Advances in opto-electronics are often led by discovery and development of materials featuring unique properties. Recently, the material class of transparent conductive oxides (TCO) has attracted attention for active photonic devices on-chip. In particular, indium tin oxide (ITO) is found to have refractive index changes on the order of unity. This property makes it possible to achieve electrooptic modulation of sub-wavelength device scales, when thin ITO films are interfaced with optical light confinement techniques such as found in plasmonics; optical modes are compressed to nanometer scale to create strong light-matter interactions. Here we review efforts towards utilizing this novel material for high performance and ultra-compact modulation. While high performance metrics are achieved experimentally, there are open questions pertaining to the permittivity modulation mechanism of ITO. Finally, we review a variety of optical and electrical properties of ITO for different processing conditions, and show that ITO-based plasmonic electro-optic modulators have the potential to significantly outperform diffractionlimited devices.

As electronic device feature sizes scale‐down, the power consumed due to onchip communications as compared to computations will increase dramatically; likewise, the available bandwidth per computational operation will continue to decrease. Integrated photonics can offer savings in power and potential increase in bandwidth for onchip networks. Classical diffraction‐limited photonics currently utilized in photonic integrated circuits (PIC) is characterized by bulky and inefficient devices compared to their electronic counterparts due to weak light–matter interactions (LMI). Performance critical for the PIC is electro‐optic modulators (EOM), whose performances depend inherently on enhancing LMIs. Current EOMs based on diffraction‐limited optical modes often deploy ring resonators and are consequently bulky, photon‐lifetime modulation limited, and power inefficient due to large electrical capacitances and thermal tuning requirements. In contrast, wavelength‐scale EOMs are potentially able to surpass fundamental restrictions set by classical (i.e. diffraction‐limited) devices via (a) high‐index modulating materials, (b) nonresonant field and density‐of‐states enhancements such as found in metal optics, and (c) synergistic onchip integration schemes. This manuscript discusses challenges, opportunities, and early demonstrations of nanophotonic EOMs attempting to address this LMI challenge, and early benchmarks suggest that nanophotonic building blocks allow for densely integrated high‐performance photonic integrated circuits.

The rapid development of wearable devices puts forward higher requirements for mass-produced integrated smart systems that incorporate multiple electric components, such as energy supplying, multisensing, and communicating. To synchronously realize continuously self-powering, multifunctional sensing, distinguish signals from different stimuli, and productively design and fabricate a large-area sensing array, an all-fabric-based self-powered pressure–temperature-sensing electronic skin (e-skin) was prepared in this study by assembling highly flexible and compressible 3D spacer fabric (SF) and the thermoelectric poly­(3,4-ethylenedioxythiophene)­poly­(styrenesulfonate) (PEDOT:PSS). The all-fabric-based e-skin can efficiently and accurately sense the temperature with a detection resolution of 0.1 K and a response time of 1 s, as well as pressure within a wide range of 200 Pa to 200 kPa and a fast response time of 80 ms. The electricity necessary for driving the sensor can be provided by the temperature difference between the body and environment. Notably, independent voltage and current signals can be generated and read out under the simultaneous temperature–pressure stimuli. For the first time, a real waistcoat-like e-skin with electricity-generating and pressure–temperature-sensing functions on the whole area was designed and prepared by a simple and easy to scale-up production method. All of these features make the developed all-fabric self-powered sensor have very promising applications.

