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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