysis and outlook we published a third paper, out this month in Optics Express, “On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses“. We were quite happy to put out this longer, technical paper, since it closed some gaps and added interesting details to the other two papers, so we felt that it provided a good complement to our other work while still being new and original.

My current work no longer involves terahertz radiation, but I learned so much during these experiments (and doing the analysis). I do always keep terahertz applications of my current work in the back of my mind, since it is such a dynamic field, and is just plain cool.

Obviously the biggest thanks go to my PhD supervisors Andreas Maier and Franz Kaertner, to my two very close collaborators Frederike Ahr and Nicholas Matlis, our collaborators in Japan Takunori Taira and Hideki Ishizuki, and the many other coauthors and colleagues mentioned in the papers.

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Photonics Publications, Q1+Q2 2019

Following up on my previous post I thought I would recount my favorite and most respected publications of the first half of 2019.

First, there are the three papers on which I’m a listed author (bias intended!):

1 April, Optics Letters
Influence of longitudinal chromatism on vacuum acceleration by intense radially polarized laser beams

I showed that if an ultrashort radially-polarized laser pulse has chromatic aberration, then the vacuum acceleration can be significantly affected. This includes a surprising increase, but in a larger parameter range it is mostly a negative effect. I expect that this has been present in past experiments, but not diagnosed.

13 June, Nature Communications
Spectral phase control of interfering chirped pulses for high-energy narrowband terahertz generation

The main work from my PhD within a collaboration at DESY. See a report on the results here.

13 June, Journal of Physics: Photonics
Spatio-temporal structure of a petawatt femtosecond laser beam

Our team at the CEA reports measurements and a detailed analysis of spatio-temporal properties of the BELLA laser system in Berkeley, USA.

Outside of my authorship of course, there are a large number of publications that have been intensely interesting and impressive to me. This is a short sampling.

18 February, Nature Photonics
Dual-energy electron beams from a compact laser-driven accelerator

25 February, Nature Communications
Optical space-time wave packets having arbitrary group velocities in free space

21 March, Physical Review X
Identification of Coupling Mechanisms between Ultraintense Laser Light and Dense Plasmas

29 March, Physical Review Letters
Upper Bound to the Orbital Angular Momentum Carried by an Ultrashort Pulse

I have a few publications waiting in the wings, and I’m sure that my colleagues across the world do as well. Here’s to a good second half of 2019.

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Favorite Photonics Publications of 2018

As is necessary this time of year, I reflected on my favorite optics/photonics and laser physics publications of this past year. Definitely not an exhaustive list, and heavily biased by my favorite topics and specialization (ultrafast optics, laser-plasma interactions, particle beam physics), these 12 articles and letters were the most interesting to me this year. I list them in chronological order of publication, to try to give a feeling of the flow of 2018.

End of year 2017, Nature Photonics
Broadband gate-tunable terahertz plasmons in graphene heterostructures

26 Feb, Nature Photonics
Giant multiphoton absorption for THz resonances in silicon hydrogenic donors

12 March, Nature Photonics
Spatiotemporal control of laser intensity

02 April, Nature Photonics
Segmented terahertz electron accelerator and manipulator (STEAM)

06 April, Nature Communications
Control of laser plasma accelerated electrons for light sources

7 May, Optics Express
Wavefront degradation of a 200 TW laser from heat-induced deformation of in-vacuum compressor gratings

29 May, Optica
Thermodynamic control of soliton dynamics in liquid-core fibers

28 June, Physical Review Letters
Extreme Ultraviolet Beam Enhancement by Relativistic Surface Plasmons

29 August, Physical Review Letters
Tilted Electron Pulses

14 Sep, Optics Express
Molecular gases for pulse compression in hollow core fibers

27 Sep, Nature
Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions

7 Nov, Optica
Ultrahigh-efficiency wavelength conversion in nanophotonic periodically poled lithium niobate waveguides

I can’t wait to see what 2019 has in store for the optics/photonics and lasers community.

