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We explore background-free options to detect mid-infrared (MIR) electric transients. The MIR field and a near-infrared probe interact via sum- (SFG) and difference-frequency generation (DFG) in an electro-optic crystal. An intuitive picture based on a phasor representation and rigorous numerical calculations are used for analysis. It turns out that separating photons generated either by SFG or DFG from the local oscillator via spectral filtering leads to a signal purely proportional the MIR intensity envelope. Background-free phase information may be extracted in a spectral window containing both SFG and DFG components and blocking the local oscillator background based on its orthogonal polarization. This variant leads to signal proportional to the square of the MIR field amplitude. It is limited by the finite efficiency of polarization filtering. The Hilbert transform as a conjugate variable to the electric field in the time domain turns out to play a fundamental role for the context discussed in this paper.

The nonlinear transformation of fluctuations by frequency broadening is found to produce strong anti-correlations in the spectral output. This effect is investigated by dispersive Fourier transform measurements. We exploit the anti-correlations in order to cancel the intensity noise in a subsequent sum-frequency mixing step. This principle allows for the generation of tunable visible pulses by cascaded nonlinear mixing whilst maintaining the same intensity noise performance as the input pulses. In addition, we demonstrate that the power fluctuations occurring in the process of passive stabilization of the carrier-envelope phase locking via difference frequency generation may be cancelled by an analogous strategy.

Femtosecond enhancement cavities (fs-EC), i.e., passive free-space resonators, are commonly used for raising the peak and average power of femtosecond laser pulses to the levels necessary for XUV frequency comb generation via high-order harmonic generation [1] and for augmenting the sensitivity of spectroscopic measurements [2]. Building on previous work on the generation of temporal dissipative Kerr-solitons in fiber- and monolithic microresonators (temporal cavity solitons) [3,4], we demonstrate efficient soliton generation in a fs-EC [5].

Temporal dissipative solitons in nonlinear optical resonators are self-compressed, self-stabilizing and indefinitely circulating wave packets. Owing to these properties, they have been harnessed for the generation of ultrashort pulses and frequency combs in active and passive laser architectures, including mode-locked lasers1–4, passive fibre resonators5 and microresonators6–11. Here, we demonstrate the formation of temporal dissipative solitons in a free-space enhancement cavity with a Kerr nonlinearity and a spectrally tailored finesse. By locking a 100-MHz-repetition-rate train of 350-fs, 1,035-nm pulses to this cavity-soliton state, we generate a 37-fs sech²-shaped pulse with a peak-power enhancement of 3,200, which exhibits low-frequency intensity-noise suppression. The power scalability unique to free-space cavities, the unprecedented combination of peak-power enhancement and temporal compression, and the cavity-soliton-specific noise filtering attest to the vast potential of this platform of optical solitons for applications including spatiotemporal filtering and compression of ultrashort pulses and cavity-enhanced nonlinear frequency conversion. Temporal dissipative soliton formation in a free-space femtosecond enhancement cavity with a thin Kerr medium is reported. Locking a 350-fs, 1,035-nm pulse train with a repetition rate of 100 MHz to this cavity-soliton state generates a 37-fs sech2-shaped pulse with a peak-power enhancement of 3,200.

Label-free optical microscopy plays an important role in biological research. Coherent Raman microscopy combines three-dimensional resolution and fast image acquisition with molecular selectivity based on the vibrational spectrum of the sample. During the last years stimulated Raman scattering (SRS) microscopy has become an important technique in this framework since it does not display an electronic nonresonant background signal and enhances quantitative data analysis. In this method, the energy difference of two quasi-monochromatic beams is tuned to be resonant with a vibrational resonance leading to an energy transfer from the pump to the Stokes field in presence of SRS. To optimize the image acquisition time, we employed Nyquist frequency modulation with boxcar averaging of the signal.

Quantum field theory offers the most exact description of the physics of microscopic interactions. The process offield quantization necessitates the existence of a vacuum field with a corresponding zero-point energy [1]. From this point of view, the ability to directly measure and manipulate electrodynamic quantum vacuum field is an attractive prospect for experimental physics. It is bound to shed new light on the processes governing macroscopic properties of quantum matter [2, 3] as well as to motivate table-top investigations of fundamental constituents of the quantum fields and their intrinsic connections the structure of spacetime [4, 5].

A highly stable setup for stimulated Raman scattering (SRS) microscopy is presented. It is based on a two-branch femtosecond Er:fiber laser operating at a 40 MHz repetition rate. One of the outputs is directly modulated at the Nyquist frequency with an integrated electro-optic modulator (EOM). This compact source combines a jitter-free pulse synchronization with a broad tunability and allows for shot-noise limited SRS detection. The performance of the SRS microscope is illustrated with measurements on samples from material science and cell biology. published

Besides their stunning physical properties which are unmatched in a classical world, squeezed states of electromagnetic radiation bear advanced application potentials in quantum information systems and precision metrology, including gravitational wave detectors with unprecedented sensitivity. Since the first experiments on such nonclassical light, quantum analysis has been based on homodyning techniques and photon correlation measurements. These methods require a well-defined carrier frequency and photons contained in a quantum state need to be absorbed or amplified. They currently function in the visible to near-infrared and microwave spectral ranges. Quantum nondemolition experiments may be performed at the expense of excess fluctuations in another quadrature. Here we generate mid-infrared time-locked patterns of squeezed vacuum noise. After propagation through free space, the quantum fluctuations of the electric field are studied in the time domain by electro-optic sampling with few-femtosecond laser pulses. We directly compare the local noise amplitude to the level of bare vacuum fluctuations. This nonlinear approach operates off resonance without absorption or amplification of the field that is investigated. Subcycle intervals with noise level significantly below the pure quantum vacuum are found. Enhanced fluctuations in adjacent time segments manifest generation of highly correlated quantum radiation as a consequence of the uncertainty principle. Together with efforts in the far infrared, this work opens a window to the elementary quantum dynamics of light and matter in an energy range at the boundary between vacuum and thermal background conditions., 19 pages, 4 figures

