Archive for the 'Optics basics' 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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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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Optics basics: Young's double slit experiment

Mar 28 2009 Published by under Optics, Optics basics

As I've so far been restricting my 'optics basics' posts to discussions of fundamental concepts related to optics, it might seem strange at first glance to dedicate a post to a single optical experiment.  What will hopefully become clear, however, is that Young's double slit experiment is connected to so many basic concepts in optical physics  (and still provides surprising new results to this day) that one post is hardly enough to describe all the interesting insights that can be gained by studying the experiment and its implications.

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Optics basics: Inverse problems

Oct 31 2008 Published by under Invisibility, Optics, Optics basics

In previous posts, I've talked at some length about computed tomography (CT) and optical coherence tomography (OCT).  Each of these is a technique for determining information about the internal structure of an object, such as the human body, from exterior measurements of the scattering of electromagnetic waves from the object.  In the case of CT, x-rays are used to measure and image a cross-sectional 'slice' of the human body, while in OCT, broadband visible light is used to probe a few millimeters into the skin or an internal organ of the human body.

Plenty of other techniques exist for measuring the internal structure of objects, using a variety of different types of waves.  Magnetic resonance imaging (MRI) subjects a patient to an intense magnetic field, and makes an image by measuring the radio waves emitted when the field is suddenly switched.  Ultrasound imaging uses ultrasonic waves to probe the soft tissues of the human body, and is used in mammography.

Each of these techniques is quite different in its range of application, but all require nontrivial mathematical techniques to reconstruct an image from the raw scattered wave data.  These mathematical techniques are broadly grouped into a class of problems known as inverse problems, and I thought it would be worth an optics basics post to discuss inverse problems, their common features, and the challenges in solving them.

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Optics basics: Coherence

Sep 03 2008 Published by under Optics, Optics basics

In previous optics basics posts, the interference of waves has played a major role.  When two or more monochromatic (single-color) waves are combined, they form a pattern of light and dark regions, in which the combined light fields have constructively or destructively interfered, respectively.  The simplest of these patterns can be created by the interference of two plane or spherical waves, and would appear as shown below:

One way of producing such a pattern is by Young's two-pinhole (or double-slit) experiment, which we will have cause to discuss in more detail below.  The actual size of the interference pattern depends on the experimental setup, including the wavelength of the light, but can be easily made visible to the naked eye.

An astute observer of nature, however, will find something fishy about this whole discussion of interference: it does not seem to manifest itself in everyday experiences with light.  Sunlight streaming through a window, for instance, doesn't interfere with the light emanating from a lamp inside the room.  Something is missing from our basic discussion of interference which explains why some light fields, such as those produced from a single laser source, produce interference patterns and others, such as sunlight, seemingly produce no interference.  The missing ingredient is what is known as optical coherence, and we discuss the basic principles of coherence theory and its relationship to interference below the fold.

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Optics basics: Polarization

Jun 11 2008 Published by under Optics, Optics basics

In a previous 'basics' post, I discussed the three major branches of optical science. My specialty, physical optics, involves the study of the wave properties of light. In particular, there are three major phenomena in physical optics: interference, diffraction, and polarization. We've talked about the first two of these in earlier posts, and it is time at last to say some words about polarization!

In essence, "polarization" is a fancy way of saying that light is a transverse wave; with that in mind, we begin with a brief discussion of transverse and longitudinal waves.

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Optics basics: Defining the velocity of a wave

Jan 17 2008 Published by under Optics, Optics basics

How do we define how fast a wave is going? The question at first glance seems obvious. When we discussed harmonic waves in a previous post, we observed that the velocity of the wave could be measured by measuring how far one of the peaks of the wave travels in a certain amount of time. There are a number of subtle points that arise when talking about wave velocity, however, including the possibility of light traveling at faster than the 'speed of light'! In this post we'll try and define the velocity of a wave, and explain why the question is not so easy to answer.

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Optics basics: What is a wave? Part IV: Important quantities

Dec 12 2007 Published by under Optics, Optics basics

To conclude my discussion of optics basics, I want to introduce some of the standard quantities used to describe waves and wave propagation. Unlike previous 'basics' posts, this one will necessarily deal with a little bit of algebra and perhaps a little trigonometry.

The simplest wave to deal with from a theoretical point of view is a harmonic wave, one which consists of an infinite sequence of regularly spaced 'ups and downs'. A portion of such a wave traveling to the right on an extremely long string would appear as:

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Optics basics: What is a wave? Part III: Diffraction

Dec 10 2007 Published by under Optics, Optics basics

In part II of my series on 'What is a wave?', I addressed one of the two most significant behaviors of waves, namely interference, the ability of a wave to 'interact' with itself. The second behavior of waves which is extremely significant is diffraction, and we will address it in this post.

Diffraction may be broadly defined as the tendency of a wave traveling in two or more dimensions to spread out as it propagates. The most significant consequence of this spreading is the ability of waves to 'bend around corners' when faced with an obstacle. We all have experienced the diffraction of sound waves: if you and a friend stand on opposite sides of a large building (say a farmhouse) in the middle of an open field, you will be able to talk to each other even though there is no direct 'line of sight' between you and your friend, and no ability for the sound waves to reflect off of intermediate surfaces. The sound waves wrap around (diffract around) the outside of the farmhouse, allowing communication.

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Optics basics: What is a wave? Part II: Interference

Nov 15 2007 Published by under Optics, Optics basics

In the first part of my series on 'What is a wave?', I attempted to give a broad definition of a wave, so that we can identify them when we see them. In this part, I will address two of the most important behaviors of waves: interference and diffraction. Interference may be loosely described as the interaction of a wave with itself, or a wave with another wave, while diffraction may be loosely described as the interaction of a wave with other objects.

We will discuss interference in this post, and consider again the wave on a string discussed in part I of this post. A pair of waves are sent down the string to a fixed end, where they are reflected and return to their point of origin. What happens when the waves pass each other? An animation of such an event is displayed below:

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