Shedding light on nanophotonics
01/01/2008
To take full advantage of light’s quantum properties, you need to understand a little quantum physics. This primer covers the basics on how light interacts with quantum dots and how quantum-based lasers work.
By Ben Rogers, Sumita Pennathur, and Jesse Adams
The photon was identified just over 100 years ago, when Albert Einstein published a paper explaining the little “bundles” of light that Max Planck had earlier observed in his famous experiments. These enigmatic and counterintuitive little bundles of energy are the best way we have to date to explain how electromagnetic radiation (including visible light) works. Nanophotonics is helping to rewrite the rules governing what can be done with photons.
The arrangement, energy-wise, of a material’s electrons sets the rules for how that material is allowed to interact with photons. While classical physical laws can explain the photonic interactions of metals, the interaction between semiconductors and photons is quantum mechanical in nature: When a photon with the right amount of energy encounters a semiconductor, it can boost an electron from an occupied state in the valence band to an unoccupied state in the conduction band. This transition requires a specific amount of energy, represented by the band gap. Want to control the type of photons with which the material can interact? Then just adjust the band gap.
When a material is reduced to a small number of atoms (think tiny particles), the energy bands spread out and break into discrete levels; that is, the band gap widens. Semiconductor particles at the size scale where this is possible are known as quantum dots, and the smaller the quantum dot, the larger its corresponding band gap. This correlation means you can adjust the boundaries of the band gap simply by adding or removing material.
When an excited electron drops from the conduction band back down to the valence band, the energy of the photon that is emitted equals the energy the electron loses in its transition; that is, it is equal to the band gap energy. So when you alter a material’s band gap energy by increasing or decreasing its size, you also change the energy of the photon it emits (the wavelength of the electromagnetic radiation). This means that you also change the energy of the photon it absorbs. The relationship between a photon’s energy, E, and its wavelength, λ, can be expressed as λ=hc/E, where c is the speed of light, and h is Planck’s constant. This relationship clarifies the reason for the shift in the optical absorption and emission of quantum dots toward shorter wavelengths (“blue shift”) as the dots get smaller.
By varying the size and composition of quantum dots, then, you can determine the color of the emitted light. For example, cadmium sulfide (CdS) and zinc selenide (ZnSe) dots can be sized to emit blue to near-UV light; cadmium selenide (CdSe) dots can emit light across the entire visible spectrum; and indium phosphide (InP) and indium arsenide (InAs) dots can emit in the far-red and near-infrared.
Quantum dots with tunable absorption/emission qualities are useful for many purposes. They can be used for tagging and tracking biological species; in anti-counterfeiting applications to create special inks; in dyes and paints; in light displays; and in chemical sensing. Studies have shown that CdSe dots can be made to seep into cancerous tumors in the body. Then, when exposed to light, the particles glow—helping surgeons zero in on sick cells and avoid healthy ones. Particles of titanium dioxide (TiO2) and zinc oxide (ZnO)—both metal oxide semiconductors—are used in sunscreens because they absorb ultraviolet radiation that damages the skin. Particles of both of these materials also scatter visible radiation, making them very good white pigments in paint, but this same property also turns skin white—or at least it used to. While conventional sunscreens use particles measuring hundreds to thousands of nanometers in diameter, TiO2 particles less than 50nm in diameter are now available. Because they are smaller, these particles have larger band gaps, so they are transparent to visible light. This makes the sunscreen go on clear, although the particles still absorb the higher energy (shorter wavelength) ultraviolet radiation.
Lasers based on quantum confinement
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Stimulated emission occurs when a photon encounters an already excited electron and causes it to emit yet another photon with the same phase and direction. (The electron then drops back to its ground state.) The original photon is not absorbed and has therefore been amplified: There are two where there was once one.
One condition under which stimulated emission can occur in a material is known as “population inversion.” This is when the excited electrons outnumber the electrons in ground states. Conventional lasers “pump” energy into the electrons using light, electricity, or chemicals, but population inversion can also be achieved using quantum-confined systems. In fact, lasers are one of the most common applications for quantum confinement: Quantum dots, quantum wires, and quantum wells have all been used to make lasers.
Quantum wells are made by constraining electrons inside regions of minimal thickness. This can be done using a sandwich-type structure, with alternating layers of different semiconductors such as gallium nitride (GaN) and indium-doped gallium nitride (InxGax-1N).
Doping involves adding a small amount of an impurity—indium metal, in this case—into a semiconductor crystal to make it a better conductor. Here, the amount of doping is indicated by x. In this way, each layer of doped GaN becomes a quantum well, electrically isolated by the less-conductive GaN layers on either side of it.
In a quantum-well laser, the InxGax-1N layers are typically 3nm to 4nm thick. A power supply electrically connected to the quantum wells supplies electrons.
Any electron that arrives in the conduction band of an isolated InxGax-1N layer is quickly trapped; it lacks the energy to surmount GaN’s larger band gap and reach the conduction band.
Together, these trapped electrons assume quantized energy levels analogous to those found in a single atom. They behave like particles in a well, whose “walls” are so “high” that the particle will never have enough energy to escape. However, particles can tunnel across the GaN barriers to reach similar energy levels in adjacent InxGax-1N wells.
Electrons usually spontaneously emit a photon within about 100 nanoseconds of becoming excited; however, in the case of quantum-well lasers the excited electrons have another option: tunneling. Tunneling happens more quickly than spontaneous emission, so instead of emitting a photon, an excited electron just keeps “vanishing” and “reappearing” in an adjacent well. Before long, there is a huge buildup of excited electrons—the population inversion a laser needs.
Like a fugitive who cannot outrun the law forever, an excited electron cannot keep tunneling indefinitely. Eventually, some of the excited electrons spontaneously emit photons and then drop back to ground states. These photons race through the well. Their electromagnetic fields perturb other excited electrons, stimulating emission of even more photons. This chain reaction produces many photons that reflect back and forth between mirrors on either end of the device, building up and emitting a coherent (single-frequency) photon stream through an aperture in one of the mirrors. This is the laser beam.
Quantum-well lasers operate over a broad spectrum of wavelengths, from 400nm to 1.5μm, and are found in many devices, including CD players and laser printers. Because of their small wavelength, blue-laser varieties enable high-density, optical information storage on DVDs and Blu-ray discs.
Quantum-wire lasers and quantum-dot lasers are made in much the same way as quantum-well lasers, though quantum-dot lasers are often made using thin films of quantum dots instead of the continuous thin films found in quantum wells.
Quantum-wire lasers and quantum-dot lasers can be switched on and off using less current, and in less time, than quantum-well lasers. This offers possibilities for high-speed digital information transfer in telecommunications.
Conclusion
Overall, you can make a very good guess as to how a material will interact with photons if you know how its electrons are organized, energy-wise. The nanoscale interaction of photons and materials—which we have come to call nanophotonics—is a field still in its infancy, and one with plenty of room to grow.
Ben Rogers, principal engineer at Nevada Nanotech Systems Inc., is lead author of this article and the book from which it was adapted: Nanotechnology: Understanding Small Systems, published by CRC Press. Contact him at [email protected].
This article was adapted from the new book, Nanotechnology: Understanding Small Systems, by Ben Rogers, Sumita Pennathur, and Jesse Adams. According to its publisher, CRC Press (www.crcpress.co.uk), the book is a “first-of-its kind, comprehensive treatment of nanotechnology.” The book includes original illustrations, hundreds of exercises, and a solutions manual.