Progress in the development of compact high repetition rate soft x-ray lasers: gain saturation at 10.9 nm and first demonstration

of an all-diode-pumped soft x-ray laser

J.J. Rocca, F.J. Furch, B.A. Reagan, Y. Wang , D. Alessi, D. Martz , B. Luther, M. Berrill , S. Domingue, D. Kemp, F. Pedaci, V.N. Shlyaptsev, M. Marconi, and C.S Menoni

NSF Center for Extreme Ultraviolet Science and Technology, Colorado State University

Fort Collins, Colorado, USA

ABSTRACT

We report new advances in the development of high repetition rate table-top soft x-ray lasers. We have extended the gain–saturated operation of these lasers to 10.9 nm demonstrating a 1 Hz repetition rate laser operation in Ni-like tellurium with an average power of 1 microwatt. In a separate development we have demonstrated the first all-diode-pumped soft x-ray laser. Lasing was achieved in the 18.9 nm line of Ni-like molybdenum in a plasma heated by a compact all-diode-pumped Yb:YAG laser. The solid state pump laser produces 8.5 ps pulses with up to 1 J energy at 10 Hz repetition rate. This diode-pumped laser has the potential to greatly increase the repetition rate and average power of soft x-ray lasers on a significantly smaller footprint. These compact soft X-ray lasers offer new scientific opportunities in small laboratory environments.

Keywords: soft x-ray lasers, diode pumped lasers

INTRODUCTION

There is significant interest in both, extending the operation of high repetition rate soft x-ray lasers to shorter wavelength, and in developing more compact high repetition rate for applications. In these proceedings we report resent advances in both areas. We have extended the range of gain-saturated table-top soft x-ray lasers capable of repetitive operation (eg. repetition rates > 1 Hz) from 13.2 nm [1] to 10.9 nm by demonstrating a saturated Ni-like Te laser that operates at 1 Hz repetition rate. In relation to the second goal, we report the first demonstration of a soft x-ray laser that is pumped by laser diodes. The next two sections report these results. The final section summarizes the measurements of the pulsewidth of an injection-seeded soft x-ray laser, which resulted in the shortest soft x-ray laser pulses reported to date from a plasma-based soft x-ray laser, 1.13 ± 0.47 ps [2]. GAIN SATURATED OPERATION OF A 10.9 NM SOFT X-RAY LASERS AT 1 HZ REPETITION RATE We report gain-saturated operation of a 10.9 nm Ni-like Te soft x-ray laser at a 1 Hz repetition rate. This is the shortest wavelength high repetition rate table-top laser reported to date. The experiments were conducted exciting a solid Te target with a sequence of laser pulses with total energy on target of up to 4.2 J from a Ti:sapphire laser. The amplified Ti:sapphire laser pulses are splitted into two beams using a 40% beamsplitter that directs that fraction of the energy into to a pre-pulse beam. From this pre pulse 2% of the energy was split to generate a pulse that is used to create an early plasma. These two pulses are focused into a ~ 34 µm x 5 mm FWHM line onto a solid Te target resulting in a Te plasma column. The remaining 60% of the laser energy is compressed to ~ 5 ps in a vacuum grating compressor. This pump pulse is directed into the plasma at a 30 degree grazing incidence angle to allows for efficient deposition of laser energy into the plasma as well as for inherent travelling wave excitation. The soft x-ray laser emission is filtered

Invited Paper

Soft X-Ray Lasers and Applications VIII, edited by James Dunn, Gregory J. Tallents, Proc. of SPIE Vol. 7451, 745106 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.826349

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by Zr foils and directed onto a grazing incidence spectrometer consisting of a variable line spaced grating and a back illuminated CCD. Figure 1 shows the measured soft x-ray laser intensity as a function of target length for target lengths between 1.8 mm and 5 mm. The total laser pump energy on target was 3.4 J. Each point represents the average of several laser shots and error bar represents one standard deviation. The solid line is a fit of the data with the expression by Tallents et al, that takes into account gain saturation [3]. Results from the fit show a small signal gain of 45.3 cm-1 and an integrated gain length product of 14.1 at 5 mm. The laser was operated at 1Hz repetition rate by moving the target at a speed of 200 μm per second. Irradiation of a 6 mm wide target with 4.2 J of laser energy generated 10.9 nm laser pulse energies of up to 2 microjoules and an average power of 1 microwatts. The shot-to-shot soft x-ray laser energy variation corresponded to a standard deviation of 35%. This relatively large variation is mostly the result of the fragile nature of the target. We observed that the damage caused to the tellurium target by the 4.2 J pulses is severe, with some laser shots visibly chipping fractions of the target that extend beyond the 200 μm distance we move the target between laser shots. This compares with results from a similar experiment using Ag target, where the energy variation of the 13.9 nm line of Ni-like Ag was 8%.

