Ultrafast Spectroscopy of Graphene-Protected Thin Copper Films

© 2016 American Chemical Society. We studied by broad-band pump-probe spectroscopy the ultrafast optical response of thin copper films covered by a monolayer of graphene. It is demonstrated that graphene protection does not alter the thermo-modulational nonlinearity of copper in the whole visible range. Also, we provide a quantitative validation of a theoretical model for this optical nonlinearity, derived from a semiclassical description of electron thermalization dynamics and subsequent modulation of copper dielectric function from 450 to 700 nm wavelength. Our results extend to the nonlinear domain the capability of graphene-protected copper nanolayers to serve as a low cost optical grade material, with major potential impact on nonlinear plasmonics and metamaterials

Ultrafast valley relaxation dynamics in monolayer MoS2 probed by nonequilibrium optical techniques

We study the exciton valley relaxation dynamics in single-layer MoS2 by a combination of two nonequilibrium optical techniques: time-resolved Faraday rotation and time-resolved circular dichroism. The depolarization dynamics, measured at 77 K, exhibits a peculiar biexponential decay, characterized by two distinct time scales of 200 fs and 5 ps. The fast relaxation of the valley polarization is in good agreement with a model including the intervalley electron-hole Coulomb exchange as the dominating mechanism. The valley relaxation dynamics is further investigated as a function of temperature and photoinduced exciton density. We measure a strong exciton density dependence of the transient Faraday rotation signal. This indicates the key role of exciton-exciton interactions in MoS2 valley relaxation dynamics

Reshaping the emission of a THz quantum cascade laser frequency comb through an on-chip graphene modulator

Graphene possesses a peculiar potential for the development of optoelectronic components capable to actively manipulate infrared light [1] . Its electrostatically tunable optical conductivity, band structure and transport characteristics, and the possibility to be grown over large areas, offer an intriguing playground for engineering, at the nanoscale, amplitude modulators, spatial light modulators and switches, combining high efficiency (> 50%) intensity modulation, reasonably high speeds (> s response times), and spectral tunability in the underexploited high (> 1.5 THz) terahertz-frequency range [2] , where frontier applications in quantum communications, quantum computing, adaptive and quantum optics are still at their infancy

Tunable, Grating-Gated, Graphene-On-Polyimide Terahertz Modulators

An electrically switchable graphene terahertz (THz) modulator with a tunable-by-design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage 1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation-optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies

All in One-Chip, Electrolyte-Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency-Tuner in the 2-5 THz Range

Layered 2D materials display unique optical and electrical properties that can enable manipulation, propagation, and detection of electromagnetic waves over a broad spectral range, with a high level of control, offering the potential to activate different functionalities, by optical or electrical means, in a single chip. Here, a compact optoelectronic device behaving as an amplitude modulator, saturable absorber mirror (SA mirror), and frequency-tuner is conceived at terahertz (THz) frequencies. It comprises a gate-tunable single layer graphene (SLG), embedded in a quarter-wave cavity, operating in the 1-5 THz range. The use of electrolyte ionic liquid gate ensures 40% optical amplitude modulation depth. Z-scan self-mixing interferometry reveals 60% reflectivity modulation, with approximate to 4.5 W cm(-2) saturation intensity. By integrating the modulator/SA mirror with a heterogeneous THz quantum cascade laser frequency comb, in an external cavity configuration, fine-tuning of the intermode beatnote frequency is also demonstrated. This opens intriguing perspectives for short pulse generation, phase-locking, frequency tuning/chirping, phase modulation, and metrological referencing, inter alia

Self-induced phase locking of terahertz frequency combs in a phase-sensitive hyperspectral near-field nanoscope

Chip-scale, electrically-pumped terahertz (THz) frequency-combs (FCs) rely on nonlinear four-wave-mixing processes, and have a nontrivial phase relationship between the evenly spaced set of emitted modes. Simultaneous monitoring and manipulation of the intermode phase coherence, without any external seeding or active modulation, is a very demanding task for which there has hitherto been no technological solution. Here, a self-mixing intermode-beatnote spectroscopy system is demonstrated, based on THz quantum cascade laser FCs, in which light is back-scattered from the tip of a scanning near-field optical-microscope (SNOM) and the intracavity reinjection monitored. This enables to exploit the sensitivity of FC phase-coherence to optical feedback and, for the first time, manipulate the amplitude, linewidth and frequency of the intermode THz FC beatnote using the feedback itself. Stable phase-locked regimes are used to construct a FC-based hyperspectral, THz s-SNOM nanoscope. This nanoscope provides 160 nm spatial resolution, coherent detection of multiple phase-locked modes, and mapping of the THz optical response of nanoscale materials up to 3.5 THz, with noise-equivalent-power (NEP) ≈400 pW √Hz−1. This technique can be applied to the entire infrared range, opening up a new approach to hyper-spectral near-field imaging with wide-scale applications in the study of plasmonics and quantum science, inter alia

Gate-tunable ultrafast optical response of single-layer graphene

© 2019 IEEE. The ultrafast relaxation dynamics in single-layer graphene (SLG) is of key importance for its applications in optoelectronic devices which rely on the dynamic response of charge carriers, such as photodetectors, saturable absorbers and modulators[1]. The absorption via interband transitions of a photon of energy in ω SLG promotes an electron from an energy -ω/2 in the valence band to an energy ω/2 in the conduction band. This strongly non-equilibrium distribution thermalizes by electron-electron scattering (t∼ 10-20 fs) to a hot Fermi-Dirac (FD) distribution, which in turn equilibrates with the cold lattice via interaction with strongly-coupled optical phonons (SCOP, t∼200-300 fs) and, on a longer time scale, via anharmonic coupling with acoustic phonons (t∼ 1-2 ps) [2]

Ultrafast spin/valley decay processes in monolayer WS<inf>2</inf>

Ultrafast spin/valley decay processes in monolayer WS2</inf

Electrically Tunable Graphene-on-Polyimide Terahertz Modulators

We report a graphene-on-polyimide THz modulator with a tunable-by-design optical bandwidth, a 90% modulation efficiency, a 20 KHz electrical bandwidth and tunable in the 1.9-2.7 THz range. The electrostatic tuning of the THz conductivity is achieved through a grating gate coupler on the top of the polyimide waveguide. By coupling the modulator with a THz quantum cascade laser frequency comb, we show it can fully compensate the cavity dispersion, resulting in stable frequency comb operation over a 45% dynamic range

Ultrafast spin/valley decay processes in monolayer WS2

The peculiar coupling of the spin and the valley degrees of freedom in Transition-Metal Dichalcogenides (TMDs) makes this class of materials very interesting for quantum information and possible valleytronics based devices. In the last year the valley depolarization dynamics in TMDs has been object of an intense study