Keith A. Wernsing

Demonstration of a 100 Hz repetition rate gain-saturated diode-pumped table-top soft x-ray laser

We demonstrate the operation of a gain-saturated table-top soft x-ray laser at 100 Hz repetition rate. The laser generates an average power of 0.15 mW at λ=18.9  nm, the highest laser power reported to date from a sub-20-nm wavelength compact source. Picosecond laser pulses of 1.5 μJ energy were produced at λ=18.9  nm by amplification in a Mo plasma created by tailoring the temporal intensity profile of single pump pulses with 1 J energy produced by a diode-pumped chirped pulse amplification Yb:YAG laser. Lasing was also obtained in the 13.9 nm line of Ni-like Ag. These results increase by an order of magnitude the repetition rate of plasma-based soft x-ray lasers opening the path to milliwatt average power table-top lasers at sub-20 nm wavelengths.

Demonstration of a compact 100 Hz, 0.1 J, diode-pumped picosecond laser

We have demonstrated an all-diode-pumped Yb:YAG chirped pulse amplification laser that produces 100 mJ pulses of 5 ps duration at 100 Hz repetition rate. The compact laser system combines a room-temperature Yb:YAG regenerative amplifier for increased bandwidth and a cryogenically cooled Yb:YAG four-pass amplifier for improved heat dissipation and increased efficiency. The optical efficiency of this amplifier is higher than that of other diode-pumped systems of comparable energy.

Development of High Energy Diode-Pumped Thick-Disk Yb:YAG Chirped-Pulse-Amplification Lasers

We discuss the results of work directed toward the development of high energy (>;1 J), high average power, diode-pumped picosecond lasers. The design and operation of diode-pumped chirped-pulse-amplification Yb:YAG lasers that combine either room temperature or cryogenically-cooled regenerative amplifiers with cryo-cooled power amplifiers for superior thermal performance and efficient energy extraction are discussed. Results obtained using thick-disk amplifiers include the generation of 100 mJ, 5-ps duration laser pulses at 100-Hz repetition rate, and 1-J pulses of 8.5-ps duration at 10-Hz repetition rate. The performance of the amplifiers in terms of pulse energy and bandwidth under a variety of pump condition is presented.

Superresolved multiphoton microscopy with spatial frequency-modulated imaging

Significance Superresolution microscopy is indispensable in biological sciences. The vast majority of superresolution imaging techniques exploit real energetic states of fluorescent molecules to break the diffraction limit. To date, superresolved imaging of second- and third-harmonic generation has been limited to specific sample preparations where the polarization state of the excitation laser can be manipulated to overcome the diffraction limit. Here, we describe a method for multiphoton superresolved imaging that does not place such restrictions on the sample and allows for simultaneous superresolved imaging of both coherent and incoherent signal light. Combined with single-element detection, this technique may allow for significant advances in multimodal multiphoton imaging of highly scattering biological tissues. Superresolved far-field microscopy has emerged as a powerful tool for investigating the structure of objects with resolution well below the diffraction limit of light. Nearly all superresolution imaging techniques reported to date rely on real energy states of fluorescent molecules to circumvent the diffraction limit, preventing superresolved imaging with contrast mechanisms that occur via virtual energy states, including harmonic generation (HG). We report a superresolution technique based on spatial frequency-modulated imaging (SPIFI) that permits superresolved nonlinear microscopy with any contrast mechanism and with single-pixel detection. We show multimodal superresolved images with two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) from biological and inorganic media. Multiphoton SPIFI (MP-SPIFI) provides spatial resolution up to 2η below the diffraction limit, where η is the highest power of the nonlinear intensity response. MP-SPIFI can be used to provide enhanced resolution in optically thin media and may provide a solution for superresolved imaging deep in scattering media.

Hour-long continuous operation of a tabletop soft x-ray laser at 50-100 Hz repetition rate.

