In this work, potential ways of accessing a greater spectral range at mid-infrared (MIR) wavelength ranges in silicon photonics are explored, in particular for sensing applications that use on-chip spectroscopy. To utilise the full low-loss transmission of silicon, silicon membranes are transfer printed onto MIR-transparent substrates for waveguides with no absorption from the substrate. A Y-junction splitter with low loss across a
1 µm optical bandwidth is also designed and experimentally demonstrated.
A CMOS compatible three-dimensional (3D) integrated photonics circuit for multilayer silicon photonics is reported. Slopes with angles between 10o and 15° were created in the oxide layer using single step wet etching to connect the two Si waveguide layers. Amorphous Si (a-Si) deposited using hot wire chemical vapor deposition (HWCVD) at a temperature of 230°C was used to fabricate the device. Losses of 0.5 dB/slope were measured in the slope waveguides at 1310 nm wavelength. As a demonstration, we propose a 4x4 network switch using a-Si based vertical directional coupler.
Germanium has become a material of high interest for mid-infrared (MIR) integrated photonics due to its complementary metal-oxide-semiconductor (CMOS) compatibility and its wide transparency window covering the 2-15 μm spectral region exceeding the 4 μm and 8 μm limit of the Silicon-on-Insulator (SOI) platform and Si material respectively. Here, we present suspended germanium waveguides operating at wavelengths of 3.8 μm and 7.67 μm with propagation losses of 2.9 ± 0.2 dB/cm and 2.6 ± 0.3 dB/cm respectively.
In recent years, we have presented results on the development of a variety of silicon photonic devices such as erasable gratings and directional couplers, tunable resonators and Mach-Zehnder interferometers, and programmable photonic circuits using germanium ion implantation and localised laser annealing. In this paper we have carried out experiments to analyse a series of devices that can be fabricated using the same technology, particularly silicon-on-insulator racetrack resonators which are very sensitive to fabrication imperfections. Simulation and experimental results revealed the ability to permanently optimise the coupling efficiency of these structures by selective localised laser annealing.
We reviewed our recent developments on the post-fabrication trimming techniques and programmable photonic circuits based on germanium ion implanted silicon waveguides. Annealing of ion implanted silicon can efficiently change the refractive index. This technology has been employed to fine-tune the optical phase, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. High accuracy phase trimming was achieved with laser annealing and a real-time feedback control system. Erasable waveguides and directional couplers were also demonstrated, which can be used to implement programmable photonic circuits with low power consumption.
The growing demand for fast, reliable and low power interconnect systems requires the development of efficient and scalable CMOS compatible photonic devices, in particular optical modulators. In this paper, we demonstrate an innovative electro absorption modulator (EAM) developed on an 800 nm SOI platform; the device is integrated in a rib waveguide with dimensions of a 1.5 μm x 40 μm, etched on a selectively grown GeSi cavity. High speed measurements at 1566 nm show an eye diagram with dynamic ER of 5.2 dB at 56 Gbps with a power consumption of 44 fJ/bit.
In this paper we present silicon and germanium-based material platforms for the mid-infrared wavelength region and we report several active and passive devices realised in these materials. We particularly focus on devices and circuits for wavelengths longer than 7 micrometers.
We review our recent developments of the trimming techniques for correcting the operating point of ring resonator and Mach-Zehnder Interferometers (MZIs). This technology has been employed to fine-tune the effective index of waveguides, and therefore the operating point of photonic devices, enabling permanent correction of optical phase error induced by fabrication variations. Large resonance wavelength shift of ring resonators was demonstrated, and the shift can be tuned via changing the laser power used for annealing. A higher accuracy trimming technique with a scanning laser was also demonstrated to fine-tune the operating point of integrated MZIs. The effective index change of the optical mode is up to 0.19 in our measurements, which is approximately an order of magnitude improvement compared to previous work, whilst retaining similar excess optical loss.
Group IV platforms can operate at longer wavelengths due to their low material losses. By combining graphene and Si and Ge platforms, photodetection can be achieved by using graphene’s optical properties and coplanar integration methods. Here, we presented a waveguide coupled graphene photodetector operating at a wavelength of 3.8 μm.
