The ground state of quantum systems is characterized by zero-point motion. This motion, in the form of vacuum fluctuations, is generally considered to be an elusive phenomenon that manifests itself only indirectly. Here, we report direct detection of the vacuum fl uctuations of electromagnetic radiation in free space. The ground·state electric-field variance is inversely proportional to the four-dimensional space·time volume, which we sampled electro-optically with tightly focused laser pulses lasting a few femtoseconds. Subcycle temporal readout and nonlinear coupling far from resonance provide signals from purely virtual photons without amplification. Our findings enable an extreme time-domain approach to quantum physics, with nondestructive access to the quantum state of light. Operating at multiterahertz frequencies, such techniques might also allow time-resolved studies of intrinsic fluctuations of elementarY excitations in condensed matter. The quantum properties of light (10) are typi calJy analyzed either by phcton oorrelation (11 14), bomodyning (15 18), or hybrid measurements (19). In those approaches, information is averaged over multiple cycles, and aocessing the vacuum state requires amplification. Femtnsecond studies still rely on pulse envelopes that vary slowly relative tn the carrier frequency (20 23). In our work, we directly probed the varuum noise of the electric field on a subcycle time scale using laser pulses lasting a few femtnseoonds. In ultrabroad band electro optic sampling (24 27), a horizon tally polarized electric field waveform (red in Fig. lA) propagates through an electro optic crystal (EOX), inducing a change Lln of the linear re fractive index 11.o that is proportional to its local amplitude Em:. (Fig. lA and fig. SI). The geometty is adjusted so that a new index ellipsoid emerges under46°tothe polarization of Ern., with nv and nr = 11.o :1:: !!:.n. An ultrashort optical probe pulse at a much higher carrier frequency vp (green in Fig. 1A; intensity, I p, electric field, E.J coprop~ with Em~ at a variable delay time td. The envelope ·of!Pbastn be on theorderofhalfacycle oflightat the highest frequencies il/2rt of En~ that are detected. We used probe pulses as short as tp = 5.8 fs, oorresponding tn Jess than L5 optical cycles at vp = 255 1Hz ( fig. 82). Upon passage through the EOX, the a! andy' components of Ep acquire a relative phase delay proportional to Lln and Eml.,td). The final polarizatim state of the probe is analyzed with ellipsometry. The differential photn rurrent 111/I is proportional tn the electric field Eml.,t,V. We used a radio frequency lock in ampli tier (R.FLA) for readout.We a

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...

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Intermetallic titanium aluminides offer an attractive combination of low density and good oxidation and ignition resistance with unique mechanical properties. These involve high strength and elastic stiffness with excellent high temperature retention. Thus, they are one of the few classes of emerging materials that have the potential to be used in demanding high‐temperature structural applications whenever specific strength and stiffness are of major concern. However, in order to effectively replace the heavier nickel‐base superalloys currently in use, titanium aluminides must combine a wide range of mechanical property capabilities. Advanced alloy designs are tailored for strength, toughness, creep resistance, and environmental stability. These concerns are addressed in the present paper through global commentary on the physical metallurgy and associated processing technologies of γ‐TiAl‐base alloys. Particular emphasis is paid on recent developments of TiAl alloys with enhanced high‐temperature capability.

In this study, we report a minimal-cost and minimal-complexity Cr:LiSAF laser that is pumped only by one inexpensive single-spatial-mode diode. The pump diode, which was originally developed for DVD-writers, provides 130 mW of output power at 660 nm with an efficiency of 30%. This simple pump source enables the construction of a Cr:LiSAF laser that (i) has an estimated total material cost below US$ 5k, (ii) has a footprint of about 20 × 30 cm, (iii) does not require active cooling, and (iv) can be driven by batteries. All of these make this system ideal for applications that require portability. In continuous-wave (cw) laser experiments, we have demonstrated lasing thresholds as low as 2 mW, slope efficiencies as high as 52%, and output powers up to 58 mW. A record cw tuning range extending from 780 to 1110 nm has also been obtained. In cw mode-locking experiments using a saturable Bragg reflector at 850 nm, the Cr:LiSAF laser produced 100 fs pulses with an average power of 38 mW at a repetition rate of 235 MHz. Using a more compact laser cavity, we have also obtained 130 fs pulses with an average power of 33 mW at a repetition rate of 757 MHz. The corresponding electrical-to-optical conversion efficiencies in cw and cw mode-locked regimes were 12.8% and 8.4%, respectively. These results show that, with the progress in laser-diode and optical mirror technology in the last decade, reasonable output powers can now be obtained from Cr:LiSAF lasers that are pumped only by one single-spatial-mode diode. We believe that this compact, low-cost, and simplistic Cr:LiSAF laser system may be an attractive source for several applications including amplifier seeding.