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

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial chargetransfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff.

Terahertz spectroscopy unravels the inner working principles of polymer:fullerene blend organic semiconductors with regard to their solar cell performance.

The standard terahertz (THz) detection mechanism known as electro-optic sampling can be improved in sensitivity by biasing the polarization of the sampling field. In this work, we show theoretically and experimentally how weak signals can be amplified without inducing distortions. Our study identifies the influence of THz field strength, the polarization quality, and biasing amplitude on signal amplification and distortion. Here we present a distortionfree amplification of a factor of 28 while at the same time reducing the measurement time significantly.

We present a new technique for single shot Terahertz detection in electro-optics sampling (EOS) with a narrowband probe pulse shaped using a Fabry-Pérot etalon. The technique allows tdetection in the frequency domain using a high-resolution CCD spectrometer. The technique is simple and sensitive. It has a high time resolution and can be simply implemented in a standard EOS scanning experiment