In recent years, we have presented results on the development of erasable gratings in silicon to facilitate wafer scale testing of photonics circuits via ion implantation of germanium. Similar technology can be employed to develop a range of optical devices that are reported in this paper. Ion implantation into silicon causes radiation damage resulting in a refractive index increase, and can therefore form the basis of multiple optical devices. We demonstrate the principle of a series of devices for wafers scale testing and have also implemented the ion implantation based refractive index change in integrated photonics devices for device trimming.
A crucial component of any large scale manufacturing line is the development of autonomous testing at the wafer scale. This work offers a solution through the fabrication of grating couplers in the silicon-on-insulator platform via ion implantation. The grating is subsequently erased after testing using laser annealing without affecting the optical performance of the photonic circuit. Experimental results show the possibility for the realisation of low loss, compact solutions which may revolutionise photonic wafer-scale testing. The process is CMOS compatible and can be implemented in other platforms to realise more complex systems such as multilayer photonics or programmable optical circuits.
Several 3D multilayer silicon photonics platforms have been proposed to provide densely integrated structures for complex integrated circuits. Amongst these platforms, great interest has been given to the inclusion of silicon nitride layers to achieve low propagation losses due to their capacity of providing tight optical confinement with low scattering losses in a wide spectral range. However, none of the proposed platforms have demonstrated the integration of active devices. The problem is that typically low loss silicon nitride layers have been fabricated with LPCVD which involves high processing temperatures (<1000 ºC) that affect metallisation and doping processes that are sensitive to temperatures above 400ºC. As a result, we have investigated ammonia-free PECVD and HWCVD processes to obtain high quality silicon nitride films with reduced hydrogen content at low temperatures. Several deposition recipes were defined through a design of experiments methodology in which different combinations of deposition parameters were tested to optimise the quality and the losses of the deposited layers. The physical, chemical and optical properties of the deposited materials were characterised using different techniques including ellipsometry, SEM, FTIR, AFM and the waveguide loss cut-back method. Silicon nitride layers with hydrogen content between 10-20%, losses below 10dB/cm and high material quality were obtained with the ammonia-free recipe. Similarly, it was demonstrated that HWCVD has the potential to fabricate waveguides with low losses due to its capacity of yielding hydrogen contents <10% and roughness <1.5nm.
This paper discusses some of the remaining challenges for silicon photonics, and how we at Southampton University have approached some of them. Despite phenomenal advances in the field of Silicon Photonics, there are a number of areas that still require development. For short to medium reach applications, there is a need to improve the power consumption of photonic circuits such that inter-chip, and perhaps intra-chip applications are viable. This means that yet smaller devices are required as well as thermally stable devices, and multiple wavelength channels. In turn this demands smaller, more efficient modulators, athermal circuits, and improved wavelength division multiplexers. The debate continues as to whether on-chip lasers are necessary for all applications, but an efficient low cost laser would benefit many applications. Multi-layer photonics offers the possibility of increasing the complexity and effectiveness of a given area of chip real estate, but it is a demanding challenge. Low cost packaging (in particular, passive alignment of fibre to waveguide), and effective wafer scale testing strategies, are also essential for mass market applications. Whilst solutions to these challenges would enhance most applications, a derivative technology is emerging, that of Mid Infra-Red (MIR) silicon photonics. This field will build on existing developments, but will require key enhancements to facilitate functionality at longer wavelengths. In common with mainstream silicon photonics, significant developments have been made, but there is still much left to do. Here we summarise some of our recent work towards wafer scale testing, passive alignment, multiplexing, and MIR silicon photonics technology.
We present three main material platforms: SOI, suspended Si and Ge on Si. We report low loss SOI waveguides (rib, strip, slot) with losses of ~1dB/cm. We also show efficient modulators and detectors realized in SOI, as well as filters and multiplexers. To extend transparency of SOI waveguides, bottom oxide cladding can be removed. We have fabricated low loss passive devices in a suspended platform that employ subwavelength gratings. Ge on Si material can have larger transparency range than suspended Si. We have designed passive devices in this platform, demonstrated all optical modulation and carried out two photon absorption measurements. We have also investigated theoretically free carrier optical modulation in Ge.
In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.
We have demonstrated a bidirectional wavelength division (de)multiplexer (WDM) on the silicon-on-insulator platform. An excellent match of the peak transmission wavelength of each channel between the two AMMIs was achieved. This type of device is ideal for integrated optical transceivers where the transmission wavelengths are required to match with the receiving wavelengths. The device also benefits from simple fabrication (as only a single lithography and etching step is required), improved convenience for the transceiver layout design, a reduction in tuning power and circuitry, and efficient use of layout space.
