Marc Hanna

Supercontinuum-seeded few-cycle mid-infrared OPCPA system

We propose and demonstrate an OPCPA architecture emitting few-cycle pulses at 3070 nm and 1550 nm based on a high-energy femtosecond ytterbium-doped fiber amplifier pump. The short pump pulse duration allows direct seeding by a supercontinuum in the 1.4 - 1.7 µm signal range, generated in bulk YAG. It also allows a simplified dispersion management along the system and broad optical gain bandwidth. The dual output system delivers 20 µJ, 49 fs signal pulses at 1550 nm and 10 µJ, 72 fs idler pulses at 3070 nm. Power scaling limitations due to beam distortion in the last MgO:PPLN-based OPCPA stage are discussed and investigated.

Enhanced nonlinear interaction in a microcavity under coherent excitation

The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.

Nonlinear temporal compression in multipass cells: theory

The use of multipass cells as a way to spatially homogenize self-phase modulation and distribute its accumulation over the propagation distance is analyzed in detail, with the aim to perform nonlinear temporal compression. In addition to the insertion of nonlinear media at specific locations in the cell, as already demonstrated, we also propose to fill the cell with a noble gas, as is done in hollow capillary-based setups. This makes the accumulation of B-integral continuous rather than discrete. In this case, analytical estimates for the B-integral per round trip and scaling rules are provided as a function of cavity geometry and gas parameters. Then, three-dimensional numerical simulations are performed to assess the spatiotemporal couplings in the output beam in various conditions. This model is checked against experimental data presented in the literature, and used to predict our proposed scheme performance. We believe that these techniques constitute a promising way to allow temporal compression at energy levels beyond 10xa0mJ, where capillary-based setups are difficult to implement.

Coherent combination of ultrafast fiber amplifiers

We review recent progress in coherent combining of femtosecond pulses amplified in optical fibers as a way to scale the peak and average power of ultrafast sources. Different methods of achieving coherent pulse addition in space (beam combining) and time (divided pulse amplification) domains are described. These architectures can be widely classified into active methods, where the relative phases between pulses are subject to a servomechanism, and passive methods, where phase matching is inherent to the geometry. Other experiments that combine pulses with different spectral contents, pulses that have been nonlinearly broadened or successive pulses from a mode-locked laser oscillator, are then presented. All these techniques allow access to unprecedented parameter range for fiber ultrafast sources.

High-energy few-cycle Yb-doped fiber amplifier source based on a single nonlinear compression stage

A simple, compact, and efficient few-cycle laser source at a central wavelength of 1 µm is presented. The system is based on a high-energy femtosecond ytterbium-doped fiber amplifier delivering 130 fs, 250 µJ pulses at 200 kHz, corresponding to 1.5 GW of peak power and an average power of 50 W. The unprecedented short pulse duration at the output of this system is obtained by use of spectral intensity and phase shaping, allowing for both gain narrowing mitigation and the compensation of the nonlinear accumulated spectral phase. This laser source is followed by a single-stage of nonlinear compression in a xenon-filled capillary, allowing for the generation of 14 fs, 120 µJ pulses at 200 kHz resulting in 24 W of average power. High-harmonic generation driven by this type of source will trigger numerous new applications in the XUV range and attosecond science.

