The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology. Field-effect transistors are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (10(12) hertz) range. All-optical injection of currents through interfering photoexcitation pathways or photoconductive switching of terahertz transients has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases--free from breakdown--the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (10(15) hertz) domain.

The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per ångström and significant modifications in their electronic system. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angström and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

Advances in optical spectroscopy and microscopy have had a profound impact throughout the physical, chemical and biological sciences. One example is coherent Raman spectroscopy, a versatile technique interrogating vibrational transitions in molecules. It offers high spatial resolution and three-dimensional sectioning capabilities that make it a label-free tool for the non-destructive and chemically selective probing of complex systems. Indeed, single-colour Raman bands have been imaged in biological tissue at video rates by using ultra-short-pulse lasers. However, identifying multiple, and possibly unknown, molecules requires broad spectral bandwidth and high resolution. Moderate spectral spans combined with high-speed acquisition are now within reach using multichannel detection or frequency-swept laser beams. Laser frequency combs are finding increasing use for broadband molecular linear absorption spectroscopy. Here we show, by exploring their potential for nonlinear spectroscopy, that they can be harnessed for coherent anti-Stokes Raman spectroscopy and spectro-imaging. The method uses two combs and can simultaneously measure, on the microsecond timescale, all spectral elements over a wide bandwidth and with high resolution on a single photodetector. Although the overall measurement time in our proof-of-principle experiments is limited by the waiting times between successive spectral acquisitions, this limitation can be overcome with further system development. We therefore expect that our approach of using laser frequency combs will not only enable new applications for nonlinear microscopy but also benefit other nonlinear spectroscopic techniques.

Laser frequency combs, sources with a spectrum consisting of hundred thousands evenly spaced narrow lines, have an exhilarating potential for new approaches to molecular spectroscopy and sensing in the mid-infrared region. The generation of such broadband coherent sources is presently under active exploration. Technical challenges have slowed down such developments. Identifying a versatile highly nonlinear medium for significantly broadening a mid-infrared comb spectrum remains challenging. Here we take a different approach to spectral broadening of mid-infrared frequency combs and investigate CMOS-compatible highly nonlinear dispersion-engineered silicon nanophotonic waveguides on a silicon-on-insulator chip. We record octave-spanning (1,500–3,300 nm) spectra with a coupled input pulse energy as low as 16 pJ. We demonstrate phase-coherent comb spectra broadened on a room-temperature-operating CMOS-compatible chip.

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Z v1,v2 , Z c1,c2 and Z v2,c2 are correct and set as above). Here we correct this error in Fig. 1 by plotting the adiabatic levels of silica for the value of Z v1,c1~3 s that we actually used. This corrected level diagram prompts a correction of the qualitative picture mediated by Fig. 1b unit cells is expected to occur, with a rate equal to the anticrossing energy gap (in frequency units) DE l =(2pB). As Fig. 1 reveals, for Dl j j §2, this rate is very small with respect to the carrier frequency, v L =2p, of the optical field (where subscript 'L' is for 'laser'). Hence, all these anticrossings are passed diabatically in the oscillating strong laser field. The system follows the diabatic terms (shown by the solid crossed arrows in Fig. 1) as the field increases or decreases without significant transfer of population between the valence band and the conduction band. This is because the system passes the anticrossings too quickly for Zener tunnelling to occur. Such an anticrossing is mostly inconsequential (almost 'ignored' by the system).As the corrected level diagram in Fig. 1 reveals, the gap becomes very large, with DE l =Bw wv L , for the ultimate level anticrossing at Dl j j~1 occurring when the field amplitude approaches and exceeds the critical field strength F crit~Dg ea where D g <9 eV is the bandgap of silica, e is the charge on the electron and a is the lattice constant). The large anticrossing gap implies a large probability of Zener tunnelling between the valence band and conduction band states, which are Wannier-Starklocalized at the nearest neighbours. This anticrossing is passed predominantly adiabatically, as indicated by the dashed parallel arrows in Fig. 1. This leads necessarily to the lower anticrossing level gradually changing its wavefunction from valence band to conduction band (the well known exchange of quantum numbers in adiabatic anticrossings) while remaining fully occupied. This gradual change can be quantified by projecting the actual filled, adiabatic field-modified valence band states to zero-field conduction-band states as given by Supplementary equation (10) These are virtual carriers similar to those responsible for virtual photoconductivity in semiconductors theoretically predicted 5-7 and experimentally observed 8 over two decades ago. However, the analogy is incomplete. In our case, the virtual carriers are produced by a nonresonant (mostly adiabatic), strong-field (and hence, necessarily, nonperturbative) excitation, in contrast with early work 5-8 , where they were excited via a near-resonant perturbative excitation. These virtual carriers make the system more polarizable in a static electric field [5][6][7][8] and likewise at optical frequencies. If so, they can account for both the transient reflectivity shown in figure 3b of ref. 2 and the transient current displayed in figure 3 of ref. 1. In both cases, a strong few-cycle field nonlinearly enhances the strong-field polarization of the dielectric by creating virtual carriers. These respond with increased excursions ...

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This work reports on the attosecond real-time observation of the electron processes [1] underlying the ability of ultrastrong few-cycle laser pulses to turn a dielectric solid from an insulating into a conducting state [2].