We report vertically illuminated, resonant cavity enhanced, graphene-Si Schottky photodetectors (PDs) operating at 1550 nm. These exploit internal photoemission at the graphene-Si interface. To obtain spectral selectivity and enhance responsivity, the PDs are integrated with an optical cavity, resulting in multiple reflections at resonance, and enhanced absorption in graphene. We get a wavelength-dependent photoresponse with external (internal) responsivity ∼20 mA/W (0.25A/W). The spectral selectivity may be further tuned by varying the cavity resonant wavelength. Our devices pave the way for developing high responsivity hybrid graphene-Si free-space illuminated PDs for optical communications, coherence optical tomography, and light-radars.

We present flexible photodetectors (PDs) for visible wavelengths fabricated by stacking centimeter-scale chemical vapor deposited (CVD) single layer graphene (SLG) and single layer CVD MoS2, both wet transferred onto a flexible polyethylene terephthalate substrate. The operation mechanism relies on injection of photoexcited electrons from MoS2 to the SLG channel. The external responsivity is 45.5A/W and the internal 570A/W at 642 nm. This is at least 2 orders of magnitude higher than bulk-semiconductor flexible membranes. The photoconductive gain is up to 4 × 10(5). The photocurrent is in the 0.1-100 μA range. The devices are semitransparent, with 8% absorptance at 642 nm, and are stable upon bending to a curvature of 1.4 cm. These capabilities and the low-voltage operation (<1 V) make them attractive for wearable applications.

We report an on-chip integrated metal graphene–silicon plasmonic Schottky photodetector with 85 mA/W responsivity at 1.55 μm and 7% internal quantum efficiency. This is one order of magnitude higher than metal–silicon Schottky photodetectors operated in the same conditions. At a reverse bias of 3 V, we achieve avalanche multiplication, with 0.37A/W responsivity and avalanche photogain ∼2. This paves the way to graphene integrated silicon photonics.

The development of CMOS compatible Si photodetectors (PDs) operating at the near-infrared (NIR) wavelength of 1550nm is attractive for on-chip optoelectronic integration, power monitoring, imaging and reflectography[1-4]. While Si PDs are widely employed in the visible spectral range (0.4-0.7 m)[2], they are not suitable for detecting NIR radiation above 1.1 m, because the energy of NIR photons at 1550nm 0.8eV) is not sufficient to overcome the Si indirect bandgap (1.12eV) and induce photogeneration of electron-hole pairs. Schottky diode PDs based on internal photoemission (IPE), whereby photoexcited carriers from a metal are emitted to Si over a potential barrier ΦB at the Si-metal interface, offer a solution for detecting sub-bandgap optical signals. The advantages of Schottky PDs are the simple material structure, easy fabrication process and straightforward integration with CMOS technology. The main disadvantage is the limited (<1%)[5] quantum efficiency, which results in limited responsivity of few mA/W for plasmonic-enhanced free-space illuminated devices[6] and 10 mA/W for waveguide integrated Schottky PD operating at 1550nm[7]. Graphene is a promising material for photonics and optoelectronics due to its light absorption over a broad spectral range, ultrafast carrier dynamics, high mobility and tunable optical properties via electrostatic doping[8]. Single layer graphene (SLG) placed at the Schottky interface increases the IPE quantum efficiency up to 7%[9,10] and therefore it is attractive to integrate SLG in Schottky PDs. However, SLG absorbs only 2.3%[11] of the incident light in the NIR, limiting the responsivity to ~10mA/W in free-space illuminated Si-graphene Schottky PDs operating at 1550nm[9]. The optical absorption in SLG can be enhanced by using a guided mode approach obtained by integrating graphene PDs with an optical waveguide[10], or using an optical cavity for the free-space illuminated devices[12]. Here, we report the integration of a FabryPerot optical cavity in Si-graphene Schottky PDs operating at 1550nm under free-space illumination. This is realized by using a 200 m Si wafer, and it enables responsivity enhancement at resonant wavelengths (Fig. 1a). We get an external responsivity Rext~0.3mA/W at 1V reverse bias. Rext can be increased by applying higher (1-10V) biases to lower ΦB, and/or integrating a gold mirror to improve reflectivity compared to the air/Si interface. The PD with a gold mirror at VR=10V reaches Rext~20mA/W, which corresponds to an internal responsivity of ~0.25 A/W (Fig. 1b) and a quantum efficiency of 20%. This is at least an order of magnitude higher compared to previously reported free-space illuminated Sigraphene PDs for NIR wavelengths[9]. [1] G. Reed, Wiley, UK (2008). [2] S. M. Sze et al., Wiley, New York (2006). [3] J. Esper et al., Acta Astronaut. 46, 287 (2000). [4] C. Daffara et al., Acc. Chem. Res. 43(6), 847 (2010). [5] I. Goykhman et al., arXiv:1401.2624. [6] B. Desiatov et al., Optica 4, 335 (2015). [7] I. Goykhman et al., Opt. Express 20, 28594 (2012). [8] F. Bonaccorso et al., Nat. Phot. 4, 611 (2010). [9] M. Amirmazlaghani et al. IEEE Quantum Electron. 49, 589 (2013). [10] I. Goykhman et al., arXiv:1512.08153v1. [11] R. R. Nair et al., Science 320, 1308 (2008). [12] M. Engel et al., Nat. Comm. 3, 1 (2012).

