Archive for the 'Optics' category

Optics basics: surface plasmons

Sep 21 2010 Published by under Optics basics

My goal in my "basics" series of posts is not just to introduce the most elementary topics in optical science, but also to give background on some of the more advanced concepts for future reference. Much of my own research, and consequently my blog interests, center on nano-optics -- the study of the behavior of light on scales much smaller than the wavelength of light -- and one specific aspect of nano-optics that has grown tremendously in importance over the past ten years is the concept of a surface plasmon.

Broadly speaking, a surface plasmon is a traveling wave oscillation of electrons that can be excited in the surface of certain metals with the right material properties. Because a plasmon consists of oscillating electric charges, they also have an electromagnetic field associated with them which also carries energy. There's a lot of terminology to explain in that short definition, and in this post I'll explain what a surface plasmon is, the properties of surface plasmons, and how those properties make them useful in nano-optical applications.

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Streets of the optical scientists!

Aug 23 2010 Published by under [PhysicalScience], Optics, Travel

This post is a repost of some proto-blogging I did on my department web page when I was a post-doc in Amsterdam.  The web page is gone, now, so I thought I'd revise the essay significantly for the blog here.

I don't think it is too much of an unfair generalization to say that science and scientists are rather unappreciated in the United States.  Folks are quite happy to reap the benefits of science and technology when it comes to their computers, iPhones, etc., but can be dismissive or indignant to scientists when their results show people truths that they are uncomfortable with, e.g. evolution and global warming.

That's not to say that other countries are necessarily much better, but I do occasionally run across pro-science efforts elsewhere that surprise me.  From 2003-2004, I did my post-doctoral work at the Vrije Universiteit in Amsterdam, The Netherlands, an experience that I will count as one of the best times of my life.  Amsterdam is just a wonderfully livable, walkable city, and even on my limited salary I was able to enjoy it immensely.  While there, I kept up my figure skating training at the Jaap Eden Ijsbanen, which is located in the neighborhood of Watergraafsmeer outside of the city center.  I would take the bus to the rink from my apartment, and every day would travel down Maxwellstraat and past Lorentzlaan, but it didn't occur to me until near the end of my time in The Netherlands that these streets are named after the physicists James Clerk Maxwell and Hendrik Antoon Lorentz!

In fact, all streets in the neighborhood of Watergraafsmeer are named after famous scientists and mathematicians, which is really a joy for a physicist like me. So after skating at the last day of the season at the Jaap Eden Ijsbanen, I decided to wander the neighborhood and hunt down the streets of those physicists whose work in the optical sciences has been a great influence on my own life's work, combining physics & travel blogging!

I present the streets in no particular order of chronology or significance; rather I present them in the order that I wandered past them. Information about the scientists themselves I gleaned from a variety of sources, including printed biographies, internet sites, and historical articles by my thesis advisor. Pictures of the various scientists were taken from Wikipedia.  So without further ado, let us begin our tour -- feel free to follow along the trail via Google maps...

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Optics basics: lasers!

Aug 02 2010 Published by under [PhysicalScience], Optics, Optics basics

One of my goals in blogging has been to run a series of posts covering the "basics" of optics, namely those concepts that form the basis of an understanding of the more advanced topics investigated by researchers today. Though I've done a pretty good job so far, I recently realized that I've left out a discussion of the most important tool of the optical scientist, and one of the most important technological advances of the modern era: the laser!

Image via Wikipedia, of an experiment at the Air Force Research Lab.

"Laser" is an acronym for "Light Amplification by Stimulated Emission of Radiation", and it refers to a device that produces light by an unusual physical process not typically found in nature.

In fact, 2010 marks the 50th anniversary of the laser, as the first functioning device was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories in California.  To draw attention to this anniversary, optics organizations have instituted a yearlong celebration called LaserFest, and many special events are planned and have already taken place; my favorite being the Lasers Rock! concert that was held in May at the CLEO/QELS conference:

Musical group "Second and Third Harmonic Generation" playing at Lasers Rock!  Picture via Ksenia's CLEO/QELS blog.

