ABSTRACT
Time-resolved optical pump-probe experiments enable the study of complex light-matter interactions on ultrafast timescales, provided that they reach sufficient sensitivity. For instance, with pump-induced ultrafast photoacoustics, probing the typically small changes in optical properties requires a high signal-to-noise ratio. Asynchronous optical sampling (ASOPS), using two separate pulsed lasers at slightly different repetition rates, can be effective at removing noise by averaging many rapidly acquired traces. However, the pump-probe delay scan with ASOPS is always as long as the pump pulse interval, which is inefficient if the delay-time range of interest is shorter. Here, we demonstrate two modified ASOPS schemes that optimize measurement efficiency by only scanning the range of interest. The modification based on frequency modulated ASOPS (MASOPS) is most efficient, especially in the presence of low-frequency flicker noise. We provide a proof-of-concept measurement of ultrafast photoacoustics in which we use MASOPS to scan a time delay of 1/20 of the pump pulse interval. The resulting noise floor is 20 times lower compared to conventional ASOPS, allowing for 20 times faster measurements. Furthermore, we show that by taking experimental noise characteristics into account, more traditional pump-probe methods can also be optimized.
ABSTRACT
Ice-nucleating proteins (INPs) from Pseudomonas syringae are among the most active ice nucleators known, enabling ice formation at temperatures close to the melting point of water. The working mechanisms of INPs remain elusive, but their ice nucleation activity has been proposed to depend on the ability to form large INP aggregates. Here, we provide experimental evidence that INPs alone are not sufficient to achieve maximum freezing efficiency and that intact membranes are critical. Ice nucleation measurements of phospholipids and lipopolysaccharides show that these membrane components are not part of the active nucleation site but rather enable INP assembly. Substantially improved ice nucleation by INP assemblies is observed for deuterated water, indicating stabilization of assemblies by the stronger hydrogen bonds of D2O. Together, these results show that the degree of order/disorder and the assembly size are critically important in determining the extent to which bacterial INPs can facilitate ice nucleation.