Next generation on-chip light sources require high modulation bandwidth, compact footprint, and efficient power consumption. Plasmon-based sources are able to address the footprint challenge set by both the diffraction limited of light and internal laser physics such as plasmon utilization. However, the high losses, large plasmonic-momentum of these sources hinder efficient light coupling to on-chip waveguides, thus, questioning their usefulness. Here we show that plasmon light sources can be useful devices; they can deliver efficient outcoupling power to on-chip waveguides and are able to surpass modulation speeds set by gain-compression. We find that waveguide-integrated plasmon nanocavity sources allow to transfer about ∼60% of their emission into planar on-chip waveguides, while sustaining a physical small footprint of ∼0.06 μm2. These sources are able to provide output powers of tens of microwatts for microamp-low injection currents and reach milliwatts for higher pump rates. Moreover, the direct modulation bandwidth exceeds that of classical, gain compression-limited on-chip sources by more than 200%. Furthermore, these novel sources feature high power efficiencies (∼1 fJ/bit) enabled by both minuscule electrical capacitance and efficient internal photon utilization. Such strong light–matter interaction devices might allow redesigning photonic circuits that only demand microwatts of signal power in the future.

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The success of information technology has clearly demonstrated that miniaturization often leads to unprecedented performance, and unanticipated applications. This hypothesis of “smaller-is-better” has motivated optical engineers to build various nanophotonic devices, although an understanding leading to fundamental scaling behavior for this new class of devices is missing. Here we analyze scaling laws for optoelectronic devices operating at micro and nanometer length-scale. We show that optoelectronic device performance scales non-monotonically with device length due to the various device tradeoffs, and analyze how both optical and electrical constrains influence device power consumption and operating speed. Specifically, we investigate the direct influence of scaling on the performance of four classes of photonic devices, namely laser sources, electro-optic modulators, photodetectors, and all-optical switches based on three types of optical resonators; microring, Fabry-Perot cavity, and plasmonic metal nanoparticle. Results show that while microrings and Fabry-Perot cavities can outperform plasmonic cavities at larger length-scales, they stop working when the device length drops below 100 nanometers, due to insufficient functionality such as feedback (laser), index-modulation (modulator), absorption (detector) or field density (optical switch). Our results provide a detailed understanding of the limits of nanophotonics, towards establishing an opto-electronics roadmap, akin to the International Technology Roadmap for Semiconductors.

Abstract:We report on a three-waveguide electro-optic switch for compact photonic integrated circuits and data routing applications. The device features a plasmonic metal-oxide-semiconductor (MOS) mode for enhanced light-matter-interactions. The switching mechanism originates from a capacitor-like design where the refractive index of the active medium, indium-tin-oxide, is altered via shifting the plasma frequency due to carrier accumulation inside the waveguide-based MOS structure. This light manipulation mechanism controls the transmission direction of transverse magnetic polarized light into either a CROSS or BAR waveguide port. The extinction ratio of 18 (7) dB for the CROSS (BAR) state, respectively, is achieved via a gating voltage bias. The ultrafast broadband fJ/bit device allows for seamless integration with silicon-on-insulator platforms for low-cost manufacturing.

X-ray phase contrast imaging (PCI) based on synchrotron radiation was introduced for the first time as an in situ imaging way to investigate sea salt phase change, i.e. deliquescence and crystallization in atmospheric environment. A performance on the deliquescence of pure NaCl, which is the dominant component in sea salt, demonstrated that this technique can directly observe the change of core particle and differentiate the outer water layer clearly in solid-aqueous system of~100 mm scale. The imaging results showed that sea salt particle deliquesced on a large scale of relative humidity (RH) between 34 and 97% RH as a solid-liquid drop, while no clear deliquescence RH was observed during the process. According to the drop size growth curve, sea salt deliquescence can be divided into three steps, namely water accumulating step (34-75% RH), bulk melting step (75-86% RH) and delay dissolving step (>86% RH), which are most probably dominated by grouped components as MgCl 2 / CaCl 2 /MgSO 4 , NaCl and Na 2 SO 4 /KCl/K 2 SO 4 , respectively. Instead at a sole RH, the crystallization of sea salt solution occurred at a range of 46-58% RH, which well agreed with the theory proposed by Ge et al. The aqueous-solid condition provided by sea salt deliquescence and crystallization may greatly enhance the heterogeneous chemical reactions in atmosphere.