Fig. 1. Measured intensity vs length for the 10.9 nm Ni-like Te laser. Each data point is an average of several laser shots and error bars show one standard deviation. A fit of the data results in a small signal gain of 45.3 cm-1 and a gain length product of 14.1.

DEMONSTRATION OF A COMPACT ALL-DIODE PUMPED SOFT X-RAY LASER Transient heating of plasmas with picosecond laser pulses impinging at grazing incidence [4-6] have produced saturated table-top lasers operating at wavelengths as short as 13.2 nm at repetition rates of 5-10 Hz [1,6]. As summarized above, in our most recent experiments we have extended gain saturated operation of high repetition rate table-top lasers to 10.9 nm, by demonstrating a Ni-like Te laser that operates at 1 Hz repetition rate. However, in order to make these soft x-ray lasers more practical and accessible tools for applications, it is important to increase their

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average power and reduce their size and complexity. So far, laser-pumped soft x-ray lasers have been built based on flashlamp-pumped chirped pulse amplification (CPA) systems. The repetition rate of these lasers systems is limited by heating of the gain material by the flashlamps to approximately 10 Hz. Moreover, flashlamp-pumped systems usually occupy several optical tables.

We report the first demonstration of an all diode-pumped soft x-ray laser. Lasing was achieved at λ= 18.9 nm by transient collisional electron-impact excitation of Ni-like Mo ions in a plasma created by heating a solid Mo target. All diode-pumped solid-state chirped-pulse-amplification pump laser systems represent a very attractive option to develop a new generation of compact high repetition rate soft x-ray lasers for applications. The small quantum defect of Yb doped materials and the much higher pumping efficiency that results from pumping with a narrow bandwidth source of the optimum wavelength will allow for soft x-ray laser operation at greatly increased repetition rates. Moreover, laser diode-pumped solid state laser systems can be dramatically more compact than equivalent flashlamp-pumped systems. Several Yb-doped materials have high quantum efficiency and absorption bands that can be well matched with commercially available high power diode lasers. In particular, Yb:YAG presents very attractive characteristics for the development of high power amplifiers: good thermal properties, high emission bandwidth and an upper-state lifetime of ~ 1 ms. Several diode-pumped chirped pulse amplification (CPA) systems based on Yb:YAG have been demonstrated [7-9]. A high repetition rate diode-pumped CPA for driving soft x-ray lasers is under development [9]. However, sub-10 ps lasers systems had not yet reached the energy necessary to efficiently pump soft x-ray lasers. Our all-diode pumped soft x-ray laser pump laser consists of a CPA laser system based on two cryo-cooled Yb:YAG amplifiers. The entire pump laser with the exception of the compressor fits in a single 5’× 12’ optical table. Cryo-cooling Yb:YAG to liquid nitrogen temperature allows to improve almost every characteristic of this material: all thermal properties are improved in almost an order of magnitude [10], the emission cross increases about 7 times (reducing the saturation fluence by the same factor) and the gain bandwidth remains relatively broad [11] to generate short pulses at high energy. Figure 2 shows a block diagram of the all-diode-pumped soft x-ray laser system, that is illustrated in Fig.3. The CPA system is conformed by a mode-locked oscillator, two stretchers, a regenerative amplifier, a multipass amplifier, and the vacuum compressor. The soft x-ray laser system is completed with a focusing optics chamber and the target chamber. A Yb:KYW oscillator generates 300 fs pulses at a repetition rate of 57 MHz with an average power of 1.2 W, and it is pumped by a 30 W fiber coupled laser diode. Similar oscillators have been previously demonstrated [12,13]. The oscillator is passively mode-locked by a semiconductor saturable absorber mirror. The output of the oscillator is divided to generate a sequence of three amplified 1.03 μm laser pulses that are used to generate the soft x-ray laser. The first and second pulses are sent through a negative GVD stretcher, while the third pulse is sent through a standard positive GVD stretcher. The pulses are then recombined with variable delays and relative energies and send to the amplifiers. After amplification the pulses go through the vacuum compressor (negative GVD grating pair) that reduces the pulsewidth of one of the pulses while further stretching the other two. This allows us to generate a sequence of two long pulses for plasma creation, followed by a short, plasma heating pulse. Furthermore, since the alignment of the three beams is determined by the cavity of the regenerative amplifier, the spatial overlap of the three pulses is ensured by design. The pulses from the stretcher are amplified to a combined pulse energy of 7 mJ in a diode-pumped Yb:YAG regenerative in which the crystal is cryo-cooled with a closed cycle helium cryostat. The crystal temperature is tuned to about 100 K utilizing an electrical heater to increase the bandwidth of the amplified pulses .Under this conditions gain narrowing reduces the duration of the stretched pulses to ~ 200 ps (positive GVD) and ~ 160 ps (negative GVD stretcher). The amplifier is pumped by 1.2 ms pulses from a 90 W fiber-coupled laser diode emitting at a wavelength of 940 nm. The repetition rate of the amplifier was varied between 10 and 100 Hz with practically no reduction in energy or beam quality.

