Giulio Maria Rossi

Toward Waveform Nonlinear Optics Using Multimillijoule Sub-Cycle Waveform Synthesizers

Waveform nonlinear optics aims to study and control the nonlinear interactions of matter with extremely short optical waveforms custom-tailored within a single cycle of light. Different technological routes to generate such multimillijoule sub-optical-cycle waveforms are currently pursued, opening up unprecedented opportunities in attoscience and strong-field physics. Here, we discuss the experimental schemes, introduce the technological challenges, and present our experimental results on high-energy sub-cycle optical waveform synthesis based on (1) parametric amplification and (2) induced-phase modulation in a two-color-driven gas-filled hollow-core fiber compressor. More specifically, for (1), we demonstrate a carrier-envelope-phase (CEP)-stable, multimillijoule three-channel parametric waveform synthesizer generating a >2-octave-wide spectrum (0.52-2.4 μm). After two amplification stages, the combined 125-μJ output supports 1.9-fs FWHM waveforms; energy scaling to >2 mJ is achieved after three amplification stages. FROG pulse characterization of all three second-stage outputs demonstrates the feasibility to recompress all three channels simultaneously close to the Fourier limit and shows the flexibility of our intricate dispersion management scheme for different experimental situations. For (2), we generate CEP-stable 1.7-mJ waveforms covering 365-930 nm (measured at 1% of the peak intensity) obtained from induced-phase modulation in a two-color-driven gas-filled hollow-core fiber. Using custom-designed double-chirped mirrors and a UV spatial light modulator will permit compression close to the 0.9-fs FWHM transform limit. These novel sources will become versatile tools for controlling strong-field interactions in matter and for attosecond pump-probe spectroscopy using VIS/IR and XUV/soft-X-ray pulses.

Spectro-Temporal Characterization of All Channels in a Sub-Optical-Cycle Parametric Waveform Synthesizer

We present FROG characterization of all three amplification channels of a two-stage sub-optical-cycle parametric waveform synthesizer covering more than two octaves in bandwidth. A flexible dispersion compensation scheme will permit compression at the multi-mJ level.

High-dynamic-range arrival time control for flexible, accurate and precise parametric sub-cycle waveform synthesis

We introduce a simple all-inline variation of a balanced optical cross-correlator (BOC) that allows to measure the arrival time difference (ATD), over the full Nyquist bandwidth, with increased common-mode rejection and long-term stability. An FPGA-based signal processing unit allows for real-time signal normalization and enables locking to any setpoint with an unprecedented accuracy of 0.07 % within an increased ATD range of more than 400 fs, resulting in attosecond resolution locking. The setup precision is verified with an out-of-loop measurement to be less than 80 as residual jitter paving the way for highly demanding applications such as parametric waveform synthesizers.

Above-Millijoule Optical Waveforms Compressible to Sub-fs Using Induced-Phase Modulation in a Neon-Filled Hollow-Core Fiber

We demonstrate 1.7-mJ optical waveforms based on induced-phase modulation for generating sub-femtosecond optical pulses. Using custom-designed double-chirped mirrors and a spatial light modulator, such optical waveforms will become a versatile tool for strong-field attoscience.

CEP dependence of signal and idler upon pump-seed synchronization in optical parametric amplifiers

We study the effect of pump-seed timing fluctuations on the carrier-envelope phase (CEP) of signal and idler pulses emerging from an OP(CP)A. A simple analytical model is derived in order to provide an intuitive explanation of the origin of CEP fluctuations, while split-step simulations are performed to cover a broad range of different seeding schemes. Finally, we compare the simulation results with real observations of the CEP of idler pulses generated by an OPA. The quantitative model presented provides a key tool for designing the next generation of low-noise CEP-stable OP(CP)A-based sources.

Optimized octave-spanning Ti:Sapphire laser oscillator characterized by novel two-dimensional shearing interferometry

