Ultrafast Primary Dynamics and Isomerization Mechanism of a Far-Red Sensing Cyanobacteriochrome.

Far-red cyanobacteriochromes (CBCRs) are bilin-based photosensory proteins that promise to be novel optical agents in optogenetics and deep tissue imaging. Recent structural studies of a far-red CBCR 2551g3 have revealed a unique all-Z,syn chromophore conformation in the far-red-absorbing Pfr state. Understanding the photoswitching mechanism through bilin photoisomerization is important for developing novel biomedical applications. Here, we employ femtosecond spectroscopy and site-directed mutagenesis to systematically characterize the dynamics of wild-type 2551g3 and four critical mutants in the 15Z Pfr state. We captured local relaxations in several picoseconds and isomerization dynamics in hundreds of picoseconds. Most mutants exhibited faster local relaxation, while their twisting dynamics and photoproducts depend on specific protein-chromophore interactions around the D-ring and C-ring. These results collectively reveal a unique dynamic pattern of excited-state evolution arising from a relatively rigid protein environment, thereby elucidating the molecular mechanism of Pfr-state photoisomerization in far-red CBCRs.

Ultrafast Nonequilibrium Dynamics of Vibrationally Hot Electron Transfer in Flavodoxin.

The understanding of ultrafast short-range electron transfer (ET) in proteins remains challenging, and thorough studies on well-defined biological systems are demanding. Here, we utilized two types of flavodoxins and designed a series of mutants on two positions to systematically characterize the complete photoinduced redox cycles. We identified one position with a favorable orientation and distance for ultrafast ET in a few femtoseconds and the other position is relatively flexible with a longer ET time scale. We found that all forward and back ET dynamics are ultrafast nonequilibrium processes, occurring through highly vibronic states and ending in vibrationally hot ground states with subsequent cooling relaxation to efficiently dissipate photon energy into the protein environment.

Optical Control of Crossing the Conical Intersection in ß-Carotene.

Optical control of dynamic processes has been challenging yet has only been demonstrated in several chemical and biological systems. The control of a reaction passing the widely present conical intersection has not been realized. Here, we modulated the phase of the excitation pulse to control the dynamics of ß-carotene through accessing the conical intersection (CI). We observed different dynamics in 110-220 fs into the CI and the consecutive process in 400-600 fs through another CI by various chirped excitation pulses. We successfully controlled those ultrafast wavepacket dynamics passing the CIs on the femtosecond time scales. The method developed here can be used to control a various of ultrafast chemical and biological reactions through the CI(s).

Ultrafast Dynamics of Nonequilibrium Short-Range Electron Transfer in Semiquinone Flavodoxin.

Short-range protein electron transfer (ET) is crucially important in light-induced biological processes such as in photoenzymes and photoreceptors and often occurs on time scales similar to those of environment fluctuations, leading to a coupled dynamic process. Herein, we use semiquinone Anabaena flavodoxin to characterize the ultrafast photoinduced redox cycle of the wild type and seven mutants by ultrafast spectroscopy. We have found that the forward and backward ET dynamics show stretched behaviors in a few picoseconds (1-5 ps), indicating a coupling with the local protein fluctuations. By comparison with the results from semiquinone D. vulgaris flavodoxin, we find that the electronic coupling is crucial to the ET rates. With our new nonergodic model, we obtain smaller values of the outer reorganization energy (λoγ) of environment fluctuations and the reaction free energy force (ΔGγ), a signature of nonequilibrium ET dynamics.

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Generation of sub-two-cycle CEP-stable optical pulses at 3.5 µm from a KTA-based optical parametric amplifier with multiple-plate compression.

We demonstrate the generation of 21 fs (1.8 optical cycle), 45-µJ, carrier-envelope phase (CEP)-stable optical pulses with an octave-spanning spectrum from 2.2 to 4.9 µm. Multi-cycle output pulses (120 fs, 149 µJ, 3.5 µm, and 300 Hz) from a KTA-based optical parametric amplifier are compressed down to the sub-two-cycle regime using YAG and Si plates. The single-shot CEP stability of the compressed pulses is measured to be 283 mrad for 500 s. This robust and compact scheme realizes a field strength of 282 MV/cm with f/11 focusing, opening a new regime of attosecond strong-field physics in solids.

Coherent soft X-ray high-order harmonics using tight-focusing laser pulses in the gas mixture.

We experimentally study the harmonics from a Xe-He gas mixture using tight-focusing femtosecond laser pulses. The spectrum in the mixed gases exhibits an extended cutoff region from the harmonic H21 to H27. The potential explanation is that the harmonics photons from Xe contribute the electrons of He atoms to transmit into the excited-state. Therefore, the harmonics are emitted from He atoms easily. Furthermore, we show that there are the suppressed harmonics H15 and H17 in the mixed gases. The underlying mechanism is the destructive interference between harmonics generated from different atoms. Our results indicate that HHG from Xe-He gas mixture is an efficient method of obtaining the coherent soft X-ray source.