Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques 1,2 , but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells 3,4 , and magnetic resonance force microscopy 5 has succeeded in detecting single electrons 6 and small nuclear spin ensembles 7 . However, this technique currently requires cryogenic temperatures, which limit most potential biological applications 8 . Alternatively, single-electron spin states can be detected optically 9,10 , even at room temperature in some systems [11][12][13][14] . Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.The nitrogen-vacancy centre in diamond is a unique solid state system that allows ultrasensitive and rapid detection of single electronic spin states under ambient conditions 12 . The nitrogen-vacancy defect is a naturally occurring impurity that is responsible for the pink colouration of diamond crystals when present in high concentration. It was demonstrated that this colour centre can be produced in diamond nanocrystals by electron irradiation. Fluorescing nitrogen-vacancy diamond nanocrystals can be used as markers for bioimaging applications 15 . Such markers have attracted widespread interest because of their unprecedented photostability and non-toxicity 16,17 . It was recognized recently that the magnetic properties of such fluorescent labels can in principle be used for novel microscopy 18,19 . Here we demonstrate the realization of a magneto-optic microscope using nitrogen-vacancy diamond as the magnetic fluorescent label that moreover does not bleach or blink. Figure 1c and d show the fluorescenc...

Individual nanometer-sized plasmonic antennas are excited resonantly with few-cycle laser pulses in the near infrared. Intense third-harmonic emission of visible light prevails for fundamental photon energies below 1.1 eV. Interband luminescence and second harmonic generation occur solely at higher driving frequencies. We attribute these findings to multiphoton resonances with the d-band transitions of gold. The strong third-order signal allows direct measurement of a subcycle plasmon dephasing time of 2 fs, highlighting the efficient radiation coupling and broadband response of the devices.

The advent of self-referenced opt' ical frequency combs,,2 has sparked the development of novel areas in ultrafast sciences such as attosecond technology3.4 and the synthesis of arbitrary optical waveforms s ,6. Few-cycle light pulses are key to these time-domain applications, driving a quest for reliable, stable and cost-efficient mode-locked laser sources with ultrahigh spectral bandwidth. Here, we present a set-up based entirely on compact erbium-doped fibre technology, which produces single cycles of light. The pulse duration of 4.3 fs is close to the shortest possible value for a data bit of information transmitted in the near-infrared regime. These results demonstrate that fundamental limits for optical telecommunications are accessible with existing fibre technology and standard freespace components.Following the report in 1987 of 6-fs optical pulses from a dye laser system 7 , the generation of few-cycle transients has been boosted by Ti:Sapphire technology. Using sophisticated intracavity dispersion control, a pulse duration of 4.4 fs has been achieved directly with a resonator 8 . Ti:Sapphire amplifiers operating at reduced repetition rates enable extreme compression in hollow fibres 9 -11 . Broadband optical parametric oscillators12 and amplifiers 13 have produced pulses as short as 3.9 fs in the visible l 4 and 8.5 fs in the near-infrared I5 . Very recently, 7.8-fs pulses at a central wavelength of 1.2 fLm were implemented with erbium-doped fibre technologyl6. All these results correspond to less than two but more than 1.3 oscillation cycles of the electromagnetic field. To synthesize even shorter pulses, the spectra from femtosecond sources may be shaped in amplitude and phase 6 or pulse trains at different wavelength may be phase-locked and combined 5 .17. In our experiment, we make use of the inherent stability of fibre laser technology18.19 to construct a Single cycle of light through the coherent superposition of two ultrabroadband spectra.The system is outlined in Fig. I, showing a mode-locked femtosecond erbium-doped fibre oscillator 20 operating at a repetition rate of 40 MHz, which provides seed pulses for two parallel femtosecond erbium-doped fibre amplifiers (EDFA; ref. 21). In each branch the average power of the fern to second pulse train is amplified to 330 mW. After coupling into free space, each output beam passes a silicon prism sequence, providing variable dispersion. To generate tailored supercontinua, both pulses are coupled into a standard telecom fibre followed by a splice to a highly nonlinear germanosilicate bulk fibre (HNF; ref. 16). In the HNF, the fundamental pump pulse at 1.55 fLm is split into two spectral components by means of the interplay of dispersion and self-phase modulation. A soli tonic part stabilizes itself by shifting to longer wavelengths. This process provides energy for a dispersive wave, which is pushed towards higher frequencies. The position and bandwidth of these spectral features is determined by the dispersion profile of the HNF and the variable amoun...

Plasmonic nanoantennas are efficient devices to concentrate light in spatial regions much smaller than the wavelength. Only recently, their ability to manipulate photons also on a femtosecond time scale has been harnessed. Nevertheless, designing the dynamical properties of optical antennas has been difficult since the relevant microscopic processes governing their ultrafast response have remained unclear. Here, we exploit frequency-resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near-infrared. Third-harmonic imaging is used in parallel to spatially monitor the plasmonic mode patterns. We find that the few-femtosecond dynamics of these nanodevices is dominated by radiative damping. A high efficiency for nonlinear frequency conversion is directly linked to long plasmon damping times. This single parameter explains the counterintuitive result that rod-type nanoantennas with minimum volume generate by far the strongest third-harmonic emission as compared to the more bulky geometries of bow-tie-, elliptical-, or disk-shaped specimens.

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).