Dual-color deep-tissue three-photon microscopy with a multiband infrared laser

Multiphoton microscopy combined with genetically encoded fluorescent indicators is a central tool in biology. Three-photon (3P) microscopy with excitation in the short-wavelength infrared (SWIR) water transparency bands at 1.3 and 1.7u2009µm opens up new opportunities for deep-tissue imaging. However, novel strategies are needed to enable in-depth multicolor fluorescence imaging and fully develop such an imaging approach. Here, we report on a novel multiband SWIR source that simultaneously emits ultrashort pulses at 1.3 and 1.7u2009µm that has characteristics optimized for 3P microscopy: sub-70u2009fs duration, 1.25u2009MHz repetition rate, and µJ-range pulse energy. In turn, we achieve simultaneous 3P excitation of green fluorescent protein (GFP) and red fluorescent proteins (mRFP, mCherry, tdTomato) along with third-harmonic generation. We demonstrate in-depth dual-color 3P imaging in a fixed mouse brain, chick embryo spinal cord, and live adult zebrafish brain, with an improved signal-to-background ratio compared to multicolor two-photon imaging. This development opens the way towards multiparametric imaging deep within scattering tissues.Microscopy: Looking deeper with three photonsResearchers in France are using a novel infra-red light source to examine both fixed and living tissue samples in deeper detail than previously possible with a technique called three-photon microscopy. The absorption of three photons at different infra-red frequencies stimulates subsequent emission of light from fluorescent molecules in the sample. Detecting the fluorescence by microscopy reveals the location and interactions of the molecules concerned. Emmanuel Beaurepaire and Frederic Duon at the University of Paris-Saclay, developed a procedure to emit ultra-short pulses of infra-red laser light with optimal characteristics for three-photon microscopy. They demonstrated their innovation by studying fluorescent proteins in brain and nerve tissue taken from mice and chicks, and also in live zebrafish brain. The procedure offers opportunities to study molecular structures and interactions more effectively than previously possible with three-photon microscopy.

Contradiction within wave optics and its solution within a particle picture: comment.

An error in the rationale presented in the paper Contradiction within wave optics and its solution within a particle picture by Altmann [Opt. Express 23, 3731 (2015)10.1364/OE.23.003731] is discussed.

Spatio-spectral structures in high harmonic generation driven by tightly focused high repetition rate lasers

We investigate the spatio-spectral properties of extreme ultraviolet (XUV) high harmonic radiation driven by high repetition rate femtosecond laser systems. In the spatio-spectral domain, ring-shaped structures at each harmonic order associated with long-trajectory electrons are found to form arrow-shaped structures at the cutoff. These structures are observed with two different laser systems: an optical parametric chirped-pulse amplifier system at a central wavelength of 1.55 μm and 125 kHz repetition rate, and a temporally compressed femtosecond ytterbium fiber amplifier at 1.03 μm wavelength and 100 kHz repetition rate. As recently pointed out, the observed structures are well explained by considering the space–time atomic dipole-induced phase for short and long electron trajectories in the generation plane. The tighter focusing geometry and longer wavelength associated with these emerging driving laser systems increase the ring-like divergence and spectral broadening for high harmonics. Cutoff energies and photon fluxes obtained in argon and neon are also reported. Overall, these results shed new light on the properties of XUV radiation driven by these recently developed high average power laser systems, paving the way to high photon-flux XUV beamlines.

Generation of few cycle pulses from a bandwidth-optimized high energy Yb-doped fiber laser source

The vast majority of HHG sources are nowadays driven using Ti:Sapphire lasers that typically generate 30 fs pulses with few mJ energy at 1 kHz repetition rate and 800 nm wavelength, corresponding to average powers limited to around 10 W. Nonlinear compression of these sources leads to the few-cycle regime with pulse durations below 5 fs [1]. Nowadays another approach is to use femtosecond rare earth-doped fiber amplifiers that are capable of generating kW average powers [2]. In this case, gain narrowing limits the pulse duration of high energy fiber CPA to above 250 fs, and accessing the few-cycle regime requires two stages of nonlinear compression [3].

Simple phase locker for coherent beam combining of multicore fiber amplifiers

A promising way for power scaling of fiber-based laser sources while maintaining a compact configuration is coherent combining of multicore fiber amplifiers, especially when the power in each core is limited by nonlinear effects. An example application is Doppler LIDAR, where long 100 ns) energetic (∼100 μ1–1 mJ) narrow-linewidth pulses are used to probe wind speed in the atmosphere. In this regime, Brillouin scattering (SBS) ultimately limits the peak power to a value that mostly depends on core diameter and fiber length. Multicore fibers can achieve the same SBS limit in each core, thus increasing the output power after combination compared to a single core fiber. If the pump is shared by all cores, this solution can also be more compact than coherent combining of several fibers.