Photodetecting fibers enable large area flexible image systems

Optical sensing and imaging typically involve optoelectronic devices and arrays that use elaborate wafer-based processes. While they can create small features, they are restricted to planar geometries and a limited coverage area.¹ The search for approaches enabling large area and flexible assemblies of optoelectronics devices with nanoscale features has recently attracted considerable interest.^2,3 However, the challenges of creating systems that can perform complex optoelectronic tasks like optical sensing and imaging remain mostly unresolved. The systems could potentially make significant contributions to the fields of medical imaging, remote sensing, industrial control, and smart fabrics.

Polymer fibers processed with conventional preform-based, fiber-drawing techniques are an attractive candidate for realizing such systems. Their fabrication is simple, low cost, and yields extended lengths of highly uniform fibers with well-controlled geometries and structures that can be woven onto arbitrary large 2D and 3D constructs and fabrics. Until recently, however, they were restricted to certain materials and larger features,⁴ which limited their use. We recently developed a fabrication approach that addresses these limitations and enables us to integrate arbitrary metal-insulator-semiconductor (MIS) structures inside thin polymer fibers over hundreds of meters in length.^5–10

Different approaches have been developed to incorporate conducting materials in fibers.^11,12 Using metallic elements in preform fiber processing, however, has been precluded due to their crystalline nature, a characteristic seemingly incompatible with fiber drawing processes. We circumvented this problem by encapsulating thin metallic ribbons that melt during the drawing process between high viscosity boundaries⁵ (see Figure 1). Their confinement prevents any leakage and enables uniform maintenance of arbitrary MIS cross sectional structures from a macroscopic preform into a long microscopic fiber.

Figure 1. (a) Fabrication of a preform containing two thin films at prescribed radial positions, each in contact with four metallic electrodes. (b) Preform-to-fiber thermal drawing. (c) Photo of a preform and the resulting fiber (top), and a scanning electron micrograph of the resulting fiber showing the cross-section structure's uniform conservation from the macroscopic preform to the microscopic fiber.

Light sensing in these novel fibers is achieved by the simple photoconducting effect in amorphous semiconductors. Photons travel through the transparent polymer cladding and excite electron-hole pairs inside the semiconductor. Applying an external electric field between adjacent electrodes contacting the semiconductor pulls apart the charges, inducing a photocurrent that signals the presence of light. The semiconductor band gap can be tuned to detect visible and thermal radiation.

The flexibility of our fabrication approach enables us to integrate several functional components into a single fiber, leading to increasingly complex capabilities. For example, spectrometric fabrics integrate a photodetecting element surrounded by an optical filter that controls their operation wavelength.⁵ Recently, we fabricated an eight-device cascaded optoelectronic fiber structure in which components down to 100nm could operate collectively to extract the direction and wavelength of incident radiation: see Figure 2(a) and 2(b).¹⁰

Figure 2. (a) Schematic of the dual-ring fibers forming the grid. Fibers are around 35cm long and 800μm in diameter, and the semiconducting layers are around 100nm thick. (b) The ratio of the current in the inner and outer layers vs. wavelength for fibers made with different film thicknesses and glass compositions: Arsenic selenide (As₂Se₃), and arsenic selenide doped with 6%of tellurium (As₄₀Se₅₄Te₆, or AST). (c) Single grid lensless imaging. An object (smiley face) is illuminated by polychromatic radiation containing two beams of intensities I₁ and I₂, and wavelengths λ₁ and λ₂, respectively. The two diffracted patterns are obtained at the grid location. The phase-retrieval algorithm is used to reconstruct the object. The theoretical calculations (top row) are compared to the experimental results (bottom row).

Such remarkable photodetecting fibers are best exploited when they are assembled into constructs and fabrics. For example, while a single photosensitive fiber cannot determine the location of an excitation point along its length, a grid of identical fibers can localize an illumination point over its entire area, which could potentially be hundreds of square meters.^5,7,8 We demonstrated more sophisticated optical processing tasks using 2D fiber webs to extract an arbitrary optical intensity distribution using a tomographic reconstruction algorithm. It can reconstruct both the amplitude and phase of an optical wave front passing through two planar fiber arrays.⁷ Recently, using these wavelength discriminating fibers, we performed lensless imaging using polychromatic light and a single fiber array: see Figure 2(c).¹⁰

The development of multimaterial fibers has heralded a novel path toward optical sensing. For the first time optoelectronic functions can be delivered at length scales and with a mechanical flexibility hitherto associated with optical fibers. The range of functionalities and applications of fiber devices is expanding through the development of new materials and structures compatible with our fabrication process. In addition to radiation, we also envision using acoustic, chemical, and particles sensing with this novel fiber platform. Our group is working toward developing active devices—such as piezoelectric, thermoelectric, logical operations, and photovoltaic—in fibers. Engineering these multiple devices inside single fibers and interweaving them into different assemblies may one day lead to truly intelligent, large-area flexible systems and fabrics.