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book reviews
photonics booksreviewed by T. Nelson |
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book reviews
photonics booksreviewed by T. Nelson |
Reviewed by T. Nelson
Optics is not ‘optics’ anymore. It’s ‘photonics’ now, and everything you knew about light is now wrong. Nanophotonics is optics at the smallest scale.
Part of it could be that the word ‘nano-optics’ without a hyphen sounds a bit funny. But funny things are happening at small scales. There is light where the electric and magnetic vectors (E and H) aren’t perpendicular to each other. There are electromagnetic waves that don’t propagate. There are microscopes that view things smaller than the wavelength of light. There is stopped (or more accurately slowed-way-down) light that just sits around doing nothing.
Nanophotonics is not just for people. Moths have a corrugated subwavelength nanostructure on their eyes that acts as a potent antireflection coating. They’re using the principles in this book to camouflage themselves at night. Butterflies and the super-black bird of paradise use deep cavity nanostructures to produce ultra-black surfaces.
Most of the interesting photonics stuff happens in the near field, so distinguishing near field from far field is important. In the near field, electromagnetic waves decay with the cube of the distance (r3). As distance increases, the wave decays with the square of r, and finally 1 / r. That makes all this interesting stuff possible.
But a better formula is needed. One formula, used in this book, says near field is when r ≪ d2 / λ, where d is the diameter of the primary lens or the source (whichever is bigger), λ is the wavelength, and r is the distance. Another formula says near field is when r ≪ λ / 2π, independent of any geometry.
Microwave engineering books and some radioastronomy books use the first one. For example, Richard C. Johnson in Antenna Engineering Handbook uses it but adds that it is “inadequate in some special situations.”
That’s an understatement. At optical wavelengths, 1/λ is a very big number and you get absurdities like the near field of the Sun being anything closer than 414,000 light years. This led to the pervasive myth (repeated on p. 32) that the Sun as observed from Earth is in near field. The other formula disagrees, saying near field is 79 nanometers. Everybody likes the simple approximations, but they can differ by 31 orders of magnitude.
The authors switch to the second criterion in later chapters.
A surface plasmon is a longitudinal acoustic surface wave coupled to a polarization charge-density wave. In other words, it is a form of energy where the wave “sloshes” between kinetic energy of electrons in an electron gas and an electric field. The frequency depends on the material: for gold and silver it’s in the visible range.
Plasmons act like radio waves that travel along the surface of the Earth. But, the authors say, the mechanism is different. Plasmons can’t exist at low frequencies—only in the visible, infrared, or near THz.
Surface plasmons are a special type of polaritons. There are several kinds of polaritons, but they are all produced by materials with complex dielectric constants. The authors say plasmon refers to a hybrid mode that is half-photon and half phonon.
Evanescent light is light emitted in the near-field zone. It is generated by nearby charges, which could be free electrons in metals or bounded polarization charges in dielectrics. That is to say, from surface plasmons. Its characteristics are (1) it decays very rapidly; and (2) it contains information on sub-wavelength confined electric fields.
This information is there because to satisfy boundary conditions the field must vary on length scales that depend on the shape, even at scales much shorter than the wavelength. It can’t stay there because the propagating wave can’t handle those high frequencies. But it’s obviously something we’d like to have, and new superresolution microscopes can now capture details that were once considered impossible. To make them work, you put a tiny fiber as close to the specimen as possible—in the near field—and capture the information-rich evanescent waves.
If readers didn’t already know what a photonic crystal is good for, they’d be utterly baffled by the chapter on photonic crystals. That’s a shame because they were a major breakthrough.
Photonic crystals are artificial crystals where two layers containing bandgaps are oriented at an angle. This gives you the equivalent of round holes or ‘rods’ in a crystal. The holes can also be made with air or vacuum, which is even better.
In a hexagonal one, you end up with only three important places: the center, called Gamma; the edge, called M, and the corner, called K. See here for an explanation. Where M and K overlap you get a omnidirectional photonic bandgap, which is a fancy way of saying light goes through it.
