Richard Soref: The new frontiers of silicon photonics
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.
Q&A with Richard Soref - Final Part
In the final part of the interview with Gazettabyte, Richard Soref talks about hybrid and monolithic integration, mid-infrared optics, how his photonics predictions made in a 2006 paper have fared, 2-micron-based optical communications, and his talk at OFC in March.
"In a rosy future, every smart phone, tablet, wrist watch, and hand-held device would contain one of these chemical-medical-physical sensors."
Richard Soref
Q: The distinction between monolithic and hybrid integration? Arguably all silicon photonics is hybrid integration.
RS: Silicon photonics is mostly monolithic and that is its advantage. Hybrid integration occurs when structures from two different material systems are combined, usually by bonding. When describing the construction of silicon photonics, if we include the germanium laser on silicon, then it is possible to make every component in a chip network from Group IV materials, and this qualifies as monolithic integration.
How important is optical performance using either hybrid or a monolithic design? Is this an academic question or are there differences and hence issues to consider when making a device monolithically or hybridly?
RS: There are real-world issues to consider when evaluating the relative performance of hybrid and monolithic. I don’t know whether hybrid integration is CMOS-foundry-compatible but I suspect that it is. I feel that hybrid integration of III-V laser diodes on silicon or germanium is quite practical, both for telecom and mid infrared. The heterogeneous approach is an excellent, viable solution until germanium-tin (GeSn) laser diodes come along.
Considerable investment is required to make monolithic real. Once actual, it should be more cost-effective than hybrid. The definition of monolithic has vague areas. Is epitaxial growth of III-V on silicon monolithic? Perhaps. If you bond a Group IV nano-membrane onto silicon or germanium, is that monolithic? Probably. We need clearer terminology.
The comparison you’re asking for probably comes down to a performance comparison of III-V and Group IV laser diodes and photo-detectors.
Regarding the economics of silicon photonics, one important issue is the cost of silicon photonics compared to copper. This looks like a major factor limiting silicon photonics adoption. Do you have a view on cost?
RS: These economics are outside my areas of expertise, but I’ll take a stab at it.
Active optical cables are one of several possible killer applications of silicon photonics I don’t know the actual numbers but am guessing that the photonic cost is already below the copper coax cost and is capable of further reduction. Data centres are validating silicon photonics adoption.
A general proposition is that ubiquitous photonics could improve our lives. Mass production at very low cost is on the horizon for silicon photonic integrated circuits (PICs). Once the high-volume, high-impact applications are known, the factory can target them. The global silicon infrastructure could push silicon PICs into the lead. Job creation, industrial competitiveness and other benefits to society will flow from high-yield PICs and opto-electronic integrated circuits (OEICs) made on large wafers. Niche applications and boutique chips may offer important functionality but at higher costs, and for those the silicon may not compete well with III-Vs.
In your paper, The Past, Present and Future of Silicon Photonics, published in 2006, you say that no one can foresee the future and that all one can do is make educated guesses. You then go on and make the following predictions shown in the table below.
What comments do you have, given the benefit of hindsight?
RS: Well, the scorecard is mixed on these prognostications. A few came true, some are on the verge, others are stuck such as the Erbium-silicon and lead-sulphide-silicon lasers. The mega-transistor OEIC has not yet arrived. The germanium-on-silicon heterolaser diode came true. The high threshold of this laser can be reduced by adding tensile strain and germanium quantum wells, but even after that has been done I feel that this laser’s efficiency will not match that of its III-V competitors.
Efficiency prospects are better for germanium-tin (GeSn). The optically-pumped GeSn laser was just demonstrated in Germany and now we are close to the monolithic on-chip germanium-tin laser diode, although it will emit at 2 microns instead of 1.55 microns.
When I read the press releases of Intel, Cisco, IBM, Mellanox, Luxtera, STMicroelectronics, PhotonIC, IME and imec, I get the impression that major progress has been made in commercial 100 Gigabit-per-second transceiver chips and active optical cables, all of these using hybrid light sources and the germanium photodiodes that I predicted.
Looking at the wide infrared spectrum, the predicted results from 5 to 100 microns have not materialised. However, results are excellent in the 1.6 to 5 micron range. The wavelength-scale components are still on the wanted list, although such modulators and detectors have appeared with a plasmonic flavour.
The GeSn quantum wells arrived in Ghent, Belgium in 2012 and were explored further in Stuttgart, Germany. I believe that a fully fledged silicon-germanium-tin (SiGeSn) technology is unfolding before our eyes today.
The competence of Group IV photonics in so many different scientific areas is its strength and survival mechanism.
In your Photonics West talk in 2013, you talk about silicon opto-electronic mid-infrared system-on-chip. You highlight several interesting application areas including chemical-bio-physical sensing, medical diagnostics, environmental monitoring and high-speed comms at 2 micron. Is sensing a distinct category, and if so, how does it differ from medical diagnostics and environmental monitoring?
RS: Medical diagnostics, industrial process control, and environmental monitoring are all within the same sensing category as chem-bio-physical sensing. That’s why on-chip sensing via photonic techniques is a very broad category. Over time, commercial sales of these photonic sensors - including disposables - could eclipse those of optical interconnects.
How should these mid-infrared applications be seen alongside SiP for telecom and datacoms? Are the two distinct or will developments in mid-infrared applications provide volumes needed that will benefit datacom and telecom silicon photonics at 1.55 microns and below?