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Fig. 2. Block diagram of the diode-pumped soft x-ray laser. The oscillator, stretchers and amplifiers are shown to fit on a single 12’ x 5’ optical table.

Fig 3. Photograph of the all-diode-pumped soft x-ray laser based on cryo-cooled Yb-YAG amplifiers. The mode locked Yb:KYW oscillator is seen on front. The cryostats of the Yb:YAG regenerative amplifier and second stage amplifier are seen at the left and center respectively. The entire pump laser with the exception of the pulse compressor fits into a single optical table. The second amplifier is schematically shown in Fig. 4. This multi-pass amplifier increases the energy to the Joule level. The gain medium is split into two 5.5 mm thick Yb:YAG crystals in an active-mirror configuration. The crystals are

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mounted on the opposite faces of a cold finger that is cooled by liquid nitrogen in an evacuated chamber. Each of the crystals is pumped by 3.5 kW pulses of 2 ms duration produced by a stack of 940 nm laser diodes. The pump beams are shaped to illuminate a circular region on each of the Yb:YAG laser crystals. The beam exiting the regenerative amplifier makes 12 passes through the amplifier. After the initial four passes the amplified pulses reach ~ 100 mJ. Subsequently, the beam is expanded to match the size of the pump beams and is sent eight additional times through the gain media (four times through each crystal) by means of changing the polarization. We measured a maximum energy of 1.45 J at 10 Hz repetition rate. The laser was operated at repetition rates up to 50 Hz with only very slight thermal lensing, but with decreased output pulse energy due to a reduction in the gain caused by localized heating of the gain medium that lowers the stimulated emission cross-section. A more rapid rise and fall times of the diode laser pulses used in the present set up combined with better thermal management will mitigate this limitation. Non-linear effects were observed to have an onset at about 1 J, but the B-integral can be reduced by further stretching the input pulse.

In the compressor the positive GVD pulse is compressed to 8.5 ps FWHM (short pulse or plasma heating pulse), while the negative GVD pre-pulses are further stretched to 350 ps FWHM. This set of collinear pulses is focused at a grazing incidence into a 4 mm wide polished Mo target to form a 3.5 mm FWHM long line with a width of ~ 35µm FWHM. The axial soft x-ray plasma emission is analyzed using a 1200 l/mm variable space grating and a back-thinned CCD. Two 0.3 µm thick aluminum filters were used to block the visible light emitted by the plasma. Fig 5 shows a single shot on-axis spectra showing lasing in the 18.9 nm line of Ni-like Mo. This particular spectrum was obtained with a total pump energy on target of about 940 mJ (short pulse 620 mJ). The non-optimized soft x-ray laser output energy was estimated to be 50 nJ. Improvement of the line focus quality can be expected to significantly increase the output energy. This new type of compact diode-pumped laser soft x-ray lasers will greatly increase the repetition rate and average power of soft x-ray lasers on a significantly smaller footprint.

