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

Laser radiation has been used to cool matter ranging from dilute gases to micromechanical oscillators. In Doppler cooling of gases, the translational energy of atoms is lowered through interaction with a laser field(1,2). Recently, cooling of a high-density gas through collisional redistribution of radiation has been demonstrated(3). In laser cooling of solids, heat is removed through the annihilation of lattice vibrations in the process of anti-Stokes fluorescence(4-6). Since its initial observation in 1995, research(7-15) has led to achieving a temperature of 208 K in ytterbium-doped glass(16). In this Letter, we report laser cooling of ytterbium-doped LiYF(4) crystal to a temperature of similar to 155 K starting from ambient, with a cooling power of 90 mW. This is achieved by making use of the Stark manifold resonance in a crystalline host, and demonstrates the lowest temperature achieved to date without the use of cryogens or mechanical refrigeration. Optical refrigeration has entered the cryogenic regime, surpassing the performance of multi-stage Peltier coolers

'Blinking', or 'fluorescence intermittency', refers to a random switching between 'ON' (bright) and 'OFF' (dark) states of an emitter; it has been studied widely in zero-dimensional quantum dots and molecules, and scarcely in one-dimensional systems. A generally accepted mechanism for blinking in quantum dots involves random switching between neutral and charged states (or is accompanied by fluctuations in charge-carrier traps), which substantially alters the dynamics of radiative and non-radiative decay. Here, we uncover a new type of blinking effect in vertically stacked, two-dimensional semiconductor heterostructures, which consist of two distinct monolayers of transition metal dichalcogenides (TMDs) that are weakly coupled by van der Waals forces. Unlike zero-dimensional or one-dimensional systems, two-dimensional TMD heterostructures show a correlated blinking effect, comprising randomly switching bright, neutral and dark states. Fluorescence cross-correlation spectroscopy analyses show that a bright state occurring in one monolayer will simultaneously lead to a dark state in the other monolayer, owing to an intermittent interlayer carrier-transfer process. Our findings suggest that bilayer van der Waals heterostructures provide unique platforms for the study of charge-transfer dynamics and non-equilibrium-state physics, and could see application as correlated light emitters in quantum technology.

Direct detection of vacuum fluctuations and analysis of sub-cycle quantum properties of the electric field are explored by a paraxial quantum theory of ultrafast electro-optic sampling. The feasibility of such experiments is demonstrated by realistic calculations adopting a thin ZnTe electrooptic crystal and stable few-femtosecond laser pulses. We show that nonlinear mixing of a short near-infrared probe pulse with multi-terahertz vacuum field modes leads to an increase of the signal variance with respect to the shot noise level. The vacuum contribution increases significantly for appropriate length of the nonlinear crystal, short probe pulse durations, tight focusing, and sufficiently large number of photons per probe pulse. If the vacuum input is squeezed, the signal variance depends on the probe delay. Temporal positions with noise level below the pure vacuum may be traced with a sub-cycle accuracy.PACS numbers: 42.50. Ct, 42.50.Lc, 42.65.Re, 78.20.Jq Finite fluctuation amplitudes in the ground state of empty space represent the ultimate hallmark of the quantum nature of the electromagnetic radiation field. These vacuum fluctuations manifest themselves indirectly in a number of phenomena that are accessible to spectroscopy such as the spontaneous decay of excited atomic states as well as the Lamb shift [1] in atoms [2] and in quantummechanical electric circuits [3]. Access to the quantum aspects of electromagnetic radiation is provided by the analysis of photon correlation [4,5] or homodyning [6][7][8][9][10][11] measurements. However, these approaches require amplification of the quantum field under study to finite intensity and averaging of the information over multiple optical cycles.On the other side, precise determination of voltage or electric field amplitude as a function of time represents a fundamental task in science and engineering. Optical techniques have to be applied when detecting electric fields oscillating in the terahertz (THz) range and above. Those approaches involve probing with ultrashort laser pulses of a temporal duration on the order of half an oscillation period at the highest frequencies under study. Far-infrared electric transients [12,13] can be characterized by photoconductive switching [14]. Electro-optic sampling in free space [15][16][17] allows field-resolved detection at high sensitivity in the entire far-and mid-infrared spectral range [18,19]. Direct studies of the complexvalued susceptibilities of materials and the elementary dynamics in condensed matter may be performed with these methods [20,21]. The time integral of near-infrared to visible electric-field wave packets is accessible with attosecond streaking [22]. So far, all those techniques were restricted to the classical field amplitude.In this Letter, we demonstrate theoretically that the quantum properties of light may be accessed directly in the time domain, i.e. with sub-cycle temporal resolution. Our considerations are based on the realistic example of electro-optic detection with zincblende-type materials T...

