Silicon photonics
Introduction
In the last few years, Si has become an important material for integrated photonics with several breakthroughs in the fields of high-speed optical modulators, integrated Ge detectors and even light sources. In combination with nano-photonic waveguide circuits in high-contrast silicon-on-insulator (SOI), many photonic functions can be integrated on a footprint order of magnitude smaller than in conventional photonic materials. This enables ultra-compact photonic integrated circuits for various telecom, datacom, sensing and other applications.
As CMOS electronics are based on the same material, the huge technology base for submicron mass fabrication can be leveraged to bring about a widespread deployment of Si-based integrated photonics.
Since 1999, IMEC has been in the forefront of Si photonics, developing wafer-scale fabrication processes for SOI photonic waveguides. The resulting low-loss waveguides form the basis of a wealth of ultra-compact photonic components, like wavelength-selective elements, biosensors and strain sensors, nonlinear functions or nano-mechanical systems.
In 2006, the Silicon Photonics Platform was created to advance the European position in the field of Si photonics. This is done by giving other parties the opportunity to fabricate Si photonic circuits on a small scale using cost-sharing mechanisms C15517, C15657, RP050.
In 2007, this platform has gained momentum and visibility and even secured independent funding through a European support action. This program is an important step towards a fables + foundry model for integrated photonics (as is now common for electronics) and results in a stronger scientific output (among many partners) for a significantly reduced cost.
In 2007, IMEC has also made significant technical advances in Si photonics. Amorphous Si has been introduced as waveguide material, bringing added flexibility to circuits, and the introduction of amorphous Si as a waveguide material (see section Clean room facilities and technology development for photonics) C15661. New fabrication processes based on 193nm lithography have been developed, with a dramatic improvement in uniformity and reproducibility (see section Clean room facilities and technology development for photonics) C15672. This has made it possible to implement polarization-independent circuits through a polarization-diversity approach P15383. The grating couplers for interfacing with optical fibers have been improved to get a higher coupling efficiency and have also been reduced in footprint P15646, P15834, P15515. Various sensor circuits have been demonstrated, including devices based on plasmonic waveguides on SOI P15664. The development of improved tools (see section Theory and numerical simulation for photonics) for mask layout and circuit design has also speeded up the design-fabrication-measurement cycles, allowing for more fabrication runs per year D15568.
Silicon-to-fiber couplers
It is clear that Si photonics is emerging as a disruptive technology for passive integrated optical functions and for active optical functions like (electronically controlled) light modulation and switching. A hurdle that has to be overcome, when implementing an optical function in SOI, is the interfacing with the outside world (i.e. the optical fiber).
Using a diffractive grating structure on top of the SOI waveguide, a highly efficient interface between an optical fiber and the integrated optical circuit can be obtained with efficiencies as high as 80% P15646, in spite of the large mismatch in size between the SOI waveguide and the core of the optical fiber.
In 2007, an experimental fiber to waveguide coupling efficiency of 55% (with a 3dB bandwidth of 100nm) and fabricated using standard CMOS technology was demonstrated for the first time.
Also, a focusing version of the 2D-polarization diversity grating has been implemented. The holes of the grating are arranged on curved (focusing) lines. The main advantage of this approach is the length reduction of the total coupling structure, since long adiabatic tapers to single mode wires can be omitted. A polarization dependent loss of around 0.5dB has been measured, without efficiency penalty. Another approach is the fabrication of these grating structures with a `direct write' technology, such as focused-ion-beam. 46% coupling efficiency for fabricated gratings with narrow slanted slits P15488 has been demonstrated.
Besides single wavelength band grating couplers, the concept of using a diffractive grating structure as a wavelength duplexer was developed in 2007. Due to its practical relevance, (especially for fiber-to-the-home transceiver applications) a patent application was filed P15651.
Wavelength division multiplexing (WDM) devices in silicon
SOI nano-photonic waveguides have already proven themselves as ideal for ultra-compact wavelength-selective functions. Because of their submicron core dimensions and high index contrast, sharp bends are supported, allowing very compact rings for filters with a large free spectral range.
With the improved fabrication processes based on 193nm lithography (see section Clean room facilities and technology development for photonics), the spectral response of neighboring rings can be controlled to within a nanometer C15672.
Also, de-multiplexers based on arrayed waveguide gratings (AWG) or planar concave gratings (PCG) now have sufficient performance to be used in practical applications. For example, an AWG in a proof-of-concept wavelength duplexer for fiber access networks was used. The polarization dependence of SOI photonic wires was addressed through a polarization diversity approach, where the fiber polarizations were split using a 2D grating coupler and routed through the circuit in opposite directions .
Figure 1: POLDIV, polarization diversity circuit for a wavelength duplex.
For PCGs, the performance has been significantly improved by including Bragg mirrors in the reflecting facets P15437. These periodic gratings can now be fabricated with the processes based on 193nm lithography (see section Clean room facilities and technology development for photonics). The example in figures 2a and 2b shows a 4-channel coarse WDM with a crosstalk better than -30dB and a footprint of only 280 x 150µm2.
Figure 2a: PCG, a top view of PCG.
Figure 2b: Detail of grating facets.
