ABSTRACT
In nature, sensory photoreceptors underlie diverse spatiotemporally precise and generally reversible biological responses to light. Photoreceptors also serve as genetically encoded agents in optogenetics to control by light organismal state and behavior. Phytochromes represent a superfamily of photoreceptors that transition between states absorbing red light (Pr) and far-red light (Pfr), thus expanding the spectral range of optogenetics to the near-infrared range. Although light of these colors exhibits superior penetration of soft tissue, the transmission through bone and skull is poor. To overcome this fundamental challenge, we explore the activation of a bacterial phytochrome by a femtosecond laser emitting in the 1 µm wavelength range. Quantum chemical calculations predict that bacterial phytochromes possess substantial two-photon absorption cross sections. In line with this notion, we demonstrate that the photoreversible Pr â Pfr conversion is driven by two-photon absorption at wavelengths between 1170 and 1450 nm. The Pfr yield was highest for wavelengths between 1170 and 1280 nm and rapidly plummeted beyond 1300 nm. By combining two-photon activation with bacterial phytochromes, we lay the foundation for enhanced spatial resolution in optogenetics and unprecedented penetration through bone, skull, and soft tissue.
Subject(s)
Phytochrome , Bacteria , Bacterial Proteins , LightABSTRACT
Self-starting pulsed operation in an electrically pumped (EP) vertical-external-cavity surface-emitting-laser (VECSEL) without intracavity saturable absorber is demonstrated. A linear hemispherical cavity design, consisting of the EP-VECSEL chip and a 10% output-coupler, is used to obtain picosecond output pulses with energies of 2.8 pJ and pulse widths of 130 ps at a repetition rate of 1.97 GHz. A complete experimental analysis of the generated output pulse train and of the transition from continuous-wave to pulsed operation is presented. Numerical simulations based on a delay-differential-equation (DDE) model of mode-locked semiconductor lasers are used to reproduce the pulse dynamics and identify different laser operation regimes. From this, the measured single pulse operation is attributed to FM-type mode-locking. The pulse formation is explained by strong amplitude-phase coupling and spectral filtering inside the EP-VECSEL.
ABSTRACT
We investigate the scaling properties of mode-locked all-normal dispersion fiber oscillators in terms of output pulse energy and compressed pulse duration. Experimental results are achieved by stepwise variation of the resonator dispersion, total fiber length, and the spectral filter bandwidth. Adjustment of these parameters enables pulse duration scaling down to 31 fs and increase of output pulse energy up to 84 nJ.
ABSTRACT
We present a mode-locked all-normal dispersion ytterbium fiber oscillator with output pulse energies beyond 0.5 µJ. The oscillator is mode-locked using nonlinear polarization rotation, and stable single-pulse operation is achieved by spectral filtering inside the resonator. The oscillator generates strongly chirped output pulses at a repetition rate of 4.3 MHz which can be compressed down to 760 fs.
ABSTRACT
We present a mode-locked, all-normal-dispersion erbium-doped fiber oscillator generating output pulses with broadband spectra covering the range from 1475 to 1620 nm. The oscillator operates at a repetition rate of 109 MHz with output pulse energies of 1.6 nJ. Mode-locked operation is achieved by use of nonlinear polarization evolution in combination with a birefringent filter. The output pulses are dechirped with an external prism compressor to a duration of 50 fs.
ABSTRACT
We demonstrate output pulse energies of 20 nJ from an erbium-doped fiber oscillator that contains only positive dispersion fibers and is mode locked by use of nonlinear polarization evolution and stabilized with a birefringent filter. The fiber oscillator operates at a repetition rate of 3.5 MHz with a central wavelength of 1550 nm. The positively chirped output pulses have a duration of 53 ps and are compressed to 750 fs. The large positive chirp of the output pulses and the steep side edges of the pulse spectrum indicate dissipative soliton operation.
ABSTRACT
A novel CPA-system with a spatially dispersive Yb:KYW-based regenerative amplifier is presented. Three prisms inside the amplifier divide the seed pulses from an Yb fiber laser system into spatially separated spectral components that pass through different volumes of the active medium and are amplified independently. This concept allows overcoming the effect of gain-narrowing. The spatially dispersive amplifier delivers laser pulses with energies of up to 30 microJ at a repetition rate of 100 kHz. Successful compression of the amplified pulses down to 171 fs is demonstrated.
