S. Witte

A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification

We demonstrate a noncollinear optical parametric chirped pulse amplifier system that produces 7.6 fs pulses with a peak power of 2 terawatt at 30 Hz repetition rate. Using an ultra-broadband Ti:Sapphire seed oscillator and grating-based stretching and compression combined with an LCD phase-shaper, we amplify a 310 nm wide spectrum with a total gain of 3x10(7), and compress it within 5% of its Fourier limit. The total integrated parametric fluorescence is kept below 0.2%, leading to a pre-pulse contrast of 2 x10(-8) on picosecond timescales.

Generation of few-cycle terawatt light pulses using optical parametric chirped pulse amplification

We demonstrate the generation of 9.8+/-0.3 fs laser pulses with a peak power exceeding one terawatt at 30 Hz repetition rate, using optical parametric chirped pulse amplification. The amplifier is pumped by 140 mJ, 60 ps pulses at 532 nm, and amplifies seed pulses from a Ti:Sapphire oscillator to 23 mJ/pulse, resulting in 10.5 mJ/pulse after compression while amplified fluorescence is kept below 1%. We employ grating-based stretching and compression in combination with an LCD phase-shaper, allowing compression close to the Fourier limit of 9.3 fs.

Label-free live brain imaging and targeted patching with third-harmonic generation microscopy

The ability to visualize neurons inside living brain tissue is a fundamental requirement in neuroscience and neurosurgery. Especially the development of a noninvasive probe of brain morphology with micrometer-scale resolution is highly desirable, as it would provide a noninvasive approach to optical biopsies in diagnostic medicine. Two-photon laser-scanning microscopy (2PLSM) is a powerful tool in this regard, and has become the standard for minimally invasive high-resolution imaging of living biological samples. However, while 2PLSM-based optical methods provide sufficient resolution, they have been hampered by the requirement for fluorescent dyes to provide image contrast. Here we demonstrate high-contrast imaging of live brain tissue at cellular resolution, without the need for fluorescent probes, using optical third-harmonic generation (THG). We exploit the specific geometry and lipid content of brain tissue at the cellular level to achieve partial phase matching of THG, providing an alternative contrast mechanism to fluorescence. We find that THG brain imaging allows rapid, noninvasive label-free imaging of neurons, white-matter structures, and blood vessels simultaneously. Furthermore, we exploit THG-based imaging to guide micropipettes towards designated neurons inside live tissue. This work is a major step towards label-free microscopic live brain imaging, and opens up possibilities for the development of laser-guided microsurgery techniques in the living brain.

Ultrafast Optical Parametric Chirped-Pulse Amplification

In recent years, optical parametric chirped-pulse amplification (OPCPA) has emerged as a powerful tool for the generation of ultrashort pulses with extreme peak intensity. It has enabled the generation of phase-controlled few-cycle pulses in widely different parts of the spectrum. For the near-infrared spectral range, OPCPA is becoming an interesting alternative to conventional Ti:Sapphire-based laser technology for various applications. In this paper, we discuss the physics behind OPCPA, as well as the practical design considerations for the development of high-intensity, phase-stable few-cycle OPCPA systems. Also, we review the experimental achievements in ultrafast OPCPA systems to date.

Emission of GHz shear waves by ferroelastic domain walls in ferroelectrics

The 90° domain walls and comparable ferroelectric domain walls are strong sound emitters in the GHz range. The elastic medium on both sides of the domain wall with its mass and elastic properties is a quasi‐infinite transmission line into which the sound is emitted. This emission causes a strong dielectric dispersion at GHz frequencies and may form the basis for many dispersive effects in this frequency range which were observed during the last 40 years in many ferroelectric and even some magnetic materials. A simple elastic‐electric equivalent circuit describes the mechanism of sound emission.

High-power parametric amplification of 11.8-fs laser pulses with carrier-envelope phase control

Phase-stable parametric chirped-pulse amplification of ultrashort pulses from a carrier-envelope phase-stabilized mode-locked Ti:sapphire oscillator (11.0 fs) to 0.25 mJ/pulse at 1 kHz is demonstrated. Compression with a grating compressor and a LCD shaper yields near-Fourier-limited 11.8-fs pulses with an energy of 0.12 mJ. The amplifier is pumped by 532-nm pulses from a synchronized mode-locked laser, Nd:YAG amplifier system. This approach is shown to be promising for the next generation of ultrafast amplifiers aimed at producing terawatt-level phase-controlled few-cycle laser pulses.

Lensless diffractive imaging with ultra-broadband table-top sources: from infrared to extreme-ultraviolet wavelengths

Lensless imaging is an elegant approach to high-resolution microscopy, which is rapidly gaining popularity in applications where imaging optics are problematic. However, current lensless imaging methods require objects to be placed within a well-defined support structure, while the light source needs to have a narrow, stable, and accurately known spectrum. Here we introduce a general approach to lensless imaging without spectral bandwidth limitations or sample requirements. We use two time-delayed coherent light pulses, and show that scanning the pulse-to-pulse time delay allows the reconstruction of diffraction-limited images for all spectral components in the pulse. Moreover, an iterative phase retrieval algorithm is introduced, which uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects without any support requirements. We demonstrate this two-pulse imaging method with octave-spanning visible light sources (in both transmission and reflection geometries), and with broadband extreme-ultraviolet radiation from a high-harmonic source. This demonstrates that our approach enables effective use of low-flux ultra-broadband sources, such as table-top soft-X-ray systems, for high-resolution imaging.

Frequency metrology on the EF (1)Sigma(+)(g)<- X (1)Sigma(+)(g)(0,0) transition in H2, HD, and D2

We present a frequency metrology study on the lowest rotational levels of the hydrogen EF Σg+1 ←X Σg+1 (0,0) two-photon transition near 202 nm. For this purpose, the fourth harmonic of an injection-seeded titanium:sapphire pulsed oscillator is employed in a Doppler-free REMPI-detection scheme on a molecular beam of hydrogen. A frequency comb laser is used to perform the absolute frequency calibration on the continuous-wave (CW) laser that injection-seeds the oscillator. Chirp-induced frequency differences between the output of the pulsed oscillator and the seeding light are monitored on-line, while possible systematic shifts related to the AC-Stark and Doppler effects are addressed in detail. The transition frequencies of the Q (0) to Q (2) lines in H2 and D2, and the Q (0) and Q (1) lines in HD are determined with an absolute accuracy at the 10-9 level.

Phase stability of terawatt-class ultrabroadband parametric amplification

The phase stability of broadband (280 nm bandwidth) terawatt-class parametric amplification was measured, for the first time to our knowledge, with a combination of spatial and spectral interferometry. Measurements at four different wavelengths from 750 to 900 nm were performed in combination with numerical modeling. The phase stability is better than 1/23 rms of an optical cycle for all the measured wavelengths, depending on the phase-matching conditions in the amplifier.

Lensless phase contrast microscopy based on multiwavelength Fresnel diffraction

We demonstrate a compact, wide-field, quantitative phase contrast microscope that does not require lenses for image formation. High-resolution images are retrieved from Fresnel diffraction patterns recorded at multiple wavelengths, combined with a robust iterative phase retrieval algorithm. Quantitative phase contrast images of living cultured neurons are obtained with a transverse resolution of <2 μm. Our system is well suited for high-resolution live cell imaging and provides a compact, cost-effective alternative to full-sized phase-contrast microscopes.