A laser pulse synthesizer with the ability to attain ultra-short pulses in the mid-IR range while maintaining favorable energy scalability could provide scientists with a more complete view of the inner workings of atoms, molecules and solids. For the past several decades, the only technology available for studying high-speed atomic processes has relied on NIR wavelengths, which work well for studying gases but can cause damage to solids before any observations are made. Much like a musical synthesizer combines notes to generate a new sound, the laser pulse synthesizer combines pulses from the range of mid-IR wavelengths to generate shorter pulses. When the pulses are combined, constructive interference in the middle of the pulses is additive while deconstructive interference cancels out the outer edges of the pulses. The pulses become shorter and shorter, until a sub-cycle pulse is created. To develop the device, researchers at the Massachusetts Institute of Technology used phase-stable 2.1-µ laser light to pump a mid-IR optical parametric amplifier (OPA) and create coherent pulses spanning either 2.5 to 4.4 µ with a pulse width of about 20 femtoseconds (fs) or 4.4 to 9 µ with a 30-fs pulse width. By maintaining pulse stability and making sure that the pulses precisely overlapped both temporally and spatially, researchers were able to combine them into synthesized pulses that were only 13 fs wide and spanned 2.5 to 9 µ with 33 microjoules (µJ) of energy. Although the 13 fs pulse duration is longer than what is possible with NIR and visible wavelengths, it corresponds to less than one optical oscillation cycle of 5 µ wavelength, enabling the sub-cycle control of electron motion. Mid-IR pulses could be used in a variety of applications and scientific research areas. For example, in non-invasive surgery these wavelengths could be used to remove diseased tissue without damaging the surrounding healthy tissue. These pulses are also useful for spectroscopy and for observing fast chemical reactions, since many biological molecules and atmospheric chemicals absorb in this wavelength range. "These mid-IR pulses will allow new types of experiments that explore dynamics taking place in atoms, molecules and solids," said researcher Kyung-Han Hong. "For example, we can use them to take a movie of how electrons behave inside of atoms and solids." The research team has explored using the high-energy, mid-IR pulses for high-harmonic generation to produce coherent pulses in the extreme UV and soft x-ray regions. "Compared to near-IR and visible light, mid-IR pulses accelerate electrons to much higher energies and, thereby, generate higher energy photons," said Hong. "Also, isolation of soft x-ray pulses becomes easier at these wavelengths." Soft x-ray and extreme UV pulses could be used to study various phenomena inside atoms and molecules at an attosecond time scale. "Scientists want to watch how electrons move around inside solids on timescales of 1,000 attoseconds or less," said Hong. "At the mid-IR wavelengths it is much easier to drive high-harmonic generation in solids because the optical damage due to multiphoton processes is much less pronounced." Adding more OPAs to the optical setup would allow higher energy scales, which could make the mid-IR pulses useful for additional experimental studies and applications. "This type of pulse synthesis has been routinely done in the microwave region, but it is much more difficult to do this in the optical region because the pulses are traveling much faster," added Hong. The research was presented at the 2016 Advanced Solid State Lasers Conference, a meeting of OSA, the Optical Society of America.