Photonic devices for future chip-integrated biosensors
On the Way to Novel Point-of-Care-Diagnostics
- Fig. 1: Scheme of optical ring resonators based on a) channel waveguides and b) slot waveguides. The light interacts with the analyte and thus, the transmission behavior of the resonator is changed. c) Simulated transmission spectrum at the output of an optical ring resonator without protein (state 1) and with protein (state 2). The wavelength shift Δλ provides information on the amount of adsorbed analyte.
- Fig. 2: Cross section and simulation of optical field distribution of a channel and slot waveguide. In contrast to the channel waveguide, the major light is confined in the vicinity of the silicon rails and can strongly interact with the environment.
- Fig. 3: Chip with more than 100 photonic devices on an area of 2 x 3 mm².
On-chip integrated photonic biosensors can lead to major advances in medical diagnostics, food and environmental monitoring through the rapid, and precise analysis of various substances. This offers the prospect of a cost effective lab-on-a-chip-platform and a reliable point-of-care-diagnostic. One research focus for future on-chip integrated biosensors is directed towards the combination of photonic devices with silicon microelectronics technology. A preferable approach for this new generation of photonic biosensors is based on optical ring resonators. In this article, concepts and functionality of such on-chip integrated ring resonators are described. In addition, we present latest research results and approaches to increase the light-matter interaction by optimized waveguide structures.
Biosensors based on photonic ring resonators enable a selective and label-free detection of various substances and thus are relevant to many areas of bio-analytics. Examples are the detection of proteins in food, toxins in the environment or blood analysis. Such biosensors represent on-chip integrated laboratories and have advantages compared to conventional point-of-care-diagnostics regarding sensitivity, miniaturization capabilities, parallelization and diversification.
The integrated photonic combines the advantages of optical sensor technology with the capabilities of microelectronic chip production. In this way novel components can be produced, suitable for a widespread use in bioanalysis, ending up in low-cost disposable chips.
At the TH Wildau research has been carried out in the field of photonic devices since 2008. It is mainly focused on the development of novel device concepts for improved sensitivity through the optimization of waveguide structures and its chip-integration. For the realization of those novel device concepts a photonic-integrated-circuit technology (PIC) provided by the institute for innovation in high performance microelectronics (IHP) is used. Certain technology developments were realized within the framework of the joint-lab between the TH Wildau and the IHP.
Photonic biosensors on a chip
In general there are two approaches for the integration of biosensors on a chip.
First the so called hybrid or heterogeneous integration and secondly the monolithic integration. In case of hybrid integration, the photonic chip and the electronic chip are manufactured separately and subsequently connected to each other. In monolithic integration both, photonic and electronic components, are integrated on a single chip.
Biosensors and corresponding lab-on-a-chip solutions can be basically realized by both approaches. The main advantage of such integrated devices is that the analysis does not require any special knowledge about bioanalysis or a laboratory environment, and that it is thus suitable for individualized point-of-care diagnostics. Silicon-based technologies are well established industrial platforms and are able to produce large quantities for the consumer market. This provides excellent conditions for future cost effective sensor solutions.
Working principle of a photonic biosensor
A new generation of biosensors for label-free detection of biomolecules is based on chip-integrated optical ring resonators. Those ring resonators consist of channel waveguides or slot waveguides. The schematics of such ring resonators is depicted in Fig. 1a) and 1b). The light of a tunable laser is coupled into the photonic chip via an optical fiber and detected by a photodiode. According to the resonance conditions, only selected wavelengths can propagate in the ring and distinct resonance peaks appear in the output spectrum. In order to realize a selective light-analyte interaction, the waveguide is functionalized with specific ligands. When the analyte interacts with the ligand, the optical wave is influenced in its propagation, the resonance condition is changed and the resonance peak is shifted by Δλ. The schematics is shown in Fig. 1c) together with calculated transmission spectra before and after reaction of the ligand and the analyte. The magnitude of the wavelength shift Δλ provides information on the quantity of the adsorbed analyte, where the detection limit of the sensor is defined by the minimum resolvable wavelength shift.
