Femtosecond fiber oscillator based on a 3×3-coupler-NALM: numerical model and realizations at 1 and 2 µm.

In this article, we present robust passively mode-locked femtosecond lasers operating at 1030 and approximately 2000 nm, respectively. The all-fiber, all-polarization-maintaining (PM) lasers are mode-locked by a nonlinear amplifying loop mirror (NALM) which is attached to the cavity by a 3×3-coupler. The NALM is phase-biased by the coupler, enabling turn-key operation of the oscillator. Femtosecond pulse generation is demonstrated using Ytterbium and Thulium doped active fibers. Depending on the wavelength and the installed dispersive elements, pulse formation can be aided by a range of attractors including self-similar pulse evolution, soliton, or dispersion-managed soliton formation.

Measurement of stress-induced birefringence in a thin-disk laser with full-Stokes polarimetry.

Stress-induced birefringence leads to losses in solid-state laser resonators and amplifiers with polarized output beams. A model of stress-induced birefringence in thin disks is presented, as well as measurements of stress-induced birefringence in a thin disk in a multi-kilowatt oscillator. A full-Stokes imaging polarimeter was developed to enable fast and accurate polarimetric measurements. Experimental and simulated results are in good agreement qualitatively and quantitatively and show that the polarization loss due to stress-induced birefringence is negligible for ytterbium-doped thin disks with a thickness around 100 µm but becomes relevant in thicker disks. It is concluded that stress-induced birefringence should be taken into consideration when designing a thin-disk laser system.

Passively Q-switched 914 nm microchip laser for lidar systems.

Passively Q-switched microchip lasers enable great potential for sophisticated lidar systems due to their compact overall system design, excellent beam quality, and scalable pulse energies. However, many near-infrared solid-state lasers operate at >1000 nm which are not compatible with state-of-the-art silicon detectors. Here we demonstrate a passively Q-switched microchip laser operating at 914 nm. The microchip laser consists of a 3 mm long Nd3+:YVO4 crystal as a gain medium while Cr4+:YAG with an initial transmission of 98% is used as a saturable absorber. Quasi-continuous pumping enables single pulse operation and low duty cycles ensure low overall heat generation and power consumption. Thus, thermally induced instabilities are minimized and operation without active cooling is possible while ambient temperature changes are compensated by adjustment of the pump laser current only. Single-emitter diode pumping at 808 nm leads to a compact overall system design and robust setup. Utilization of a microchip cavity approach ensures single-longitudinal mode operation with spectral bandwidths in the picometer regime and results in short laser pulses with pulse durations below 10 ns. Furthermore, beam quality measurements show that the laser beam is nearly diffraction-limited. A 7% output coupler transmittivity is used to generate pulses with energies in the microjoule regime and peak powers of more than 600 W. Long-term pulse duration, pulse energy, and spectral wavelength measurements emphasize excellent system stability and facilitate the utilization of this laser in the context of a lidar system.

Thin-disk laser system operating above 10 kW at near fundamental mode beam quality.

We report on a thin-disk laser system with more than 10 kW of output power and a beam quality of M2=1.76 at an overall optical-to-optical efficiency of 51%. The system consists of two thin-disk laser oscillators and a thin-disk multi-pass amplifier system. To reach high output powers while maintaining good beam quality, the output beams of two identical laser oscillators are polarization-combined. Subsequently, the beam is amplified in a multi-pass system. To the best of our knowledge, this is the highest output power achieved for a thin-disk laser system with a beam quality close to fundamental mode.

Design concepts in absorbance optical systems for analytical ultracentrifugation.

Analytical ultracentrifugation is a powerful technique for analyzing particles in solution, and has proved valuable for a wide range of applications in chemistry, biochemistry and material sciences for many years. The field is presently seeing a resurgence of instrument development from commercial and academic groups. To date, no modern optical modeling techniques have ever been applied to the basic imaging properties of the optical system in analytical ultracentrifugation. In this manuscript we provide a contextual framework for the application of such techniques, including an overview of the essential optical principles. The existing commercial and open source detection systems are evaluated for imaging performance, highlighting the limitations of chromatic aberration for broadband acquisitions. These results are the inspiration for a new mirror-based design, free of chromatic aberration. Our findings present a path forward for continued development in imaging and detector technology, where improved data quality will now push the limits of detection and resolution of analytical ultracentrifugation for years to come.

Thin-film filter wavelength-stabilized, grating combined, high-brightness kW-class direct diode laser.

We report on kW-class dense wavelength beam combining of a laser diode module consisting of ten broad-area laser diode bars by using a novel multi-laser cavity approach based on a thin-film filter (TFF) as a dispersive optical element. The wavelength-stabilized output of the TFF cavity is beam combined upon a -1st order transmission grating. Hereby, a cylindrical telescope is used for linear dispersion-matching between the TFF and the combiner grating. On the basis of simulations of the resulting beam quality deterioration, we are able to optimize the cavity and the combiner setup for optimal beam quality preservation. We demonstrate a highly efficient direct diode laser with 1.1-kW output power and a symmetrical beam parameter product of about 6mm × mrad (95 % power content) in both beam axis.