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Nobel Prize in Physics 2018

Two weeks ago, the Nobel prize in physics was announced to be awarded to three physicists for two topics, both dealing with laser physics. One half was awarded to Arthur Ashkin for his invention and perfection of the optical tweezers, which many feel to be long overdue since he was not included in the prize for laser cooling with Steven Chu. The second half was awarded to Gerard Mourou and Donna Strickland together for their invention of the method of Chirped Pulse Amplification (CPA) in the 80s, which revolutionized high-power lasers since then enabling countless discoveries.

I can’t say much personally about the award to Ashkin except for what I’ve read. But regarding the award to Mourou and Strickland I can say quite a lot. For my Bachelor’s, master’s, PhD, and current PostDoc I’ve always and exclusively used lasers that rely crucially on the CPA technique, and my PhD work relied very specifically on technical aspects of such a laser system. Even further, Mourou founded Center for Ultrafast Optical Sciences (CUOS) at Michigan where I did my Bachelor’s work, was a intellectual founder of the Extreme-Light Infrastructure (ELI) project in Europe where I was associated for my PhD, and is currently at Ecole Polytechnique nearby where I currently am doing a PostDoc. I’ve seen him present once in 2015 and many of the people where I currently work have interacted with him much more.

The positives of this award are great. Donna Strickland is the first female physics laureate in 55 years and received the award for work she did during her PhD; both challenges to the view that you need to be a famous powerful male physicist to win the prize. This prize brings a lot of attention to the field of ultrafast laser physics, which there hasn’t been much in the past.

In fact, if you were to ask me before the prize was awarded which people in my field were most likely to win the prize I would not have included Mourou or Strickland. They are both great physicists of course, but I can name many others, including women, who I would say have a more prolific and impactful career over many many years including more than one discovery. This would include Ursula Keller, Margaret Murnane and Henry Kapetyn, Ferenc Krausz, Paul Corkum, and many others. But the fact is that all of those mentioned rely on lasers using the CPA technique to do their work.

So overall it is a reminder that the prize is not given, at least this year, for a career, but for a single worthy discovery. But secondly, I would predict that the prize will be given in the coming years to one of those I mentioned, or someone else that uses the CPA technique. They had to give the prize for the enabling discovery first, but more are coming in the field.

It’s an exciting (and motivating) prize for me, to be involved both technically with the work that I have done, but also to be involved just very indirectly with the individuals to have gotten the award. It means nothing about me personally, but is still exciting. It’s also a reminder that a prize given to one or a few people is in no way a summary of the best researchers in the field, and is just the opposite: a prize given specifically to one or a few people in recognition of a specific work. And there are a limited number of these prizes, but many many more scientists doing important work that will never be recognized with such a prize.

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What is Climate?

This is an answer to The Flame Challenge™ 2018 – What is Climate? – by the Alan Alda Center for Communicating Science at Stony Brook University.

Let’s talk about basketball for a second. Ignore the title and bear with me, OK?

I guess most of you play sports, and are at least familiar with how basketball works. Shooting the ball is one of the most important skills, and there is a shot that is worth one point more than the others, the three-pointer. When you are good at shooting 3s, then you can be really unstoppable. I’m not so good at it, but let’s assume you are. Overall you make half of all the 3s you shoot (this would be REALLY good actually). You are the terror of your opponents. You have some friends that are just horrible and barely make any, and there is one kid in school who just seems to make every single one.

This makes sense. Some of you are better than others, and some are worse than others. After playing a game you have probably made a few, and your friend who is pretty bad made only one. But let’s imagine that a new kid comes to school, and you decide to play with him/her to try to get to know them. They make their first 3! So they are 1-for-1 as far as you know. Would it then make sense to assume that they make every single 3 they ever take? No! Of course not, you and I know that would be silly. They could be really bad at it and just got super lucky (but they would never tell you), or more likely they are pretty good. But still, it would be weird to assume that they make every single shot, right?

Well, what is climate? In basketball, there are two different things: your skill at making 3s, and the outcome of a single shot. Everybody makes a shot every once in a while, but you don’t judge how good someone is just on that shot. You have to know more. How good they are depends on if they practice, or if they work hard, or maybe they just have that natural talent. Climate is exactly like that. Climate is not one single event, climate is the overall state of the earths Temperature/winds/storms bla bla. Weather is different from climate, and is just the actual outcome.