RS: Ultra-fast data transmission in the 2 micron band for long-haul and short-haul links arises from the new generation of hollow-core photonic bandgap fibres currently with a loss of 2dB per kilometer. The idea here is that the new 2 micron room-temperature fibre communications will be a practical supplement to current 1.55 micron equipment, and that the various 2 micron connections are not intended to replace existing 1.55 micron infrastructure. In other words, 2 microns expands the global network capacity and does so with low energy consumption. The chip volumes needed at 2 microns will benefit 1.55 microns and vice versa.
What are the main challenges as you see it for these applications? And are these applications a decade behind datacom and telecom because there are no Intels, Ciscos or silicon photonics start-ups driving them?
RS: The main challenge I see for 2 microns is developing the high performance foundry-based room-temperature opto-electronic transceiver chips. I visualise cost-effective germanium-tin (GeSn) photo-detectors and soon GeSn laser diodes integrated in silicon-on-insulator (SOI) waveguide technology with SOI free-carrier modulators. This is a natural sweet-spot for monolithic Group IV photonics.
And no, these applications are not a decade behind because an excellent start has already been made on sensors and 2 microns by researchers in the US, Europe and Asia, even without those start-ups.
The challenge I see in sensors is for practitioners to thread their way through the thicket of reported results on chip-scale refractometers, spectrometers, microfluid channels, label-free detection, resonators, trace-gas concentrators, and plasmonic reflectors, so as to converge upon sensitive, practical solutions.
Parts-per-billion photonic sensors can compete with electronic sensors. Financial investment will actualise the sensor vision and the 2-micron dream. A mixture of technical demonstrators, faith, and venture capital should suffice. Which comes first? Is this a chicken-and-egg question?
At the upcoming OFC 2015 in March, you are to give a talk. What will be the theme?
RS: My mid-infrared talk has a fibre theme and a sensor theme—both about Group IV foundry chips made at low cost. I just described how the fast, energy-efficient 2 micron fibre-optic transceiver chip could be developed with GeSn and SOI. Also, I feel that photonic sensing has tremendous untapped potential including disposables and tiny sensors enabled by battery-powered on-chip nano-lasers.
In a rosy future, every smart phone, tablet, wrist watch, and hand-held device would contain one of these chemical-medical-physical sensors. There are terrific network possibilities. A group of such portable, co-operating sensors, widely dispersed in geography, could be linked by the global cell-phone network or the internet cloud because these photonics will become mainstays of the internet-of-things.
You said as an aside in your 2013 talk that "germanium is the new silicon". What did you mean by that and should we all be looking at 'Germanium Photonics'?
RS: We should be. Germanium has a short wave and a long wave role to play. Germanium shines at 1.6 microns with the laser, the famous photo-detector, and the modulators employing free-carrier, Franz-Keldysh and quantum-confined Stark effects. Germanium is a fine buffer layer on silicon, functioning as an epitaxy platform for SiGeSn. With its high transparency over 1.8 to 15 microns, germanium is superior to silicon over the 8 to 15 micron range where silicon has 2 to 10 dB/cm of loss.
Successful waveguiding experiments in germanium over 2 to 6 microns have confirmed its infrared value for filters, resonators, interconnects and photonic crystals, while waveguide demonstrations over 6 to 14 micron are expected. Germanium is a stellar nonlinear optical material because its third-order susceptibility is much larger than that of silicon. Hence germanium offers superior four-wave mixing, Raman lasing and Brillouin gain, assuming pump wavelengths longer than 3.4 microns. Germanium serves in heterostructures and quantum structures as a barrier to GeSn. Quantum dots are available. Doped germanium can be a plasmonic conductor. Overall, self-contained photonics.
Lastly, what opportunities excite you most and if you were giving advice to young engineers and researchers looking at silicon photonics/ Group IV photonics, what would you recommend they focus on over the next 10 years?
RS: I think that young scientists and engineers entering this field should aim at ambitious, modern goals. They could investigate significant trends-in-motion or exotic emerging directions.
Regarding trends, the young engineers have opportunities for impact in photonic sensors, terabit-per-second optical interconnect devices, analogue-and-digital mixed signal chips, microwave photonics of several kinds, space-division and mode-division multiplexing, night-vision imaging, beam steering, and control of microwave antennas. Their talents could create new PICs and OEICs for practical outcomes.
For the more speculative category, the young scientists could focus on pathfinding forms of optical computing, as opposed to types discredited in the past. They could make advances in quantum integrated photonics, to use Professor Ben Eggleton’s term, getting results in quantum computing, secure communications and metrology.
In a similar vein, some good targets are the Ising machine and the neural-network computer with its training sets. Linear-and-nonlinear optics at Terahertz and long wave infrared could be explored.
I’m excited by the topic of nano-laser diodes; an all-Group IV photonics version rather than the metal-coated kind. I visualise a two-dimensional germanium photonic crystal grown directly on an oxidised SOI waveguide that possess a surface grating. A wavelength-scale GeSn gain region is embedded in the line-defect cavity zone where lateral PIN injection takes place. Then, this surface emitting nano laser is vertically coupled to the SOI beneath it.
For the first part of the interview, click here
Between 1998 and 2003, Professor Soref wrote 36 short poems that were published in internet magazines, some are which are still online. He is also a distinguished photographer, click here to see some of his work. "I gravitate to art forms of which science is one," he says.