Silicon photonic thermal photodetectors for mid-infrared sensing
Lead Research Organisation:
University of Southampton
Department Name: Optoelectronics Research Centre (ORC)
Abstract
Mid-infrared (mid-IR) absorption spectroscopy is a well-known and versatile analytical technique for uniquely identifying and measuring the concentrations of gases, chemicals, and biological molecules by measuring which wavelengths of mid-IR light an analyte absorbs. Existing mid-IR absorption sensors are however either bulky and expensive (e.g. benchtop spectrometers), or have poor sensitivity and specificity (e.g. LED based sensors). Miniaturising such sensors could be transformative for diverse medical, industrial, and environmental sensing scenarios.
High performance, low cost, and small spectroscopic sensors could be created using mid-IR optical circuits on silicon chips. These chips would ideally combine all of the required optical functions of the sensor (i.e. light source, waveguides for routing light, interaction between the light and the analyte, and light detection), and could be fabricated at low cost in high volumes, thanks to existing silicon manufacturing infrastructure that has been developed for electronics and for near-infrared optical communications.
The last few years have seen rapid development of many of the components that are needed to create these sensor systems: silicon photonic waveguides that can transmit light with low loss at almost any mid-IR wavelength have been developed, while lasers emitting high powers in the mid-IR are now readily available and have been successfully integrated with silicon waveguides.
However, there remains a crippling lack of practical photodetector technologies; those that have already been integrated wilth optical circuits on silicon chips are either expensive to manufacture, are impractical because they have to be cooled to cryogenic temperatures, or do not work at all required wavelengths. This project will develop new waveguide integrated thermal photodetectors, which work by converting the incoming light into a temperature change that can be measured with an electronic circuit. They will be able to operate at room temperature at any mid-IR wavelength, and will be manufactured using low cost techniques. This project will also demonstrate that sensors employing these photodetectors can reach the sensitivities required for clinical and industrial uses, by using them to measure low concentrations of artificial sweeteners in soft drinks - an industrially important example application.
These detectors will potentially transform mid-infrared sensor systems from an academic curiosity into a commercially viable technology.
High performance, low cost, and small spectroscopic sensors could be created using mid-IR optical circuits on silicon chips. These chips would ideally combine all of the required optical functions of the sensor (i.e. light source, waveguides for routing light, interaction between the light and the analyte, and light detection), and could be fabricated at low cost in high volumes, thanks to existing silicon manufacturing infrastructure that has been developed for electronics and for near-infrared optical communications.
The last few years have seen rapid development of many of the components that are needed to create these sensor systems: silicon photonic waveguides that can transmit light with low loss at almost any mid-IR wavelength have been developed, while lasers emitting high powers in the mid-IR are now readily available and have been successfully integrated with silicon waveguides.
However, there remains a crippling lack of practical photodetector technologies; those that have already been integrated wilth optical circuits on silicon chips are either expensive to manufacture, are impractical because they have to be cooled to cryogenic temperatures, or do not work at all required wavelengths. This project will develop new waveguide integrated thermal photodetectors, which work by converting the incoming light into a temperature change that can be measured with an electronic circuit. They will be able to operate at room temperature at any mid-IR wavelength, and will be manufactured using low cost techniques. This project will also demonstrate that sensors employing these photodetectors can reach the sensitivities required for clinical and industrial uses, by using them to measure low concentrations of artificial sweeteners in soft drinks - an industrially important example application.
These detectors will potentially transform mid-infrared sensor systems from an academic curiosity into a commercially viable technology.
Publications
Rowe D
(2023)
Group IV Mid-Infrared Photonic Devices and Applications
Mitchell C
(2024)
Mid-infrared silicon photonics: From benchtop to real-world applications
in APL Photonics
Stirling C
(2024)
Sub-wavelength gratings in silicon photonic devices for mid-infrared spectroscopy and sensing
in Photonics and Nanostructures - Fundamentals and Applications
Stirling C
(2024)
Sub-wavelength gratings in silicon photonic devices for mid-infrared spectroscopy and sensing
in Photonics and Nanostructures - Fundamentals and Applications
| Description | 1) A new simulation model for was created for simultaneous modelling of light propagation, thermal dissipation, and electrical current flow, which can be applied to any waveguide integrated bolometer geometry. It enabled us to accurately simulate the performance of new waveguide integrated bolometer designs and assess them against each other. 2) Two new waveguide integrated thermal photodetectors were designed: a) a suspended-Si based bolometer for the 3-6µm wavelength range, and b) a Ge-on-Si waveguide bolometer for the 6-10µm wavelength range. The designs of both bolometers were significant improvements on previously published work, in several respects: (A) The wavelength range of operation was significantly extended, in that the Ge-on-Si device can operate at much longer mid-IR wavelengths (at which previous Si waveguide based devices could not reach), and in that the bandwidth of operation of a single device was much larger than in previous devices (i.e. several micrometres vs a few hundred nanometres). (B) The new designs are much better suited to wafer-scale fabrication and mass production, because the minimum feature sizes of the new devices are large enough to be fabricated by deep-UV lithography (rather than e-beam lithography), and because the metals used as electrodes and as absorbers in the new designs are CMOS compatible. (C) The predicted responsivity of the suspended-Si device was >100x higher than published devices. The improvement was achieved by engineering the thermal characteristics of the device. On the other hand, we learned that designing a highly responsive Ge-on-Si bolometer is challenging, because of the high thermal conductance of the Si substrate. 