I hardly need to describe the impact lasers have had on our society, and it is hardly possible to list all of the applications!  Among other things, lasers are used to read CDs, DVDs and Blu-ray discs, they form the basis of the fiber-optic communications systems by which you are probably reading this post, they are used in medicine both to diagnose problems as well as to perform laser surgery, they are used to cut material in industrial fabrication.  Their properties make them ideal for doing optics research of all sorts, and they are now an essential tool for researchers.

In this post I would like to describe the physics of lasers.  This is no mean feat, because there is a lot to say about how they work, and many variations on the fundamental idea that was first proposed by Charles Townes in the 1950s¹.  I will proceed somewhat carefully:

  • First, I will discuss what a laser is, and what properties a laser has that distinguishes it from "ordinary" light sources like light bulbs.
  • Second, I will describe the fundamental physics behind the lasing effect, in particular the process of stimulated emission.
  • Finally, I will explain the engineering that is used to take advantage of stimulated emission and make a laser.

Let's take a look...

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The Discovery Place does optics!

Jul 29 2010 Published by under Optics, Personal

This post involves a little bit of boasting!  For the past month, the Discovery Place science museum in Charlotte has been displaying a small interactive optics exhibit targeted at 8-14 year-olds as part of their "Explore More Stuff" series.  The kicker is that I played a small part in the exhibit, suggesting an idea for one of the interactive "stations"!

The museum contacted our department a couple of months ago and a few faculty, including me, went to brainstorm with their staff for their exhibit.  They did a great job quickly turning the ideas that came out of the session into kid-resistant displays.  The exhibit gets phased out next week, but I stopped downtown this week to take a few quick pics!

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Freaks & geeks: optical freak waves in the laboratory

Jul 13 2010 Published by under Optics, Physics

ResearchBlogging.orgOne of the most fruitful and intriguing avenues for developing novel scientific research is through cross-pollination with other fields of study.  This is one of the reasons I'm proud of my excessively liberal arts-focused education, as well as one of the reasons I like reading blogs on diverse subjects outside of my field: interesting ideas can often come from unexpected sources.

An example of this I found a few months ago in Physical Review Letters, in an article entitled, "Freak waves in the linear regime: a microwave study," by Höhmann, Kuhl, Stöckman, Kaplan and Heller.  Freak waves, also known as rogue waves*, are anomalously large -- and deadly -- isolated oceanic waves that can shatter and overturn ships, and they have only been acknowledged relatively recently as a genuine and unusual phenomenon, albeit one that is still not completely understood.

Hokusai's 1832 The Great Wave off Kanagawa, via Wikipedia.   Not necessarily a freak wave, but probably close to what most people would envision one to be.

It was probably inevitable that researchers in optics would become interested in freak waves: broadly speaking, a wave is a wave, and an effect that appears in water waves is likely reproducible in electromagnetic waves.  Experts on oceanic freak waves have even been invited to speak at optics meetings; a session at the 2009 Optical Society of America's Frontiers in Optics meeting was opened with the invited talk, "Freak Ocean Waves in One and Two Dimensions," by Peter Janssen and Jean-Raymond Bidlot of the European Center for Medium-Range Weather Forecasts.

In this post I thought I would take a look at the phenomenon of freak waves, the physical origins of said waves, and methods that physicists have used to create electromagnetic versions of them in the laboratory.

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Invisibility physics: Kerker's "invisible bodies"

Jul 02 2010 Published by under Invisibility, Optics

(This is a continuation of my “history of invisibility physics” series of posts.  The earlier posts are: Part I, Part II, Part III, Part IV, Part V, Part VI)

The history of invisibility physics truly began with the concept of radiationless motions of charged particles, as described by Ehrenfest in 1910 and Schott in 1933.  There are many more discoveries associated with these and related phenomena, which would eventually be referred to as nonradiating sources.

I would like to jump ahead in the history a little bit, however, and discuss a paper published in the Journal of the Optical Society of America in 1975 by Milton Kerker, entitled, "Invisible bodies".  The article, relatively unknown today, is the first article to describe an object which is invisible in the true sense of the word -- although the object itself is microscopic!

