Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm–2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.

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Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces

We present a tutorial review of all-optical switching in fibre and integrated optics waveguides. These switching devices require non-linear refractive index changes in single or coupled waveguides, and involve either the low-power guided modes of the structure or soliton-type waves guided, emitted or captured by waveguides. We discuss the physical principles involved in these all-optical switching schemes, material requirements, recent experiments and limitations. A representative rather than comprehensive list of references is provided.

All-optical waveguide switching

Semiconductors provide some of the most promising materials for nonlinear optics, because of large resonant nonlinearities, control of recombination time (from milliseconds to femtoseconds), well-developed fabrication technologies, and compatibility with other optoelectronic devices. The paper reviews some of the concepts and results that have come from the study of nonlinear optics in semiconductors. The emphasis is on nonlinear absorption and the refractive index that arises from the photo-induced excitation of free carriers. Mechanisms described include state-filling, carrier transport, and photorefractivity. Devices include optical bistability, reflective asymmetric Fabry-Perot all-optical absorptive switches, optically addressed spatial light modulators, and real-time holography. The paper's approach is to provide a basic engineering understanding of the principles, some of the historical details, and a snapshot of the state of the field today.

Resonant optical nonlinearities in semiconductors

It is shown theoretically that the photonic density of states (DOS) of three-dimensional semiconductor photonic crystals can be dramatically changed on ultrafast time scales through two photon excitation of free carriers. Calculations for GaAs inverse opals show that the photonic band gap exhibits a large shift in frequency and a change in width with an appropriate excitation pulse. At certain frequencies, the DOS can be switched from a high value to zero, from zero to a high value, and from a high to zero to a high on 100-fs time scales, independent of the relaxation time of the semiconductor. This technique allows for ultrafast control of spontaneous emission and trapping of photons.

Ultrafast switching of photonic density of states in photonic crystals

Time‐division interferometry with 430 fs tunable laser pulses is used for direct femtosecond measurements of the wavelength dependence of the nonresonant nonlinear index of refraction, n2, in AlGaAs waveguides at room temperature. Below band‐gap n2 values of ∼10−12 cm2/W are observed with resonant enhancement as the laser wavelength is tuned toward the band edge.

Femtosecond measurements of the nonresonant nonlinear index in AlGaAs

Room‐temperature all‐optical switching has been achieved in single‐mode strip‐loaded nonlinear directional couplers with a GaAs/AlGaAs multiple quantum well guiding layer. For low input intensities nearly complete (1:3–1:5) cross coupling occurs, while at high input intensities switching occurs so that most (>3:1) of the light stays in the input guide. The effects of carrier diffusion are minimized by the use of picosecond pulses. The response time and the recovery time of the nonlinear directional couplers are measured with pump‐probe experiments, and the origin of the nonlinearity is attributed to fast electronic effects.

Picosecond all‐optical switching in single‐mode GaAs/AlGaAs strip‐loaded nonlinear directional couplers

The build-up of exciton populations in resonantly laser excited semiconductors is studied micro-scopically. For excitation at the 2 s -exciton resonance, it is shown that polarization with a strict s -type radial symmetry can be eﬃciently converted into an incoherent p -type population. As a consequence, inversion between the 2 p and 1 s exciton states can be obtained leading to the appearance of signiﬁcant terahertz gain.

2004 Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors

First principle DFT-based microscopic many-body models are used to investigate inter- and intra-valley carrier dynamics in the monolayer transition-metal dichalcogenide MoTe2. Electron-electron and electron-phonon scatterings are calculated for transitions within the full Brillouin zone to determine overall carrier relaxation timescales as well as intra- and inter-valley transition rates. For excitation above the barriers separating bandstructure valleys carriers are found to relax on a ten femtosecond timescale into hot quasi-Fermi distributions at the band minima. Subsequently, the hot carrier plasma is cooled down on a picosecond timescale predominantly through emission of optical phonons. Local carrier occupations lead to strong energy renormalizations in momentum space. However, for the material investigated here, the global energy minimum remains at the K-points once carriers relax into global quasi-Fermi distributions. No transition from a direct to an indirect bandgap is observed.

S. W. Koch

Papers

Picosecond all‐optical switching in single‐mode GaAs/AlGaAs strip‐loaded nonlinear directional couplers

2004 Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors

Influence of inter- and intra-valley carrier dynamics on the optical properties of monolayer TMDCs