Ultrafast lasers, also known as ultrashort pulse lasers, are characterized by their pulse widths in the range of tens of picoseconds (1 ps = 10^-12 seconds) and below. Since the inception of the first laser in 1960, achieving shorter pulse widths and higher power has been a crucial development goal for laser technology. Over the years, ultrafast laser technology has rapidly advanced and is now widely used in advanced manufacturing, ultrafast imaging, information storage, and clinical medicine.
Compared to continuous-wave lasers or long-pulse lasers, ultrafast lasers exhibit unique characteristics such as short pulse durations, high peak power, and minimal thermal effects. The brief pulse duration allows laser energy to be deposited in an extremely localized region during photon-electron interactions, significantly reducing defects such as recast layers and micro-cracks. Additionally, ultrafast laser pulses can mitigate the plasma shielding effects often seen in long-pulse laser processing. With peak powers reaching up to 100 terawatts (TW), ultrafast lasers can process nearly all materials, including metals, semiconductors, dielectrics, and biological tissues. Due to their high peak power, the interaction of ultrafast lasers with atoms, free electrons, ions, and plasmas is nonlinear and non-equilibrium, rather than thermal. This reduces statistical uncertainties in the laser damage threshold, allowing femtosecond laser processing to achieve sub-micron precision.
Traditional Gaussian beams from ultrafast lasers have limitations in their spatial and temporal energy distributions during processing. Single-point focusing scanning methods often fall short in meeting the precision, efficiency, and multi-scale processing requirements. Consequently, researchers have turned their attention to methods for beam shaping in ultrafast lasers. Laser beam shaping can typically be divided into spatial shaping, temporal shaping, and spatiotemporal shaping. Spatial shaping refers to altering the distribution of laser energy in space, while temporal shaping involves changing the distribution of laser energy over time. Compared to traditional Gaussian beams, shaped beams offer new spatial and temporal energy distributions, providing significant advantages in processing quality, precision, and efficiency.
This article provides a brief overview of temporal beam shaping methods and their applications for ultrafast lasers.
Temporal Beam Shaping Methods
Temporal beam shaping involves generating arbitrary pulse shapes or pulse sequences using methods such as the 4f system, gratings/prisms, or controllable diffractive elements. Temporal beam shaping transforms a single ultrafast pulse into a pulse sequence, with each sequence consisting of several sub-pulses spaced from femtoseconds to picoseconds apart, and each sub-pulse’s energy ratio can also be freely designed. Common methods for temporal beam shaping include Fourier transform methods, spatial light modulators (SLM), metasurfaces, and thin-film techniques.
One method involves using an optical 4f system to temporally shape picosecond lasers. The basic principle is to convert the incident laser from the time domain to the frequency domain, modulate it with a frequency modulator, and then restore it to the time domain to achieve the desired temporal pulse shaping.
Another method is based on diffraction, using a 2D phase SLM to simultaneously shape both the phase and amplitude of femtosecond laser pulses. This approach suppresses certain types of temporal replication features caused by modulator defects in femtosecond pulse shaping and allows for multi-channel outputs suitable for various applications. Dynamic dielectric metasurfaces are used to manipulate the phase and amplitude of frequency components for near-infrared ultrashort (femtosecond) pulse shaping.
Specific Applications
Case Study 1: Chemical Etching of Micromachined Channels in Quartz Glass
Femtosecond pulse sequences are used for chemical etching to create microchannels in quartz glass. The impact of pulse delay and energy distribution on the micromachining results is studied. Compared to traditional femtosecond pulses, temporally shaped femtosecond laser pulse sequences can significantly enhance the etching rate. This enhancement is primarily achieved by controlling the local transient electronic dynamics of the material through ultrafast laser pulse shaping, resulting in higher photon absorption efficiency and a more uniform modified zone. The use of temporally shaped femtosecond pulse sequences allows for non-polarized etching of quartz glass. With pulse delays greater than 1 picosecond, no coherent field vector coupling occurs, leading to the formation of disordered interconnected nanostructures rather than the nanograting structures commonly seen with non-shaped pulses.
Optical microscopy images of femtosecond laser-assisted chemical etching of microchannels: (a) Conventional pulse; (b) Double pulse.
Case Study 2: Polarization-Dependent Anisotropy in Silicon Processing
By varying the pulse delay and scanning speed of femtosecond laser pulse sequences, the polarization-dependent anisotropy during the fabrication of laser-induced periodic surface structures (LIPSS) on silicon is utilized to achieve adjustable scanning line widths. Using orthogonally polarized femtosecond laser double pulse sequences on the silicon surface, the orientation of LIPSS can be controlled such that it remains perpendicular to the scanning direction regardless of the laser path. Adjusting the pulse delay influences the particle size, as shown in the images. Compared to single pulses, femtosecond laser double pulses with a pulse delay of 20 picoseconds can increase the yield by 2.6 times and reduce the average size of silicon nanoparticles by approximately one-fifth.
Transmission electron microscopy (TEM) images and corresponding size distributions of silicon nanoparticles prepared with different pulse delay times: (a) 0 fs; (b) 100 fs; (c) 200 fs; (d) 1000 fs.
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