Ismo Vartiainen

Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser

We report a first dissipative dispersive-managed soliton fiber laser operating at 2 μm. The cavity comprised of all-anomalous-dispersion fiber employs chirped fiber Bragg grating, which ensures net-normal cavity dispersion and semiconductor saturable absorber for mode-locking.

A micro-patterned silicon chip as sample holder for macromolecular crystallography experiments with minimal background scattering

At low emittance synchrotron sources it has become possible to perform structure determinations from the measurement of multiple microcrystals which were previously considered too small for diffraction experiments. Conventional mounting techniques do not fulfill the requirements of these new experiments. They significantly contribute to background scattering and it is difficult to locate the crystals, making them incompatible with automated serial crystallography. We have developed a micro-fabricated sample holder from single crystalline silicon with micropores, which carries up to thousands of crystals and significantly reduces the background scattering level. For loading, the suspended microcrystals are pipetted onto the chip and excess mother liquor is subsequently soaked off through the micropores. Crystals larger than the pore size are retained and arrange themselves according to the micropore pattern. Using our chip we were able to collect 1.5 Å high resolution diffraction data from protein microcrystals with sizes of 4 micrometers and smaller.

Laboratory demonstration of a mid-infrared AGPM vector vortex coronagraph

Context. Coronagraphy is a powerful technique to achieve high contrast imaging, hence to image faint companions around bright targets. Various concepts have been used in the visible and near-infrared regimes, while coronagraphic applications in the mid-infrared nowadays remain largely unexplored. Vector vortex phase masks based on concentric subwavelength gratings show great promise for such applications. Aims. We aim at producing and validating the first high-performance broadband focal plane phase mask coronagraphs for applications in the mid-infrared regime, and in particular the L band with a fractional bandwidth of ∼16% (3.5–4.1 μm). Methods. Based on rigorous coupled wave analysis, we designed an annular groove phase mask (AGPM) producing a vortex effect in the L band, and etched it onto a series of diamond substrates. The grating parameters were measured by means of scanning electron microscopy. The resulting components were then tested on a mid-infrared coronagraphic test bench. Results. A broadband raw null depth of 2 × 10 −3 was obtained for our best L-band AGPM after only a few iterations between design and manufacturing. This corresponds to a raw contrast of about 6 × 10 −5 (10.5 mag) at 2λ/D. This result is fully in line with our projections based on rigorous coupled wave analysis modelling, using the measured grating parameters. The sensitivity to tilt and focus has also been evaluated. Conclusions. After years of technological developments, mid-infrared vector vortex coronagraphs have finally become a reality and live up to our expectations. Based on their measured performance, our L-band AGPMs are now ready to open a new parameter space in exoplanet imaging at major ground-based observatories.

Room-temperature macromolecular crystallography using a micro-patterned silicon chip with minimal background scattering.

A micro-patterned sample holder of single-crystalline silicon, loaded with multiple protein crystals which are exposed to a humidified gas stream, allows high-quality room-temperature data collection.

Concentric ring metal grating for generating radially polarized light

A subwavelength concentric ring metal grating for visible light (λ=632.8 nm) is designed and fabricated by electron-beam lithography to transform circularly polarized light into radially polarized light. Experimental results are compared to theoretical predictions and the advantages and disadvantages of the element with alternative methods are discussed.

Interlaced zone plate optics for hard X-ray imaging in the 10 nm range

Multi-keV X-ray microscopy has been particularly successful in bridging the resolution gap between optical and electron microscopy. However, resolutions below 20 nm are still considered challenging, as high throughput direct imaging methods are limited by the availability of suitable optical elements. In order to bridge this gap, we present a new type of Fresnel zone plate lenses aimed at the sub-20 and the sub-10 nm resolution range. By extending the concept of double-sided zone plate stacking, we demonstrate the doubling of the effective line density and thus the resolution and provide large aperture, singlechip optical devices with 15 and 7 nm smallest zone widths. The detailed characterization of these lenses shows excellent optical properties with focal spots down to 7.8 nm. Beyond wave front characterization, the zone plates also excel in typical imaging scenarios, verifying their resolution close to their diffraction limited optical performance.

A beam branching method for timing and spectral characterization of hard X-ray free-electron lasers

We report a method for achieving advanced photon diagnostics of x-ray free-electron lasers (XFELs) under a quasi-noninvasive condition by using a beam-splitting scheme. Here, we used a transmission grating to generate multiple branches of x-ray beams. One of the two primary diffracted branches (+1st-order) is utilized for spectral measurement in a dispersive scheme, while the other (−1st-order) is dedicated for arrival timing diagnostics between the XFEL and the optical laser pulses. The transmitted x-ray beam (0th-order) is guided to an experimental station. To confirm the validity of this timing-monitoring scheme, we measured the correlation between the arrival timings of the −1st and 0th branches. The observed error was as small as 7.0 fs in root-mean-square. Our result showed the applicability of the beam branching scheme to advanced photon diagnostics, which will further enhance experimental capabilities of XFEL.

High-speed fixed-target serial virus crystallography

We report a method for serial X-ray crystallography at X-ray free-electron lasers (XFELs), which allows for full use of the current 120-Hz repetition rate of the Linear Coherent Light Source (LCLS). Using a micropatterned silicon chip in combination with the high-speed Roadrunner goniometer for sample delivery, we were able to determine the crystal structures of the picornavirus bovine enterovirus 2 (BEV2) and the cytoplasmic polyhedrosis virus type 18 polyhedrin, with total data collection times of less than 14 and 10 min, respectively. Our method requires only micrograms of sample and should therefore broaden the applicability of serial femtosecond crystallography to challenging projects for which only limited sample amounts are available. By synchronizing the sample exchange to the XFEL repetition rate, our method allows for most efficient use of the limited beam time available at XFELs and should enable a substantial increase in sample throughput at these facilities.

High-efficiency zone-plate optics for multi-keV X-ray focusing

High-efficiency nanofocusing of hard X-rays using stacked multilevel Fresnel zone plates with a smallest zone width of 200 nm is demonstrated. The approach is to approximate the ideal parabolic lens profile with two-, three-, four- and six-level zone plates. By stacking binary and three-level zone plates with an additional binary zone plate, the number of levels in the optical transmission function was doubled, resulting in four- and six-level profiles, respectively. Efficiencies up to 53.7% focusing were experimentally obtained with 6.5 keV photons using a compact alignment apparatus based on piezoelectric actuators. The measurements have also been compared with numerical simulations to study the misalignment of the two zone plates.

Inverse polarizing effect of subwavelength metallic gratings in deep ultraviolet band

Using subwavelength structures to manipulate the polarization of deep ultraviolet light is generally known as difficult with the current nano-fabrication technologies. An aluminum grating with period close to the incident wavelength is designed and experimentally evidenced to exhibit a pronounced inverse polarizing effect in the deep ultraviolet band. By using the Fourier modal method and planar waveguide theory, we show that the main contributor to the inverse polarizing effect is the excitation of surface plasmons on the front surface of the grating.