Laser Science Laboratory is a new laboratory started in April 2019. We have knowledge and technology to design and build lasers with top-class performance in the world. For example, we have developed a laser which generates the world shortest 7 femtosecond infrared light pulses, and a high-performance ultrafast fiber laser which uses a fluoride fiber as a laser medium.
We also have a unique technology which allows us to directly measure light waves oscillating with a period of several femtoseconds. In addition, we are conducting joint research projects with life science laboratories and companies.
When high-intensity light is focused into a medium, the increase in the refractive index due to some nonlinear effects and the decrease of it due to the plasma generated by the multiphoton ionization are balanced, and the light travels a rather long distance with the focused condition. This phenomenon is called filamentation. Using this filamentation effect, we have succeeded in generating the world's shortest 7 femtosecond mid-infrared light pulse[for example, IEEE J. Sel. Top. Quantum Electron. 21 8700612 (2015), Opt. Express 28 36527 (2020)]. This is an epoch-making technique that can rather easily generate extremely short pulses, in which the electromagnetic field oscillates only once. Using the 7 femtosecond mid-infrared light pulse as a light source, we are developing high-speed infrared spectrum measurement, femtosecond pump-probe spectroscopy, and hyperspectral imaging spectroscopy[for example, J. Opt. 17 094004 (2015), arXiv:2209.06372]. In the future, we would like to apply the technology to various scientific and industrial fields, for example, environment science, biology, and medical applications.
It is very well-known that light has wave nature. However, it is still very difficult to directly measure the wave of light even using the latest state-of-the-art technology.
The principal investigator of this laboratory has developed a new method for measuring the waveforms of light [for example, Nat. Commun. 4 2820 (2013), Optica 10 302 (2023)]. It is considered to be a useful method for the studies on such as high field physics, ultrafast optical signal processing, and development of synchrotron facilities. Currently, we are developing the technology so that this method can be used in various wavelength regions.
Broadband, highly coherent mid-infrared (3–20μm) light sources have a wide range of applications such as environmental science, medicine, and life science. High-intensity lasers with the wavelength of 2 μm are expected to have very high efficiency for the wavelength conversion to the mid-infrared range.
In our laboratory, we have developed a solid-state laser that generates 2 μm pulses with the duration of 265 fs and the pulse energy of 1 mJ [Opt. Express 30 7332 (2022)]. Moreover, this laser was used to generate coherent mid-infrared light. Besides, the 2 μm source can also be useful for laser processing. Currently, we are working on commercializing the 2 μm laser in cooperation with FiberLabs Inc.
Nano and micro particles can be pushed, trapped and manipulated by using a focused laser beam. This is due to a "radiation force" which is induced on the particles by the laser. Its magnitude and direction actually depend on an induced polarization (how the electrons in the particles are oscillated by the electromagnetic wave laser). A transparent, non-resonant, laser is typically used for this optical trapping research. Prof. T. Kudo et al. have been working on the optical trapping researches when the laser is resonant to the particles in various ways.
We conducted the optical trapping theory and experiment when the trapping laser is resonant to electronic transition of the particles, especially in nonlinear optical regions [PRL 109, 087402 (2012),Opt. Exp. 25, 4655 (2017)]. We also found unprecedented assembling phenomena, called optically evolved assembly, when the trapping laser is resonant to surface plasmon resonance of metallic nanoparticles [Nano Lett. 18, 5846 (2018)], photonic bandgap of colloidal photonic crystals [Nano Lett. 16, 3058 (2016)] and whispering gallery mode of microspheres [J. Phy. Chem. Lett. 11, 6057 (2020)]. All of these results are based on the strong light and matter interaction under the optical trapping. Currently, we are working on resonant optical trapping using the laser techniques in our laboratory.
Midinfrared light is extensively used for identifying molecules because their characteristic vibrational modes commonly exist in the midinfrared spectral region.
We recently found that optical force can be resonantly enhanced when vibrational modes of target particles are excited by mid-infrared laser[Phys. Rev. Applied 18, 054041 (2022)]. Silica particles rapidly transport in the 9 μm mid-infrared evanescent wave (top video) since their vibrational mode (Si-O-Si bonds) is excited, while the polystyrene particles are not transported (bottom video) because their vibrational mode is not excited by the laser. We believe that this technique can be used for optical force chromatography according to molecular structures.
Different from the optical force, particles are migrated along the temperature gradient which is caused by laser absorption. This is known as opto-thermophoresis and/or optothermal trapping. 1.4 to 1.5 μm lasers were used for these studies in the past. In the spectrum, water has absorption peaks around 1.5, 2 μm, respectively, related to O-H bonds.
Here, we used 2 μm infrared laser to excite the vibrational modes of water for heating water[Opt. Exp. 29, 38314 (2021)]. Currently we are developing further efficient optothermal trapping with fiber based laser system.