2Coherent states represent the closest counterpart to a classical electromagnetic wave that exists in quantum electrodynamics. The quantum noise amplitudes of their electric and magnetic fields coincide precisely with those of the vacuum state 19 . Recently, we have succeeded to directly detect the bare vacuum fluctuations of the mid-infrared electric field with highly sensitive electro-optic sampling based on ultrashort laser pulses 15,16 . One key aspect of this technique is that it operates out of a time-domain perspective. Therefore, it should provide a resolution substantially below the duration of an oscillation period of any quantum field under study. Naturally, it is tempting to think about an experiment that synchronously couples a nonclassical state of light into the space-time volume which is probed, thus providing a quantum noise amplitude that deviates from pure vacuum fluctuations. Especially, it would be an attractive manifestation of quantum physics if less noise as compared to the quantum vacuum could be localized in time and space. In conventional homodyning studies, the carrier wave of a local oscillator needs to be phaselocked to a quantum state 11,16 . Instead, we have to prepare a squeezed electromagnetic transient with a noise pattern that is synchronized with the intensity envelope of an ultrashort probe pulse. This tightly focussed few-femtosecond optical wave packet then defines a subcycle space-time segment in which the quantum statistics of a mid-infrared nonclassical signal is sampled. Our scheme to implement such an experiment is sketched in Fig. 1(a). We send an intense near-infrared pump pulse (red-yellow envelope) with duration of 12 fs and centre frequency of 200 THz into a thin generation crystal (GX). In a first step, a carrier-envelope phase-locked electric field transient 20 is generated by optical rectification (red line). Once built up, it starts to locally phase shift the co-propagating multi-terahertz vacuum fluctuations (green shaded band) by means of the electro-optic effect in the GX which establishes a change in refractive index n(t) proportional to the mid-infrared electric field amplitude E THz (t). In a simplified picture, the resulting local anomalies in the speed of light might induce depletion of vacuum amplitude at certain space-time regions (blue shaded sections), piling it up in others (stained in red). A high efficiency for this two-step mechanism to squeeze the mid-infrared vacuum is 3 ensured by the large second-order nonlinearity of the 16-m-thick exfoliated piece of GaSe we employ as GX 20 . Tight focussing of the pump to a paraxial spot radius w pump of 3.6 m also defines the transverse spatial mode for the nonclassical electric field pattern. After the GX, the squeezed vacuum is collimated and residual pump is removed by a 70-m-thick GaSb filter inserted under Brewster's angle. A mode-matched 5.8 fs probe pulse (blue envelope) is then superimposed onto the multi-terahertz field and focussed to w probe = 3.6 m in a AgGaS 2 detector crystal (DX) of...

This review discusses the progress and ongoing efforts in optical refrigeration. Optical refrigeration is a process in which phonons are removed from a solid by anti-Stokes fluorescence. The review first summarizes the history of optical refrigeration, noting the success in cooling rare-earth-doped solids to cryogenic temperatures. It then examines in detail a four-level model of rare-earth-based optical refrigeration. This model elucidates the essential roles that the various material parameters, such as the spacing of the energy levels and the radiative quantum efficiency, play in the process of optical refrigeration. The review then describes the experimental techniques for cryogenic optical refrigeration of rare-earth-doped solids employing non-resonant and resonant optical cavities. It then examines the work on laser cooling of semiconductors, emphasizing the differences between optical refrigeration of semiconductors and rare-earth-doped solids and the new challenges and advantages of semiconductors. It then describes the significant experimental results including the observed optical refrigeration of CdS nanostructures. The review concludes by discussing the engineering challenges to the development of practical optical refrigerators, and the potential advantages and uses of these refrigerators.

Doc. ID 157833)We review the field of laser cooling of solids, focusing our attention on the recent advances in cryogenic cooling of an ytterbium-doped fluoride crystal (Yb 3+ :YLiF 4 ). Recently, bulk cooling in this material to 155 K has been observed upon excitation near the lowest-energy (E4-E5) crystal-field resonance of Yb 3+ . Furthermore, local cooling in the same material to a minimum achievable temperature of 110 K has been measured, in agreement with the predictions of the laser cooling model. This value is limited only by the current material purity. Advanced material synthesis approaches reviewed here would allow reaching temperatures approaching 80 K. Current results and projected improvements position optical refrigeration as the only viable all-solid-state cooling approach for cryogenic temperatures.

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The THz radiation power efficiency of these structures is about 15 times higher compared to a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast THz spectroscopy and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.Comment: 12 pages, 7 figure