The combination of plasmonic nanoparticles and graphene enhances the responsivity and spectral selectivity of graphene-based photodetectors. However, the small area of the metal-graphene junction, where the induced-electron hole pairs separate, limits the photoactive region to submicron length scales. Here, we couple graphene with a plasmonic grating and exploit the resulting surface plasmon polaritons to deliver the collected photons to the junction region of a metal-graphene-metal photodetector. This gives a 400% enhancement of responsivity and a 1000% increase in photoactive length, combined with tunable spectral selectivity. The interference between surface plasmon polaritons and the incident wave introduces new functionalities, such as light flux attraction or repulsion from the contact edges, enabling the tailored design of the photodetector's spectral response. This architecture can also be used for surface plasmon biosensing with direct-electric-redout, eliminating the need of bulky optics.

We design, optimize and demonstrate a silicon-graphene avalanche Schottky photodetector with photoresponsivities of 1A/W and 0.5A/W for visible and telecom wavelengths respectively.

Plasmonic coupling of gold nanoparticles to a gold surface creates intense plasmonic hot spots with large electromagnetic field-enhancements within the cavity formed by the two metallic surfaces. The localised field in such structures is extremely sensitive to morphological fluctuations and subtle changes in the dielectric properties of the cavity contents. Here, we present an optical method that pins down the properties of the gap contents with high sensitivity, termed normalising plasmon resonance (NPR) spectroscopy. We use this on a variety of ultrathin molecular spacers such as filled and empty cucurbiturils, and graphene. Clear differences in the spectral positions and intensities of plasmonic modes observed in the scattering spectrum resolve thickness differences of 0.1 nm, and refractive index changes from molecular filling.

Ultra-thin flakes of 2D organic-inorganic perovskite (C6H9C2H4NH3)2PbI4 are produced using micromechanical exfoliation. Mono- and few-layer areas are identified using optical and atomic force microscopy, with an interlayer spacing of 1.6 nm. Refractive indices extracted from the optical spectra reveal a sample thickness dependence due to the charge transfer between organic and inorganic layers. These measurements demonstrate a clear difference in the exciton properties between “bulk” (>15 layers) and very thin (<8 layer) regions as a result of the structural rearrangement of organic molecules around the inorganic sheets.

Graphene's high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal-graphene-metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo- and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.

Graphene is used as the thinnest possible spacer between gold nanoparticles and a gold substrate. This creates a robust, repeatable, and stable subnanometer gap for massive plasmonic field enhancements. White light spectroscopy of single 80 nm gold nanoparticles reveals plasmonic coupling between the particle and its image within the gold substrate. While for a single graphene layer, spectral doublets from coupled dimer modes are observed shifted into the near-infrared, these disappear for increasing numbers of layers. These doublets arise from charger-transfer-sensitive gap plasmons, allowing optical measurement to access out-of-plane conductivity in such layered systems. Gating the graphene can thus directly produce plasmon tuning.

Bubble formation and behaviour have been studied over decades, but the complex two-phase flow phenomena involved are still not fully understood. In view of the importance of two-phase flow processes in a broad range of industrial applications, such as the chemical process industry, food industry and aerospace applications, it is crucial to obtain a detailed understanding of single and multiple bubble dynamics. Gravity plays an important role in bubble formation and behaviour. Several studies have been conducted on single bubble formation under microgravity conditions, but the effects of gravitational accelerations much larger than on Earth have not been previously documented. In order to gain a full understanding of the effect of gravity on the bubble dynamics and in view of industrial applications, particularly aerospace applications, it is essential to examine bubble formation and behaviour under hypergravity conditions. Bubble formation and behaviour at the surface of a porous material and at a nozzle were investigated at hypergravity levels of 1–20g using the Large Diameter Centrifuge (LDC) at ESA/ESTEC. The formation of air bubbles through a porous filter into a water column was recorded under hypergravity conditions and the obtained data were analysed qualitatively. A decrease in bubble size and an increase in bubble formation frequency with increasing hypergravity levels could be clearly observed. Data for the experiments on air and oil bubble formation at a nozzle into a water column were recorded under hypergravity conditions using a high speed camera (for different nozzle sizes and air/oil flow rates). For the recorded data from the experiments on air and oil bubble formation at a nozzle, a decrease in bubble size and an increase in bubble formation frequency with increasing gravitational acceleration could be observed qualitatively. Quantitative analysis of the data obtained for the experiments on air bubble formation at a nozzle clearly showed a decrease in average bubble diameter with increasing hypergravity levels. The effect of the nozzle diameter on the bubble size was shown to be small and the bubble diameter was larger for higher flow rates.