Moderate repetition rate ultra-intense laser targets and optics using variable thickness liquid crystal films

Ultra-intense laser technology improvements allow new facilities to achieve progressively higher repetition rates, soon to exceed 1 Hz with petawatt power.¹ Increased shot rates are necessary to probe new physics regimes² and also promise higher average flux for experiments such as ion acceleration, neutron radiography, and energetic beam generation,^3–5 and related applications like proton cancer therapy,⁶ but only if the problems of target insertion and pulse contrast enhancement at higher repetition rates can be solved. Many approaches originally developed for single-shot lasers will not work in this regime. Presented here is a method of inserting low-Z targets at sustained, moderate repetition rates in the form of freely suspended films made of a commonly available liquid crystal.⁷ The film thickness can be tuned over three orders of magnitude to access multiple experimental and optical application regimes. The basic approach described is also scalable to much higher repetition rates.

A successful insertion mechanism for ultra-intense laser experiments and applications must meet multiple requirements. First, targets must be thin to enable efficient laser interaction, e.g., 10 nm to above 1 μm for fundamental ion acceleration experiments;^3,6 changing thickness to optimize these interactions with the current norm of pre-manufactured, fixed thickness targets is cumbersome for single-shot experiments and impossible above low repetition rate. Second, ultra-high intensities above 10²⁰ $W/cm2$ require few μm-accurate target alignment methods that currently impede moderate repetition rate operation. Third, targets must have sufficiently large extent to minimize plasma damage to the supporting structures. Finally, the target expense must be low to enable practical, sustained runs at moderate repetition rates. Other requirements, for example, involving debris management, may arise beyond this minimal set.

Several current methods satisfy some of these requirements. Gas and liquid sprays allow for kHz repetition rate shots at mJ pulse energies,^8,9 but are non-ideal for some applications due to their dispersed nature⁸ and spherical droplet expansion reducing ion energy and yield.⁹ High rate liquid jets of cryogenic hydrogen¹⁰ and water¹¹ currently cannot achieve sub-μm thicknesses and have relatively high vapor pressure near the target that disrupts ultra-intense laser propagation. A third technique passes a ribbon target between two motors;¹² despite enabling shot rates near 1 Hz, a several μm thick tape is required to survive laser interaction, and the motors cause positional jitter of tens of μm. While improvement of these technologies continues, currently none are feasible for moderate repetition rate intense laser experiments using ultra-thin targets such as are needed for fundamental laser interaction studies or investigation of the full range of ion acceleration mechanisms.

Laser pulse contrast enhancement also presents difficulties at moderate repetition rate. Laser interactions can be critically affected by pre-plasma target expansion arising from poor intensity contrast.¹³ This is often resolved with a plasma mirror device, typically anti-reflection (AR) coated fused silica, which initially transmits pre-pulse light before the main pulse leading edge forms a highly reflective plasma surface. Locally the AR coating is destroyed on shot, and the necessary mirror shift encumbers moderate repetition rate operation. Additionally, laser interactions may benefit¹⁴ from polarizations other than that easily generated from modern high intensity systems. Sufficiently large aperture zero-order waveplates are expensive and difficult to obtain, especially since they must be thin enough (sub-μm) to prevent nonlinear phase accumulation (B-integral) which otherwise degrades a short pulse focus.

A thin, tunable thickness medium provides a solution to these issues—optimizing ion acceleration from thickness-dependent processes such as ion acceleration,³ enhancing pulse contrast by selecting the appropriate thickness for interferometric pre-pulse rejection, and polarization adjustment with minimal pulse degradation using a zero-order waveplate film. Liquid crystal membranes preserve planar geometry while adding the considerable benefit of on-demand thickness variation between a few nm and several tens of μm,⁷ a range not possible with any other target material. Very low vapor pressure allows formation at typical experimental vacuum levels and low volume per film renders them ideal for long-term moderate repetition rate use. A device has been developed that forms such films under vacuum with the temperature and volume control necessary for thickness selection. The design passively maintains excellent alignment even for tight focus geometries, and has been used to demonstrate rapid, thin film target insertion and high power optics formation.