So, just like your friend that usually makes every single 3 can sometime miss one, it can sometimes be warm in the winter, or cold in the summer. The climate describes the state of the earth as a system, but not one single day or place. There are a lot of talks about climate change these days, and when they say that they mean that the overall state of the world is changing, but it may not affect you in the same way.

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Why the Laser is Important

As I head down the rabbit hole of my own bias, I hope to explain why the laser (Light Amplification by Stimulated Emission of Radiation) is so important. I am a practicing laser physicist, and have already given short arguments for why science in general is important and why physics specifically is important. Now, why is the laser important, and why is it something special? The answer to this lies in what light is exactly and how it interacts with the world around us, and exactly what a laser allows us to do. The unique qualities of a laser make it the most important tool for advanced scientific discovery outside of the computer.

I have often heard the declaration “A lab is not a real lab unless it has a laser!” I may not go that far just so I don’t offend anyone, but I certainly agree with the sentiment at least for the physical sciences. Since the invention and rapid advancement of the laser in the 1960s, they have spread to all sciences (not just the study of light, optics) to help study materials and samples of all kinds, and to make precise measurements.

Most laser light is in the optical frequency range, which is the same range of colors (different frequencies are seen as different colors by our eyes) that we see every day. And even the laser light outside of the visible range is usually only just outside of it (infrared and ultraviolet, like what the Predator sees). This is not an accident. We see these ranges of colors because they are what come from the sun, and we evolved to see the light that is most relevant. They come from the sun because they are related to the temperature of the sun. The reason that most lasers produce light in the same range is because the sources of lasers are usually some type of atom or molecule (either in a gas or most commonly in a crystal). The laser material, the atom or molecule in whatever form, relaxes from an excited state when stimulated and produces more light than what came in.

Because of how lasers work, being produced from excited states of atoms or molecules, they also are uniquely suited to study atoms and molecules. We use the light from lasers to probe all different types of new atoms, molecules, materials, biological samples, etc. to learn more about their structure, properties, and behavior in all types of interesting states and configurations. This is why well outside of a lab that is actually studying light, scientists use laser light as a tool for their own research.

Another property of laser light is that the wavelengths are usually somewhere around 1 micrometer (a human hair is around 100 micrometers wide, so think 100 times smaller than that). This wavelength is the distance between the peak of each oscillation of the ‘wave’ that makes up the light. This is such a small distance, smaller than most rulers of course, that scientists can use light to make measurements of physical sizes and distances to a much higher precision than ever before. Not only that, but scientists can also measure frequency and time with laser light, to incredible accuracy. Many of you have heard of the recent, Nobel Prize winning, discovery of gravity waves by LIGO. Well, the L in LIGO stands for laser, since the detection of the incredibly small changes in distance over mile-long arms of the LIGO machines is measured by a laser.

One more ability of lasers has to do with what light actually is. Light is not only something that we can see, but it is also a fundamental part of the laws of physics. Light is made up of photons, the same particle that is responsible for the electromagnetic force, that together form electromagnetic waves. These electromagnetic waves, of which laser light is a certain form, can exert forces on particles. Basically, laser light can exert force on electrons just like the voltage in all of our wall sockets causes electrons to travel through our cables and power all of our devices. However, laser light can do this in a very special way. Laser light can cool atoms, and can manipulate and accelerate particles, atoms, and molecules in a variety of ways. There is even something called ‘optical tweezers’ where you can trap (‘hold’) and move microscopic objects with laser light.

Because of these special properties of light and especially laser light, the laser has become an incredible workhorse of the scientific world. I wouldn’t say myself that a lab is not a lab without a laser, but at least most of the cool labs do have lasers.

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Why Physics is Important

I already made the case for why science is important, but why is physics specifically important? All of the same arguments for science apply to physics just as well. However, physics by its nature has a unique place within science that makes it especially crucial.

The first reason that physics by itself is especially important is the role that it plays in basic research – the same basic research that I spoke about previously. The research that pushes the boundaries of what we know about basic laws (in the case of physics, physical laws) is done in a disproportionate amount by physicists and physics researchers. This is almost included in the definition of physics itself and is part of what is required to call certain lines of research “physics research”. Although sometimes the line blurs between physics and engineering depending on where you are, who you are talking to, and what lab or research group you are in, more often than not physics research can be categorized as basic, fundamental research.

