A Dual-Sideband Attosecond Interferometry Setup

Abstract

We present the development and implementation of an experimental setup designed to investigate attosecond photoionization delays using a dual-sideband RABBITT (Reconstruction of Attosecond Beating By Interference of Two-Photon Transitions) technique. The setup utilizes an attosecond extreme ultraviolet source from high-harmonic generation driven by a carrier-envelope-phase-stabilized Ti:sapphire laser centered at 800 nm. The extreme ultraviolet radiation is synchronized with a 1200 nm infrared probe pulse generated via a non-collinear optical parametric amplifier. Active delay stabilization by means of a spectrally resolved interferometer signal achieves 45 as root-mean-square timing precision and enables the observation of sideband oscillations. Taking advantage of the dependence of the sideband signal on the carrier-envelope phase of the driving field, we report sideband-yield oscillations as a function of this parameter.

Figures (7)

• Figure 1: (a) Schematic illustration of the paths leading to the formation of the sidebands $S^{(\pm)}_{q-1,q+1}$. Adjacent odd-order, XUV harmonics (light and dark purple arrows) are coupled to the same final continuum energy by absorption or emission of one and two NIR photons (red arrows), giving rise to indistinguishable quantum pathways (I-IV). (b-c) Numerical simulation of the sideband intensity oscillations as a function of the delay $\tau$ between the attosecond pulse train and the NIR field for photoelectrons emitted in opposite direction along the laser polarization direction ($\theta = 0^{\circ}$ (b), and $\theta = 180^{\circ}$ (c)). $H_{q-1}$ and $H_{q+1}$ indicate the energy of the main photoelectron lines. GS and $I_p$ indicate the energy position of the ground state and the ionization potential of the target atom, respectively. Parameters used in the simulations: $q=18$, $I_{\mathrm{NIR}}=6\times10^{11}\,\mathrm{W/cm^2}$, target atom: Helium.
• Figure 2: Schematic of the complete experimental setup. The laser output is split by a 50:50 beam splitter ($\mathrm{BS}_1$) into two arms. The first arm is directed to the HHG chamber, where the 800 nm driving pulse is focused into an argon gas jet to generate an attosecond XUV pulse train. The second arm drives a two-stage NOPA. Within the NOPA, the beam is further divided into three branches by using the beam splitters $\mathrm{BS}_2$ and $\mathrm{BS}_3$, respectively. The first branch (B1) generates a white-light continuum in a sapphire crystal (SAP) for seeding the NOPA stages; a long-pass filter placed after the WLG stage suppresses unwanted spectral components and is removed when operating the 800 nm stabilization scheme. $L_1$ indicates the lens used to focus the 800 nm radiation in the sapphire plate. The remaining two branches B2 and B3 serve as pump beams for the first and second NOPA stages, respectively. In the branch B2, the 800 nm pump is focused by the lens $L_2$ in the BBO crystal. A variable neutral density (ND) filter is placed immediately after the NOPA to tune the NIR intensity. The XUV and NIR pulses are recombined using a double-holey mirror (DHM) with reflective coating on both surfaces and focused by a toroidal mirror (TM) in the interaction region of a VMI spectrometer, consisting of the three electrodes repeller (R), extractor (E) and ground (G) that guide the photoelectrons towards the detector. A small fraction of the co-propagating beams is extracted through the DHM and routed to the delay stabilization setup for active interferometric phase locking. The delay stages $DS_1$, $DS_2$ were used to optimize the temporal overalp required for efficient amplification in the first and second NOPA stage. The piezo stage $\tau$ was used to optimize, stabilize and control the delay $\tau$ between the XUV and NIR pulses in the VMI region.
• Figure 3: (a) Normalized NOPA spectrum at the output of the second amplification stage demonstrating the central wavelength tunability from approximately 1100 nm to 1700 nm. (b) Pulse duration as a function of the number of chirped mirror pairs used for dispersion compensation. (c) Second-order autocorrelation trace recorded with three pairs of chirped mirrors demonstrating temporal compression down to approximately 19 fs (FWHM), close to the Fourier-transform limit (18 fs). (d) CEP stability of the NOPA obtained from an $f$--$2f$ spectral interferometer, yielding an RMS CEP phase jitter of 244 mrad.
• Figure 4: Recombination of the two interferometer arms using a double-coated, double-holey mirror. Panel (a) shows the 800 nm stabilization scheme, while panel (b) shows the 1200 nm stabilization scheme, which requires a sapphire (SAP) crystal for WLG. The red and dark yellow lines indicate the radiation at 800 nm and 1200 nm, respectively. BS: beam splitter; DBS: dichroic beam splitter.
• Figure 5: Long-term delay stabilization performance for the 800 nm and 1200 nm schemes. The interferometer was locked to a fixed phase setpoint (red line) and monitored over 90 min. The phase stability corresponded to an RMS timing jitter of 64.18 as for the 800 nm scheme (panel a) and 94.76 as for the 1200 nm scheme (panel b). The green lines indicate the $\pm\sigma_{\tau}$ stability bounds.