In case of channel waveguides the light is guided in the high-index silicon and interacts with the analyte by the evanescent field only, i.e. a fraction of about 20 % of the total intensity of the guided light. Slot waveguides, as shown in Fig. 2, provide much higher light-analyte interaction, since the intensity of the guided light can be more than three times higher in the immediate vicinity of the silicon ridges. The disadvantage of current slot waveguides, however, consists in relatively high optical losses, mainly induced by scattering effects at the sidewalls of the silicon ridges.
As a consequence, slot waveguide based ring resonators have typically small optical quality factors, also called Q-factor. The width δλ (full width at half maximum) of the resonance line is directly coupled to the Q-factor. At increased Q-factor δλ is decreased. Thus smaller wavelength shift Δλ can be resolved and lower analyte concentrations are detectable. Resonators for optical sensors require extremely high Q-factors of up to 100,000. So far, these values can be obtained only with channel waveguides.
State of the art
Label-free binding detection by surface plasmon resonance has become a standard in bioanalysis. Such devices are relatively expensive. Therefore, chip-integrated chemical and biological molecular sensors employing optical resonators have gained increasing research interests. Especially silicon photonic sensors have become very attractive for various optical sensing applications, combining high sensitivity with potentially low cost. Fig. 3 shows a chip with more than 100 photonic devices, fabricated on the base of silicon-on-insulator (SOI) technology.
A biosensor for the detection of proteins, based on slot waveguides, was demonstrated for the first time, in 2009 , where an avedin-biotin system has been used. The experiments revealed a 3.5-times stronger light-analyte interaction compared to a ring resonator consisting of a channel waveguide. However, with a value of about 5000, the Q-factor was relatively small. A recently presented hybrid solution, consisting of a combination of channel waveguide and slot waveguide, provided a significantly increased Q-factor and thus may represent a promising alternative approach to future bio-sensing applications . Besides the Q-factor, the light analyte interaction is an important feature which should be subject to further research activities. According to recent simulations studies [3, 4] up to 75 % of the light can be guided in a slot waveguide at a rail width around w = 180 nm, a slot width of s = 180 nm and a waveguide height of h = 220 nm (see Fig. 2).
Chip-integrated photonic biosensors are promising candidates for future lab-on-a-chip solutions. Silicon based photonic devices have the advantage that they can be produced by common semiconductor industry processes. This approach profits from synergy effects, achieved by combining highly sensitive bio analytics with the advantages of well-established semiconductor technologies and its capabilities of mass production. Current research activities are directed towards the further optimization of the underlying photonic waveguide structures, mainly in terms of sensitivity and integration.
Parts of this work has been supported by the Federal Ministry for Research and Technology (BMBF) under grant number 03FH086PX2.
Patrick Steglich1,*, Silvio Pulwer1, Claus Villringer1, Joachim Bauer1, Friedhelm Heinrich1, Birgit Dietzel1, Andreas Mai2, Sigurd Schrader1
1TH Wildau, Research Group for Photonics, Laser and Plasmatechnologies, Wildau, Germany
2IHP, Institute for Innovation in High Performance Microelectronics, Frankfurt (Oder), Germany
Faculty of Engineering and Natural Science
Photonic, Laser and Plasmatechnologies
Technical University of Applied Sciences
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 Patrick Steglich, Christian Mai, David Stolarek, Stefan Lischke, Sebastian Kupijai, Claus Villringer, Silvio Dümecke, Friedhelm Heinrich, Joachim Bauer, Stefan Meister, Dieter Knoll, Mauro Casalboni, Sigurd Schrader: “Novel ring resonator combining strong field confinement with high optical quality factor”, Photonics Technology Letters, IEEE, vol. 27, no. 20, (2015): 2197–2200, DOI: 10.1109/LPT.2015.2456133.
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 Patrick Steglich, Claus Villringer, Silvio Dümecke, Yazmin Padilla Michel, Mauro Casalboni, Sigurd Schrader: “Silicon-on-insulator slot-waveguide design trade-offs”, in PHOTOPTICS 2015, P. A. Ribeiro and M. Raposo, Eds., vol. 2. SCITEPRESS, 3 (2015): 47–52, DOI: 10.5220/0005336200470052.