According to our Special Topic on photonic crystals,
the work of Professor Steven Johnson ranks at #8 by total
citations, based on 29 papers with "photonic crystal" in
the title cited a total of 1,751 times. InEssential
Science Indicators^{SM}from
Thomson
Reuters, Professor Johnson's record includes 95 papers,
the majority of which are classified in the field of
Physics, cited a total of 3,668 times between January 1,
1998 and June 30, 2008.

Johnson is Assistant Professor of Applied
Mathematics at MIT. In the interview below, he talks
with ScienceWatch.com about his work with
photonic crystals.

Please tell us a little about your research
and educational background.

I received three bachelor-of-science degrees in 1995 from MIT, in physics,
mathematics, and computer science, and got my Ph.D. in the MIT physics
department in 2001. I joined the MIT applied mathematics faculty in 2004
after postdoctoral positions at MIT and Harvard.

My research, since my days as a graduate student with Prof. John
Joannopoulos at MIT, has centered around
nanophotonics—electromagnetism in materials that have structures on
the same scale as the wavelength. This can make light behave in very
unusual ways compared to how it propagates through a mostly uniform medium
like air or solid glass. Even though the basic physical laws of
electromagnetism are well understood, their consequences can be complex and
unexpected in the context of such structures. The challenge is twofold:
first, to develop techniques, both involving large computers and involving
pencil-and-paper analysis, to understand the behavior of light in these
circumstances; and second, to design structures that lead to new phenomena
and devices.

What first interested you in photonic
crystals?

"...I think that nanophotonic
control of light has taken on a new urgency,
with a focus on discovering the fundamental
limits of optical design."

My interest in physics has always centered on things that you can see and
touch, and in the ways that combining simple interactions can lead to
surprising effects. Even though the equations of classical electromagnetism
have been around since the work of Maxwell in 1865, it is amazing to see
that they can have so many unexpected consequences in nanostructured
materials. And it is an area where you can literally see the results, from
iridescent butterfly wings and peacock feathers in nature to synthetic
materials that are leading to exciting new applications such as medical
fibers, more efficient solar cells, or ultra-bright LEDs.

A key paper in your publications is the 2001
Optics Express paper, "Block-iterative frequency-domain
methods for Maxwell's equations in a planewave basis," (8: 173-90,
2001). Would you talk about the significance of this paper for
photonic crystals?

That paper describes an efficient computational technique to find the
allowed behaviors of light in a complicated three-dimensional structure,
such as a three-dimensional photonic crystal or a traditional dielectric
waveguide. A large part of the impact of this paper is due to the fact that
it is paired with a free/open-source software package called "MPB" that I
developed as an MIT graduate student.
MPB has enabled many researchers to enter this field
and begin doing calculations quickly, without having to develop a
simulation tool themselves or purchase a commercial package that they
cannot modify to suit their needs.

Your most-cited paper in our analysis is the 1999
Phys. Rev. B paper, "Guided modes in photonic crystal slabs,"
(60[8]: 5751-8, 15 August 1999). Would you talk a little bit about
this paper's methods, findings, and conclusions?Actually, quite a few of your papers deal with photonic
crystal slabs. Would you talk about what these are, and any advantages
they possess?

Photonic crystals, in general, are periodic arrangements of two or more
optical materials (e.g., glass and air). One of the exciting properties
that a photonic crystal can possess is a photonic band gap, a range of
wavelengths in which light cannot penetrate the crystal—it acts like
a kind of "optical insulator." In order to obtain a true photonic band gap
in three dimensions, however, one needs to build a complicated structure
that has periodic patterns in all three dimensions. Although this is
possible, and amazing progress has been made in three-dimensional
nanofabrication, it is still challenging. A photonic-crystal slab is a
simpler structure, a thin slab of material with only a two-dimensional
pattern, which is much easier to fabricate using traditional lithography
(like the processes used to make computer chips), while still approximating
some of the useful properties of a three-dimensional crystal.

"My interest in physics has always
centered on things that you can see and
touch, and in the ways that combining simple
interactions can lead to surprising
effects."

In the 1990s, many researchers were interested in photonic crystals
fabricated via two-dimensional patterns, but there was a lot of confusion
about the properties and designs of such structures, especially in how they
compared to simple two-dimensional models (in which light propagating
vertically is not considered). My 1999 paper was one of the early
theoretical papers to describe the correct three-dimensional analysis of
photonic-crystal slabs, and gave a thorough survey of their properties and
design principles. Some of my later papers extended this work to include
"defects" designed into the slab to introduce waveguides and resonant
cavities.

How far has this work come since you entered the
field? Where do you see it going in the next decade?

Offhand, I can think of more than half a dozen start-up companies that have
been founded since I joined the field, all based in one way or another on
ideas from photonic crystals. Here in Cambridge, I've been directly
involved with one called OmniGuide, making a new kind of fiber that carries
high-power lasers for endoscopic surgeries; it's been immensely gratifying
to have some of my theoretical work directly translated into technology
that is literally saving lives.

The sheer number of people now working in this field is staggering; it used
to be possible to personally know almost every principal scientist working
on photonic-crystal problems, but now there are far too many groups to keep
track of.

There is an increasing shift in the electronics industry from focusing on
speed to focusing on power. In optical devices, this translates into
increasing concern with slowing down and trapping light via nanostructures
to achieve more efficient nonlinear modulation, light emission, and light
absorption—even pushing towards considerations on a single-photon
scale. All of these concerns were present in past work, but I think that
nanophotonic control of light has taken on a new urgency, with a focus on
discovering the fundamental limits of optical design.

What should the "take-home" lesson be about your
research?

Instead of finding exotic materials to achieve some particular optical
property, the problem is now to take existing materials and find new
geometries that have the properties we need. Many of the limits imposed by
natural materials are removed, and replaced by only the limitations of our
imaginations.

Professor Steven G. Johnson, Ph.D.
Assistant Professor of Applied Mathematics
Massachusetts Institute of Technology
Cambridge, MA, USA

Johnson SG, Joannopoulos JD, "Block-iterative
frequency-domain methods for Maxwell's equations in a
planewave basis," Opt. Express 8(3): 173-90, 29
January 2001. Source:
Essential Science Indicators from
Thomson
Reuters.