Close to transform-limited, few-cycle 12 µJ pulses at 400 kHz for applications in ultrafast spectroscopy

Towards high power broad-band OPCPA at 3000 nm

High-energy femtosecond laser pulses in the mid-infrared (MIR) wavelength range are essential for a wide range of applications from strong-field physics to selectively pump and probe low energy excitations in condensed matter and molecular vibrations. Here we report a four stage optical parametric chirped pulse amplifier (OPCPA) which generates ultrashort pulses at a central wavelength of 3000 nm with 430 μJ energy per pulse at a bandwidth of 490 nm. Broadband emission of a Ti:sapphire oscillator seeds both the four stage OPCPA 800 nm and the pump line at 1030 nm. The first stage amplifies the 800 nm pulses in BBO using a non-collinear configuration. The second stage converts the wavelength to 1560 nm using difference frequency generation in BBO in a collinear geometry. The third stage amplifies this frequency non-collinearly in KTA. Finally, the fourth stage generates the 3000 nm radiation in a collinear configuration in LiIO ₃ due to the broad amplification bandwidth this crystal provides. We compress these pulses to 65 fs by transmission through sapphire. Quantitative calculations of the individual non-linear processes in all stages verify that our OPCPA architecture operates close to optimum efficiency. Low absorption losses suggest that this particular design is very suitable for operation at high average power and multi kHz repetition rates.

Single-step fabrication of surface waveguides in fused silica with few-cycle laser pulses

Direct laser writing of surface waveguides with ultrashort pulses is a crucial achievement towards all-laser manufacturing of photonic integrated circuits sensitive to their environment. In this Letter, few-cycle laser pulses (with a sub-10 fs duration) are used to produce subsurface waveguides in a non-doped, non-coated fused-silica substrate. The fabrication technique relies on laser-induced microdensification below the threshold for nanopore formation. The optical losses of the fabricated waveguides are governed by the optical properties of the superstrate. We have measured losses ranging from less than 0.1 dB/mm (air superstrate) up to 2.8 dB/mm when immersion oil is applied on top of the waveguide.

Ion-ion coincidence imaging at high event rate using an in-vacuum pixel detector

A new ion-ion coincidence imaging spectrometer based on a pixelated complementary metal-oxide-semiconductor detector has been developed for the investigation of molecular ionization and fragmentation processes in strong laser fields. Used as a part of a velocity map imaging spectrometer, the detection system is comprised of a set of microchannel plates and a Timepix detector. A fast time-to-digital converter (TDC) is used to enhance the ion time-of-flight resolution by correlating timestamps registered separately by the Timepix detector and the TDC. In addition, sub-pixel spatial resolution (<6 μm) is achieved by the use of a center-of-mass centroiding algorithm. This performance is achieved while retaining a high event rate (10⁴ per s). The spectrometer was characterized and used in a proof-of-principle experiment on strong field dissociative double ionization of carbon dioxide molecules (CO₂), using a 400 kHz repetition rate laser system. The experimental results demonstrate that the spectrometer can detect multiple ions in coincidence, making it a valuable tool for studying the fragmentation dynamics of molecules in strong laser fields.

CEP-stable few-cycle pulses with more than 190 μJ of energy at 100 kHz from a noncollinear optical parametric amplifier

Noncollinear optical parametric amplifiers (NOPAs) have become the leading technique for the amplification of carrier-envelope phase (CEP)-stable, few-cycle pulses at high repetition rate and high average power. In this Letter, a NOPA operating at a repetition rate of 100 kHz delivering more than 24 W of average power before compression is reported. The amplified bandwidth supports sub-7 fs pulse durations and pulse compression close to the transform limit is realized. CEP stability after amplification is demonstrated. The system paves the way to attosecond pump-probe spectroscopy with electron-ion coincidence detection.

Numerical study of spatiotemporal distortions in noncollinear optical parametric chirped-pulse amplifiers

During amplification in a noncollinear optical parametric amplifier the spatial and temporal coordinates of the amplified field are inherently coupled. These couplings or distortions can limit the peak intensity, among other things. In this work, a numerical study of the spatiotemporal distortions in BBO-based noncollinear optical parametric chirped-pulse amplifiers (NOPCPAs) is presented for a wide range of parameters and for different amplification conditions. It is shown that for Gaussian pump beams, gain saturation introduces strong distortions and high conversion efficiency always comes at the price of strong spatiotemporal couplings which drastically reduce the peak intensity even when pulse fronts of the pump and the signal are matched. However, high conversion efficiencies with minimum spatiotemporal distortions can still be achieved with flat-top pump beam profiles.