Claudius Riek

All-passive phase locking of a compact Er:fiber laser system

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.

Direct sampling of electric-field vacuum fluctuations

Probing the fluctuating vacuum According to quantum mechanics, a vacuum is not empty space. A consequence of the uncertainly principle is that particles or energy can come into existence for a fleeting moment. Such vacuum or quantum fluctuations are known to exist, but evidence for them has been indirect. Riek et al. present an ultrafast optical based technique that probes the vacuum fluctuation of electromagnetic radiation directly. Science, this issue p. 420 Ultrafast optics can directly probe the electric-field vacuum fluctuations. 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.

Subcycle quantum electrodynamics

Squeezed states of electromagnetic radiation have quantum fluctuations below those of the vacuum field. They offer a unique resource for quantum information systems and precision metrology, including gravitational wave detectors, which require unprecedented sensitivity. Since the first experiments on this non-classical form of light, quantum analysis has been based on homodyning techniques and photon correlation measurements. These methods currently function in the visible to near-infrared and microwave spectral ranges. They require a well-defined carrier frequency, and photons contained in a quantum state need to be absorbed or amplified. Quantum non-demolition experiments may be performed to avoid the influence of a measurement in one quadrature, but this procedure comes at the expense of increased uncertainty 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 using electro-optic sampling with few-femtosecond laser pulses. We directly compare the local noise amplitude to that of bare (that is, unperturbed) vacuum. Our nonlinear approach operates off resonance and, unlike homodyning or photon correlation techniques, without absorption or amplification of the field that is investigated. We find subcycle intervals with noise levels that are substantially less than the amplitude of the vacuum field. As a consequence, there are enhanced fluctuations in adjacent time intervals, owing to Heisenberg’s uncertainty principle, which indicate generation of highly correlated quantum radiation. Together with efforts in the far infrared, this work enables the study of elementary quantum dynamics of light and matter in an energy range at the boundary between vacuum and thermal background conditions.

Paraxial Theory of Direct Electro-optic Sampling of the Quantum Vacuum.

Direct detection of vacuum fluctuations and analysis of subcycle 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 the multiterahertz vacuum field 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 pulse duration, tight focusing, and a 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 a noise level below the pure vacuum may be traced with subcycle resolution.

Ultrashort pulse characterization with a terahertz streak camera

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.

Stimulated Raman scattering microscopy by Nyquist modulation of a two-branch ultrafast fiber source

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.

Characterization of ultrashort laser pulses with a terahertz streak camera

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.

Boxcar detection for high-frequency modulation in stimulated Raman scattering microscopy

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...

Subcycle multi-terahertz quantum electrodynamics

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].

Stimulated Raman scattering microscopy with a Nyquist modulated two-branch fiber laser and boxcar signal recovery

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.