Liquid crystals exhibit mesophases incorporating features of conventional solid and liquid phases, characterized by degrees of molecular structure. The smectic phase of 4′-octyl-4-cyanobiphenyl (8CB) has molecular layering that allows thickness control and sufficient surface tension for freely suspended film formation.⁷ Previous experiments were single-shot, with films formed manually in individual copper frames before being transferred to the vacuum chamber for pumping, alignment, and the laser shot. Faster operation necessitates in-vacuum film formation, and so the Linear Slide Target Inserter (LSTI) shown in Fig. 1(a) was designed to be installed in-situ.

The LSTI consists of a wiper that moves vertically over a single aperture within a copper frame, guided by a thermoplastic polymer bridge (not shown to reveal detail) fitted with spring-loaded plungers to vary the force pressing the wiper down. Aperture diameters from 1 mm to over 11 mm are used for various applications and to prevent plasma damage to the edge. Surface smoothness is critical to thickness control, so the copper frame is polished to a mirror finish and the wiper is fitted with a Teflon bottom to prevent scratching. Temperature regulation is necessary for precise thickness control⁷ and is achieved by water cooling the frame.

A liquid crystal charge is initially applied between the LSTI wiper and frame. A single sub-μm thick film requires volume ∼100 nl, challenging to dispense alone. Instead 1–10 μl or more is applied as a reservoir, and other formation parameters are varied to control thickness. In this way one application of liquid crystal can provide hundreds or thousands of films, enabling prolonged target formation. Proper control of temperature, volume, and LSTI surface polish results in films of uniform thickness that are extremely smooth across their entire area, as shown in Fig. 1(b). Film thickness is monitored by a commercial white light spectral reflectance device relayed from outside the target chamber.⁷ 

Of note is the LSTI formation plane repeatability. Figure 1(c) shows the 45° bevel behind the frame aperture. The smectic phase surface tension moves a forming film to the front bevel edge, resulting in formation to within 2 μm of the same position each time as measured with a μm resolution confocal target positioner.¹⁵ Film formation is illustrated in Fig. 1(c): films begin stretched out of plane between the wiper and aperture edge, causing film curvature which appears dark in this image. As the wiper proceeds through the down-stroke the film lowers into the aperture plane. Targets form well within even small Rayleigh ranges (⁠$F/2$⁠), eliminating the significant problem of repeated target alignment above low repetition rate.

Although temperature, volume, and surface polish all affect film formation, for practical, long-term film repeatability the quickest and most convenient thickness-tuning variable is wiper speed. For example, while the volume applied sets an upper thickness limit and in general a higher temperature results in thicker films, sub-100 nm films can still be produced under the conditions of large volume and high temperature with a sufficiently fast wiper speed. Figure 2 highlights two regions of film formation behavior. The vertical bars indicate the range of film thicknesses produced at the given wiper speed, while the circles show the average of these thicknesses. Region II, at the fastest speeds, produces sub-100 nm films with precision within 10 nm each time—ideal for applications where a consistent, thin target is desired, as is the case of some ion acceleration mechanisms. The maximum repetition rate is limited currently only by the motor, which permits rates as high as 0.3 Hz.

Film thickness increases with decreasing wiper speed, but Region I has less precision due chiefly to the difficulty of controlling film volume on the necessary 100 nl level: slower speeds allow more time for volume flow from the meniscus to the forming film. While thickness scans are a typical tool for many experiments, in some a specific thickness is required for each shot. This requirement merely extends the time between shots and can be achieved in one of two ways: one approach is to apply exactly the correct volume from a precision syringe pump through a wiper hole designed for this purpose. A second approach, and the one used here, is to start with a thicker film and reshape it with additional wiper draws.

Wiping from further above the film aperture brings additional liquid crystal from the reservoir, preferentially accessing the thicker section of a given wiper speed range. Then redrawing at a slightly higher speed (typically a few $μm/s$ increase) effectively wipes away some smectic layers to reduce film thickness. Several redraws may be required depending on the desired accuracy—however, the rapid LSTI draws allow for multiple iterations even on shot/minute systems. Finally, smectic phase stability combined with low vapor pressure enables films formed by the LSTI to maintain their thickness nearly indefinitely, making them ideal as well for laser systems with low repetition rates of several shots per hour or day.