Since their inception, metamaterial fishnet structures have frequently been used to exhibit a negative refractive index. Their shape and structure make it possible to independently produce both a negative permeability (μ) and a negative permittivity (ε). Fishnets that display this characteristic can be referred to as a double negative metamaterial. Although other techniques have been demonstrated, fishnets are commonly fabricated using electron-beam lithography (EBL) or focused ion-beam (FIB) milling. In this paper we demonstrate the fabrication of fishnets using nano-imprint lithography (NIL). Advantages associated with NIL include a shorter fabrication time, a larger feasible pattern area and reduced costs. In addition to these advantages, the quality of the fabricated structures is excellent. We imprint a stamp directly into a metal-dielectric-metal stack which creates the fishnet and, as an artifact of the technique, a periodic array of nanopillars. Two different designs of the fishnet and nanopillar structure have been fabricated and optical measurements have been taken from both. In addition to the experimental measurements the structures have also been extensively simulated, suggesting a negative refractive index with a real part as large in magnitude as five can be achieved.
In this paper we will discuss recent results in our work on Silicon Photonics. This will include active and passive devices for a range of applications. Specifically we will include work on modulators and drivers, deposited waveguides, multiplexers, device integration and Mid IR silicon photonics. These devices and technologies are important both for established applications such as integrated transceivers for short reach interconnect, as well as emerging applications such as disposable sensors and mass market photonics.
We report on the fabrication and characterisation of fishnet structures of various dimensions on a polymer layer. The fabrication process causes metal-dielectric-metal rectangular pillars to be compressed to the bottom of fishnet structures. The metamaterial structures are fabricated using nanoimprint lithography, allowing large areas to be patterned quickly and good reproducibility through multiple use of the nanoimprint stamp. A tri-layer comprising of silver (Ag) and magnesium fluoride (MgF2) was deposited on a thick polymer layer, in this instance PMMA, before being directly imprinted by a stamp. When the metal-dielectric layered pillars are imprinted to a sufficient depth in the PMMA below the fishnet, distinct resonance peaks can be measured at both visible and near-infrared frequencies. The precise wavelength of the resonant peak at near-infrared and its Q-factor can be changed by altering the physical dimensions and number of metal and dielectric layers of the fishnet respectively. The response viewed at visible frequencies is due to the pillars that sit in the PMMA, below the fishnet. Silver and magnesium fluoride layers that comprise the suppressed pillars are crushed during the imprinting process but still allow for light to be transmitted. Despite imprinting directly into multiple metal and dielectric layers, high quality structures are observed with a minimum feature size as low as 200 nm. Resonance peaks are measured experimentally in reflectance using an FTIR spectrometer with a calcium fluoride (CaF2) beam-splitter and a visible wavelength range spectrometer with a silicon (Si) detector.
We present the most recent results of EU funded project P3SENS
(FP7-ICT-2009.3.8) aimed at the development of a
low-cost and medium sensitivity polymer based photonic biosensor for point of care applications in proteomics. The
fabrication of the polymer photonic chip (biosensor) using thermal nanoimprint lithography (NIL) is described. This
technique offers the potential for very large production at reduced cost. However several technical challenges arise due
to the properties of the used materials. We believe that, once the NIL technique has been optimised to the specific
materials, it could be even transferred to a kind of roll-to-roll production for manufacturing a very large number of
photonic devices at reduced cost.
We report on the fabrication of 70 nm wide, high resolution rectangular U-shaped split ring resonators (SRRs) using
nanoimprint lithography (NIL). The fabrication method for the nanoimprint stamp does not require dry etching. The
stamp is used to pattern SRRs in a thin PMMA layer followed by metal deposition and lift-off. Nanoimprinting in this
way allows high resolution patterns with a minimum feature size of 20 nm. This fabrication technique yields a much
higher throughput than conventional e-beam lithography and each stamp can be used numerous times to imprint patterns.
Reflectance measurements of fabricated aluminium SRRs on silicon substrates show a so-called an LC resonance peak in
the visible spectrum under transverse electric polarisation. Fabricating the SRRs by NIL rather than electron beam
lithography allows them to be scaled to smaller dimensions without any significant loss in resolution, partly because
pattern expansion caused by backscattered electrons and the proximity effect are not present with NIL. This in turn helps
to shift the magnetic response to short wavelengths while still retaining a distinct LC peak.
