Researchers create nanoscale waveguide for next gen photonics

A team, led by Xiang Zhang from the US Department of Energy's Lawrence Berkeley National Laboratory, demonstrated the nanoscale waveguiding of light at visible and near infrared frequencies in a metal insulator semiconductor device featuring low loss and broadband operation. "The novel mode design of our nanoscale waveguide holds great potential for nanoscale photonic applications, such as intrachip optical communication, signal modulation, nanoscale lasers and biomedical sensing," said Zhang. Zhang is a principle investigator with Berkeley Lab's Materials Sciences Division and director of the University of California at Berkeley's Nanoscale Science and Engineering Center (SINAM). He is the corresponding author of a paper published by Nature Communications that describes this work titled 'Experimental Demonstration of Low Loss Optical Waveguiding at Deep Sub wavelength Scales'. The paper describes the use of the hybrid plasmon polariton, a quasiparticle the researchers conceptualised and created, in a nanoscale waveguide system. The system is said to be capable of shepherding light waves along a metal dielectric nanostructure interface over sufficient distances for the routing of optical communication signals in photonic devices. The key is the insertion of a thin low dielectric layer between the metal and a semiconductor strip. "We reveal mode sizes down to 50 by 60nm²nm using near field scanning optical microscopy at optical wavelengths," said Volker Sorger, a graduate student in Zhang's research group and one of the two lead authors on the paper. "The propagation lengths were 10 times the vacuum wavelength of visible light and 20 times that of near infrared." To meet an increasing demand for higher data bandwidth and lower power consumption, the energy required to create, transmit and detect each bit of information needed to be reduced. "This requires reducing physical photonic component sizes down beyond the diffraction limit of light while still providing integrated functionality," explained Sorger. Typically, the size and performance of photonic devices is constrained by interference that arises between closely spaced light waves. This diffraction limit results in weak photonic electronic interactions that can only be avoided through the use of devices much larger than today's electronic circuits. A breakthrough came with the discovery that it is possible to couple photons with electrons by squeezing light waves through the interface between a metal/dielectric nanostructure whose dimensions are smaller than half the wavelengths of the incident photons in free space. Directing waves of light across the surface of a metal nanostructure generates electronic surface waves called plasmons that roll through the metal's conduction electrons - those loosely attached to molecules and atoms. The resulting interaction between plasmons and photons creates a quasiparticle called a surface plasmon polariton(SPP) that can serve as a carrier of information. Hopes were high for SPPs in nanoscale photonic devices because their wavelengths can be scaled down below the diffraction limit, but problems arose because any light signal loses strength as it passes through the metal portion of a metal dielectric interface. "Until now, the direct experimental demonstration of low loss propagation of deep subwavelength optical modes was not realised due to the huge propagation loss in the optical mode that resulted from the electromagnetic field being pushed into the metal," Zhang said. "With this trade off between optical confinement and metallic losses, the use of plasmonics for integrated photonics, in particular for optical interconnects, has remained uncertain." To solve the problem of optical signal loss, Zhang and his group proposed the hybrid plasmon polariton (hpp) concept. A semiconductor (high dielectric) strip is placed on a metal interface, barely separated by a thin oxide (low dielectric) layer. This new metal oxide semiconductor design results in a redistribution of an incoming light wave's energy. Instead of being concentrated in the metal, where optical losses are high, some of the light wave's energy is squeezed into the low dielectric gap where optical losses are substantially less compared to the plasmonic metal. "With this design, we create an hpp mode, a hybrid of the photonic and plasmonic modes that takes the best from both systems and gives us high confinement with low signal loss," said Ziliang Ye, the other author of the paper, who is also a graduate student in Zhang's research group. "The hpp mode is not only advantageous for down scaling physical device sizes, but also for delivering novel physical effects at the device level that pave the way for nanolasers, as well as for quantum photonics and single photon all optical switches." The hpp waveguide system is compatible with current semiconductor/cmos processing techniques, as well as with the Silicon on Insulator platform used today for photonic integration. This should make it easier to incorporate the technology into low cost, large scale integration and manufacturing schemes. Sorger believes that prototypes based on this technology could be ready within the next two years and the first products could be on the market within five years. "We are already working on demonstrating an all optical transistor and electro optical modulator based on the hpp waveguide system," Sorger noted. "We're also now looking into bio medical applications, such as using the hpp waveguide to make a molecular sensor."