arXiv subjects

Qiaoliang Bao

Publications and source records attributed to Qiaoliang Bao.

32 records · Page 2Linked to original sources

arXiv2018

Long Range Intrinsic Ferromagnetism in Two Dimensional Materials and Dissipationless Future Technologies

The inherent susceptibility of low-dimensional materials to thermal fluctuations has long been expected to poses a major challenge to achieving intrinsic long-range ferromagnetic order in two-dimensional materials. The recent explosion of interest in atomically thin materials and their assembly into van der Waals heterostructures has renewed interest in two-dimensional ferromagnetism, which is interesting from a fundamental scientific point of view and also offers a missing ingredient necessary for the realization of spintronic functionality in van der Waals heterostructures. Recently several atomically thin materials have been shown to be robust ferromagnets. Such ferromagnetism is thought to be enabled by magneto crystalline anisotropy which suppresses thermal fluctuations. In this article, we review recent progress in two-dimensional ferromagnetism in detail and predict new possible two-dimensional ferromagnetic materials. We also discuss the prospects for applications of atomically thin ferromagnets in novel dissipationless electronics, spintronics, and other conventional magnetic technologies. Particularly atomically thin ferromagnets are promising to realize time reversal symmetry breaking in two-dimensional topological systems, providing a platform for electronic devices based on the quantum anomalous Hall Effect showing dissipationless transport. Our proposed directions will assist the scientific community to explore novel two-dimensional ferromagnetic families which can spawn new technologies and further improve the fundamental understanding of this fascinating area.

cond-mat.mtrl-sci↗

arXiv2018

Wafer-scale fabrication of 2D van der Waals heterojunctions for efficient and broadband photodetection

A variety of fabrication methods for van der Waals heterostructures have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we report few-layer van der Waals PtS2/PtSe2 heterojunction photodiodes fabricated on a 2" SiO2/Si substrate that is only limited by the size of work chamber of the growth equipment, offering throughputs necessary for practical applications. Theoretical simulation results show that the bandgap of PtS2 is shrunk to half of its original size in the PtS2/PtSe2 heterostructures, while PtSe2 exhibits a limited response to the coupling. Both PtSe2 and PtS2 layers in the coupled system are still semiconductors. Dynamic photovoltaic switching in the heterojunctions is observed at zero-volt state under laser illuminations of 532 to 2200 nm wavelengths. The PtS2/PtSe2 photodiodes show excellent characteristics in terms of a high photoresponsivity of 361 mAW-1, an external quantum efficiency (EQE) of 84%, and a fast response speed (66 ms). The wafer-scale production of 2D photodiodes in this work accelerates the possibility of 2D materials for practical applications in the next-generation energy-efficient electronics.

cond-mat.mes-hall↗

arXiv2017

Dipole-field-assisted charge extraction in metal-perovskite-metal back-contact solar cells

Hybrid organic-inorganic halide perovskites are low-cost solution-processable solar cell materials with photovoltaic properties that rival those of crystalline silicon. The perovskite films are typically sandwiched between thin layers of hole and electron transport materials, which efficiently extract photogenerated charges. This affords high-energy conversion efficiencies but results in significant performance and fabrication challenges. Herein we present a simple charge transport layer-free perovskite solar cell (PSC), comprising only a perovskite layer with two interdigitated gold back-contacts. Charge extraction is achieved via self-assembled molecular monolayers (SAMs) and their associated dipole fields at the metal/perovskite interface. Photovoltages of approximately 600 mV generated by SAM-modified PSCs are equivalent to the built-in potential generated by individual dipole layers. Efficient charge extraction results in photocurrents of up to 12.1 mA/cm2 under simulated sunlight, despite a large electrode spacing.

physics.app-ph↗

arXiv2016

High Performance Atomically Thin Flat Lenses

We experimentally demonstrate ultrathin flat lenses with a thickness of 7 Å, which corresponds to the fundamental physical limit of the thickness of the material, is fabricated in a large area, monolayer, CVD-prepared tungsten chalcogenides single crystals using the low-cost flexible laser writing method. The lenses apply the ultra-high refractive index to introduce abrupt amplitude modulation of the incident light to achieve three-dimensional (3D) focusing diffraction-limited resolution (0.5λ) and a focusing efficiency as high as 31%. An analytical physical model based diffraction theory is derived to simulate the focusing process, which shows excellent agreement with the experimental results.

physics.optics↗

arXiv2015

Black Phosphorus-Polymer Composites for Pulsed Lasers

Black phosphorus is a very promising material for telecommunication due to its direct bandgap and strong resonant absorption in near-infrared wavelength range. However, ultrafast nonlinear photonic applications relying on the ultrafast photo-carrier dynamics as well as optical nonlinearity in black phosphorus remain unexplored. In this work, we investigate nonlinear optical properties of solution exfoliated BP and demonstrate the usage of BP as a new saturable absorber for high energy pulse generation in fiber laser. In order to avoid the oxidization and degradation of BP, we encapsulated BP by polymer matrix which is optically transparent in the spectrum range of interest to form a composite. Two fabrication approaches were demonstrated to produce BP-polymer composite films which were further incorporated into fiber laser cavity as nonlinear media. BP shows very fast carrier dynamics and BP-polymer composite has a modulation depth of 10.6%. A highly stable Q-switched pulse generation was achieved and the single pulse energy of ~194 nJ was demonstrated. The ease of handling of such black phosphorus-polymer composite thin films affords new opportunities for wider applications such as optical sensing, signal processing and light modulation.

