Session N3: Ultrafast Measurements and Techniques

Next Generation Instrumentation: LAMP~--~\emph{L}CLS~-~\emph{A}SG~-~\emph{M}ichigan~-~\emph{P}roject for Novel Science with the LCLS~FEL

We are designing and building the next generation multi-purpose instrumentation especially adapted to accommodate unique large-area, single-photon counting pnCCD detectors together with advanced many-particle ion and electron imaging spectrometers (reaction microscope, REMI; velocity map imaging, VMI; magnetic bottle) for simultaneous detection of scattered and fluorescent photons and charged particles in experiments at the LCLS FEL. The new end-station presents improvements to the existing CAMP [1] instrument, such as extended range and flexibility of detector positioning and control, better vacuum level, more convenient sample changing procedure, better temperature control, more versatility with pump-probe laser in- and out-coupling, etc. The instrument will be available to any scientist and is planned to be commissioned in the second half of 2012.\\[4pt] [1] L. Str\"{u}der et al., Nucl. Instr. Meth. Phys. Res. A \textbf{614}, \textbf{483}(2010)   

Measurement of the world's shortest X-ray pulses

One of the essential characteristics of LCLS and other FELs that are currently in operation or still under development is their ultrashort pulse duration, further enhanced just recently by novel schemes of electron bunch compression, which opens up unprecedented opportunities for the detailed investigation of reaction dynamics. However, to date there is no measuring device or concept which is able to determine precisely the pulse duration of the generated X-ray pulses. By overlapping the FEL with a synchronized optical laser in a gas target and measuring the energy of the IR laser dressed photoelectrons ('streaking spectroscopy') we were able to determine the pulse duration of the shortest FEL pulses available at LCLS to be not more than 4 fs. In addition, an analysis of the pulse substructure yields an estimation for the length of the underlying single-spikes in the order of 600 as.   

Power Scaling and Stability of Intracavity High Order Harmonic Generation

We generate high order harmonics of a femtosecond frequency comb at the focus of a high finesse optical cavity with 150 MHz repetition rate. The resulting table top high average brightness extreme ultraviolet (XUV) light source has promising applications in XUV frequency metrology, strong field and molecular physics studies, and more traditional XUV applications currently served by synchrotron light sources. We will discuss our recent technical achievements and detailed understandings of the intracavity extreme nonlinear processes that have led to XUV output power beyond the10 $\mu $W per harmonic level and reduced high frequency optical phase noise. We will also present the latest measurement on the coherence properties of VUV/XUV frequency combs.   

Ionization dynamics inside femtosecond enhancement cavities

Intra-cavity high harmonic generation utilizing femtosecond enhancement cavities (fsEC) has been shown as a route to generate frequency combs in the vacuum-ultraviolet. Such VUV frequency combs have the potential to enable precision spectroscopy in this otherwise difficult to access spectral region. Pulse energies exceeding 25 $\mu $J are achievable inside a fsEC with peak intensities at the intracavity focus above 1$\times $10$^{14}$W/cm$^{2}$. At these intensities, we identify fundamental limitations to the intracavity pulse evolution due to ionization induced phase shifts and spectral blue shifting. Numerical simulations and experimental measurements of the intra-cavity ionization dynamics will be presented. We show that the fsEC can itself be used for precise measurements of extreme optical nonlinearities.   

Isolated 80 as XUV pulses characterized by PROOF

Attosecond extreme ultraviolet (XUV) pulses are a useful tool for studying electron dynamics. Double optical gating (DOG) was developed to generate isolated attosecond pulses with broad XUV spectra, violating the central momentum approximation (CMA) of FROG-CRAB (frequency-resolved optical gating for complete reconstruction of Attosecond bursts) for characterizing attosecond pulses. For broadband pulses, PROOF (phase retrieval by omega oscillation filtering) was developed. The quantum interference of the continuum states in the dressing laser field in a streak camera was utilized to retrieve the spectral phase of the XUV pulses. PROOF does not rely on the CMA and sets no limit on the bandwidth. In the experiments, isolated attosecond pulses with continuous spectra from 25 to 80 eV were generated with DOG. The bandwidth is larger than the photoelectron center energy. The pulses are retrieved by FROG-CRAB and PROOF. While the two methods retrieve same 80 as pulses for a nearly transform-limited spectrum, they deviate significantly for a chirped spectrum due to the violation of the CMA in FROG-CRAB.   

Active pulse synchronization for OPCPA systems

The parametric amplification in nonlinear crystals requires both spacial and temporal stability of pump and seed pulses to attain stability of the amplified pulse. Especially the development of thin disk pump sources with pulse lengths down to $2\,ps$ requires a temporal stability well beyond $100\,fs$. To reduce the timing shifts between pump and seed pulses in OPCPA systems we introduce a novel, active pulse synchronization system combining a high precision translation stage and a piezo-electric driven mirror. The timing jitter reduction of OPCPAs with $kHz$ repetition rate demands a fast detection system allowing nearly shot to shot correction. Therefore the spectrum of a cross-correlation between the $1030\,nm$, $1\,nm$ bandwidth pump and a broad bandwidth Ti:Sa seed pulse stretched to $10\,ps$ in a BBO crystal is directly and in real time measured using a position sensitive detector. This method can easily be adopted to other OPA/OPCPA systems giving the chance to correct not only for slow drifts but also for fluctuations up to $300\,Hz$.   

Continuous-Wave Light Modulation at Molecular Frequencies

We use continuous-wave (CW) stimulated Raman scattering inside a hydrogen-filled, high-finesse cavity as a wavelength-independent molecular modular for optical light. CW laser beams whose frequency difference is slightly detuned from a molecular Raman resonance are used to drive rotational transitions in the hydrogen. The high intensity of these fields inside the cavity induces coherent rotations, and in this coherent state the molecules act as a CW modulator. Thus, any wavelength of optical light can be modulated by a single pass through the cavity. In our proof of principle experiment, we use Raman beams of wavelengths 1064 nm and 1135 nm that are resonant with the cavity to create a molecular coherence that modulates a 785 nm beam in a single pass. The modulation frequency is approximately 18 THz, which corresponds to a rotational transition in molecular hydrogen.   

Dynamical image-charge effects in attosecond time-resolved streaked photoelectron spectra of metal surfaces

The release of conduction-band electrons from a metal surface by a sub-femtosecond extreme ultraviolet (XUV) pulse, and their propagation through and near the solid [1], provokes a dielectric response in the solid that acts back on the photoelectron wave packet. We calculated the (wake) potential associated with this photoelectron self-interaction in terms of bulk and surface plasmon excitations and show that it induces a considerable, XUV-frequency-dependent temporal shift in laser-streaked XUV-photoemission spectra [2], suggesting the observation of the ultrafast solid-state dielectric response in contemporary streaked photoemission experiments. \\[4pt] [1] C.-H. Zhang and U. Thumm, Phys. Rev. Lett. 102, 123601(2009); Phys. Rev. A 80, 032902 (2009).\\[0pt] [2] C.-H. Zhang and U. Thumm, Phys. Rev. A 82, 043405(2010).