Designer molecules promise all-optical processing

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

Supporters of all-optical switching say it will allow dramatic speed increases in data communications by eliminating the need to convert photonic signals to electronic signals – and back. All-optical processing – switching light using light – would also facilitate photonic computers, which promise much higher data throughput and speed than electronic processing.

In another step toward all-optical switching, a team at Georgia Institute of Technology has synthesized a material optimized for photonic switching applications. The organic dye material features a combination of large nonlinear properties on the one hand, and low nonlinear and linear losses on the other. Optical media with these features offer the potential to develop all-optical switching devices that require only low switching power but offer high contrast – boosting efforts to realize all-optical computing and communication.

However, big challenges remain for all-optical processing. Switching means manipulating one signal with another. Electrons used for today’s electronic processors come with an electrical charge and therefore intrinsically interact with each other, but optical signals – or photons – do not. This lack of interaction has the advantage of supporting parallel transmission of huge amounts of information, but when one light beam has to change another from “on” to “off,” such independence is not a plus – at least not by default.

Nevertheless, independent propagation of light waves works only for linear systems, so one way to circumvent the lack of interaction needed for switching is making the medium nonlinear where needed. This way, a nonlinear phase shift (or refractive index change) can be introduced by one beam on another, also known as cross-phase modulation. It has been demonstrated that this works in various switching configurations, such as interferometers (where the phase of one arm is delayed), or microdisk or microring resonators.

Introducing phase shifts in isotropic (noncrystalline) materials requires the so-called third-order nonlinear susceptibility to be large. Finding materials of this type is not a problem in itself, but it usually comes at the cost of high linear and nonlinear losses, which limit the performance of the switch.

This is the problem that the researchers set out to confront. “There are various problems to be solved [finding suitable materials for all-optical switching], but we have decided to tackle the problem by going down the properties of the organic molecules,” said Joseph Perry, a professor in the school of chemistry and biochemistry. “We used a mixture of intuitive chemical models and high-performance simulations to crack the problem.”

Georgia Tech professor Joseph Perry, left, is part of the team that developed a new photonic material that could facilitate all-optical signal processing. Courtesy of Georgia Institute of Technology.

The starting points were carbon-based polymethine dye molecules, well known for long extension in one direction – and, thus, “being one of the most polarizable molecules we know of.” Large polarizability is good, as it is known to give large third-order nonlinearities (susceptibility). On the other hand, large molecular dimensions in one direction mean low-energy one- and two-photon absorption bands, which are not favorable from a linear and nonlinear loss perspective. So the goal for the team at Georgia Tech’s Center for Organic Photonics and Electronics, led by Perry and professor Seth Marder, was to increase polarizability – but without making the molecules too long.

The clue came from two-photon absorption spectra, which exhibit two peaks with a low absorption gap between them – i.e., a window with low loss that can be tuned to match the telecommunications wavelength window, said Perry. To do so, the polymethine molecules were “fitted” with selenium atoms at their ends, which are known to be polarizable themselves. This helped increase the nonlinearity – without increasing the length. As a result, the photon energy of the switched light is smaller than the lowest-lying single-photon transition, while twice this photon energy falls between transitions that would excite detrimental two-photon absorption.

Based on this strategy, published in the March 19, 2010, issue of Science, the next challenge will be making these molecules suitable for denser packing – as to date they have been studied only in solution. Although spin-coating techniques have been demonstrated, where materials of this type have been applied to other materials to modify their nonlinear properties, the ultimate goal is to incorporate them in a solid phase for use in optical waveguides. Making the molecules suitable for denser operation will be a challenge because their behavior is likely to change if they are densely packed. This will require the molecule designers to make further adjustments, “but evidence from second-order materials shows that this is possible,” Perry said.