cond-mat.mtrl-sci↗

arXiv2015

Graphene Nanobubble: A New Optical Nonlinear Material

Graphene is a rising star in nonlinear optics due to its saturable absorption and giant Kerr nonlinearity, these properties are useful in digital optics based on optical nonlinear devices. However, practical applications require large optical nonlinearities and these are inherently limited by the interaction length of atomically thin graphene. Here, we demonstrate optical bistability in a Fabry Perot cavity containing monolayer and bilayer graphene which have been restructured to form nanobubbles. We find that graphene nanobubble can act as a new type of optical nonlinear media due to its vertical side wall as well as added curvature, which enable strong non linear dispersive effects leading to a large optically induced phase change. Unlike thermally induced bistability, the all optical switching between two transmission states happens within a time scale of tens of nanoseconds. Nanobubble based optical devices with intrinsic optical nonlinearity help to overcome the optical path length limitation of atomically thin two dimensional films and allow us to explore the promise of using such elements as the building block of digital all-optical circuitry.

cond-mat.mes-hall↗

arXiv2013

Achieving Giant Magneto-optic Effects with Optical Tamm States in Graphene-based Photonics

We manipulate optical Tamm states in graphene-based photonics to achieve and steer large magneto-optical effects. Here we report the presence of a giant Faraday rotation via a single graphene layer of atomic thickness while keeping a high transmission. The Faraday rotation is enhanced across the interface between two photonic crystals due to the presence of an interface mode, which presents a strong electromagnetic field confinement at the location of the graphene sheet. Our proposed scheme opens a promising avenue to realize high performance graphene magneto-optical devices that can be extended to other two-dimensional structures.

physics.optics↗

arXiv2012

Large nonlinear Kerr effect in graphene

Under strong laser illumination, few-layer graphene exhibits both a transmittance increase due to saturable absorption and a nonlinear phase shift. Here, we unambiguously distinguish these two nonlinear optical effects and identify both real and imaginary parts of the complex nonlinear refractive index of graphene. We show that graphene possesses a giant nonlinear refractive index n2=10-7cm2W-1, almost nine orders of magnitude larger than bulk dielectrics. We find that the nonlinear refractive index decreases with increasing excitation flux but slower than the absorption. This suggests that graphene may be a very promising nonlinear medium, paving the way for graphene-based nonlinear photonics.

physics.optics↗

arXiv2010

Monolayer Graphene as Saturable Absorber in Mode-locked Laser

We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can be generated using monolayer graphene as saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability and output energy.

physics.optics↗

arXiv2010

Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion towards all normal dispersion

Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated under various cavity dispersion conditions. It was shown that not only wide range soliton wavelength tuning but also soltion pulse width variation could be obtained in the fiber lasers. Our results show that the graphene mode locked erbium-doped fiber lasers provide a compact, user friendly and low cost wavelength tunable ultrahsort pulse source.

physics.optics↗

arXiv2010

Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser

Atomic layer graphene possesses wavelength-insensitive ultrafast saturable absorption, which can be exploited as a full-band mode locker. Taking advantage of the wide band saturable absorption of the graphene, we demonstrate experimentally that wide range (1570 nm - 1600nm) continuous wavelength tunable dissipative solitons could be formed in an erbium doped fiber laser mode locked with few layer graphene.

physics.atom-ph↗

arXiv2010

Vector Dissipative Solitons in Graphene Mode Locked Fiber Lasers

Vector soliton operation of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated. Either the polarization rotation or polarization locked vector dissipative solitons were experimentally obtained in a dispersion-managed cavity fiber laser with large net cavity dispersion, while in the anomalous dispersion cavity fiber laser, the phase locked NLSE solitons and induced NLSE soliton were experimentally observed. The vector soliton operation of the fiber lasers unambiguously confirms the polarization insensitive saturable absorption of the atomic layer graphene when the light is incident perpendicular to its 2D atomic layer.

physics.optics↗

arXiv2009

Atomic layer graphene as saturable absorber for ultrafast pulsed lasers

The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the thickness of graphene. Our results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth and wideband tuneability.

physics.optics↗

arXiv2009

Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker

Due to its unique electronic property and the Pauli Blocking Principle, atomic layer graphene possesses wavelength-independent ultrafast saturable absorption, which can be exploited for the ultrafast photonics application. Through chemical functionalization, a graphene-polymer nanocomposite membrane was fabricated and firstly used to mode lock a fiber laser. Stable mode locked solitons with 3 nJ pulse energy, 700 fs pulse width at the 1590 nm wavelength have been directly generated from the laser. We show that graphene-polymer nanocomposites could be an attractive saturable absorber for high power fiber laser mode locking.

nlin.PS↗