Center for Ultrafast Optical Science

The Center for Ultrafast Optical Science (CUOS) is an interdisciplinary research center in the College of Engineering at the University of Michigan in Ann Arbor. CUOS was sponsored as a Science and Technology Center by the National Science Foundation during 1990-2001, and as a College of Engineering Center continues its research in ultrafast optics with funding from a variety of government agencies and industry. Its mission is to perform multidisciplinary research in the basic science and technological applications of ultrashort laser pulses, to educate students from a wide variety of backgrounds in the field, and to spur the development of new technologies.

HERCULES laser

The HERCULES is a 300 TW Ti:Sapphire repetative laser with a peak focused intensity of 2×10²² Wcm^-2 and the ASE ns temporal intensity contrast of up to 10^-15. 

The HERCULES laser design is based on chirped-pulse amplification with cleaning of amplified spontaneous emission (ASE) noise after the first amplifier. The output pulse of the short pulse oscillator (12 fs-pulsewidth, Femtolasers) of the HERCULES laser is preamlified in the two-pass pre-amplifier to the microjoule energy level. The ASE added by the two-pass amplifier is removed by the cleaner based on cross-polarized-wave generation providing the ASE contrast of 10^-11. The clean microjoule energy pulse is stretched to ~0.5 ns by the stretcher based on a modified mirror-in-grating design. The high-energy ring regenerative amplifier and a 4-pass amplifier bring the pulse energy to a joule energy level with nearly diffraction-limited beam quality. Two sequential a 4-pass and a 3-pass Ti:sapphire amplifiers of 25 mm and 50 mm beam diameter respectively raise the output energy to 17 J.

The output pulse is compressed in a 4-grating compressor to ~30 fs. The compressor is based on two 42×21 cm-size and two 22×16.5 cm-size 1200 l/mm-gold-coated holographic gratings (Jobin Yvon). Because the beam size in the compressor is rather large (150 mm diameter), achromatic lenses are used in the final relays to prevent spatially varying group delay across the beam. After the beam compression, it is down-collimated by the all-reflective telescope to 100 mm-diameter and is sent to the interaction chambers located in two separate experimental areas to perform research on (i) laser wakefield electron acceleration and (ii) ion acceleration and high-order harmonic generation from solid density targets.

At present the Hercules laser facility undergoes the upgrade as funded by the National Science Foundation MRI program. This includes (i) the renovation of the laser rooms to ISO class 7 cleanroom; (ii) the replacement of the pump lasers for the 2-nd, 3-rd and 4-th stage of amplification with the commercial lasers GAIA HP and ATLAS 100 from Thales Optronique SA which allows to bring the output power of the Hercules to 500 TW at 1 shot/min and to ~100 TW at 5 Hz repetition rate in a burst mode; (iii) improvements to the radiation shielding in the gas target area; (iv) the modification to the gas target chamber allowing for the use of f/40 focusing optics for an increased output laser power.

HERCULES Target Areas

Two experimental areas of the HERCULES house two separate experimental vacuum chambers for gas and solid targets interaction experiments. Both chambers are connected to the Hercules compressor chamber with vacuum beam lines and are equipped with vacuum pumps, target positioning stages, plasma diagnostics and data acquisition equipment.

Both experimental areas have radiation shielding. 60 cm thick cement walls surround the solid target chamber (Fig. 1) while the gas target chamber has 60 cm thick cement walls and local shielding made of lead bricks. During the facility upgrade the shielding walls in the whole room for the laser wakefield experiments will be made of the high-density concrete blocks 45 cm thick. There are a few primary experiments in solid target chamber (Fig. 2a): ion acceleration, high energy electron transport and high harmonics generation. All these experiments require intensities in excess of 10²¹ W/cm² and high temporal contrast of the laser.

High intensities are achieved by using a short focal length parabolic mirror (f/1 or shorter) to focus laser light into a diffraction limited spot size. For this purpose, we use a 4-inch diameter dielectric coated deformable mirror (DM) (Xinetics Inc.) (Fig. 2b) with a feedback from the wavefront sensor. To achieve high temporal intensity contrast of 10^-11 a special pulse cleaning technique is implemented at the front end of Hercules. Further improvement of the laser contrast by 5 orders of magnitude will be achieved by using 2 plasma mirrors in a separate vacuum chamber. After these mirrors the laser pulse is sent to the interaction chamber where it is corrected for any arising wavefront errors using the DM.

