Quantum dots can be used to create a nanometric optical switch, Tadashi Kawazoe and a research team from the Japan Science and Technology Corporation (JST) and the Tokyo Institute of Technology (TIT) recently demonstrated. "To realize the nanometric optical switch, we had to have a new switching mechanism, near-field signal waveguide technologies, and process technology by which to produce devices at nanometric levels," says Kawazoe, who concentrated on the optical near field, looking for breakthroughs that would overcome the problems the team faced.
Using near-field optical chemical vapor deposition to build its 10 nm quantum dots, the switch uses three quantum dots: cube 1, with a side length of L; cube 2 with a length of , and cube 3 with a length of 2L. Cube 1 has a single energy sublevel: (1,1,1), cube 2 has two sublevels: (2,1,1) and (1,1,1), and cube 3 has four sublevels: (2,2,2), (2,2,1), (2,1,1), and (1,1,1). Kawazoe says the quantized carrier energy sublevels in the three cubes resonate with each other. Under this resonant condition, a coupling energy of near-field interaction can be given by following Yukawa function: V(r)=A(exp(-µ.r)/r), in which V is magnitude of interaction, A is the separation between two quantum cubes, µ is the effective mass of the Yukawa function,, and r is a coupling coefficient. To put the process simply, almost all the energy of the excitation of cube 1 transfers to the lowest carrier energy level in the neighboring cube 2, and that energy in turn transfers to the lowest energy level in cube 3.
Figure 1. When the switch is off, an incoming signal passes into cube 1 (input), propagates directly to the lowest energy level in cube 2 (output), and continues to the lowest level in cube 3 (control). When the switch is on, the control cube is optically excited to prevent signal transfer from cube 2. The signal thus passes directly out of the switch.
In switching terms, cube 1 acts as an input, cube 2 acts as an output, and cube 3 acts as control. If the switch is off, input energy escapes to control cube 3, which obstructs the output signal from cube 2. If the switch is on, control cube 3 is excited with light, which blocks energy coming from input cube 1, allowing that energy to flow to cube 2 and exit as output (see figure).
Nanometer-scale optical waveguides make the far/near-field conversions necessary to couple nanometric devices with conventional external photonic devices. For optimal operation, the waveguides must offer high conversion efficiency; a guide beam of less than 100 nm for efficient coupling to sub-100 nm dots; and a guide propagation length on the order of the optical wavelength to avoid direct coupling of the propagating far-field light to the dots.
Kawazoe's team accomplished the coupling requirement with a plasmon waveguide scheme that used a silicon dielectric wedge coated with a thin metal film. The waveguide transforms incoming far-field light into 2-D surface plasmon mode. It then further converts the 2-D plasmon mode to a 1-D TM-plasm on mode that propagates along a plateau at the top of the waveguide wedge where the metal coating is thicker and acts like a metal core waveguide. At the waveguide outlet, the TM-plasmon mode is converted to the optical near-field so that it couples with the nanometric quantum dots close by.
"Our waveguide satisfies all three conditions," Kawazoe says. "First, the scattering coupling at the edge means a very high conversion efficiency from 2-D surface plasmon mode to 1-D TM-plasmon mode. Second, the beam width narrows to as little as 1 nm as the core diameter decreases, because this waveguide has no cutoff. Third, the propagation length of 2.5 nm (λ=830 nm) for a TM-plasmon with a gold core insulated by air is sufficient." whither nanometric devices?
"I do believe that nanoscale optical device technologies and near-field optics may play an important role in future optical systems in 10 years or so," says Sadik Esener, professor at the University of California at San Diego.
Max Lagally of the University of Wisconsin agrees. "Certainly I would think that near-field technology at some wavelengths, perhaps not visible, will see major advances."
Kawazoe's group is concentrating on the optical near field as a possible way to overcome the difficulties in creating nanometric devices. Nevertheless, he says, studies must proceed carefully, as new technical inventions and new physical phenomena are constantly surfacing.