arXiv subjects

Marco Romagnoli

Publications and source records attributed to Marco Romagnoli.

10 recordsLinked to original sources

arXiv2026

C-band 160 Gbs-1 Zero-bias Graphene Photodetectors: Breaking the Responsivity-Bandwidth Trade-off by Heterostructure Engineering

Graphene photodetectors offer ultrafast response and broadband operation, but their responsivity is typically limited by rapid hot-carrier cooling, leading to a trade-off between sensitivity and speed. Here, we demonstrate that modifying the dielectric environment provides an effective route to control hot-carrier cooling and enhance device performance. By employing a WSe2 encapsulation architecture, we suppress out-of-plane energy dissipation, leading to an increased cooling length (~2.68 um) and a reduced heat-exchange coefficient. As a result, we obtain zero-bias graphene photodetectors with responsivities up to ~0.12 A/W (potentially ~0.4 A/W) while maintaining ultrafast operation beyond the setup-limited 110 GHz bandwidth. The devices enable direct detection at data rates of 120 Gb s-1 (NRZ) and 160 Gb s-1 (PAM-4), with performance achieved using minimal digital signal processing. These results establish dielectric engineering as a key design axis for controlling hot-carrier dynamics, enabling energy-efficient, high-speed optical receivers for next-generation interconnects and AI-driven data systems.

physics.optics↗

arXiv2024

High-speed Source-Device-Independent Quantum Random Number Generator on a Chip

A wide range of applications require, by hypothesis, to have access to a high-speed, private, and genuine random source. Quantum Random Number Generators (QRNGs) are currently the sole technology capable of producing true randomness. However, the bulkiness of current implementations significantly limits their adoption. In this work, we present a high-performance source-device independent QRNG leveraging a custom made integrated photonic chip. The proposed scheme exploits the properties of a heterodyne receiver to enhance security and integration to promote spatial footprint reduction while simplifying its implementation. This characteristics could represents a significant advancement toward the development of generators better suited to meet the demands of portable and space applications. The system can deliver secure random numbers at a rate greater than 20 Gbps with a reduced spatial and power footprint.

quant-ph↗

arXiv2021

Ultra-clean high-mobility graphene on technologically relevant substrates

Graphene grown via chemical vapour deposition (CVD) on copper foil has emerged as a high-quality, scalable material, that can be easily integrated on technologically relevant platforms to develop promising applications in the fields of optoelectronics and photonics. Most of these applications require low-contaminated high-mobility graphene (i.e., approaching 10 000 $cm^2 V^{-1} s^{-1}$) at room temperature) to reduce device losses and implement compact device design. To date, these mobility values are only obtained when suspending or encapsulating graphene. Here, we demonstrate a rapid, facile, and scalable cleaning process, that yields high-mobility graphene directly on the most common technologically relevant substrate: silicon dioxide on silicon (SiO$_2$/Si). Atomic force microscopy (AFM) and spatially-resolved X-ray photoelectron spectroscopy (XPS) demonstrate that this approach is instrumental to rapidly eliminate most of the polymeric residues which remain on graphene after transfer and fabrication and that have adverse effects on its electrical properties. Raman measurements show a significant reduction of graphene doping and strain. Transport measurements of 50 Hall bars (HBs) yield hole mobility $μ_h$ up to 9000 $cm^2 V^{-1} s^{-1}$ and electron mobility $μ_e$ up to 8000 $cm^2 V^{-1} s^{-1}$, with average values $μ_h$ 7500 $cm^2 V^{-1} s^{-1}$ and $μ_e$ 6300 $cm^2 V^{-1} s^{-1}$. The carrier mobility of ultraclean graphene reach values nearly double of that measured in graphene HBs processed with acetone cleaning, which is the method widely adopted in the field. Notably, these mobility values are obtained over large-scale and without encapsulation, thus paving the way to the adoption of graphene in optoelectronics and photonics.

cond-mat.mtrl-sci↗

arXiv2021

Wafer-scale integration of graphene-based photonic devices

Graphene and related materials can lead to disruptive advances in next generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (>5,000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapour deposition (CVD) of single layer graphene (SLG) matrices comprising up to ~12000 individual single crystals (SCs), grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ~80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ~5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ~0.25, 0.45, 0.75, 1 dB V-1 for device lengths ~30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hBN is used to protected SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. Our full process flow (from growth to device fabrication) enables the commercial implementation of graphene-based photonic devices.

physics.app-ph↗

arXiv2020

Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene-polymer-graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity 8 x 10$^{10}$ $cm^{-2}$ at the charge neutrality point, and a large Seebeck coefficient 140 $μ$V K$^{-1}$, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices.