Ultrafast time-domain spectroscopy system using 10 GHz asynchronous optical sampling with 100 kHz scan rate.

An ultrafast time-domain spectroscopy system employing asynchronous optical sampling at a repetition rate of 10 GHz is presented. Two ultra-compact Ti:sapphire femtosecond ring lasers allow to achieve scan rates as high as 100 kHz for a 100 ps long time window and a time-delay resolution of 100 fs. The feasibility of this high-speed ASOPS system is evaluated by performing THz time domain spectroscopy on molecular gases where signal-to-noise ratios exceeding 30 dB for averaging times in the millisecond range have been obtained. In order to demonstrate the benefits of this system for ultrafast pump-probe spectroscopy we demonstrate the high-sensitivity detection of coherent acoustic phonons with dephasing times in the range of the 100 ps time window.

Mode-locking of 2 µm Tm,Ho:YAG laser with GaInAs and GaSb-based SESAMs.

We have investigated passive mode-locking of Tm,Ho:YAG lasers with GaInAs- and GaSb-based semiconductor saturable absorber mirrors (SESAMs). With a GaInAs-based SESAM, stable dual-wavelength mode-locking operation was achieved at 2091 nm and 2097 nm, generating pulses with duration of 56.9 ps and a maximum output power of 285 mW. By using the GaSb-based SESAMs, we could generate mode-locked pulses as short as 21.3 ps at 2091 nm with a maximum output power of 63 mW. We attribute the shorter pulse duration obtained with the GaSb SESAMs to the ultrafast recovery time of the absorption and higher nonlinearity compared to standard GaInAs SESAMs.

Mode-locked Yb:YAG thin-disk oscillator with 41 µJ pulse energy at 145 W average infrared power and high power frequency conversion.

We demonstrate the generation of 1.1 ps pulses containing more than 41 µJ of energy directly out of an Yb:YAG thin-disk without any additional amplification stages. The laser oscillator operates in ambient atmosphere with a 3.5 MHz repetition rate and 145 W of average output power at a fundamental wavelength of 1030 nm. An average output power of 91.5 W at 515 nm was obtained by frequency doubling with a conversion efficiency exceeding 65%. Third harmonic generation resulted in 34 W at 343 nm at 34% efficiency.

Passively mode-locked Tm,Ho:YAG laser at 2 microm based on saturable absorption of intersubband transitions in quantum wells.

We report the first demonstration of a solid state laser passively mode-locked through the saturable absorption of short-wavelength intersubband transitions in doped quantum wells: a continuous wave Ti:sapphire laser end-pumped Tm,Ho:YAG laser at the center wavelength of 2.091 mum utilizing intersubband transitions in narrow In(0.53)Ga(0.47)As/Al(0.53)As(0.47)Sb quantum wells. Stable passive mode-locking operation with maximum average output power of up to 160 mW for 2.9 W of the absorbed pump power could last for hours without external interruption and a mode-locked pulse with duration of 60 ps at repetition rate of 106.5 MHz was generated.

Subpicosecond thin-disk laser oscillator with pulse energies of up to 25.9 microjoules by use of an active multipass geometry.

The pulse shaping dynamics of a diode-pumped laser oscillator with active multipass cell was studied experimentally and numerically. We demonstrate the generation of high energy subpicosecond pulses with a pulse energy of up to 25.9 microJ at a pulse duration of 928 fs directly from a thin-disk laser oscillator. These results are achieved by employing a selfimaging active multipass geometry operated in ambient atmosphere. Stable single pulse operation has been obtained with an average output power in excess of 76 W and at a repetition rate of 2.93 MHz. Self starting passive mode locking was accomplished using a semiconductor saturable absorber mirror. The experimental results are compared with numerical simulations, showing good agreement including the appearance of Kelly sidebands. Furthermore, a modified soliton-area theorem for approximating the pulse duration is presented.

Topological guiding of elastic waves in phononic metamaterials based on 2D pentamode structures.

A topological state with protected propagation of elastic waves is achieved by appropriately engineering a phononic metamaterial based on 2D pentamode structures in silicon. Gapless edge states in the designed structure, which are characterized by pseudospin-dependent transport, provide backscattering-immune propagation of the elastic wave along bend paths. The role of the states responsible for forward and backward transfer can be interchanged by design.

Viscoelastic properties and efficient acoustic damping in confined polymer nano-layers at GHz frequencies.

