Generally and in one form of the invention this is a periodic surface filter comprising at least one element at a surface of the filter and electronic controls to change the optical characteristics of the element. Other methods and devices are disclosed.

There is a significant gap between the internal efficiency of light-emitting diodes (LEDs) and their external efficiency. The reason for this shortfall is the narrow escape cone for light in high refractive index semiconductors. We have found that by separating thin-film LEDs from their substrates (by epitaxial lift-off, for example), it is much easier for light to escape from the LED structure and thereby avoid absorption. Moreover, by nanotexturing the thin-film surface using "natural lithography," the light ray dynamics becomes chaotic, and the optical phase-space distribution becomes "ergodic," allowing even more of the light to find the escape cone. We have demonstrated 30% external efficiency in GaAs LEDs employing these principles.High efficiency light-emitting diodes (LEDs) are desired for many applications such as displays, printers, short-haul communications, and optoelectronic computer interconnects. However, there is a significant gap between the internal efficiency of LEDs and their external efliciency. The internal quantum yield of good quality double heterostructures' can exceed 99%, as we have demonstrated recently.2 On the other hand ordinary LEDs are usually only a few percent efficient. The reason for this shortfall is the difficulty for light to escape from high refractive index semiconductors. The escape cone for internal light in a semiconductor of refractive index n,=3.5 is Only -16", as imposed by Snell's Law. This narrow escape cone for spontaneous emission covers a solid angle of z ( l/4$> x4rr sr. A mere 2% of the internally generated light can escape into free space, the rest suffering total internal reflection and risking reabsorption.A number of schemes have partially overcome this problem, based on the old idea3 of coupling the light out of the semiconductor by means of a high refractive index hemispherical dome. Optimally, the hemispherical lens material should be index matched to the semiconductor refractive index. Failing that, the output coupling efficiency will be substantially diminished: -(nT/4nz), where nl is the refractive index of the lens. (For simplicity, nl is assumed +z,. ) This formula is actually a general upper limit since it can be derived by statistical mechanical phase-space arguments4 without reference to a specific lens geometry. Therefore it applies to inverse Winston concentrators and other types of optical schemes. For a matching refractive index, the "lens" structure needs to be a thick, transparent, semiconductor layer' which can add to the cost. The present state-of-the-art6 is -30% external efficiency in AlGaAs-based LEDs, employing a thick transparent semiconductor superstrate, and total substrate etching in a particularly low-loss optical design.The key to increasing the escape probability is to give the photons multiple opportunities to find the escape cone. In other words there should be an efficient mechanism that ')Present address: Caltech, Pasadena, CA 91125. redirects photons which were originally emitted out of the escape cone, back into the es...

Epitaxial liftoff is an alternative to lattice-mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin-film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self-bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin-film atomic lattices can be simultaneously imaged, showing only a thin (20–100 Å) amorphous layer in between.

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