physics.app-ph↗

arXiv2020

High-responsivity graphene photodetectors integrated on silicon microring resonators

Graphene integrated photonics provides several advantages over conventional Si photonics. Single layer graphene (SLG) enables fast, broadband, and energy-efficient electro-optic modulators, optical switches and photodetectors (GPDs), and is compatible with any optical waveguide. The last major barrier to SLG-based optical receivers lies in the low responsivity - electrical output per optical input - of GPDs compared to conventional PDs. Here we overcome this shortfall by integrating a photo-thermoelectric GPD with a Si microring resonator. Under critical coupling, we achieve $>$90% light absorption in a $\sim$6 $μ$m SLG channel along the Si waveguide. Exploiting the cavity-enhanced light-matter interaction, causing carriers in SLG to reach $\sim$400 K for an input power of $\sim$0.6 mW, we get a voltage responsivity $\sim$90 V/W, demonstrating the feasibility of our approach. Our device is capable of detecting data rates up to 20 Gbit/s, with a receiver sensitivity enabling it to operate at a 10$^{-9}$ bit-error rate, on par with mature semiconductor technology. The natural generation of a voltage rather than a current, removes the need for transimpedance amplification, with a reduction of the energy-per-bit cost and foot-print, when compared to a traditional semiconductor-based receiver.

physics.app-ph↗

arXiv2020

A liquid nitrogen cooled superconducting transition edge sensor with ultra-high responsivity and GHz operation speeds

Photodetectors based on nano-structured superconducting thin films are currently some of the most sensitive quantum sensors and are key enabling technologies in such broad areas as quantum information, quantum computation and radio-astronomy. However, their broader use is held back by the low operation temperatures which require expensive cryostats. Here, we demonstrate a nitrogen cooled superconducting transition edge sensor, which shows orders of magnitude improved performance characteristics of any superconducting detector operated above 77K, with a responsivity of 9.61x10^4 V/W, noise equivalent power of 15.9 fW/Hz-1/2 and operation speeds up to GHz frequencies. It is based on van der Waals heterostructures of the high temperature superconductor Bi2Sr2CaCu2O8, which are shaped into nano-wires with ultra-small form factor. To highlight the versatility of the detector we demonstrate its fabrication and operation on a telecom grade SiN waveguide chip. Our detector significantly relaxes the demands of practical applications of superconducting detectors and displays its huge potential for photonics based quantum applications.

physics.app-ph↗

arXiv2020

High-speed double layer graphene electro-absorption modulator on SOI waveguide

We report on a C-band double layer graphene electro-absorption modulator on a passive SOI platform showing 29GHz 3dB-bandwith and NRZ eye-diagrams extinction ratios ranging from 1.7 dB at 10 Gb/s to 1.3 dB at 50 Gb/s. Such high modulation speed is achieved thanks to the quality of the CVD pre-patterned single crystal growth and transfer on wafer method that permitted the integration of high-quality scalable graphene and low contact resistance. By demonstrating this high-speed CVD graphene EAM modulator integrated on Si photonics and the scalable approach, we are confident that graphene can satisfy the main requirements to be a competitive technology for photonics.

physics.app-ph↗

arXiv2016

Deterministic patterned growth of high-mobility large-crystal graphene: a path towards wafer scale integration

We demonstrate rapid deterministic (seeded) growth of large single-crystals of graphene by chemical vapour deposition (CVD) utilising pre-patterned copper substrates with chromium nucleation sites. Arrays of graphene single-crystals as large as several hundred microns are grown with a periodicity of up to 1 mm. The graphene is transferred to target substrates using aligned and contamination-free semi-dry transfer. The high quality of the synthesised graphene is confirmed by Raman spectroscopy and transport measurements, demonstrating room-temperature carrier mobility of 21 000 cm2 / V s when transferred on top of hexagonal boron nitride. By tailoring the nucleation of large single-crystals according to the desired device geometry, it will be possible to produce complex device architectures based on single-crystal graphene, thus paving the way to the adoption of CVD graphene in wafer-scale fabrication.

cond-mat.mtrl-sci↗

arXiv2013

Graphene Based Optical Phase Modulation

In this paper we report phase modulation obtained by inducing a capacitive charge on graphene layers embedded in the core of a waveguide. There is a biasing regime in which graphene absorption is negligible but large index variations can be achieved with a voltage-length product as small as $V_π\,L_π\simeq 0.04 $\,V\,cm . Examples of phase induced changes are computed for straight waveguides and for microring resonators showing the possibility to implement several optoelectronic functionalities as modulators, tunable filters, and switches.

physics.optics↗