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You could learn a lot from a ducky: the van Cittert-Zernike theorem

Jun 12 2010 Published by under Animals, Optics

(Alternate titles considered for this post: Ducktoral degree, Send in the ducks, Proof by in-duck-tion, Duck Tales, Duck-ing the issue.)

One of my specializations in optics is the theory of optical coherence, which is the theory that characterizes the random fluctuations of light, and the consequences of said fluctuations.  It is typically one of the most difficult optics topics to teach beginning optics students, probably because it combines two challenging bodies of mathematics: wave theory and probability theory.  Any teaching tool that can be used to help students visualize and understand the basics is welcome, though such tools are few and far between in coherence.

Enter the ducks!  Early this year, some colleagues of mine published a short note pointing out that one can visualize a fundamental result from optical coherence theory, the van Cittert-Zernike theorem, by watching the waves a group of ducks generate when they splash into a pond!

The letter is by W.H. Knox, M. Alonso and E. Wolf, "Spatial coherence from ducks," Physics Today, March 2010, p. 11; it can be freely read here.  Though the letter describes the connection between coherence and ducks, it doesn't explain what the van Cittert-Zernike theorem is, so I thought I'd fill in a bit of detail with this post!

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Singular Optics: Light chasing its own tail

May 04 2010 Published by under Optics

(Title stolen shamelessly from my postdoctoral advisor, who I assume will forgive me.)

As I've noted numerous times in previous posts, one of the fundamental properties that characterizes wave behavior (i.e. that makes a wave a wave) is wave interference.  When two or more waves combine, they produce local regions of higher brightness (constructive interference) and lower brightness (destructive interference), the latter involving a partial or complete "cancellation" of the wave amplitude.

Researchers have long noted that the regions of complete destructive interference of wavefields, where the brightness goes exactly to zero, have a somewhat regular geometric structure, and that the wavefield itself has unusual behavior in the neighborhood of these zeros.   In the 1970s this structure and behavior was rigorously described mathematically, and further research on this and related phenomena has become its own subfield of optics known as singular optics.  Singular optics has introduced a minor "paradigm shift" of sorts to theoretical optics, in which researchers have learned that the most interesting parts of a light wave are often those places where there is the least amount of light!

In this post we'll discuss the basic ideas of singular optics; to begin, however, we must point out that most people have the wrong idea of what a "typical" interference pattern looks like!

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Michelson and the President (1869)

Apr 12 2010 Published by under History of science, Optics

I'm currently working my way through the book The Master of Light: a Biography of Albert A. Michelson (1973), written by one of his daughters, Dorothy Michelson Livingston.  I typically find the beginnings of biographies to be rather slow-moving, with some sort of statement like, "There was little to indicate in his/her childhood what a great scientist he/she would become," but this is definitely not the case for Michelson -- his life story is interesting starting pretty much at birth!

I thought I'd share another anecdote from the book that I found fascinating: Michelson's meeting, at a young age before he was famous, with the President of the United States Ulysses S. Grant!

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Wave interference: where does the energy go?

Apr 07 2010 Published by under Optics, Physics

Last week was a relatively lousy one for me, but it was made up in part by getting a good question from a student on waves and interference after class.  It's really nice to get a question that indicates a genuine interest in the science (as opposed to just wanting an answer to homework), and I thought I'd discuss the question and its answer as a post.

The situation in question is as follows: suppose you have a harmonic wave on a string traveling to the right such that in a snapshot of time, the string looks as follows:

This wave carries energy, and there is a net flow of energy to the right.  Now suppose we excite the string with an additional wave of the same frequency and amplitude, but completely out of phase.  The sum of the two waves then vanishes:

The two waves cancel each other out, leaving a completely unmoving string due to destructive interference.  My student asked me: what happens to the energy?  As posed, it seems that we started with two waves carrying energy, but they canceled each other out, leaving no energy!  This interpretation cannot possibly be correct, so where is the flaw in our description?

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