Liquid-crystal-on-silicon
By adding a liquid-crystal (LC) layer on top of SOI chips, extra functionality to these chips can be added. When an electric field is applied over the LC layer, the molecules rotate and the effective refractive index changes. This effect can be used to tune the wavelength of certain optical devices. The tunability of SOI micro-ring resonators with a top layer of LC was investigated. The entirely manual fabrication process of these cells was optimized in order to maximize the yield.
A tuning range of a few nanometers in standard micro-rings was achieved. Currently, investigations are ongoing to find ways to increase this range and steps are being taken to increase the uniformity of the LC layer C16025, C15387.
Optical nano-electromechanical system (NEMS) in silicon
Silicon photonics C15429 also offers a great platform for optical NEMS with a large range of possible applications: photonic-on-chip switching, routing and filtering, free-space spatial light modulators C16016 and small displacement sensing. In the past year, a reliable fabrication process for optical NEMS was demonstrated. This fabrication process made it possible to demonstrate the potential of optical NEMS for experimental small displacement sensing C14228. A NEMS device detecting small displacements with a responsivity of 10µm-1 and detectable vibration amplitudes in the order of nanometers P14469 was demonstrated.
Silicon-based biosensors
Integrated optical devices offer an enormous potential for miniaturized, compact and highly sensitive biosensors. The prospect of fabricating multiple sensors on one chip is an important step towards the realization of the lab on a chip concept. The SOI material platform for label-free detection of bio-molecular interactions is used. Several approaches are being investigated: ring resonators, surface plasmon interferometers, slotted waveguides. All approaches require chemical modification of the waveguide and cavity surfaces, which is developed in collaboration with the Polymer Research Group of Ghent University. Collaboration with the Molecular Biology Group of Ghent University and VIB offers access to the necessary biotechnology facilities.
Current plasmonic based biosensor research is directed towards providing an integrated, low-cost, reusable and sensitive device. A fully integrated waveguide based surface plasmon biosensor is under development. A thin gold layer, chemically modified with detector molecules, is embedded into a Si waveguide. Depending on the refractive index in the vicinity of the detector molecules, the plasmonic modes that are supported by this layer interfere constructively or destructively. Simulation results show this device to be capable of detecting bulk refractive index changes as small as 10-6 refractive index units. Measurements on fabricated samples have shown proof-of-principle for this device P15664, C15388, C15435, C15671.
A SOI micro-ring resonator is a high Q-factor-factor cavity with very small dimensions. Its resonance wavelength shifts when bio-molecular interaction takes place on the surface, which is used for bio-molecular detection. The sensitivity of the SOI micro-ring increases as the quality factors of the cavity increases.
With optimized design, it is possible to fabricate micro-rings with a radius of 5μm and Q factors over 30,000. A wavelength shift of 70nm/RIU (refractive index units) is measured for bulk refractive index changes. Proof-of-principle biosensing experiments of protein interaction using the high-affinity protein couple avidin/biotin showed that the device has a sensitivity of 10ng/ml P15500. Reduction of noise, optimization of receptor density and efficient sample fluid delivery using micro-fluidics helps to further reduce this number. By placing the resonators in an array and using a system for simultaneous read-out, it is possible to develop a micron-size biochip for the detection and negative control of antibody/antigen interaction.
In previously described biosensors, the bio-molecules interact with the evanescent field in the cladding of the waveguide, where the light concentration is rather low. By etching an approximately 100nm slot in the high index core of a photonic wire, light can be concentrated in the slot area. By introducing a biosensitive layer in that slot, it is believed that the sensitivity of Si-based biosensors can be promoted by an increased interaction between light and selectively bound bio-molecules.
Currently, both optical cavity sensors and interferometric sensors based on this principle are being designed.
Nanocrystals on silicon
A cheap, reliable and robust light source integrated on Si is often identified as a major missing link in Si photonics research. A few options already exist, such as the continuous wave Si Raman laser or a heterogeneously integrated III-V microdisk laser on Si, but all have major drawbacks, like complex processing and weak temperature robustness. A different approach is being used by depositing colloidal quantum dots, produced in suspension through wet chemical synthesis. These quantum dots have a very efficient photoluminescence and can be tuned to a specific emission wavelength by controlling their size. The synthesis of these IV-VI nanocrystals (PbSe, PbS) has been perfected at the Physics and Chemistry of Nanostructyres Group of Ghent University, but will be adapted to a core/shell PbSe/CdSe quantum dot synthesis to improve long-term stability in films exposed to air. The films can be deposited through langmuir-blodgett monolayer deposition, drop-casting or spin-coating. Deposition and processing is hence extremely easy. The quantum dots can be deposited very locally by ultra-violet (UV)-contact lithography. Experiments indicate drop-casting to be the preferred deposition method, because of its simplicity and ability to form uniform films of desired thickness. Further improvements need to be made to increase uniformity and decrease scattering at the film interface. First tests show a modest coupling of the photo-luminescent light to the SOI waveguides below the quantum dot film, indicating that luminescence is still present after deposition and luminescent light can be coupled to a photonic waveguide. The luminescence properties of different films will be investigated more thoroughly, in parallel with more coupling and interaction experiments on SOI photonic waveguides.