Fig. 4. Schematic layout of multi-pass amplifier. The beam from the regenerative amplifier makes four passes through the gain medium changing the polarization from p to s after the second pass. Next the beam is expanded, sent through the Faraday rotator, and further amplified in additional four passes. Subsequently the polarization changes from s to p, the beam goes through TFP1 and is sent back on itself to make four more passes. Finally the beam switches polarization back to s, and is ejected by TFP2. FR: Faraday Rotator; WP: Waveplate; TFP: Thin Film Polarizer.

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Fig. 5. On-axis single-shot soft x-ray spectra of plasma created by an all-diode-pumped pump laser showing lasing in the 18.9 nm laser line of Ni-like Mo.

GENERATION OF 1 PS SOFT X-RAY LASER PULSES BY INJECTION SEEDING OF A

PLASMA AMPLIFIER Table-top soft x-ray lasers (SXRL) based on the amplification of spontaneous emission in plasma amplifiers produce energetic laser pulses with duration determined by the duration of the gain. Compact soft x-ray lasers generating intense pulses with 5 picoseconds to nanoseconds duration have been used for a wide range of applications. The recent demonstration of injection-seeded SXRLs [14-18] creates a fundamentally new regime for the generation of short soft x-ray laser pulses in which the pulsewidth is governed by the amplifier bandwidth and is independent of the gain duration. In those experiments high harmonic (HH) pulses were amplified in laser transitions from multiply charged ions in plasmas created by either optical field ionization of a gaseous media [144], or by laser irradiation of solid targets [15-18]. The higher density of the latter gain media allows for broader amplifier bandwidths that can support the generation of shorter soft x-ray laser pulses.

We have measured the duration of phase-coherent SXRL pulses from an injection-seeded plasma amplifier created irradiating a solid target. An ultrafast streak camera was used to make single-shot measurements of soft x-ray pulses generated by seeding the 32.6 nm line of Ne-like Ti with the 25th harmonic of a Ti:Sapphire laser. The measured pulse duration of 1.13 ± 0.47 ps is to our knowledge the shortest soft x-ray pulse duration demonstrated to date from a plasma amplifier.

High resolution single shot laser pulse measurements were made possible by an electrostatically focused ultrafast soft x-ray streak camera developed by Zenghu Chang et al at Kansas State University [19]. The SXRL amplifier consisted of a 3 mm long Ti plasma column producing gain in the 32.6 nm J=0-1 line of Ne-like Ti. This transient gain amplifier plasma was created by laser heating a Ti slab with pulses from a table-top Ti:Sapphire laser operating at 800 nm. The amplifier was seeded with pulses from the 25th harmonic of the same laser generated by focusing into an argon gas cell 20 mJ pulses, separately compressed to ~ 50 fs. The HH output of the gas cell was relay imaged onto a ~ 100 µm diameter spot at the input of the plasma amplifier using a gold-coated toroidal mirror designed to operate at a grazing incidence angle of 10 degrees [15]. The Ti soft x-ray amplifier was excited with a sequence of pre-pulses followed by a ~ 0.9 J heating pulse of 6.7 ps duration impinging at a grazing incidence angle of 23 degrees [2]. The streak camera was synchronized with the soft x-ray laser pulse by closing a photoconductive switch using 50 fs, ~ 80 µJ light pulses from the same Ti:Sapphire laser. The resolution of the camera was determined in-situ by measuring the pulse duration of the HH seed pulse, which pulse duration of ~ 50 fs is much shorter than the camera resolution.

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The upper part of Fig. 6 (left) shows the measured distribution of detected photoelectrons at the camera screen for three typical single-shot HH streaks. Each spot is caused by the arrival of a single electron to the camera screen followed by amplification in the microchannel plate. The plot in the lower left part of Fig. 6 shows the measured pulsewidth, which is in effect the camera resolution, as a function of the number of detected photoelectrons in the range of 20-50. Here the pulsewidth is defined as the time interval containing 76% of the electrons.. A linear least square fit of the data shows the pulse width slightly increases as a function of the number of detected photoelectrons. Simulations of the camera response shows this effect is dominantly caused by an slightly larger under-sampling error at low number of detected photoelectrons, and that the spread of the pulsewidth data is dominated by the statistical energy distribution of the electrons emitted by the photocathode. Figure 6 (right) summarizes the streak camera measurement of the amplified seed pulses. The frames on the upper part of the figure show three individual streaks. Simulations of the camera response shows this effect is dominantly caused by an slightly larger under-sampling error at low number of detected photoelectrons, and that the spread of the pulsewidth data is dominated by the statistical energy distribution of the electrons emitted by the photocathode. The linear fit of the measured pulsewidth dependence as a function of detected photoelectrons number shows again a small positive slope. The pulse width of the seeded

