Research Highlights

In two-photo polymerization (TPP), also commonly called multiphoton lithography (MPL), ultrashort laser pulses trigger photo-chemical reaction that forms cross linking of monomer molecules to create polymer. It is common practice to use "weak" pulses with pulse energies in nanojoule range due to availability of laser oscillators and detrimental effects such as laser damage at with "strong" pulses. From an energy efficiency point of view, using strong pulses is beneficial due to nonlinear scaling of two-photon absorption with respect to laser intensity. We model TPP in the strong-pulse regime with pulse energy in the microjoule range and show that there exists an "optimal" writing condition for a specific photopolymer where total energy is minimized. This is important for upscaling TPP for practical use in applications such as photonics, electronics and healthcare. Additive Manufacturing 60, 103241 (2022).

Based on our previous work on the Bessel beam, we use a superposition of high-order Bessel modes to achieve "helical beams" that have intensity rotating along the propagation direction. Pitch, handedness and rotation rate can be tuned by phase modulation on a spatial light modulators. With such beams, micro-helices are fabricated "volumetrically" at a speed >100 faster than conventional methods. This work is a step toward mass-production of micro- and nanostructures for manufacturing devices for photonic, medical and other applications. Photonics Research (2021).

The complexity of the interaction between ultrashort laser pulse and solid material creates a possibility to modify energy coupling and tailor material response using temporal and spectral techniques. We used a two-color, double-pulse approach to reduce material's damage threshold, making it easier to process with low-energy, short-wavelength laser. We are investigating and integrating these techniques, to optimize material processing for better quality, resolution, and productivity.

In a recent paper, we utilize the excitation of free carriers to achieve feature size below the diffraction limit. In this study, two ultrashort laser pulses are spatially separated by a small amount, and by tuning the timing (delay) between the two pulses, features can be formed in overlapped region with size smaller than that permitted by the diffraction limit.

A pulse compressor based on the multi-plate continuum (MPC) design has been constructed. It consists of two compression stages and outputs pulses with 12 fs pulse duration at 1 μm central wavelength, corresponding to <4 optical cycles. Such few-cycles pulses have been used to study laser damage in semiconductors.