A computer simulation of the optical nano antenna that Harvard researchers have fabricated shows two gold-coated nano rods seperated by a 30-nanometer gap. (Credit: Ertugrul Cubukcu)
Burning Terabyte CDs
A new device that tightly focuses
laser light could increase the density of optical data storage
Source: “Plasmonic Laser Antenna”
Ertugrul Cubukcu et al.
Applied Physics Letters 89: 093120
Results: By building a nano antenna directly onto a commercial semiconductor laser, Ken Crozier and Federico Capasso of Harvard University were able to focus light with a wavelength of 830 nanometers to a spot 40 nanometers wide. The experimental work was done by graduate students Ertugrul Cubukcu and Eric Kort.
Why It Matters: Optical discs such as CDs and DVDs are written and read using laser light. A smaller wavelength produces a smaller spot size, which allows more data to be crammed onto a disc. For instance, CDs are written and read using light with a wavelength of 780 nanometers; for DVDs, the wavelength is 650 nanometers, and for Blu-ray discs, it’s 405 nanometers. That’s why Blu-ray discs store so much data–up to 50 gigabytes for dual-layer discs. Traditional optical techniques use mirrors and lenses to further shrink the spot, but at best they can shrink it to half the light’s wavelength. The researchers’ antenna sidesteps the limits of traditional optics to produce ultrasmall spots of light that could increase storage density to about three terabytes (3,000 gigabytes) on a disc the size of a CD. Moreover, the fabrication process they developed makes it easy and inexpensive to integrate the antenna into a commercial laser.
Methods: The antenna is made of two gold-coated nanosize rods separated by a 30-nanometer-wide gap. When light from the laser hits the nano rods, it applies a force to the electrons in the gold, nudging them out of place. The electrons oscillate back and forth, causing electrical charges to build up on both sides of the gap–positive charges on one side and negative charges on the other. The rods and the gap act as a tiny capacitor, which effectively concentrates the energy from the laser light into a spot about the size of the gap.
Next Steps: The researchers are exploring fabrication techniques that can decrease the gap between the rods–and the spot size–to 20 nanometers. They are also exploring alternatives to the gold that coats the rods; silver, say, could focus light more efficiently than gold at the wavelengths used in the consumer electronics industry