Optical harmonic generation occurs when high intensity light (>10 W m) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

We present a micrometer scale, on-chip integrated, plasmonic enhanced graphene photodetector (GPD) for telecom wavelengths operating at zero dark current. The GPD is designed and optimized to directly generate a photovoltage and has an external responsivity∼12.2V/W with a 3dB bandwidth∼42GHz. We utilize Au split-gates with a∼100nm gap to electrostatically create a p-n-junction and simultaneously guide a surface plasmon polariton gap-mode. This increases light-graphene interaction and optical absorption and results in an increased electronic temperature and steeper temperature gradient across the GPD channel. This paves the way to compact, on-chip integrated, power-efficient graphene based photodetectors for receivers in tele and datacom modules.The ever-growing demand for global data traffic[1] is driving the development of next generation communication standards [2,3]. The increasing numbers of connected devices[4], the need for new functionalities, and the development of high-performance computing [5,6] require optical communication systems performing at higher speeds, with improved energy-efficiency, whilst maintaining scalability and cost-effective manufacturing. Si photonics[7-9] offers the prospect of dense (nanoscale) integration[10] relying on mature, low-cost (based on complementary metal-oxide-semiconductor (CMOS) fabrication processes) manufacturing [8,9], making it one of the key technologies for short-reach (<10km) optical interconnects[11] beyond currently employed lithium niobate[12] and indium phosphate[13]. A variety of functionalities have been developed and demonstrated in Si photonics for local optical interconnects[11]. Electro-optic modulators based on carrier-depletion (phase-modulation) in Si[14, 15] or the Franz-Keldysh effect[16] (amplitude-modulation) in strained Si-Ge[17, 18] encode information into optical signals at telecom wavelengths (λ =1.3-1.6µm). On the receiver side, Ge[19] or bonded III-V[20, 21] photodetectors (PD) are needed for optical-to-electrical signal conversion, since the telecom photon energies are not sufficient for direct (band-to-band) photodetection in Si[22].On-chip integrated Ge PDs [23][24][25][26][27] are standard components in Si photonics foundries [8,9,22]. Their external responsivities (in A/W), R I = I ph /P in , where I ph is the photocurrent and P in is the incident optical power, can exceed 1A/W [8,23] and their bandwidth can reach 60GHz [25][26][27]. Following the development of high temperature (> 600 • C) [19] heterogeneous integration of Ge-on-Si using epitaxial growth and cyclic thermal annealing [19,28,29], the concentration of defects and threading dislocations in Ge epilayers and at Si/Ge interfaces can be reduced [19], resulting in low (<10nA[9, 27]) dark current in waveguide integrated Ge p-i-n photodiodes [24,27]. However, Ge-on-Si integration is a complex process [19,22,29], as the lattice mismatch between Si and Ge [19], ion implantation [23,25], thermal budget (i.e. thermal energy transfer to the wafer) management [22], and the non-plan...

The mid-infrared (MIR) spectral range is of immense use for civilian and military applications. The large number of vibrational absorption bands in this range can be used for gas sensing, process control and spectroscopy. In addition, there exists transparency windows in the atmosphere such as that between 3.6-3.8 µm, which are ideal for free-space optical communication, range finding and thermal imaging. A number of different semiconductor platforms have been used for MIR light-emission. This includes InAsSb/InAs quantum wells 1 , InSb/AlInSb 2 , GaInAsSbP pentanary alloys 3 , and intersubband transitions in group III-V compounds 4 . These approaches, however, are costly and lack the potential for integration on silicon and silicon-on-insulator platforms. In this respect, two-dimensional (2D) materials are particularly attractive due to the ease with which they can be heterointegrated. Weak interactions between neighbouring atomic layers in these materials allows for deposition on arbitrary substrates and van der Waals heterostructures enable the design of devices with targeted optoelectronic properties. In this Letter, we demonstrate a light-emitting diode (LED) based on the 2D semiconductor black phosphorus (BP). The device, which is composed of a BP/molybdenum disulfide (MoS 2 ) heterojunction emits polarized light at l = 3.68 μm with room-temperature internal and external quantum efficiencies (IQE and EQE) of ~1% and ~3×10 -2 %, respectively. The ability to tune the bandgap, and consequently emission wavelength of BP, with layer number, strain and electric field make it a particularly attractive platform for MIR emission.Electroluminescence (EL) from 2D transition metal dichalcogenides (TMDs) was observed shortly after monolayers from this class of materials were first isolated 5,6,7,8,9 . In monolayer TMD crystals, the formation of a direct bandgap allows for reasonable light-emission efficiencies to be achieved. The high degree of confinement in monolayers also ensures a large exciton binding

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS2, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A ′ 1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS2 band structure induced by the A ′ 1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.Coherent modulation of the optical properties of a material, following impulsive photo-excitation of the lattice, is fundamentally interesting and technologically relevant because it can be used for applications in sensors[1], actuators and transducers [2][3][4], that can be operated at extremely high frequencies (up to several THz[5]). In order to exploit this effect, it is necessary to understand the mechanism underlying the coherent phonon (CP) generation process and to identify the physical parameters (such as the pump pulse photon energy) that allow their efficient excitation. In view of possible device applications, it is of paramount importance to detect the spectral dependence of the CP amplitude, in order to identify in which photon energy window the optical response of the material can be efficiently modulated.We present a novel transient absorption (TA) setup, combining broadband detection from 1.8 to 3eV, with extremely high temporal resolution (∼20fs). This is ideally suited to trigger and detect vibrational coherences in different classes of materials. We use it to generate and detect CPs in 1L-MoS 2 , as a representative semiconducting 1L-transition metal dichalcogenide (TMD). We focus on TMDs because they support strongly bound excitons with unique physical properties, enabling novel applica-tions in optoelectronics and photonics [6][7][8]. However, in TMDs, the dynamical interaction between excitons and phonons, when constrained to 1L, is unexplored.When TMDs are exfoliated down to 1L, they undergo a transition from indirect to direct band gap[9], accompanied by a strong enhancement of the photoluminescence (PL) quantum yie...

We propose and simulate the characteristics of optical filters based on subwavelength gratings. In particular, we demonstrate through numerical simulations the feasibility of implementing SWG Bragg gratings in silicon-on-insulator (SOI). We also propose SWG ring resonators in SOI and verify their operation using numerical simulations and experiments. The fabricated devices exhibit an extinction ratio as large as 30 dB and a Q-factor as high as ~20,000. These fundamental SWG filters can serve as building blocks for more complex devices.

An optical true time delay line (OTTDL) is a basic photonic building block that enables many microwave photonic and optical processing operations. The conventional design for an integrated OTTDL that is based on spatial diversity uses a length-variable waveguide array to create the optical time delays, which can introduce complexities in the integrated circuit design. Here we report the first ever demonstration of an integrated index-variable OTTDL that exploits spatial diversity in an equal length waveguide array. The approach uses subwavelength grating waveguides in silicon-on-insulator (SOI), which enables the realization of OTTDLs having a simple geometry and that occupy a compact chip area. Moreover, compared to conventional wavelength-variable delay lines with a few THz operation bandwidth, our index-variable OTTDL has an extremely broad operation bandwidth practically exceeding several tens of THz, which supports operation for various input optical signals with broad ranges of central wavelength and bandwidth.

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Black phosphorus (BP) has emerged as a promising two-dimensional (2D) semiconductor for applications in electronics, optoelectronics, and energy storage. As is the case for many 2D materials, the fabrication of...