We report the uninterrupted operation of an 18.9 nm wavelength tabletop soft x-ray laser at 100 Hz repetition rate for extended periods of time. An average power of about 0.1 mW was obtained by irradiating a Mo target with pulses from a compact diode-pumped chirped pulse amplification Yb:YAG laser. Series of up to 1.8 x 10(5) consecutive laser pulses of ~1 µJ energy were generated by displacing the surface of a high shot-capacity rotating molybdenum target by ~2 µm between laser shots. As a proof-of-principle demonstration of the use of this compact ultrashort wavelength laser in applications requiring a high average power coherent beam, we lithographically printed an array of nanometer-scale features using coherent Talbot self-imaging.

1 Joule, 100 Hz Repetition Rate, Picosecond CPA Laser for Driving High Average Power Soft X-Ray Lasers

A diode-pumped cryogenic Yb:YAG CPA laser that produces 1J, 5ps pulses allowed for the first time the uninterrupted generation of 1.8×105 sub-20nm wavelength laser pulses with microjoule energy at 100Hz repetition rate on a table-top.

What role do defects play in the laser damage behavior of metal oxides

We have investigated the role of native point defects in the laser damage behavior of amorphous thin films of Sc2O3 deposited by ion beam sputtering. Through systematic characterization and detailed modeling we show that native defects influence the 1-on-1 laser induced damage threshold fluence of the Sc2O3. This effect, as shown by the model and confirmed by experiments, is pulse duration dependent.

Analysis of Recombination in CdTe Heterostructures With Time-Resolved Two-Photon Excitation Microscopy

We used time-resolved photoluminescence micro-scopy to analyze charge carrier transport and recombination in CdTe double heterostructures fabricated by molecular beam epitaxy (MBE). This allowed us to determine the charge carrier mobility in this system, which was found to be 500-625 cm2/(V·s). Charge carrier lifetimes in the 15-100 ns range are limited by the interface recombination, and the data indicate higher interface recombination velocity near extended defects. This study describes a new method to analyze the spatial distribution of the interface recombination velocity and the interface defects in semiconductor heterostructures.

Point spread function engineering with multiphoton SPIFI

MultiPhoton SPatIal Frequency modulated Imaging (MP-SPIFI) has recently demonstrated the ability to simultaneously obtain super-resolved images in both coherent and incoherent scattering processes — namely, second harmonic generation and two-photon fluorescence, respectively.1 In our previous analysis, we considered image formation produced by the zero and first diffracted orders from the SPIFI modulator. However, the modulator is a binary amplitude mask, and therefore produces multiple diffracted orders. In this work, we extend our analysis to image formation in the presence of higher diffracted orders. We find that tuning the mask duty cycle offers a measure of control over the shape of super-resolved point spread functions in an MP-SPIFI microscope.

Two-dimensional single-pixel superresolution imaging by conjugate-domain computed tomography (Conference Presentation)

Superresoltuion (SR) microscopy is a valuable tool for biological studies. While the ability to resolve features to 20 nm and below is now routine in transparent specimens, such as cell cultures and cleared specimens, many areas of biological study have not been probed with SR imaging. Further, SR microscopy has thus far been limited primarily to contrast mechanisms that rely on real energy states of a target molecule, with fluorescence being the dominant modality. We recently demonstrated that spatial-frequency modulated imaging (SPIFI) enables superresolved imaging for both multiphoton fluorescence and nonlinear coherent scattering with single-pixel detection. The technique operates by projecting a set of spatial frequencies in one dimension along a spatiotemporally modulated line-focus that illuminates the specimen. Harmonics of the spatial frequencies projected onto the specimen encode spatial information beyond the diffraction limit of the illumination light. This additional information scales with the order of the nonlinearity, but is limited to the single dimension in which the grating sequence is projected. Consequently, 2D images collected with SR SPIFI are diffraction limited in the dimension perpendicular to the line focus. In this work, we extend our technique to a two-dimensional resolution enhancement with an inverse-domain lateral computed tomography. By enabling 2D SR SPIFI while maintaining single-pixel detection, we anticipate more widespread use of this method for imaging in turbid media.