This was an interesting bit of science news from last week: according to an article on optics.org (free registration required), a research team from Rensselaer Polytechnic Institute and Rice University has fabricated the ‘blackest’ material ever known, which reflects just 0.045% of the light incident upon it. This beats the previous record of 0.16% that was set by a nickel and phosphorous alloy.

Though most people have probably not given it much thought before now,  it is obvious on even a cursory inspection of objects in daily life that there is no such thing as a perfect absorber (i.e. an object of perfect ‘blackness’).  Looking about my office, the keyboard, stapler and textbook on my desk all reflect a significant amount of light, even though they are all officially black in color.  One can show via the Fresnel formulas for reflection and transmission at an interface that one cannot make a simple material which absorbs light for all directions of illumination.  This is, in essence, a consequence of the wave nature of light.

This reflection has been a  problem ever since people started studying optics.  For telescopes, light reflection from a lens reduces the efficiency of the system and the reflected light can add to the system noise.  For eyeglasses, light reflection hides the eyes of the wearer (a cosmetic problem) and can also increase eyestrain.  To circumvent these difficulties, scientists have used antireflection coatings for quite some time.  The principle is based on interference, illustrated roughly below: a plane wave normally incident upon a thin transparent layer of material will reflect partially off the front and rear boundaries of the layer.  If the layer is chosen to have an optical thickness of a quarter wavelength, and a refractive index less than the medium it is deposited on, the wave reflecting from the rear layer will be a half-wavelength out of phase with the front reflected wave and there will be destructive interference between them!

This reflection can be made perfect, but only for a single direction of incidence.  If light is coming in from multiple directions (at it always invariably is), one finds imperfect but still typically reduced antireflection.

It’s interesting to note that the ideas of reflectionless coatings and perfect absorbers were combined in one of the first attempts to make ‘stealth’ aircraft, back in WWII.  From Arnold Sommerfeld’s Optics (Academic Press, 1964),

The more complete solutions… are of some historic interest.  During the war the problem arose to find, as a counter measure against allied radar, a largely non-reflecting (“black”) surface layer of small thickness.  This layer was to be particularly non-reflecting for perpendicular or almost perpendicular incidence of the radar wave… But thereby the problem is not yet solved.  For at its back surface the layer borders on the object (metal) which is to be camouflaged, and this second surface still reflects strongly.  Hence, the further condition must be imposed that the layer should absorb sufficiently strongly.

This is not how stealth aircraft work their magic today, but it is still an interesting historical footnote!  The solution Sommerfeld posed could not rightly be said to fall into the category of metamaterials, as its required a precise tuning of both the electric and magnetic properties of the material.

 So how does the Rice/RPI material achieve its high degree of blackness?  Their material is made of an extremely low density collection of carbon nanotubes.  These tubes of carbon are a billionth of a meter in diameter and upwards of a millimeter long.  The tubes are aligned vertically like blades of grass, and only fill 3-5% of the total volume of the sample.  The effect seems to be somewhat like an ‘optical jungle’, a complicated structure that light can pass into easily, on account of its low density, but must spend an appreciable amount of time (for light) to reemerge from.  During that time, it is much more likely to be absorbed.

The news article contains a comparative image which shows the difference between the new material and the NIST reflectance standard with 1.4 percent reflectance (the link also contains what must be a subtle reference/joke).