Richard Soref: The new frontiers of silicon photonics

Silicon photonics luminaries series

Interview 4: Professor Richard Soref

John Bowers acknowledges him with ‘kicking off’ silicon photonics some 30 years ago, while Andrew Rickman refers to him as the ‘founding father of silicon photonics’. An interview with Richard Soref. 

It was fibre-optic communications that started Professor Richard Soref on the path to silicon photonics.

“In 1985, the only photonic chip that could interface to fibre was the III-V semiconductor chip,” says Soref. He wondered if an elemental chip such as silicon could be used, and whether it might even do a better job. He had read in a textbook that silicon is relatively transparent at the 1.30-micron and 1.55-micron wavelengths used for telecom and it inspired him to look at silicon as a material for optical waveguides.

Soref's interest in silicon was a combination of the potential of using the chip industry’s advanced manufacturing infrastructure for electro-optical integration and his own interest in materials. “I’m a science guy and I have curiosity and fascination with what the world of materials offers,” he says. “If I have an avenue like that, I like to explore where the physics takes us.”

In 1985 Soref constructed and did experiments on waveguides based on un-doped silicon resting upon a doped silicon substrate. It turned out not to be the best choice for a waveguide and in 1986 Soref proposed using a silicon-on-insulator waveguide instead, what has become the mainstream approach for the silicon photonics industry.

Silicon-on-insulator had a far greater refractive index contrast between the waveguide core and its cladding and is far less lossy. And while Soref didn’t build such structures, “it stimulated others to develop that major, major waveguide, so I’m proud of that”.

The original waveguide idea was not a wasted one, though. Soref and then research assistant, Brian Bennett, used the undoped-on-doped silicon waveguide structure to study and quantify free-carrier electro-modulation effects. These effects underpin the workings of the bulk of current silicon photonic modulators. Soref says their published academic paper has since been cited over 1,800 times.

Soref is approaching his 80th birthday and is a research professor at the University of Massachusetts in Boston. He has spent over 50 years researching photonics, silicon photonics and the broader topic of mid-infrared wavelengths and Group IV photonics, as well as spending five years researching liquid crystals for displays and electro-optical switching. For 27 years he was employed at the Air Force Research Laboratory. He has also worked at the Sperry Research Center and the MIT Lincoln Laboratory.

Applications go beyond telecom and optical interconnect, and perhaps the most important application is sensing

Group IV photonics

Soref’s research interests are broad as part of his fundamental interest in material science. In more recent years he has focused on Group IV photonics but not exclusively so.

The term silicon-photonics is firmly entrenched in the global community, he says, a phrase that includes on-chip germanium photo-detectors and even, with heterogeneous integration, III-V materials. Group IV photonics is a superset of silicon photonics and includes silicon-germanium-tin materials (SiGeSn) and well as silicon carbide. Such materials will likely be used in the monolithic silicon chip of the future, he says.

He has published papers on alloys such as silicon germanium carbon and silicon germanium tin. “I was estimating what these never-before-seen materials would do; you could create new alloys and how would those alloys behave,” says Soref.

Silicon germanium tin offers the possibility of a direct bandgap light emitter. “It is a richer material science space, with independent control of the bandgap and the lattice parameter,” says Soref.

Adding tin to the alloy lengthens the wavelength of operation, typically in the 1.5-micron to 5-micron range, the near infra-red and part of the mid infra-red part of the spectrum. “Applications go beyond telecom and optical interconnect, and perhaps the most important application is sensing,” says Soref.

The applications in this wavelength range include system-on-a-chip, lab-on-a-chip, sensor-on-a-chip and sensor-fusion-on-a-chip for such applications as chemical, biological, medical and environmental sensing. Such sensor chips could be in your smartphone and play an important role in the emerging Internet of Things (IoT). “Sensing could be a very important economic foundation for Group IV photonics,” says Soref.

And Soref does not stop there. He is writing a paper on Group III nitrides for ultra violet and visible-light integrated photonics: “I think silicon and Group IV are limited to the near-, mid- and longwave infra red”.

Challenges

Soref points to the work being done in developing commercial high-volume manufacturing: the use of 300mm silicon wafers, developing process libraries and perfecting devices for volume manufacturing. He welcomes AIM Photonics, the US public-private venture investing $610 million in photonics and manufacturing.

But he argues that there should also be an intellectual space for growth, “a wider space which is not so practical but which will become practical”. He cites the emerging areas of sensing and microwave photonics. “That is the frontier,” says Soref. “And the foundry work should not prevent that intellectual exploration.”

An important application area for microwave photonics is wireless, from 5GHz to 90GHz. Soref envisages a photonic integrated circuit (PIC), or an opto-electronic IC (OEIC) that features electronics and optics on-chip, that communicates with other entities often via fibre but also wirelessly.

“That means RF (radio frequency) or microwave, and for microwave that requires a transmitter and receiver on the chip,” says Soref. Such a device would find use in the IoT and future smartphones.

Microwave designs in the past used an assemblage of discrete components that makes a system on a board. These new microwave PICs or OEICs could perform many of the classical functions such as spectral analysis, optical control of a phased array microwave antenna, microwave signal processing, and optical analogue to digital conversion (ADC) and optical digital to analogue conversion (DAC).

This is analogous to the convergence of computing and photonics, says Soref. In computing, the signal goes from the electrical domain to the optical and back, while for microwave photonics it will be conversions between the microwave and photonic domains on the chip.

There are also quantum-photonic applications: quantum computing, quantum cryptography and quantum metrology where photonic devices could play a role.

Opportunities

These are the three emerging opportunities areas Soref foresees for Group IV photonics emerging in the next decade: sensors, microwave photonics and the quantum and computing worlds in addition to the existing markets of telecom and optical interconnect.

Soref is not sure that silicon photonics has yet reached its tipping point. “To make silicon photonics and Group IV photonics ubiquitous and pervasive, it takes a lot of investment and a lot of commercial results,” he says. “We have not yet arrived at that stage of economic foundation.”

New optical devices

Soref also highlights how continual advances in CMOS feature size, from 45nm down to 7nm, promise new photonic components that could become commonplace.

Soref cites the example of a silicon-on-isolator nanobeam. The nanobeam is a strip waveguide with air holes, in effect a one-dimensional photonic crystal lattice in a waveguide.

The nanobeam structure is of interest as it performs the same role as the micro-ring resonator, a useful optical building block used in such applications as modulation.

“The photonic crystal structure requires extreme control of dimensions to reduce unwanted scattering, so it needs very fine lithography,” says Soref. People have argued such structures are impractical due to the unrealistic dimensional control needed.

“But foundries have shown you can get a very high-quality photonic crystal in a silicon fab,” he explains. “This foundry advantage would enable new components that might have seemed too difficult or marginal on paper.”

Significant progress in silicon photonics may have been achieved since his first work in 1985, but as Soref highlights, it is still early when assessing the full significance of the technology.