Daniel Träutlein

Compact coherent anti-Stokes Raman scattering microscope based on a picosecond two-color Er:fiber laser system

We present a compact coherent anti-Stokes Raman scattering microscope based on a widely tunable picosecond Er:fiber laser. Intense and bandwidth-limited 1 ps pump pulses at a center wavelength of 775 nm are generated via frequency mixing within the broadband fundamental at 1.55 microm. Narrowband Stokes pulses are obtained by frequency shifting of solitons in a highly nonlinear bulk fiber and subsequent second-harmonic generation. The tuning range from 850 nm to 1100 nm gives access to vibrational resonances between 1150 cm(-1) and 3800 cm(-1). A first imaging application in the spectral region of CH stretch vibrations is demonstrated.

Two-stage dynamic DNA quality check by xeroderma pigmentosum group C protein

Xeroderma pigmentosum group C (XPC) protein initiates the DNA excision repair of helix‐distorting base lesions. To understand how this versatile subunit searches for aberrant sites within the vast background of normal genomic DNA, the real‐time redistribution of fluorescent fusion constructs was monitored after high‐resolution DNA damage induction. Bidirectional truncation analyses disclosed a surprisingly short recognition hotspot, comprising ∼15% of human XPC, that includes two β‐hairpin domains with a preference for non‐hydrogen‐bonded bases in double‐stranded DNA. However, to detect damaged sites in living cells, these DNA‐attractive domains depend on the partially DNA‐repulsive action of an adjacent β‐turn extension that promotes the mobility of XPC molecules searching for lesions. The key function of this dynamic interaction surface is shown by a site‐directed charge inversion, which results in increased affinity for native DNA, retarded nuclear mobility and diminished repair efficiency. These studies reveal a two‐stage discrimination process, whereby XPC protein first deploys a dynamic sensor interface to rapidly interrogate the double helix, thus forming a transient recognition intermediate before the final installation of a more static repair‐initiating complex.

Multimilliwatt ultrashort pulses continuously tunable in the visible from a compact fiber source

We report on a single-pass device that efficiently converts the broadband near-infrared output from a femtosecond fiber laser into a narrow spectrum in the visible. With fan-out poled MgO:LiNbO3 we obtain sub-picosecond, continuously tunable pulses in the 520-700 nm range. Conversion efficiencies as high as 30% are observed at typical pump power levels of 30 mW, corresponding to average output powers up to 9.5 mW. The specifications of our device are ideal for applications in confocal microscopy and frequency metrology.

Specific local induction of DNA strand breaks by infrared multi-photon absorption

Highly confined DNA damage by femtosecond laser irradiation currently arises as a powerful tool to understand DNA repair in live cells as a function of space and time. However, the specificity with respect to damage type is limited. Here, we present an irradiation procedure based on a widely tunable Er/Yb : fiber femtosecond laser source that favors the formation of DNA strand breaks over that of UV photoproducts by more than one order of magnitude. We explain this selectivity with the different power dependence of the reactions generating strand breaks, mainly involving reactive radical intermediates, and the direct photochemical process leading to UV-photoproducts. Thus, localized multi-photon excitation with a wavelength longer than 1 µm allows for the selective production of DNA strand breaks at sub-micrometer spatial resolution in the absence of photosensitizers.

Highly versatile confocal microscopy system based on a tunable femtosecond Er: Fiber source

The performance of a confocal microscopy setup based on a single femtosecond fiber system is explored over a broad range of pump wavelengths for both linear and nonlinear imaging techniques. First, the benefits of a laser source in linear fluorescence excitation that is continuously tunable over most of the visible spectrum are demonstrated. The influences of subpicosecond pulse durations on the bleaching behavior of typical fluorophores are discussed. We then utilize the tunable near-infrared output of the femtosecond system in connection with a specially designed prism compressor for dispersion control. Pulses as short as 33 fs are measured in the confocal region. As a consequence, 2 mW of average power are sufficient for two-photon microscopy in an organotypic sample from the mouse brain. This result shows great prospect for deep-tissue imaging in the optimum transparency window around 1100 nm. In a third experiment, we prove that our compact setup is powerful enough to exploit even higher-order nonlinearities such as three-photon absorption that we use to induce spatially localized photodamage in DNA.

Few-Cycle Nonlinear Optics with Single Plasmonic Nanoantennas

Optical antennas are excited resonantly with sub-10-fs pulses in the near infrared. Intense third harmonic emission allows measurement of a sub-cycle plasmon dephasing time of 2 fs, demonstrating efficient radiation coupling of these broadband nanodevices.

Nonlinear emission from a single metal nanoantenna excited by 8-fs laser pulses

While the linear optical properties of metal nanoantennas have been studied in detail over the past few years, the origin of incoherent and coherent nonlinear emission is still controversial. Up to now, laser pulses ranging from picoseconds down to 100 fs have been employed for excitation.

Ultrabroadband Er:fiber Systems and Applications

Compact and low-noise Er:fiber lasers allow efficient frequency conversion from the near ultraviolet into the mid infrared. These widely tunable sources enable multi-color experiments in applications ranging from precision metrology via bioimaging to pump-probe measurements on single electron systems.

Generation of multiple DNA lesions at subnuclear resolution by multi-photon irradiation

In the field of DNA repair, multi-photon absorption is becoming increasingly popular as a tool to induce DNA damage with high spatial resolution. Typically, cell nuclei of live cells are subjected to a defined pattern of irradiation at a wavelength around 800 nm, which is the characteristic output of femtosecond Ti:Sapphire lasers. This allows analyzing the spatial and temporal aspects of the recruitment of nuclear proteins, in particular repair factors, at the site of damage. Various types of DNA lesions, like UV-photoproducts [1], strand breaks [2] and reactive oxygen species [3, 4] were reported by different studies using femtosecond pulses with comparable parameters. This method thus appears to lack specificity with respect to the type of DNA damage inflicted [5] hampering the analysis of individual DNA repair pathways. In this study we take advantage of a tunable Er:fiber laser source [6] to investigate DNA damage induction by multiphoton absorption at λ > 800 nm and assess its specificity. To this end, we have increased the output power of our system by integrating a homebuilt Yb:fiber amplifier and a subsequent grating compressor. The output of the Yb-amplifier has a center wavelength of 1050 nm with a full width half maximum (FWHM) of 50 nm, the pulse duration in the focal plane of the objective-lens is sub-100 fs and the average power after the grating compressor is 250 mW. We present a comparative analysis of DNA-damage induced by multi-photon absorption at 774 nm (Figure 1.) and 1050 nm based on damage-specific immunostainings and the recruitment of various DNA repair factors. Potential mechanisms underlying the difference in the observed spectrum of lesions will be discussed.

Confocal Microscopy and Micromanipulation Based on a Femtosecond Fiber Laser with Ultrawide Tuning Range

We present a highly versatile confocal microscopy setup based on a single mode-locked erbium-doped fiber laser. This system provides ultrashort pulses for nonlinear microscopy at 1550 nm (fundamental), from 1050 nm to 1400 nm (tailored supercontinuum) and at 775 nm (second harmonic). Pulse durations in the focal plane are 200 fs at 775 nm and less than 40 fs for the tunable infrared. Instead of using a photonic crystal fiber (PCF), a low-noise supercontinuum is generated in a conventional single-mode fiber.