Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Organic Photodetectors for Next-Generation Wearable Electronics.

Mechanically flexible active multielectrode arrays (MEA) have been developed for local signal amplification and high spatial resolution. However, their opaqueness limited optical observation and light stimulation during use. Here, we show a transparent, ultraflexible, and active MEA, which consists of transparent organic electrochemical transistors (OECTs) and transparent Au grid wirings. The transparent OECT is made of Au grid electrodes and has shown comparable performance with OECTs with nontransparent electrodes/wirings. The transparent active MEA realizes the spatial mapping of electrocorticogram electrical signals from an optogenetic rat with 1-mm spacing and shows lower light artifacts than noise level. Our active MEA would open up the possibility of precise investigation of a neural network system with direct light stimulation.

https://www.pnas.org/content/pnas/114/40/10554.full.pdf

Transparent, conformable, active multielectrode array using organic electrochemical transistors

Recent developments in optical biosensors based on integrated photonic devices are reviewed with a special emphasis on silicon-on-insulator ring resonators. The review is mainly devoted to the following aspects: (1) Principles of sensing mechanism, (2) sensor design, (3) biofunctionalization procedures for specific molecule detection and (4) system integration and measurement set-ups. The inherent challenges of implementing photonics-based biosensors to meet specific requirements of applications in medicine, food analysis, and environmental monitoring are discussed.

Optical Biosensors Based on Silicon-On-Insulator Ring Resonators: A Review

Solution-processed phototransistors can substantially advance the performance of image sensors. Phototransistors exhibit large photoconductive gain and a sublinear responsivity to irradiance, which enables a logarithmic sensing of irradiance that is akin to the human eye and has a wider dynamic range than photodiode-based image sensors. Here, we present a novel solution-processed phototransistor composed of a heterostructure between a high-mobility organic semiconductor and an organic bulk heterojunction. The device efficiently integrates photogenerated charge during the period of a video frame then quickly discharges it, which significantly increases the signal-to-noise ratio compared with sampling photocurrent during readout. Phototransistor-based image sensors processed without photolithography on plastic substrates integrate charge with external quantum efficiencies above 100% at 100 frames per second. In addition, the sublinear responsivity to irradiance of these devices enables a wide dynamic range of 103 dB at 30 frames per second, which is competitive with state-of-the-art image sensors. A solution-processed organic phototransistor is operated at 100-frame-per-second rates with external quantum efficiencies above 100%. Dynamic range as high as 103 dB was shown for 30-frame-per-second operation.

/pdf/charge-integrating-organic-heterojunction-phototransistors-28dv3mv9mf.pdf

Charge-integrating organic heterojunction phototransistors for wide-dynamic-range image sensors

Silicon photonic biosensors hold the potential for highly accurate, yet low cost point-of-care devices. Maximizing the sensitivity of the sensing chips while reducing the complexity and cost of the read-out system is pivotal to realize this potential. Here we present an extensive analysis, both from a practical and a theoretical perspective, of current biosensors, and analyze how subwavelength structures can be exploited to enhance their sensitivity. This study is not restricted just to the near-infrared band as we also determine the sensing capabilities of the suspended silicon waveguides with subwavelength metamaterial cladding working in the mid-infrared range. These waveguides have been recently proposed to cover the full transparency window of silicon ( λ ∼ 8.5  μm), where the fingerprint spectral region of many molecules takes place and so a plethora of evanescent field absorption-based applications will be developed in the near future.

/pdf/invited-subwavelength-structures-for-silicon-photonics-1z5abyu5td.pdf

[INVITED] Subwavelength structures for silicon photonics biosensing

Flexible low-voltage organic phototransistors based on air-stable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

Waveguide-based biochemical sensors exploit detection of target molecules that bind specifically to a functionalized waveguide surface. For optimum sensitivity, the waveguide should be designed to mediate maximum influence of the surface layer on the effective refractive index of the guided mode. In this paper, we define a surface sensitivity metric which quantifies this impact and which allows to broadly compare different waveguide types and integration platforms. Focusing on silicon nitride and silicon-on-insulator (SOI) as the most common material systems, we systematically analyze and optimize a variety of waveguide types, comprising simple strips, slot and double slot structures, as well as sub-wavelength gratings (SWG). Comparing the highest achievable surface sensitivities, we provide universal design guidelines and physically interpret the observed trends and limitations. Our findings allow to select the appropriate WG platform and to optimize sensitivity for a given measurement task.

