Ultrafast Amplifiers: a Brighter Future with Great Potential

Ultrafast amplifiers can help steer chemical reactions with more powerful pulses. Courtesy of Google.

What are Ultrafast Amplifiers?

For scientists and engineers, it is always exciting to have their laser machines empowered, and today a variety of ultrafast amplifiers are built right for this purpose. Ultrafast amplifiers are used either to scale up the average output power of an ultrafast laser pulse train, or to intensify individual laser pulses to attain tremendously high peak powers. Nevertheless, the development of ultrafast amplifiers is not merely a pursuit of higher figures of power metrics. Instead, light sources aided by ultrafast amplifiers exhibit enormous potential for diverse applications, and have evolved into powerful tools for a wide spectrum of scientific fields.

Four Applications of Ultrafast Amplifiers

1. Fundamental Physics and Chemistry

By the time the first ultrafast laser emerged, people had managed to establish the grand picture about the cause-and-effect of many physical and chemical processes. However, largely unknown were a plethora of details regarding the intermediate stages between the two ends of the timelines. To study these dynamic processes, the rule of thumb is that the probing systems must rely on signals at approximately the same time regime. Hence, the earliest attention drawn by ultrafast amplifiers were from the physicists and chemists who were interested in those incredibly fast femtosecond to picosecond reactions, such as molecular vibration, charge transfer, molecular conformation changes, and electron-hole scattering.

Perhaps the area that benefits most directly from the invention of ultrafast laser systems is femtochemistry, which offers a window to observe the dynamics of chemical reactions, or more specifically, to understand why the reactions follow one path but not the other. The typical method employed is the so-called pump-probe spectroscopy, in which people initiate a reaction with a pumping pulse, and detect the reaction evolution via subsequent probing pulses. This area has presented fruitful results such as the demonstration of the conformational dynamics of stem-loop RNA structures.

Furthermore, greater potential of ultrafast amplifiers lies in its ability to initiate high harmonic generation, which provides an access to attosecond (10^-18 s) time scale. By isolating short laser pulses at this level, which is already achievable right now via Ti-Sapphire laser and regenerative amplifier, scientists expect to develop advanced spectroscopic techniques for the investigation of even briefer processes, such as the charge transfer in photosynthetic reaction center, and the excited states of DNA. The former can possibly pave a more efficient way of harvesting solar energy and the latter has implications in diseases such as skin cancer.

2. Material Processing

In addition to academic areas, the influence of ultrafast laser systems has also penetrated into the material processing industry, such as photolithography and micro-machining. Using ultrafast amplifiers, semiconductor chip makers can create nanoscale patterns on photoresists with solid state laser. As in micro-machining field, by combining femtosecond pulses with ultrafast amplifiers, engineers can guarantee that the targets are processed intense enough to remove the unwanted materials (millijoule pulse energy), and quick enough before collateral damage occurs (given that heat diffusion and electron-phonon coupling are in picosecond to microsecond scale). Perhaps the most spectacular example is cutting of explosives. It has been demonstrated that the pulsed femtosecond laser can minimize the thermal transfer and shock waves caused by cutting, and the explosive was hence ablated even before any chemical reactions took place.

Fine microstructures made by femtosecond laser micromachining. Courtesy of Google.

Moreover, greater advantages of this technique is that the laser energy is delivered to the material via nonlinear optical absorption, a scenario that allows creating features that are smaller than the diffraction limit. Thus three dimensional submicron structures can be produced for a number of applications, and the most encouraging one, clinical trial of femtosecond LASIK eye surgery is already commercially available. Other applications include photonic devices, read only memory chips, and hollow channel waveguides that may be of significant merit for optical communication network, biological optical chips, optical data memory and more. Thus, even though the associated cost and complexity made ultrafast laser systems difficult for industrial applications, people are still exploring new techniques to improve the situation.

3. Bio-Imaging

In recent year, more functionality of ultrafast amplifiers are exploited for bio-imaging applications. Ultrafast lasers systems are highly favored for their ability to construct 3D image of thick specimens. While the images produced by ordinary fluorescence microscopes may suffer from blurry caused by out-of-focal-plane signals, ultrafast amplifiers can boost the peak power to megawatt (MW) level, where non-linear processes such as two-photon excitation of fluorescent dyes can be driven. Because of this mechanism, the signals above and below the focal plane will be largely eliminated, hence obtaining much better image qualities.

A multicolor image of live mouse brain, made by ultrafast laser systems. Courtesy of Naoki Honkura and Takeshi Imamura

In addition, when performing fluorescence imaging such as multiphoton-excited fluorescence (MPEF) microscopy on living organisms, the critical point is to reach high peak powers for higher image brightness and contrast, and at the meantime limit average power so that the organism will not be damaged by heat or photo-toxicity. For this reason, ultrafast amplifiers are highly desired in that they can offer sustainable path to ultrahigh peak power (100MW) while operating at moderate average power (1-5 W).

More importantly, the ultrafast laser techniques have made possible the generation and detection of terahertz radiation, which has consequently led to terahertz pulse imaging (TPI), an advanced diagnostic measure for epithelial cancer. Traditionally, the cancer diagnosis is performed visually, followed by biopsy for any suspicious lesions. However, with TPI the biopsies may be unnecessary in that TPI in reflection mode exhibits a contrast between cancerous tissue and healthy tissue. Hence, this technique is paving a way to a non-invasive cancer diagnostic method.

4. Nuclear “Alchemy”

It may look absurd to mention alchemy, but with enormously high laser intensity, it become real science. In 1999, researchers in Lawrence Livermore National Laboratory shoot a thin gold foil with laser pulses of 1.5 petawatt (10¹⁵ watt) power and 0.5 femtosecond duration. The laser pulses were so strong that one out of very 10 billion of gold nuclei was induced to decay to platinum. This was the world first nuclear reaction solely driven by laser light, and it also demonstrated the vast potential of laser with ultra-high power.

Conclusion

In a word, ultrafast amplifiers are not only pushing the limit of out ability to harness energy, but also expanding the boundaries of our knowledge territory.

For more information about ultrafast amplifiers, please refer to the FindLight ultrafast laser amplifiers page.