(Nanowerk Spotlight) Photonic integrated circuits, which use light instead of electricity to transmit and process information, have emerged as a critical technology for fields such as telecommunications, artificial intelligence, sensing, and astronomy. By leveraging the well-established silicon fabrication processes used in electronic circuits, silicon photonics has become a leading platform for state-of-the-art photonic integrated circuits. However, despite their widespread deployment, silicon photonic integrated circuits face inherent limitations due to silicon’s indirect bandgap and nonlinear optical properties, hindering their ability to meet growing demands for device functionality and performance.
Over the past two decades, since the first experimental isolation of graphene in 2004, two-dimensional materials with atomically thin structures and exceptional properties have garnered significant attention as a means to overcome these limitations. Among various two-dimensional materials, graphene oxide stands out as a highly promising candidate for hybrid integrated photonic devices with superior performance.
Graphene oxide exhibits many attractive optical properties, such as an ultra-high optical nonlinearity, significant material anisotropy, and a broadband response. Moreover, its properties can be flexibly altered through reduction and doping methods, substantially expanding the range of functionalities and devices that can be developed.
Crucially, graphene oxide also features facile synthesis processes and transfer-free film coating with precise control over thickness, showing strong potential for large-scale on-chip integration.
Now, in a pioneering study published in Advanced Materials (“2D Graphene Oxide Films Expand Functionality of Photonic Chips”), a research team led by David J. Moss at Swinburne University of Technology in Australia, has harnessed unique property changes induced by photothermal effects in two-dimensional graphene oxide films to demonstrate novel functionalities that extend beyond the capabilities of conventional photonic integrated circuits.
By integrating graphene oxide films onto silicon waveguides with precise control over their thickness and size, the researchers achieved all-optical control and tuning, optical power limiting, and nonreciprocal light transmission – all featuring very wide operational optical bandwidths.
The team’s approach relied on the integration of graphene oxide films onto silicon waveguides fabricated on a silicon-on-insulator wafer using complementary metal-oxide-semiconductor (CMOS) compatible fabrication technologies.
The coating of the graphene oxide film was achieved using a solution-based self-assembly method that enables transfer-free and layer-by-layer film deposition, offering both high repeatability and compatibility with various integrated material platforms.
Crucially, this approach can yield conformal film coating with direct contact and envelopment of graphene oxide films around the silicon waveguides, resulting in efficient light-matter interaction that is superior to typical film transfer methods used for other two-dimensional materials like graphene and transition metal dichalcogenides.
Silicon (Si) waveguides integrated with 2D graphene oxide (GO) films. a) Schematic illustration of a Si waveguide integrated with a 2D GO film. Insets show schematic of GO’s atomic structure and transverse electric (TE) mode profile of the hybrid waveguide with a monolayer of GO. b) Microscopic image of fabricated Si chip coated with a monolayer of GO. Inset shows a scanning electron microscope (SEM) image of a 2D layered GO film coated on a Si substrate. The numbers 1−3 refer to the number of GO layers for that part of the image. c) Measured Raman spectra of the uncoated Si chip (Si) and the chip coated with a monolayer of GO (GO-Si). (Image: Reproduced from DOI:10.1002/adma.202403659, CC BY)
The researchers first demonstrated efficient all-optical control and tuning in nonresonant waveguides with continuous-wave light. By combining a high-power pump light and a low-power probe light at different wavelengths, they showed that the loss experienced by the probe light could be controlled by the pump light power. Notably, this control was achieved over a very wide operational bandwidth, enabled by the broadband optical response of the graphene oxide films.
Next, they reported effective optical power limiting for continuous-wave light propagation through the hybrid waveguides. While light at low power experienced linear propagation loss, light at high power underwent a strong additional loss due to graphene oxide’s photothermal effects, limiting the output power. This optical limiting capability functioned as a “fuse” to prevent damage from excessive light power, similar to the role of fuses in electronic circuits.
Finally, in the first demonstration of its kind, the research team achieved broadband nonreciprocal light transmission with high nonreciprocal transmission ratios exceeding 10 decibels. By partially reducing a segment of the graphene oxide film on the waveguide, they ensured that light traveling in the forward direction experienced lower loss than light in the backward direction, which first encountered the unreduced segment of the film where it suffered additional loss from photothermal effects.
Remarkably, this nonreciprocal transmission spanned the entire telecommunications C-band and potentially beyond, a bandwidth far exceeding previous reports of nonreciprocal transmission in integrated photonic devices.
Underpinning these three functionalities was a fascinating physical mechanism: the photothermal effects in graphene oxide films, which include self-heating, thermal dissipation, and photothermal reduction. As light propagates through the graphene oxide films, it induces localized heating that can trigger the reduction of graphene oxide through the removal of oxygen-containing functional groups, leading to increased optical absorption.
Intriguingly, this photothermal reduction exhibits a reversible characteristic within a certain range of light power, where the reduced graphene oxide can revert to its initial state upon cooling. By carefully analyzing their experimental results, the researchers revealed insights into how graphene oxide’s properties, such as its extinction coefficient and thermal conductivity, evolve during these processes.
The research team identified four key advantages of two-dimensional graphene oxide films that make them uniquely suited for enabling these functionalities:
The material property changes induced by reversible photothermal effects provide a novel underlying mechanism.
The films’ broadband optical response yields a significantly wider operational bandwidth compared to bulk materials.
The relatively low loss of unreduced graphene oxide helps minimize additional insertion losses.
The ease of graphene oxide fabrication with precise control, along with its high compatibility with integrated platforms, is beneficial for practical device production and performance optimization.
Looking ahead, the functionalities demonstrated in this study are fundamental building blocks for photonic integrated circuits, with the potential to impact a wide range of applications. All-optical control and tuning could enable signal multicasting from a pump light to multiple probe lights. Optical power limiting could serve as a safeguard against power overload in sensitive laser systems. Nonreciprocal light transmission might facilitate optical signal processing and enhance light detection and ranging (LiDAR) systems.
This study stands out as pioneering in the field of silicon photonics due to its innovative use of graphene oxide films to overcome intrinsic limitations of silicon-based photonic integrated circuits. Unlike previous approaches that struggled with silicon’s indirect bandgap and limited nonlinear optical properties, the integration of GO films enables unprecedented functionalities. Specifically, the research demonstrates all-optical control, power limiting, and nonreciprocal light transmission across an exceptionally wide optical bandwidth.
These capabilities are achieved through precise manipulation of photothermal effects in GO, a mechanism not previously harnessed to this extent in photonic devices. By achieving these functionalities without the need for external power sources or complex device architectures, this work sets a new benchmark for the design and performance of hybrid photonic devices, opening new avenues for advancements in telecommunications, sensing, and beyond.
The work of Moss and colleagues marks a significant step forward in harnessing the unique properties of two-dimensional materials to overcome the limitations of conventional silicon photonics. By seamlessly integrating graphene oxide films with silicon waveguides, they have unlocked new capabilities in all-optical control, power limiting, and nonreciprocal transmission, paving the way for a new generation of photonic integrated circuits.
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