Stable ultra-broadband pulse generation via Kerr-lens mode locking (KLM) paves the way for scientific explorations in many fields, such as optical metrology, study of ultrafast dynamics, and nonlinear light microscopy. For example, Ti:sapphire laser oscillators, capable of generating octave-spanning spectra with potentially close-to-single-cycle waveforms, have been employed as optical flywheels providing a very precise timing reference [1], and to study waveform-sensitive interactions with solid-state nanostructures [2]. Despite the experimental demonstrations of octave-spanning spectra from Ti:sapphire oscillators, the intracavity spatiotemporal pulse dynamics has not been completely understood and optimized, and the pulse characterization still remains challenging. Although the use of double-chirped mirror (DCM) pairs provide octave-spanning dispersion compensation, the residual dispersion oscillations, originating from the chirped mirrors, play adverse roles in few-cycle KLM dynamics. Residual intracavity phase oscillations tend to generate satellite pulses, which hamper the onset of stable mode locking. Moreover, the interplay between such residual phase oscillations and optical Kerr nonlinearity limits the output spectral bandwidth, as well as leading to strong modulations in both longitudinal and spatial modes. Therefore, the improvements of the output spectrum, beam quality, and mode-locking stability require a more precise intracavity dispersion control. A phase-optimized cavity has been designed and fabricated delivering <0.1 rad of intracavity residual phase over the whole resonating bandwidth of 0.65 μm–1.14 μm. Fig. 1(a) shows the experimental spectrum (blue), in excellent agreement with the numerical simulation (grey), from the optimized cavities with 5% output coupling, as well as the previous state-of-the-art output, having a 3.6-fs transform-limited (TL) pulse duration (green). >10-dB enhancements in power spectral density around both the center gain region and 1140 nm are obtained, suggesting the improvements in terms of both power and precision locking control to be beneficial for the use as optical frequency comb. In the time domain (see the inset of Fig. 1(a)), a compressed pulse with 4.3-fs duration (3.9-fs TL from the 5% spectrum) is characterized by a newly designed two-dimensional shearing interferometry (2DSI) scheme. Instead of chirping two replicas of the initial pulse [3], the narrowband ancillary pulses are generated by means of inteference filters inside a Michelson interferometer featuring a piezo-controlled arm length, allowing to generate the 2DSI fringe pattern shown in the inset of Fig. 1(b). The filtering approach, avoiding stretching-ratio calculations, gives a precise control over the shear frequency that plays a key role in the reconstruction of the electric field. In addition, the simplicity in choosing the wavelengths of the ancillae just by replacing the filters makes our 2DSI setup flexible to characterize pulses with different parameters. A better result toward single-optical-cycle pulses is expected for optimized cavities using 1% output coupling. The compressed pulses will be used to study field-controlled, carrier-envelope phase-sensitive photoemission from arrays of tailored metalhc nanoparticles [2]. We will provide experimental results and detailed discussions in the presentation.

Ellipticity dependence of higher-order harmonics in solids: Unraveling the interplay between intraband and interband dynamics

Recently, we introduced an ab-initio time-dependent density-functional theory (TDDFT) framework that allows us to investigate the coupled interplay between the interband and intraband mechanisms of high-harmonic generation (HHG) from solids [1] without making a-priori model assumptions or strong approximations. Here, using HHG experiments on bulk silicon samples combined with TDDFT simulations, we study the complex physics underlying anisotropic harmonic emission, as reported by You et al. [2] for the strongly anisotropic ellipticity dependence of the 19th harmonic (HH19) generated in bulk MgO. In [2], the observed anisotropy was explained with real-space trajectories in a 2D one-band model including scattering from neighboring atomic sites. Our TDDFT simulations [3] and HHG experiments reveal that the various higher-harmonic orders generated in solids exhibit qualitatively different sensitivity to driver-pulse ellipticity ε (not displayed here), resulting from a different response of intraband and interband dynamics [3], in contradiction with the model proposed in [2]. In fact, band-structure and joint-density-of-states (JDOS) effects become important [1].

Shot-to-shot and long-term CEP-stable front-end for a parallel optical waveform synthesizer

We present a highly automated front-end of a Ti:Sa-based optical waveform synthesizer capable of driving multiple supercontinua with stable and controlled carrier-envelope phases. The remarkable CEP-stability of 217 mrad rms single-shot will allow shot-to-shot and long-term stable waveform synthesis.

High-dynamic-range relative arrival time measurement for accurate and precise parametric waveform synthesis

We introduce an all-inline scheme for relative timing measurement of ultrabroadband pulses, featuring FPGA-based detection/feedback. High common-mode rejection and long-term stability were validated by out-of-loop measurement, paving the way for ultrabroadband optical parametric waveform synthesis.

Orthogonal control for stable parallel waveform synthesis

We present a control scheme for parallel optical parametric waveform synthesizers. Orthogonalization of the relative arrival time and relative phase between all individual channels promises to achieve shot-to-shot stable and controlled ultra-broadband pulse synthesis.