laser, Tp, can be obtained from the data in Fig. 6 by de-convolution using 22HHmp TTT −= , where Tm and THH are

the measured seeded laser pulsewidth and the streak camera resolution respectively (dashed line shown at the bottom of Fig 3). The flat slope shows that the resulting seeded laser pulsewidth, 1.13 ± 0.47 ps, is independent from the number of detected photoelectrons within the 20-50 photoelectron range of the measurements.

Fig. 6. Top left: Examples of single shot streaks of high harmonic pulses used to determine the resolution of the camera. The vertical axis is in the direction of the slit. Bottom Left: measured pulsewidth as a function of number of photoelectrons recoded on the camera screen. The letters identify the individual streaks shown on the top of the figure. Top Right: Examples of single shot streak-camera measurements of the amplified seed pulses. Bottom Right: Recorded seeded laser pulsewidth as a function of number of photoelectrons recoded on the camera screen. The solid line is a linear fit to the data. The dashed line is the deconvoluted real pulsewidth as a function of the number of electrons. The resulting measured pulsewidth is 1.13 +- 0.47 ps.

The measured laser pulse duration is in good agreement with model simulations performed using a 3D propagation code that self consistently computes the seed intensity increase considering gain saturation due to population inversion depletion by both the amplified spontaneous emission and the amplified seed pulse in a fully transient approach. The injected seed pulse is computed to initially broaden as its bandwidth narrows dramatically in the much narrower bandwidth plasma amplifier. Subsequently at ~ 2.5 mm within the amplifier the effect of gain saturation starts to contribute to additional pulse broadening. The simulated pulse duration of 1.2 ps at 3 mm is in good agreement with the measured pulsewidth. Simulations suggest that SXRL pulse durations of a few hundred femtosecond should be achieved by seeding a higher density plasma amplifier in which collisional broadening increases the gain bandwidth.

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For example, gain-saturated laser pulses of ~ 290 fs duration could be obtained seeding a 3.5 mm long λ = 10.9 nm Ni-like Te plasma amplifier with an electron density of ~ 2 x 1021 cm-3. These high brightness ultrafast table-top soft x-ray lasers will make possible the diagnostics of very rapidly evolving laser-created plasmas near the critical surface, ultrafast imaging with nanometer-scale resolution, and other ultrafast dynamic applications requiring a large number of photons per pulse.

SUMMARY

Recent progress in the development of high repetition rate soft x-ray lasers at Colorado State University has extended the wavelength range of gain-saturated table-top lasers to 10.9 nm. More than a microwatt average power was obtained at this wavelength exciting a Ni-like Te plasma at 1 Hz repetition rate. Injection seeding of this type of amplifier could result in the generation of sub-picosecond soft x-ray laser pulses. In separate work we have developed a compact Yb:YAG amplifier that has allowed us to demonstrated the first all-diode-pumped soft x-ray laser. In an initial demonstration of this system we have achieved lasing at 18.9 nm in Ni-like Mo at a repetition rate of 10 Hz. This diode-pumped laser has the potential to greatly increase the repetition rate and average power of soft x-ray lasers on a significantly smaller footprint.

Acknowledgement. We acknowledge the important contribution of Zenghu Chang, Mahendra Man Shakya ,Steve Gilbertson and Eduardo Granados to the pulsewidth measurement of the seeded soft x-ray laser. This work was supported by the NSF Engineering Research Centers Program of the National Science Foundation under NSF Award Number EEC-0310717 and by NSF grant # 0521649. . REFERENCES 1. J.J. Rocca, Y. Wang, M.A. Larotonda, B.M. Luther, M. Berrill, and D. Alessi, “Saturated 13.2 nm high-repetition-

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