3) It was found that strips of metal deposited and patterned on top of waveguides acting act as plasmonic absorbers can absorb light very strongly in an extremely broadband wavelength range. In the bolometer devices these absorbers convert light into heat, therefore strong and broadband absorption is desirable. For example, a 10um long TiN strip (with a 2um long taper at its input) patterned directly on top of a 500nm thick SOI strip waveguide can absorb > 88% of light over the whole wavelength range between 3um and 6um. These can replace resonant nano-antennas that we used in previous work, which have a relatively narrow absorption bandwidth. 4) We developed (i.e. designed, fabricated, and tested) a variety of new components that can be used in photonic integrated circuits for mid-infrared sensing, in combination with the new photodetectors. These include suspended Si components (i.e. waveguides, bends, edge couplers, grating couplers) for 3-6um wavelengths, and Ge-on-Si components (i.e. single mode waveguides, edge couplers, waveguide bends, waveguide crossings, 1x2 and 2x2 splitters) for the 5-11um wavelength range. We also implemented Ge-on-Si waveguide based microfluidic channels (with PDMA microfluidic cell bonded to the surface of the chip) and measured absorption of mid-IR light by aqueous analytes deposited as droplets on top of the waveguides. This work paves the way for future developments of mid-infrared photonic integrated circuits operating at wavelengths in the 3-10um wavelength range in which waveguide integrated bolometers are used as photodetection elements. |
| Exploitation Route | The newly designed photodetectors could in the long term be used within miniature, mass-producible sensors of gasses and chemicals, which work by absorption spectroscopy in the mid-infrared wavelength range. In the short to medium term, the device designs could be included into the offering of the Silicon Photonics multi-project wafer service called Cornerstone, which is based at the University of Southampton, which would allow external academic or industrial users of Cornerstone to obtain manufactured chips that include these photodetectors incorporated into their own circuit designs. As the maturity level of mid-infrared Silicon Photonics increases, it is anticipated that these or similar thermal photodetectors will be applied in demonstrator sensing systems that are directly relevant to specific real-world sensing applications, which would eventually lead to more widespread adoption. |
| Sectors | Agriculture Food and Drink Chemicals Electronics Environment Healthcare Manufacturing including Industrial Biotechology |
| Description | Collaboration with STFC RAL Space |
| Organisation | Rutherford Appleton Laboratory |
| Department | RAL Space |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | - We have provided advice to the Spectroscopy group at RAL Space concerning the design of Germanium-on-Silicon waveguide chips for spectroscopic sensing. - We have sent existing waveguide chips to RAL Space, which they used to verify that the new characterisation setup that they built could be used to couple mid-infrared light into waveguides. - We have fabricated waveguide chips according to the new design developed by RAL Space, and shipped them. |
| Collaborator Contribution | - RAL Space built a new experimental setup for characterising the transmission of mid-infrared photonic integrated circuits. - They designed a new photonic integrated circuit, for spectroscopic sensing. - They performed sensing experiments using these chips. |
| Impact | No public outputs yet. |
| Start Year | 2021 |
| Title | A model for optical, thermal, and electrical simulations of waveguide integrated bolometers. |
| Description | A software tool was developed which is able to run optimisations of waveguide integrated bolometers. The tool is scripted in Matlab, and is used to control optical, thermal, and electrical simulations of a device in different Lumerical software packages (i.e. FDTD and HEAT), link them together, and then process the results to extract key figures of merit. It allows for the bolometer device geometry to be set from the Matlab script, and then proceeds to 1) simulate how light will travel through the device, 2) how much heat would be generated from optical absorption in the device, 3) heat transfer through the device, and 4) the effect of the changed temperature distribution on the electrical current passing through the device. The tool was applied to the design of mid-infrared waveguide bolometers in suspended-Si and Ge-on-Si, but could be applied to any wavelength or waveguide platform. |
| Type Of Technology | New/Improved Technique/Technology |
| Year Produced | 2024 |
| Impact | The software tool has been used within this project for the design of new bolometer devices. |
| Description | Southampton Science and Engineering Festival |
| Form Of Engagement Activity | Participation in an open day or visit at my research institution |
| Part Of Official Scheme? | No |
| Geographic Reach | Local |
| Primary Audience | Public/other audiences |
| Results and Impact | Southampton Science and Engineering Festival (SOTSEF) is one of the annual flagship events of the Public Engagement with Research unit, with thousands of visitors joining researchers, staff and students at the University of Southampton for ten days of scientific discovery and celebration. As part of the day we set up a display to teach visiting public (largely parents with children) about spectroscopy, photonic integration, and applications of optical sensing. |
| Year(s) Of Engagement Activity | 2023 |
| URL | https://www.sotsef.co.uk/ |