/pdf/surface-sensing-with-integrated-optical-waveguides-a-design-4qxd8fbrgc.pdf

Surface sensing with integrated optical waveguides: a design guideline.

Photonic integration has seen tremendous progress over the previous decade, and several integration platforms have reached industrial maturity. This evolution has prepared the ground for miniaturized photonic sensors that lend themselves to efficient analysis of gaseous and liquid media, exploiting large interactions lengths of guided light with surrounding analytes, possibly mediated by chemically functionalized waveguide surfaces. Among the various sensor concepts, phase-sensitive approaches are particularly attractive: offering a flexible choice of the operation wavelength, these schemes are amenable to large-scale integration on mature technology platforms such as silicon photonics or silicon nitride (Si3N4) that have been developed in the context of tele- and data-communication applications. This paves the path toward miniaturized and robust sensor systems that offer outstanding scalability and that are perfectly suited for high-volume applications in life sciences, industrial process analytics, or consumer products. However, as the maturity of the underlying photonic integrated circuits (PICs) increases, system-level aspects of mass-deployable sensors gain importance. These aspects include, e.g., robust system concepts that can be operated outside controlled laboratory environments as well as readout schemes that can be implemented based on low-cost light sources, without the need for benchtop-type tunable lasers as typically used in scientific demonstrations. It is, thus, the goal of this tutorial to provide a holistic system model that allows us to better understand and to quantitatively benchmark the viability and performance of different phase-sensitive photonic sensor concepts under the stringent limitations of mass-deployable miniaturized systems. Specifically, we explain and formulate a generally applicable theoretical framework that allows for a quantitatively reliable end-to-end analysis of the overall signal chain. Building upon this framework, we identify and explain the most important technical parameters of the system, comprising the photonic sensor circuit, the light source, and the detector, as well as the readout and control scheme. We quantify and compare the achievable performance and the limitations that are associated with specific sensor structures based on Mach–Zehnder interferometers (MZIs) or high-Q optical ring resonators (RRs), and we condense our findings by formulating design guidelines both for sensor concepts. As a particularly attractive example, we discuss an MZI-based sensor implementation, relying on a vertical-cavity surface-emitting laser (VCSEL) as a power-efficient low-cost light source in combination with a simple and robust readout and control scheme. In contrast to RR-based sensor implementations, MZIs can be resilient to laser frequency noise, at the cost of a slightly lower sensitivity and a moderately increased footprint. To facilitate the application of our model, we provide a MATLAB-based application that visualizes the underlying physical principles and that can be readily used to estimate the achievable performance of a specific sensor system. The system-level design considerations are complemented by an overview of additional aspects that are important for successful sensor system implementation such as the design of the underlying waveguides, photonic system assembly concepts, and schemes for analyte handling.

Integrated phase-sensitive photonic sensors: a system design tutorial

We demonstrate a robust concept for instantaneous extraction of fringe order and unwrapped phase in integrated Mach-Zehnder sensors without continuous tracking. The scheme exploits a frequency-modulated probe laser and a 2×3 MMI at the sensor output and allows for continuous self-calibration and high-resolution phase detection.

Johannes Milvich

Papers

Flexible low-voltage organic phototransistors based on air-stable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

Surface sensing with integrated optical waveguides: a design guideline.

Integrated phase-sensitive photonic sensors: a system design tutorial

Mach-Zehnder interferometer readout for instantaneous sensor calibration and extraction of endlessly unwrapped phase