Well, I hear you saying, light goes through a lot of things, so what?
After baffling us with many pages of math, the authors finally tell us. In a photonic crystal, almost all the light goes through the little holes instead of the glass as happens in an optical fiber. This makes possible a special kind of laser called a supercontinuum laser. The only problem is that they cost 1,468 times as much as a regular optical fiber and they’re very fragile.
The last part of the book shows the applications, which include flat optical lenses, metamaterials, and nanoantennas. For instance, the common wire-grid polarizers act by funneling TM-mode light into the gaps, while TE-mode light is converted into evanescent waves and reflected away, so you get good polarization and almost no absorption.
The writing style in this book is reasonably clear, with minor ambiguities from terms like “large spatial frequency,” by which they mean high frequency, not large size.
But there’s a lot of jargon to wade through. I’d recommend writing the symbol names down for each chapter, as the same symbols have different meanings than in other fields (m is not mass, for instance, and ν [nu] is not frequency). Their meanings change from chapter to chapter.
Nanophotonics is a big deal these days. With some persistence you will come away with a solid understanding of weird light tricks at the smallest scales. I’d recommend reading one of these first if you haven’t done so.
A clue is that if a book has ‘introduction’ in the title, it's not one. That’s even more so in elementary particle physics: the ‘elementary’ refers to the particles, not the physics.
aug 26 2026
Reviewed by T. Nelson
In this one, Zhe-Yu Jeff Ou of Indiana University says his students kept asking questions about photons such as how a photon can pass through a Fabry-Perot filter. He says that the solution is mode theory. With the concept of modes, he says, all those questions are easy to answer.
The only problem is that ‘mode’ doesn’t seem to mean anything. Just about anything can belong to a mode. There are temporal modes that determine wavelength and coherence. Polarization can have a mode. The optical field itself is a sum of eigensolutions, all of which are modes. There are Gaussian modes, temporal modes and mode functions like u(r, t) = ∑λ cλuλ(r, t), which the author says is a superposition of a set of orthogonal modes, the character ‘λ’ indicating ‘mode.’ That’s in addition to the ordinary use of modes in a fiber and mode-locked laser. It sounds great at first, but eventually the reader realizes that ‘mode’ is just another way of using up your supply of lambdas. The author stops using them after the first few chapters.
As you might imagine, this book is mostly math, but math is reasonably simple with the occasional how-the-hell-did-they-get-that step thrown in. As usual in this field, symbols, even important ones like aˆ† (which normally means ‘creation operator’), mean different things in different chapters. This is a good way to baffle a student. But the formulas really do relate to experiments.
If you persevere you’ll get a better understanding of the math that’s used in quantum optics and how all the concepts derive from each other. For example, astronomers can’t measure interference fringes from stars at optical wavelengths because of atmospheric turbulence, but they can measure intensity, so they use intensity correlations instead of phase correlations. You need formulas for that.
There’s also a big section on squeezed states and a good discussion of entanglement. The author frequently cites R. J. Glauber, who was awarded a Nobel prize along with two other physicists for his work on quantum optics. By the time we get to the experimental section, the student is probably champing at the bit to learn how to build that box on page 156 with a χ(3) in it, which the author says is a photon converter. It turns out to be just an ordinary SPDC (spontaneous parametric down-conversion) crystal . . . but with lots of formulas.
The most interesting thing in this book is on page 218 where he quotes P.A.M. Dirac as saying something profound:
Each photon only interferes with itself. Different photons never interfere.
This was a brilliant observation on Dirac's part, even though we now know that you can also get interference patterns from pairs of photons using a laser and a coincidence detector.
The idea of discussing experiments was a good one, but after reading this book I wasn’t convinced that adding an abstraction like modes helps as much as the author says. There must have been some other reason the author's students were getting confused.
Nearly useless index. No table of symbols. Diagrams are very small but nice and sharp.
sep 14 2026