Saying that femtosecond laser technology has undergone changes in recent years is an understatement. This overlooks not only the significant technical advancements but also improvements in accessibility. Complex desktops crowded with user-built components and countless discrete optical devices requiring daily attention have given way to single-box systems tailored to meet the rapidly evolving demands of femtosecond applications. An early example of this transformation is the tunable lasers used for multiphoton microscopy, followed closely by powerful industrial all-in-one lasers supporting micromachining applications from stent cutting to OLED processing.
Femtosecond laser pulses can generate a small amount of second harmonic light from the interface between two materials or any non-centrosymmetric material. The generated second harmonic light signal can non-destructively detect and image features above and below the surface of semiconductor wafers, such as structural defects, thin film quality, and even trace metal contamination.
Today, the latest examples of this trend include a range of shoebox-sized sealed lasers with power levels below 5W, featuring fixed wavelengths at key operating points, including 780, 920, and 1064 nm. These user-friendly lasers further offer application-specific parameters such as short pulse width, high beam quality, optimized pre-compensation for final pulse width, and rapid modulation and control of output power.
A new generation of ultrafast lasers is specifically optimized to support the needs of end-market users, including additive manufacturing, medicine, semiconductor metrology, and applied research.
01 Nanomanufacturing
Lasers can be used in many additive manufacturing (AM) processes, including metal laser sintering and polymer stereolithography. Each of these processes offers a way to create complex and unique structures without the need for masks or molds. Additive manufacturing is particularly valuable for small-scale production applications such as rapid prototyping of parts or personalized medical implants.
An emerging AM method known as two-photon polymerization stereolithography is gaining attention for multiple reasons. First, it achieves higher spatial resolution than any other AM method. Second, it is a three-dimensional freeform process, meaning it is not constrained by the layer-by-layer creation limitations of laser sintering or single-photon stereolithography.
The advent of compact, hands-free femtosecond lasers has made technologies like two-photon polymerization more economically viable across numerous industries and applications.
How does laser technology achieve this? In stereolithography, the laser beam is focused into a bath of photosensitive resin. When light of a suitable wavelength (typically ultraviolet) irradiates this resin, it breaks the polymer bonds, causing the material to become reactive and form solid polymers from liquid monomer chemicals.
Two-photon polymerization is a three-dimensional freeform additive manufacturing technology that offers higher spatial resolution and can produce extremely small parts and features. New femtosecond lasers have made two-photon polymerization economically viable.
This process allows the creation of nearly any shape directly from CAD files, and the raw materials are inexpensive. In the two-photon method, the ultrafast laser is tuned to twice the wavelength normally absorbed by the resin. By using high numerical aperture (NA) optics, the beam is focused to a narrow waist. Only at this waist — and only at this point — is the peak power of the ultrafast pulse high enough to drive two-photon absorption.
This method provides unparalleled resolution for two reasons. First, using high NA optics creates a tightly focused micron-scale waist. Second, since two-photon absorption depends on the square of the peak power, the transmitted laser power can be adjusted so that polymerization occurs only in a small central region within the laser beam waist. This process enables submicron spatial resolution, with researchers in Hong Kong reporting the creation of features measuring approximately 100 nm. By utilizing a programmable mirror array, they further accelerated the process to enable a multi-beam technique.
A new class of femtosecond lasers is ideally suited for this application. These lasers operate at 780 nm, combining high power, short pulse width, and dispersion pre-compensation to deliver high throughput at the focal plane. Compared to longer pulse width lasers, these parameters result in a more efficient polymerization process with higher resolution. User-friendly power control features further enhance precise control over the process. Early applications of these new lasers include manufacturing lab-on-chip products, creating microstructured surfaces, and developing new photonic products such as micro-patterned crystals.
02 Label-Free In Vivo Imaging
Multiphoton excitation microscopy is widely used across life science research. Like two-photon polymerization, it relies on the high peak power of femtosecond pulses to interact with the sample with spatial selectivity only at the tightly focused beam waist.
A key trend here involves translational research, where scientists are gradually shifting multiphoton technology toward clinical laboratory applications and eventually real-time applications such as intraoperative biopsy.
For obvious reasons, the target technologies are those that do not require fluorescent labels or transgenic proteins like green fluorescent protein to generate images. These technologies include second harmonic generation (SHG) for collagen imaging, where 920 nm is a suitable wavelength; third harmonic generation (THG) for membrane imaging, where 1064 nm is an excellent match; and endogenous fluorescence excitation for imaging various biomolecules and metabolites, where 780 to 800 nm works well.
High numerical aperture optics focus the femtosecond laser beam into a tiny waist, with ultrafast pulses achieving peak power high enough to drive two-photon absorption. Additive manufacturing technology offers submicron spatial resolution and can create features as small as 100 nm.
03 Advanced Wafer Metrology
Ultrafast lasers are increasingly important in advanced wafer metrology. A mature technique known as picosecond laser acoustics (PLA) measures layer thickness and images critical alignment marks under opaque layers. The latter capability is crucial in multilayer lithography processes.
In the PLA method, the absorption of a laser pulse (i.e., the pump) generates an acoustic wave that propagates inward from the laser surface. Some of this acoustic energy is reflected back to the surface by underlying layers and structures, where it is detected through changes in reflectivity caused by a second laser pulse (i.e., the probe).
PLA benefits from a new generation of compact femtosecond lasers, which enable higher-resolution imaging and improved overall measurements.
04 Terahertz Generation and Detection
Terahertz radiation can provide unique spectral or imaging information in solid and liquid materials. Frequencies in this range are linked to the vibrations of nanoparticles, macromolecules such as polymers and proteins, and phonon vibrations in crystal structures.
Today, femtosecond laser pulses can be used to generate and detect terahertz radiation through multiple mechanisms.
One method involves focusing femtosecond laser pulses on a photoconductive antenna (or switch) composed of a dielectric material strip, such as gallium arsenide (GaAs), sandwiched between two metal conductors (e.g., gold) with an applied bias voltage. Similar structures are also used as terahertz detectors.
Another method of generating terahertz radiation is called optical rectification, where the laser is focused into a nonlinear crystal, such as gallium phosphide (GaP) or zinc telluride (ZnTe), generating difference frequency components within the terahertz pulse.
Femtosecond laser pulse generation of terahertz pulses offers several advantages over continuous wave methods. The terahertz radiation produced by ultrashort laser pulses has higher intensity. It simultaneously covers a broad and continuous part of the terahertz spectrum, and its pulse nature supports analytical techniques such as time-resolved spectroscopy.
Future Outlook
Although femtosecond lasers are often considered one of the most sophisticated types of coherent light sources, their development and application follow patterns common to all other laser technologies. They have successively transformed from research subjects to research tools and ultimately to components used in other tools and systems. Like other laser technologies, the evolution of femtosecond sources is driven by rapidly expanding practical applications ranging from life sciences to industrial diagnostics and manufacturing processes.
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