Stimulated Raman scattering (SRS) microscopy is an important non-linear optical technique for the investigation of unlabeled samples. The SRS signal manifests itself as a small intensity exchange between the laser pulses involved in coherent excitation of Raman modes. Usually, high-frequency modulation is applied in one pulse train, and the signal is then detected on the other pulse train via lock-in amplification. While allowing shot-noise limited detection sensitivity, lock-in detection, which corresponds to filtering the signal in the frequency domain, is not the most efficient way of using the excitation light. In this manuscript, we show that boxcar averaging, which is equivalent to temporal filtering, is better suited for the detection of low-duty-cycle signals as encountered in SRS microscopy. We demonstrate that by employing suitable gating windows, the signal-to-noise ratios achievable with lock-in detection can be realized in shorter time with boxcar averaging. Therefore, high-quality images are recorded at a faster rate and lower irradiance which is an important factor, e.g., for minimizing degradation of biological samples.Stimulated Raman scattering (SRS) microscopy is an important non-linear optical technique for the investigation of unlabeled samples. The SRS signal manifests itself as a small intensity exchange between the laser pulses involved in coherent excitation of Raman modes. Usually, high-frequency modulation is applied in one pulse train, and the signal is then detected on the other pulse train via lock-in amplification. While allowing shot-noise limited detection sensitivity, lock-in detection, which corresponds to filtering the signal in the frequency domain, is not the most efficient way of using the excitation light. In this manuscript, we show that boxcar averaging, which is equivalent to temporal filtering, is better suited for the detection of low-duty-cycle signals as encountered in SRS microscopy. We demonstrate that by employing suitable gating windows, the signal-to-noise ratios achievable with lock-in detection can be realized in shorter time with boxcar averaging. Therefore, high-quality images are...

We generate few-cycle nonclassical transients by optical rectification of femtosecond pulses and subsequent index modulation in the multi-THz range. Subcycle analysis of quantum statistics by ultrabroadband electro-optic sampling reveals substantial squeezing below the vacuum level.

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 electro-optic 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., 10 pages, 6 figures

Electro-optic sampling at center frequency and bandwidth around 70 THz allows measuring the vacuum fluctuations of the electric field in free space. Distinction from detector shot noise is accomplished by modifying the space-time volume probed.

Phase-stable functional waveforms covering the entire mid infrared are synthesized with amplitudes up to 1 V/A. Few-femtosecond gate pulses allow direct readout of the electric field with sensitivity approaching the single-photon regime.

A phase-locked terahertz transient is exploited as an ultrafast phase gate for femtosecond optical pulses. We directly map out the group delay dispersion of a low-power near-infrared pulse by measuring the electro-optically induced polarization rotation as a function of wavelength. Our experiment covers the spectral window from 1.0 to 1.4 μm and reaches a temporal precision better than 1 fs. A quantitative analysis of the detector response confirms that this streaking technique requires no reconstruction algorithm and is also well suited for the characterization of pulses spanning more than one optical octave.

The generation of phase-locked pulses with a well-defined electric field is essential for applications like high harmonic pulse generation [1] or precision metrology [2]. Active and passive locking schemes have been widely demonstrated utilizing Ti:sapphire technology. However, active stabilization requires cumbersome feedback loops and a locking of f CEO = 0 is very challenging at the full repetition rate of the laser.

Precise characterization techniques for ultrashort optical pulses have become indispensible with the emergence of ever shorter waveforms. Typically the pulse under study is overlapped with an optical gate and analyzed after a nonlinear interaction with the latter. If the duration of the pulse to be characterized is much shorter or longer than available gate sources, streak cameras [1, 2] or electro-optic sampling [3] are used, respectively. If the duration of both pulses is similar, auto- and cross-correlation techniques [4] are required, which need complex deconvolution algorithms and are often limited to bandwidths below one optical octave.

A passively phase-locked laser source based on compact femtosecond Er:fiber technology is introduced. The carrier-envelope offset frequency is set to zero via difference frequency generation between a soliton at a wavelength of 2 μm and a dispersive wave at 860 nm generated in the same highly nonlinear fiber. This process results in a broadband output centered at 1.55 μm. Subsequently, the 40 MHz pulse train seeds a second Er:fiber amplifier, which boosts the pulse energy up to 8 nJ at a duration of 125 fs. Excellent phase stability is demonstrated via f-to-2f spectral interferometry.

We demonstrate a technique for the characterization of ultrabroadband and low-intensity laser pulses using spectrally resolved THz polarization streaking. Our method yields the spectral amplitude and phase without resorting to complex algorithms.

We demonstrate a passively phase-locked seed source for fs-Er:fiber technology with CEO-frequency set to zero. The broadband output is reamplified and phaselocking is monitored by means of an f-2f interferometer.

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 fluctuations 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.