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This review article presents the generation and control of polarization-shaped femtosecond laser pulses. Some applications of these pulses to coherent control are described, including multiphoton photoionization, forced molecular rotation, optical control of lattice vibrations, nano-optics, and wavelength conversion. Theoretical expectations for enantiomeric purification and magnetization are also discussed.

The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.

The excited-state intramolecular proton transfer (ESIPT) phenomenon is nowadays widely acknowledged to play a crucial role in many photobiological and photochemical processes. It is an extremely fast transformation, often taking place at sub-100 fs timescales. While its experimental characterization can be highly challenging, a rich manifold of theoretical approaches at different levels is nowadays available to support and guide experimental investigations. In this perspective, we summarize the state-of-the-art quantum-chemical methods, as well as molecular- and quantum-dynamics tools successfully applied in ESIPT process studies, focusing on a critical comparison of their specific properties.

High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

A broadband, compact, all-electrically driven mid-infrared frequency comb based on a quantum cascade laser widens the scope of application of combs in this frequency range beyond that of sources which depend on a chain of optical components. A mid-infrared frequency comb Optical frequency combs are light sources that produce a comb-like spectrum, with sharp equidistant frequency modes, and have many uses in metrology and spectroscopy applications. The mid-infrared regime is particularly important for molecular fingerprinting, but so far the comb sources in this wavelength regime are bulky and rely on a chain of optical components. For wide practical applications, an electrically injected, compact scheme is desired. Andreas Hugi et al . now demonstrate a mid-infrared frequency comb generator based on a semiconductor device, a continuous-wave quantum cascade laser. Optical frequency combs 1 act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational–vibrational bands of most light molecules are found 2 . Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals 3 . Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity 4 , 5 , 6 , 7 . Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser 8 , 9 , 10 . However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser 11 are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7 micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10 hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200 hertz. The intermode beat can be varied over a frequency range of 65 kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers.

Solid attosecond science Attosecond techniques exploit the electric field surrounding femtosecond laser pulses to steer electrons, and are widely applied to atoms or molecules in the gas phase. Electrons liberated by few-cycle laser pulses from solids are also predicted to show strong sensitivity to the phase of the light, but observation of this effect has been elusive. Krüger et al . demonstrate the phenomenon in the spectra of electrons laser-emitted from a nanoscale tungsten tip; current modulation of up to 100% and interference are observed, depending on the carrier envelope phase of the laser. This work should facilitate sub-femtosecond, sub-nanometre probing of collective electron dynamics in a range of solid-state systems. Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses 1 . It has led to the generation of extreme-ultraviolet pulses 2 with a duration of less than 100 attoseconds (ref. 3 ; 1 as = 10 −18  s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny 4 , 5 ) and to ultrafast electron holography 6 . All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted 7 , 8 to show a strong light-phase sensitivity, but only very small effects have been observed 14 . Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons 9 ) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms.

Optical arbitrary waveform generation will allow waveforms to be synthesized at optical frequencies but with the flexibility currently available at radiofrequencies. This technique is enabled by combining frequency comb technology, which produces trains of optical pulses with a well-defined frequency spectrum, with pulse shaping methods, which are used to transform a train of ultrashort pulses into an arbitrary waveform. To produce a waveform that fills time, the resolution of the shaper must match the repetition rate of the original pulse train, which in turn must have a comb spectrum that is locked to the shaper. Here, we review the current efforts towards achieving optical arbitrary waveform generation and discuss the possible applications of this technology.

A new multiplex technique of coherent anti-Stokes Raman spectro-imaging with two laser frequency combs is shown to record molecular spectra of broad bandwidth on a microsecond scale. Raman spectroscopy with laser frequency combs Advances in optical spectroscopy and microscopy have had a profound impact throughout the physical, chemical and biological sciences. Particularly valuable are label-free methods capable of probing complex systems in a non-destructive and chemically sensitive manner, ideally with high spatial and temporal resolution. This is offered by coherent Raman spectroscopy, and here Takuro Ideguchi et al now show that it can be implemented using two laser frequency combs and thereby allow spectra covering a wide bandwidth to be measured with high resolution on a single detector on the microsecond timescale. With further system development, the method is expected to offer exciting new possibilities not only in spectroscopy but also for real-time microscopy observations of, for example, biological processes. 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 1 , 2 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 3 , 4 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 5 or frequency-swept laser beams 6 , 7 , 8 , 9 . Laser frequency combs 10 are finding increasing use for broadband molecular linear absorption spectroscopy 11 , 12 , 13 , 14 , 15 . Here we show, by exploring their potential for nonlinear spectroscopy 16 , 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.

Photons as information carriers have the potential to meet the ever-increasing demands on bandwidth and information density in fields such as information and communication technology, biomedicine and computing. Organic semiconductors may be well-suited to such applications, thanks to their ability to transmit, modulate and detect light in an architecture that is low cost, flexible, lightweight and robust. Here we review recent breakthroughs in organic photonics, including ultrafast all-optical modulation in polymer photonic crystals, silicon/organic hybrid systems, gain switching in polymer amplifiers and lasers, and new devices such as hybrid organic/inorganic electrically pumped lasers. The increasing capability for manufacturing a wide variety of optoelectronic devices from polymer and polymer–silicon hybrids, including transmission fibre, modulators, detectors and light sources, suggests that organic photonics has a promising future in communications and other applications.

Integrated switching devices comprise the building blocks of ultrafast optical signal processing 1 , 2 . As the next stage following intensity switching 1 , 3 , 4 , circular polarization switches 5 , 6 , 7 , 8 , 9 are attracting considerable interest because of their applications in spin-based architectures 10 . They usually take advantage of nonlinear optical effects, and require high powers and external optical elements. Semiconductor microcavities provide a significant step forward due to their low-threshold, polarization-dependent, nonlinear emission 11 , 12 , fast operation 13 and integrability. Here, we demonstrate a non-local, all-optical spin switch based on exciton–polaritons in a semiconductor microcavity. In the presence of a sub-threshold pump laser (dark regime), a tightly localized probe induces the switch-on of the entire pumped area. If the pump is circularly polarized, the switch is conditional on the polarization of the probe, but if it is linearly polarized, a circularly polarized probe fully determines the final polarization of the pumped area. These results set the basis for the development of spin-based logic devices, integrated in a chip 14 . An all-optical spin switch based on exciton–polaritons in a semiconductor microcavity is demonstrated. These results may lead to small and fast spin-based on-chip logic devices.