Fig. 3 shows the gas target chamber experimental area where we investigate laser wakefield electron acceleration using gas jets and capillary discharge plasmas and radiation generation. This experimental chamber (Fig. 4) is equipped with a  magnetic spectrometer to study spectral characteristics of the accelerated electrons, a probe line to monitor plasma dynamics and magnetic field generation due to electron current, x-ray CCDs to study betatron radiation from a laser wakefield and various optical diagnostics. The bright multi-keV x-rays produced through betatron oscillations of the electron beam in a laser wakefield accelerator have a unique combination of properties such as an ultra-short duration, small source size and broad spectral coverage.  These properties make this betatron x-ray source ideal for femtosecond temporally resolved phase contrast imaging of rapid phenomena in plasmas as well as in ultrafast x-ray diffraction and absorption spectroscopy.  

The T-cubed laser

The 20 TW peak power CPA hybrid Ti:sapphire/Nd:phosphate glass system delivers 8 J in 400 fs with focused intensity up to 3x10¹⁹ W/cm² (λ=1.053 μm) with the ASE energy contrast of better than 10^-5.  After the pulse compression, the laser is sent to the plasma mirror chamber, where contrast is improved by another 2.5 orders of magnitude. After that, the pulse is delivered to the experimental chamber, equipped with vacuum pumps, target positioning system, optics and different plasma and optical diagnostics apparatus including spectrometers for visible to NIR range, CCDs, x-ray pin diodes, PMTs with scintillators, optical plasma interferometer, Thomson parabola ion spectrometer, etc.

The T-cubed oscillator system “Mira-900” by Coherent utilizes a Kerr mode-locked Ti:sapphire laser producing a 76 MHz train of 100 fs pulses operating at a wavelength of 1.053 microns with an average power of 200 mW. This oscillator is pumped with CW DPSS Sprout-G green laser from Lighthouse Photonics with an average power of ~10  W.  The short pulse train from the oscillator is sent to the stretcher, where it is stretched out temporally by 4 passes on a single 1740 lines/mm grating to approximately 1 ns (Fig. 1). After the selection of a single pulse with the Pockels cell at 10 Hz repetition rate it is amplified in the Ti:sapphire regenerative amplifier to a 1 mJ level. The regenerative amplifier is pumped with frequency doubled 10 Hz repetition rate Quantel “Smart” laser. The pump laser has a wavelength of 0.53 microns, a duration of 6 ns and delivers ~40 mJ/arm of pump energy to both ends of Ti:sapphire crystal.

The regen output is coupled into the amplification chain, where it is amplified in Neodymium doped glass rod (Nd:Glass) amplifiers up to 10 Joules. The chain currently consists of two heads with rods of Ø16×500 mm and one head with a rod of Ø 45×500 mm. Vacuum spatial filters after each Nd:Glass amplifier are used for beam expansion, relay imaging and beam smoothing by elimination of local modulations of its intensity. The operational repetition rates of the final amplifiers are 3 min for the Ø 16 mm heads and 8 min for the Ø 45 mm head. The amplified pulse is delivered to the vacuum compressor chamber, where it is compressed by a dielectric grating pair (1740 l/mm) to duration of 400 fs and has a maximum energy of 8 J. The 10 Hz Ti:Sapphire regenerative amplifier is used as an alignment laser for the amplifier chain, gratings pulse compressor and the experimental setup. From the output of the compressor chamber, the beam is delivered to the plasma mirror chamber and later to the experimental chamber where the pulse is focused on a solid or gas target to produce high-temperature relativistic plasmas.

In the past this laser has been successfully used for the discoveries of many high-field science effects, such as wakefield electron acceleration and observation of collimated electron beam, observation of nonlinear Thomson scattering, observation of Coulomb explosion and ion acceleration in underdense plasma, observation of proton acceleration, radioactive isotope production with accelerated ions, observation of relativistic harmonics generation and others. More recently the T-cubed laser was used to study the front vs rear light ion acceleration, for development of a high purity deuteron beams, neutron source generation, study of the proton conversion efficiency in isolated reduced mass targets, and investigation of  hot electron dynamics via copper K_α imaging.