For the same reasons as I stated previously, basic research is absolutely necessary. Although even scientists cannot tell you what basic research might be useful for, that is exactly the point. A famous saying is that “innovation and engineering alone cannot take one from the candle to the light bulb”. Something more fundamental about how our world works must be discovered. This is basic research and often physics research.

The second quality of physics research that is important (and somewhat related to the first reason) is the role that physics plays in relation to other sciences. Sciences like biology, chemistry, neuroscience, among others. Often these fields discover and characterize phenomenon, without a complete or fundamental knowledge of why these phenomena occur or why they occur in exactly that way. Physics often can collaborate with these fields to provide the explanation. This is also sheds light on why these other sciences are important, and again the lines can be blurred between fields. But once the underlying mechanisms of phenomena are known they can be significantly expanded. This is the role physics plays.

As a physicist, I am certainly biased in this discussion (and some physicists may call what I do engineering), but I believe that physics serves a unique role within the sciences that is especially crucial. Of course, the relevance of chemistry and biology to human health research is a damning counter, and these topics will surely be under-represented in the blog. Physics is important, but of course all sciences are as well.

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Why Science is Important

To some of the people who are interested in this blog, or maybe most of the people, the answer to the question “why is science important?” might seem painfully obvious. Although I may agree at some times, the discussion of the answer is still important. Recent questioning of scientifically backed techniques or theories in the media and the public at large demonstrates that importance (read: climate change, vaccinations, evolution). Science is a system of investigation of problems, ideas, mechanisms, behaviors, trends, and at the most basic level laws of nature that has a self-consistent system of criticism built-in. Although science may not be perfect, it is in all of its areas the closest thing we have to truth, and the most significant driver of our increasing knowledge of the world around us.

Science is such an important and ever-present good that it can satisfy the questions of various personalities. The first of those is the philosopher, or in some way those who are least concerned with practical details. Although I have no intent to get in to epistemology, it must be said – without any concern for philosophical precision – that science is the closest we get to truth. The results of science are the closest thing we have in our whole existence to “knowing” something. Of course we can “know” that the grass is green, or “know” that we love our mothers, but these are different types of knowledge. To know that something is the case not only in our own experience or from our own perspective, but in an objective way is the knowledge that science provides us.

There can be a much longer discussion on why this is the case. Of course to make the assertion that science provides objective knowledge without any proof seems to be an affront to science itself. However of course, this is a discussion that can become very long and convoluted. The basic reasoning is as follows: Science uses logic and reason to either connect theories, whether just ideas or more concrete things such as axioms, equations, etc. to observed behaviors, or too predict observations that we have not yet made. These ideas are proposed in a precise way, and the observations are quantified mathematically and statistically in order to have a compact description. These scientific results are then generally attempted and hopefully replicated by independent researchers across the world and scrutinized by experts of various types.

Finally, after some combination of a large amount of scrutiny, a number of replicated results, a certain amount of time, and a lack of any counter-examples, something in science moves from just an idea to a real result, or law. These laws are what are really the output of science and are what I refer to when I say ”knowledge”. This is the self-consistent system of scrutiny, and is a big reason why science is a success in its modern form.

I am willing to bet that many of you, and most of the people on earth, are not satisfied with the argument just laid out. It may be just ignorance of the details of the scientific process, or just a lack of satisfaction with an increase in knowledge really being important. Both of those are understandable, and especially for science like the LHC at CERN in Geneva, this increase in knowledge actually doesn’t really seem that relevant. People who have these criticisms are more concerned with practical details, but the beauty of science is that it has answers to that as well.

The most beautiful example of this is the device that you are reading this on right now. The modern computing and internet revolution is wholly thanks to science that was only 100 years ago considered bizarre, crazy, somewhat tenuous, and not connected to anything useful in our daily lives.