We investigate the viscoelastic properties of confined molecular nano-layers by time resolved optical pump-probe measurements. Access to the elastic properties is provided by the damping time of acoustic eigenmodes of thin metal films deposited on the molecular nano-layers which show a strong dependence on the molecular layer thickness and on the acoustic eigen-mode frequencies. An analytical model including the viscoelastic properties of the molecular layer allows us to obtain the longitudinal sound velocity as well as the acoustic absorption coefficient of the layer. Our experiments and theoretical analysis indicate for the first time that the molecular nano-layers are much more viscous than elastic in the investigated frequency range from 50 to 120 GHz and thus show pronounced acoustic absorption. The longitudinal acoustic wavenumber has nearly equal real and imaginary parts, both increasing proportional to the square root of the frequency. Thus, both acoustic velocity and acoustic absorption are proportional to the square root of frequency and the propagation of compressional/dilatational acoustic waves in the investigated nano-layers is of the diffusional type, similar to the propagation of shear waves in viscous liquids and thermal waves in solids.

Generation and detection of gigahertz acoustic oscillations in thin membranes.

Single crystalline membranes are a perfect model system for the study of coherent acoustic phonon generation and decay in the time domain. Coherent acoustical modes are excited and detected in thin single-crystalline silicon and gallium arsenide membranes with femtosecond pulses in the ultraviolet and infrared wavelength region using the asynchronous optical sampling technique. The measured acoustic spectra are compared with each other and are discussed in terms of different generation and detection mechanisms. A clear dependence of the generated spectra on the absorption length of the pump and probe pulses is observed. It is shown that a short absorption length for the pump pulse leads to the generation of coherent high frequency phonons up to several 100 GHz frequencies. Membranes are demonstrated to be useful as broadband acoustic cavities and can help to disentangle details of high frequency phonon dynamics. Two-layer membrane systems offer additional insight into energy transfer in the GHz frequency range and adhesion properties.

A single-source precursor route to anisotropic halogen-doped zinc oxide particles as a promising candidate for new transparent conducting oxide materials.

Numerous applications in optoelectronics require electrically conducting materials with high optical transparency over the entire visible light range. A solid solution of indium oxide and substantial amounts of tin oxide for electronic doping (ITO) is currently the most prominent example for the class of so-called TCOs (transparent conducting oxides). Due to the limited, natural occurrence of indium and its steadily increasing price, it is highly desired to identify materials alternatives containing highly abundant chemical elements. The doping of other metal oxides (e.g., zinc oxide, ZnO) is a promising approach, but two problems can be identified. Phase separation might occur at the required high concentration of the doping element, and for successful electronic modification it is mandatory that the introduced heteroelement occupies a defined position in the lattice of the host material. In the case of ZnO, most attention has been attributed so far to n-doping via substitution of Zn(2+) by other metals (e.g., Al(3+)). Here, we present first steps towards n-doped ZnO-based TCO materials via substitution in the anion lattice (O(2-) versus halogenides). A special approach is presented, using novel single-source precursors containing a potential excerpt of the target lattice 'HalZn·Zn3O3' preorganized on the molecular scale (Hal = I, Br, Cl). We report about the synthesis of the precursors, their transformation into halogene-containing ZnO materials, and finally structural, optical and electronic properties are investigated using a combination of techniques including FT-Raman, low-T photoluminescence, impedance and THz spectroscopies.

Correction: A single-source precursor route to anisotropic halogen-doped zinc oxide particles as a promising candidate for new transparent conducting oxide materials.

[This corrects the article DOI: 10.3762/bjnano.6.222.].

Passively mode-locked Yb:YAG thin-disk laser with pulse energies exceeding 13 microJ by use of an active multipass geometry.

We demonstrate the generation of high-energy picosecond pulses directly from a thin-disk laser oscillator by employing a self-imaging active multipass geometry. Stable single-pulse operation has been obtained with an average output power in excess of 50 W, excluding a cw background of 8%, at a repetition rate of 3.8 MHz. Self-starting passive mode locking was accomplished using a semiconductor saturable absorber mirror. The maximum pulse energy was 13.4 microJ at a pulse duration of 1.36 ps with a time-bandwidth product of 0.34. Single-pass external frequency doubling with a conversion efficiency of 60% yielded >28 W of average power at 515 nm.

A surface phase transition of supported gold nanoparticles.

A thermal phase transition has been resolved in gold nanoparticles supported on a surface. By use of asynchronous optical sampling with coupled femtosecond oscillators, the Lamb vibrational modes could be resolved as a function of annealing temperature. At a temperature of 104 degrees C the damping rate and phase changes abruptly, indicating a structural transition in the particle, which is explained as the onset of surface melting.

Optimum excitation conditions for the generation of high-electric-field terahertz radiation from an oscillator-driven photoconductive device.

We report the impulsive generation of terahertz (THz) radiation with a field amplitude of more than 1.5 kV/cm at megahertz repetition rates, using an interdigitated photoconducting device. The approach provides an average THz power of 190 microW, corresponding to an optical-to-THz conversion efficiency of 2.5 x 10(-4). Optimum conditions are achieved when the excitation spot size is of the order of the THz wavelength.