Ultra fast laser for attosecond science

Ultra fast laser for attosecond science
At present, attosecond pulses are mainly obtained through high-order harmonic generation (HHG) driven by strong fields. The essence of their generation can be understood as electrons being ionized, accelerated, and recombined by a strong laser electric field to release energy, thereby emitting attosecond XUV pulses.
Therefore, attosecond output is extremely sensitive to the pulse width, energy, wavelength, and repetition rate of the driving laser(Ultra fast laser): shorter pulse width is beneficial for isolating attosecond pulses, higher energy improves ionization and efficiency, longer wavelength raises cutoff energy but significantly reduces conversion efficiency, and higher repetition rate improves signal-to-noise ratio but is limited by single pulse energy. Different applications (such as electron microscopy, X-ray absorption spectroscopy, coincidence counting, etc.) have different emphases on the attosecond pulse index, which puts forward differentiated and comprehensive requirements for driving lasers. Improving the performance of driving lasers is crucial for use in attosecond science.

Four core technological routes to enhance the performance of driving lasers(Ultra fast laser)
1. Higher energy: Designed to overcome the low conversion efficiency of HHG and obtain high-throughput attosecond pulses. The technological evolution has shifted from traditional chirped pulse amplification (CPA) to the optical parametric amplification family, including optical parametric chirped pulse amplification (OPCPA), dual chirped OPA (DC-OPA), frequency domain OPA (FOPA), and quasi phase matching OPCPA (QPCPA). Further combining coherent beam synthesis (CBC) and pulse splitting amplification (DPA) synthesis techniques to overcome the physical limitations of single channel amplifiers, such as thermal effects and nonlinear damage, and achieve Joule level energy output.
2. Shorter pulse width: Designed to generate isolated attosecond pulses that can be used to analyze electronic dynamics, requiring few or even sub periodic driving pulses and stable carrier envelope phase (CEP). The main technologies include using nonlinear post compression techniques such as hollow core fiber (HCF), multi thin film (MPSC), and multi-channel cavity (MPC) to compress pulse width to extremely short lengths. CEP stability is measured using an f-2f interferometer and achieved through active feedback/feedforward (such as AOFS, AOPDF) or passive all-optical self stabilization mechanisms based on frequency difference processes.
3. Longer wavelength: Designed to push attosecond photon energy to the “water window” band for biomolecule imaging. The three major technological paths are:
Optical parametric amplification (OPA) and its cascade: It is the mainstream solution in the 1-5 μ m wavelength range, using crystals such as BiBO and MgO: LN; >Crystals such as ZGP and LiGaS ₂ are required for the 5 μ m wavelength band.
Differential Frequency Generation (DFG) and Intra Pulse Differential Frequency (IPDFG): can provide seed sources with passive CEP stability.
Direct laser technology, such as Cr: ZnS/Se transition metal doped chalcogenide lasers, is known as the “mid infrared titanium sapphire” and has the advantages of compact structure and high efficiency.
4. Higher repetition rate: aimed at improving signal-to-noise ratio and data acquisition efficiency, and addressing the limitations of space charge effects. Two main paths:
Resonance enhanced cavity technology: using high-precision resonant cavities to enhance the peak power of megahertz level repetitive frequency pulses to drive HHG, has been applied in fields such as XUV frequency combs, but generating isolated attosecond pulses still poses challenges.
High repetition rate and high-power laser direct drive, including OPCPA, fiber CPA combined with nonlinear post compression, and thin film oscillator, has achieved isolated attosecond pulse generation at a repetition rate of 100 kHz.