Femtosecond supercontinuum generation in water in the vicinity of absorption bands

High stability white light generation in water at multi-kilohertz repetition rates

An efficient supercontinuum (SC) generation featuring high spectral intensity across a large bandwidth requires high peak powers of several megawatts from pulsed lasers. Under these conditions and at multi-kilohertz (kHz) repetition rates, the SC generated in most materials is unstable due to thermal effects. In this work, we leverage the superior dispersion properties of water to maximize the spectral width of the SC, while avoiding stability issues due to thermal loading by means of a constant laminar flow of the liquid. This flow is controlled by a differential pressure scheme that allows to precisely adjust the fluid velocity to an optimum value for maximum stability of the SC. This approach is successfully implemented for repetition rates of 50 kHz and 100 kHz and two different pump wavelengths in the visible (VIS) and near infrared (NIR) spectral region with stability of the SC signal only limited by the driving pulses. The resulting water SC spans more than one octave covering the VIS to NIR range. Compared to established materials, such as yttrium aluminum garnet (YAG) and sapphire, the spectral bandwidth is increased by 60% and 40% respectively. Our scheme has the potential to be implemented with other liquids such as bromine or carbon disulfide (CS2), which promise even wider broadening and operation up to the mid-infrared.

Terahertz aqueous photonics

Developing efficient and robust terahertz (THz) sources is of incessant interest in the THz community for their wide applications. With successive effort in past decades, numerous groups have achieved THz wave generation from solids, gases, and plasmas. However, liquid, especially liquid water has never been demonstrated as a THz source. One main reason leading the impediment is that water has strong absorption characteristics in the THz frequency regime.A thin water film under intense laser excitation was introduced as the THz source to mitigate the considerable loss of THz waves from the absorption. Laser-induced plasma formation associated with a ponderomotive force-induced dipole model was proposed to explain the generation process. For the one-color excitation scheme, the water film generates a higher THz electric field than the air does under the identical experimental condition. Unlike the case of air, THz wave generation from liquid water prefers a sub-picosecond (200-800 fs) laser pulse rather than a femtosecond pulse (~50 fs). This observation results from the plasma generation process in water.For the two-color excitation scheme, the THz electric field is enhanced by one-order of magnitude in comparison with the one-color case. Meanwhile, coherent control of the THz field is achieved by adjusting the relative phase between the fundamental pulse and the second-harmonic pulse.To eliminate the total internal reflection of THz waves at the water-air interface of a water film, a water line produced by a syringe needle was used to emit THz waves. As expected, more THz radiation can be coupled out and detected. THz wave generation from other liquids were also tested.

Evaluation of the potential eye hazard at visible wavelengths of the supercontinuum generated by an ultrafast NIR laser in water

Lasers with ultrashort pulse durations have become ubiquitous in various applications, including ocular surgery. Therefore, we need to consider the role of nonlinear optical effects, such as supercontinuum generation during propagation within the ocular media, when evaluating their potential hazard. We used a NIR femtosecond laser to generate a supercontinuum within an artificial eye. We recorded the visible spectra of the supercontinuum generated and calculated the energy contained within the visible band. Our results indicate that for wavelengths between 1350 nm and 1450 nm the energy contained within the visible band of the generated white light supercontinuum may surpass current safety exposure limits, and pose a risk of injury to the retina.

Spectral characterization of a supercontinuum source based on nonlinear broadening in an aqueous K2ZnCl4 salt solution

We report on investigations concerning the shot-to-shot spectral stability properties of a supercontinuum source based on nonlinear processes such as self-phase modulation and optical wave-breaking in a highly concentrated K₂ZnCl₄ double salt solution. The use of a liquid medium offers both damage resistance and high third-order optical nonlinearity. Approximately 40 μJ pulses spanning a spectral range between 390 and 960 nm were produced with 3.8% RMS energy stability, using infrared input pulses of 500±50  fs FWHM durations and 2.42±0.04  mJ energies with an RMS stability of 2%. The spectral stability was quantified via acquiring single-shot spectra and studying shot-to-shot variation across a spectral range of 200-1100 nm, as well as by considering spectral correlations. The regional spectral correlation variations were indicative of nonlinear processes leading to sideband generation. Spectral stability and efficiency of energy transfer into the supercontinuum were found to weakly improve with increasing driver pulse energy, suggesting that the nonlinear broadening processes are more stable when driven more strongly, or that self-guiding effects in a filament help to stabilize the supercontinuum generation.