Two-photon polymerization (TPP) usually uses a femtosecond laser as a light source. This type of laser has the following key features:

1. Short pulse width: The pulse width generated by a femtosecond laser is extremely short, usually in the range of tens to hundreds of femtoseconds (fs). Such a pulse width can ensure a very high instantaneous power density at the focus, which is crucial for two-photon absorption.

2. High repetition rate: In order to ensure sufficient excitation probability and imaging speed, femtosecond lasers usually operate at a higher repetition rate, such as 50-100 MHz, which helps to generate a large number of pulses in a short time, thereby achieving efficient two-photon polymerization.

3. Tunable wavelength: Two-photon polymerization usually uses lasers in the near-infrared band, because near-infrared light has strong tissue penetration ability and can reduce phototoxicity to samples. The common wavelength range is about 680-1080 nanometers, and in some cases wavelengths around 1550 nanometers are also used.

4. Moderate average power: In order to protect biological samples and avoid thermal effects, the average power of the femtosecond laser needs to be moderate, which can ensure sufficient two-photon excitation without causing damage to the sample.

5. Mode-locking mechanism: In order to produce ultrashort pulses, femtosecond lasers usually use mode-locking technology. This technology can stably produce a series of very short pulse sequences.

Common types of femtosecond lasers include:

• Titanium sapphire laser: This is one of the most commonly used femtosecond lasers and is favored for its wide wavelength tuning range (about 680-1080 nanometers). For example, Coherent's Chameleon series lasers are one of the Ti:Sapphire lasers widely used in two-photon polymerization.

• Fiber lasers: In recent years, fiber lasers have also been widely used in the field of two-photon polymerization, especially in situations where high average power and good stability are required. The advantages of fiber lasers also include compact design, low maintenance and high reliability.

• Other types of lasers: In addition to the above two, there are other types of lasers used for two-photon polymerization, such as some specially designed solid-state lasers or semiconductor lasers.

For example, researchers at Jinan University used multi-wavelength femtosecond lasers for two-photon polymerization. This laser can provide multiple wavelength options, which are suitable for different polymer materials and application requirements.

In addition, scientists at Purdue University in the United States have developed a new two-photon polymerization technology that reduces the required femtosecond laser power by cleverly combining two lasers, thereby reducing costs while maintaining the ability of high-resolution 3D printing.

In summary, femtosecond lasers are an indispensable part of two-photon polymerization technology, especially those that can provide short pulses, high repetition rates, tunable wavelengths, and moderate average powers.