Continued advances in laser technologies have enabled the development of lasers with high pulse energies and high peak powers, which are essential for scientific fields including relativistic plasma physics, astrophysics, and fusion science. However, the use of kilojoule (kJ)-class high-peak-power lasers is often limited by conventional optics made of metals or dielectrics due to their breakdown under high laser intensities. For tackling the challenge, the idea of plasma lenses has been proposed. They have three-dimensional (3D) geometries and designed density distributions, which are ablated by a laser pre-pulse to form a shaped plasma gradient.  After suitable expansion, the shaped plasma gradient creates a transparent “optic” with a defined refractive index profile, which can then modify the focal properties of kJ laser pulses. Such plasma lenses have tight requirements on materials and 3D structures, making their fabrication extremely challenging.

    Laser direct writing via two-photon polymerization (TPP) has been established as one of the most promising micro/nanofabrication techniques. We succeeded in using TPP processes to fabricate low-density plasma lenses with 3D structures in millimeter-scale sizes and submicron resolutions. Four main technical challenges have been addressed to fabricate such plasma lenses. First, low-density microstructures were designed to achieve desired plasma density distributions (i.e. refractive index profiles) upon pre-ablation and plasma expansion. A lens structure resembling a bicycle wheel was designed with the spokes comprising the low-density portion of the lens and the rim providing the mechanical support needed. The second challenge was the selection of low-atomic-number resins (mainly composed of C, H, O elements) capable of generating strong submicron spokes as well as a mechanically robust rim during TPP while keeping the resultant plasma density low. The third challenge was to determine the optimum TPP writing conditions needed to create the submicron spokes in lengths up to 500 µm with minimum deformation. The final challenge was to establish procedures to release the plasma lenses from the substrates and precisely mount them into mechanical holders for insertion into the kJ laser systems.

    The performance of the plasma lenses fabricated was tested using the OMEGA EP laser system at the Laboratory for Laser Energetics at the University of Rochester. These tests demonstrated the performance of the plasma lenses produced via TPP and the numerous potential structures and density distributions that can be rapidly created using the TPP approaches.