The LSTI was used in several experiments performed with the Scarlet laser, a 400 TW titanium:sapphire based system firing once per minute with 10 J on target in 30 fs pulses, using an $F/2$ off-axis parabola that results in a 2 μm FWHM focal spot.¹⁶ An initial data set used two LSTIs: a 11 mm aperture as a plasma mirror and a 4 mm aperture as the target. The plasma mirror LSTI was placed in the beam bath from the final focusing optic; as such it needed to be quite close (10 mm) to the laser focus to experience sufficient intensity for plasma generation and reflection at high field. The angle of incidence onto both devices was 45°. Proper thickness tuning of the plasma mirror allows an etalon minimum reflection of <0.2% at low incident intensity (pre-pulse rejection) and over 75% at high intensity, in principle enhancing the contrast by a factor of >350.¹⁷ For this experiment the energy in the target chamber was ∼5 J, so the shots with a plasma mirror saw slightly less on-target energy.

Figure 3 shows the maximum proton energy recorded by a Thomson parabola spectrometer observing the target normal direction. Target thickness could be varied to optimize ion acceleration while plasma mirror thickness could be adjusted to change the laser contrast in real time. Films shot without the plasma mirror exhibit decreased proton energy for targets thinner than 1 μm, while cleaned shots showed optimum energy for thicknesses below 200 nm, accessing more efficient thin target acceleration. The uncleaned thick target shots are more resilient to laser prepulse and so have comparable energy with the cleaned thin targets, and further cleaning (from a second plasma mirror) should again enhance the thin target ion acceleration.

Freely suspended liquid crystal films can also be used as waveplates due to their uniaxial birefringence. The 8CB molecular arrangement results in efficient polarization rotation for light transmitting off normal incidence. On-demand thickness tuning allows different polarization effects to be chosen with the same setup. To demonstrate this, an alignment laser was oriented through crossed cube polarizers with a LSTI placed in between, as shown in Fig. 4(a). The results are shown in Fig. 4(b), where the amount of light transmitting through the film and both polarizers is shown as a function of film thickness. For the 632 nm laser incident on the film at 58°, quarter wave rotation was observed at 4.5 μm, and half wave at 9 μm. At Brewster's angle for an 800 nm central wavelength, a 5.5 μm film is required for quarter wave rotation. Though short pulse tests have not been performed yet, calculations of the efficiency for a broadband short pulse indicate liquid crystals should perform similarly to other zero-order waveplate materials. Additionally, the films have demonstrated higher than 100 $mJ/cm2$ damage threshold, and even so film cost and ease of formation make sacrificial waveplate functionality a possibility.

Critically, these films are true zero-order waveplates with the first quarter wave rotation occurring for films on the order of a few microns, in contrast to much thicker compound zero-order waveplates. This is necessary for short pulse polarization rotation, since even short propagation distances through media will cause nonlinear phase accumulation from the intense light that will degrade the focused laser mode. Another unusual and helpful aspect is that, unlike traditional waveplates, the films must be operated off-normal incidence to achieve birefringence, which means potentially damaging back-reflections are of no concern. The exact angle is not critical, so they are easy to align. The LSTI used for this test had a 4 mm aperture, but others have been developed to accommodate larger beam sizes, up to 50 mm currently.

We present an in-situ, liquid crystal based thin film target inserter with on-demand thickness variation from 10 nm to over 50 μm at a repetition rate as high as 0.3 Hz for the thinnest films and better than one shot per minute in general. This range, exceeding three orders of magnitude, gives unprecedented access to the study of physical processes that depend on thickness such as laser-based ion acceleration. The films are formed automatically aligned, and the use of wiper speed as a control mechanism enables large volumes of liquid crystal to be used such that several hundred films can be formed before venting the vacuum chamber. Implementations of plasma mirrors and zero-order waveplates have been demonstrated. Future work includes extending this technique to much higher repetition rates.

We thank Hiroshi Yokoyama and Peter Palffy-Muhoray of the Kent State University Liquid Crystal Institute and Michael Storm for fruitful discussions. This work was supported by the DARPA PULSE program through AMRDEC and by the NNSA under Contract No. DE-NA0001976.