Planar devices that can be categorised as having a nanophotonic dimension constitute an increasingly important area of
photonics research. Device structures that come under the headings of photonic crystals, photonic wires and
metamaterials are all of interest - and devices based on combinations of these conceptual approaches may also play an
important role. Planar micro-/nano-photonic devices seem likely to be exploited across a wide spectrum of applications
in optoelectronics and photonics. This spectrum includes the domains of display devices, biomedical sensing and sensing
more generally, advanced fibre-optical communications systems - and even communications down to the local area
network (LAN) level. This article will review both device concepts and the applications possibilities of the various
In this paper we discuss theoretical modelling methods for the design of photonic crystal and photonic quasi-crystal
(PQC) LEDs - and apply them to the analysis of the extraction enhancement performance and shaping of the emitted
beam profile of PQC-LED structures. In particular we investigate the effect of the pitch of the PQC patterning, and
consider the physical mechanisms giving rise to performance improvements. In addition, we examine the relative
contributions to performance improvements from effective index reduction effects that alter the conditions for total
internal reflection at the device air interface, and from photonic crystal scattering effects that give rise to radically
improved extraction performance. Comparisons are made with the performance of recently fabricated devices.
The response of metallic split ring resonators (SRRs) scales linearly with their dimensions. At higher frequencies, metals
do not behave like perfect conductors but display properties characterized by the Drude model. In this paper we compare
the responses of nano-sized gold-based SRRs at near infra-red wavelengths. Deposition of gold SRRs onto dielectric
substrates typically involves the use of an additional adhesion layer. We have employed the commonly used metal
titanium (Ti) to provide an adhesive layer for sticking gold SRRs to silicon substrates - and have investigated the effect
of this adhesion layer on the overall response of these gold SRRs. Both experimental and theoretical results show that
even a two nm thick titanium adhesion layer can shift the overall SRR response by 20 nm.
Metamaterials based on single-layer metallic Split Ring Resonators (SRR) and Wires have been
demonstrated to have a resonant response in the near infra-red wavelength range. The use of
semiconductor substrates gives the potential for control of the resonant properties of split-ring
resonator (SRR) structures by means of active changes in the carrier concentration obtained using
either electrical injection or photo-excitation. We examine the influence of extended wires that are
either parallel or perpendicular to the gap of the SRRs and report on an equivalent circuit model that
provides an accurate method of determining the polarisation dependent resonant response for
incident light perpendicular to the surface. Good agreement is obtained for the substantial shift
observed in the position of the resonances when the planar metalisation is changed from gold to
We report a novel method for modeling the resonant frequency response of infra-red light, in the range of 2 to 10
microns, reflected from metallic spilt ring resonators (SRRs) fabricated on a silicon substrate. The calculated positions of
the TM and TE peaks are determined from the plasma frequency associated with the filling fraction of the metal array
and the equivalent LC circuit defined by the SRR elements. The capacitance of the equivalent circuit is calculated using
conformal mapping techniques to determine the co-planar capacitance associated with both the individual and the
neighbouring elements. The inductance of the equivalent circuit is based on the self-inductance of the individual
elements and the mutual inductance of the neighboring elements.
The results obtained from the method are in good agreement with experimental results and simulation results obtained
from a commercial FDTD simulation software package. The method allows the frequency response of a SRR to be
readily calculated without complex computational methods and enables new designs to be optimised for a particular
frequency response by tuning the LC circuit.
Gold Split Ring Resonators (SRRs) were fabricated on silicon substrates by electron beam lithography and lift-off, with overall dimensions of approximately 200 nm. Reflectance spectra from the SRRs are similar to those published elsewhere. New devices are proposed based on the additional functionality afforded by the use of a silicon substrate.
We describe a simple technique for the selective area modification of the bandgap in planar 3-D photonic crystals (PhC). The PhCs are grown by controlled drying of monosized polystyrene spheres. Uniaxial pressure of 41 MPa can produce a shift in the bandgap of ~90 nm from 230 nm spheres. An unexpected broadening of the bandgap is attributed to the change in topology associated with large necks formed between spheres at pressures greater than 10 MPa.