The Transistor, which is responsible for the operation of our computer processors at the most basic level (and has been decreasing in size exponentially) is a device explained by what is called Solid-State Physics, an extension of Quantum Mechanics to specific systems. Quantum Mechanics was only solidified in the 1920s, and still is considered strange and bizarre (and is, in my opinion).

The GPS systems that we depend on so much rely on GPS satellites (yes, actual satellites in space) that are only accurate because of Relativistic corrections to their internal clocks. Einstein’s theories of relativity were absolutely groundbreaking results in the beginning of the 20th century, but also involved math that a very small percentage of physicists could even understand (still true).

The fiber optic communications systems that give us such fast internet speeds rely on the bouncing of pulses of light through glass rods the size of a human hair. Those light pulses are created and then received by devices dependent again on Solid-State Physics.

The screens we look at for so many hours a day were only a futuristic dream before scientists developed new optical theories and systems that could then be perfected by engineers.

I could continue ad-nauseum about such examples, but the point is this. Science is the most important driver of innovation in many sectors of our lives. This innovation increases quality of life, increases the ability of the human race, and spurs economic growth year after year after year. BUT, this is not just the science that is familiar to us, or that is sure to result in something tangible. This is all science. Science at the most basic level, in our time seeming to have absolutely no connection to anything practical may be what revolutionizes society in the next generations.

This is important. This is absolutely the most relevant and stunning claim that science has to necessity. But there is one last property of science that must be mentioned. It is just undeniably cool. I for one have worked on some of the most intense lasers in the world, and for a small amount of time on fusion energy. We are in labs all over the world recreating and observing phenomenon that have never been witnessed on earth and only in the far reaches of space in the most exotic scenarios. And, as Carl Sagan once said and Niel DeGrasse Tyson has reminded us, we are all made of star stuff.

The heavy atoms that make life possible and exist within our bodies were born in stars billions of years ago. Physicists and Cosmologists have shown that when the universe was young certain stars were the only place where these heavy elements could possibly be created. If you could trace back the iron in our blood, billions of years ago it was formed in some of the most extreme conditions ever seen in the history of the universe.

Science has so much to offer society, so much innovation to excite, and so much meaning to bring to our existence here. Sometimes scientists seem arrogant. Sometimes scientific research seems too farfetched or too isolated. And sometimes what scientists expect or predict turns out to be wrong. However, the case for science being important and integral to our society is absolutely closed. We must  science as paramount, and we must continue to do so for as long as there are questions to be answered.

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Introduction to the Ponderomotive.com Blog

Welcome to Ponderomotive.com, and the first post on the blog, the main component of the site. My name is Spencer Jolly and I am currently a PhD student in Physics studying at The University of Hamburg, and working at DESY in the west of the city. DESY is a large facility that is one of the best in the world in the areas of particle acceleration, light-source physics, and has some of the most advanced light sources for users in the world.

I studied in Michigan, in Texas, and now in Germany and always did research on high-intensity laser-plasma interaction. Basically, shooting powerful lasers at gases! When I think now about how much I knew when I first started 6 years ago (or even one year ago…) I realize I have learned a lot. I will probably say the same thing about now in a few years, but I find myself thinking that this stuff is so cool and important that it needs to be communicated. This blog will try to do that job.

So, Ponderomotive.com is just that, a lasers blog, but also a physics and general science blog. I hope to add some more unique parts to the site in the future, but for now the blog is the main feature. I hope to write technical articles, general articles about recent results or otherwise, to filter and gather other science news every week, and rarely to express some views about how science interacts with the world at large. Hopefully this is without too much controversy.

In the beginning, there will be no strict schedule as I populate the blog slowly, but the plan is to trend towards regular content.

The technical articles will start out with a 16-part series on how a laser works, accessible to a range of audiences. I will also rate each article with how technical it really is so that no reader is scared away, but one also knows how worth it the article may be.

There will also be ‘soft’ articles meant to be very accessible, ‘filter’ articles of the weeks or months news, and historical or biographical articles. I foresee these which require more writing skill to be of evolving quality.

Please join me in an interesting new website with hopefully a unique perspective and a unique niche to cater to. I hope it finds all of my readers well and becomes something that contributes to our science knowledge and our view of science in some small positive way.

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