Tunable frequency-controlled isolated attosecond pulses characterized by either 750 nm or 400 nm wavelength streak fields.

A compact and robust Mach-Zehnder type interferometer coupled with the double optical gating technique provides tunable isolated attosecond pulses and streak field detection with fields centered at either 750 nm or 400 nm. Isolated attosecond pulses produced at 45 eV (with measured pulse duration of 114 ± 4 as) and 20 eV (with measured pulse duration of 395 ± 6 as) are temporally characterized with a 750 nm wavelength streak field. In addition, an isolated 118 ± 10 as pulse at 45 eV is measured with a 400 nm wavelength streak field. The interferometer design used herein provides broad flexibility for attosecond streak experiments, allowing pump and probe pulses to be specified independently. This capability is important for studying selected electronic transitions in atoms and molecules.


Introduction
The generation of isolated attosecond pulses in the extreme ultraviolet (XUV) region, 30-250 eV, and the vacuum ultraviolet (VUV) region, 6-30 eV [1], has a profound impact on the study of dynamics of electrons in atoms [2][3][4][5][6], molecules [7,8] and solids [6,9].Such pulses are produced by high harmonic generation (HHG) from few-cycle Ti:sapphire laser pulses, To date, the shortest isolated attosecond pulses produced, with a duration of 80 as (bandwidth 55-110 eV), were generated with linearly polarized 3.5 fs driving laser pulses and characterized by attosecond streaking [10].Although this generation scheme extends to the harmonic cutoff region, isolated attosecond pulses can only be produced near the cutoff region of the harmonics.Double optical gating (DOG) [11,12] and polarization gating [13] with elliptically polarized fields allow the generation of isolated attosecond pulses in either the plateau region or the cutoff region of the harmonic spectra because the HHG driving field is effectively gated to one half-cycle (1.3 fs).As demonstrated here, the spectral bandwidth can be filtered so that the frequency of the pulses can be controlled.
Various experimental schemes for measuring atomic and molecular dynamics by coupling an isolated attosecond pulse and the HHG-driving near-infrared pulse have been devised to observe time dependences of photons, ions, or photoelectrons [2][3][4][5]9].Atomic and molecular dynamics observed in these experiments depend on which states are accessed via interaction with the light pulses.Frequency control of both pulses will thus allow the temporal structures and the relative phases of a broader range of ultrafast electronic and molecular dynamics to be determined.For example, a photoelectron wave packet ionized from a localized electronic state [7,14] can be controlled or probed by taking advantage of the phase and frequency of the optical probe field.As a first demonstration of frequency modification of the optical probe field, attosecond streaking with ultraviolet pulses of 400 nm wavelength is shown here.The streaking technique [15] is a powerful method to measure the temporal structure and phase of a photoelectron wave packet [2,3,6,9].For example, a plasmon resonance in a gold nanoparticle can be excited with a pulse at 400 nm.A streaking measurement can measure the plasmon dephasing time by observing the enhancement of the photoelectron momentum shift due to the field of the plasmon resonance [6].Frequency control of the excitation pulse allows studying nanoparticles of varying structures and plasmon resonance frequencies.
In order to create and utilize variable-wavelength probe fields, a compact Mach-Zehnder (MZ) interferometer configuration is useful, because a collinear configuration with copropagating harmonic and probe beams does not allow the probe field to be modified independently from the driving HHG field [10].In previous work using MZ interferometers [15][16][17], the optical path length between the pump and probe arms has typically been as long as several meters so that the beams can be recombined after HHG.This extended configuration requires considerable effort to stabilize the relative timing jitter.This instability is exacerbated when producing harmonics with intensity sufficient for XUV nonlinear spectroscopy [18,19].To achieve higher stability and flexibility, the size of the interferometer can be greatly reduced, as described here, by combining all optical fields before the HHG region.
In this paper, we report the generation and characterization of isolated attosecond pulses centered at two different XUV and VUV frequencies produced by DOG in combination with metal filters.In addition, an isolated attosecond pulse is characterized with both 750 nm and 400 nm wavelength streak fields for the first time.The optical fields necessary for the measurement are formed using a compact and robust MZ type interferometer located completely outside of the vacuum chamber.

Overview of experimental setup
A carrier-envelope phase (CEP) stabilized Ti:Sapphire oscillator/chirped pulse amplifier followed by a hollow-core fiber compressor produces 8 fs, 350 µJ pulses centered at 750 nm wavelength at a 3 kHz repetition rate.In this laser system [20,21], the CEP stability is ~150 mrad RMS using a 30 Hz feedback loop.Figure 1(a) shows the optical and vacuum components after the hollow-core fiber.The novel feature of this setup is the compact MZ interferometer, shaded in Fig. 1(a) and shown in more detail in Fig. 1(b).The driving and gating fields needed for DOG are generated in one arm of the interferometer, while the optical field for streaking is generated in the other arm.The optical fields from both arms are collinearly combined, pass though a β-BaB 2 O 4 (BBO) crystal and gas jet, in which the DOG fields generate an isolated attosecond pulse, and are focused into the interaction region of a time-of-flight (TOF) photoelectron spectrometer.The interferometer thus controls the timedelay between the attosecond pulse and streaking field.The interferometer occupies only 290 cm 2 on the optical table and all components are outside the vacuum chamber.The BBO crystal can also be used to produce the second harmonic of the streak field, whose central wavelength is shifted to 400 nm from the expected value of 375 nm due to the phase matching angle of the BBO.

Compact Mach-Zehnder interferometer
The output beam from the fiber compressor is introduced through an annular hole mirror [HM1, Fig. 1(b)] which splits the beam into the inner and outer arms of the interferometer.The inner beam (the HHG arm), with 300 µJ pulse energy (85%), passes through two fused silica plates (FS1, FS2), 2 mm and 1 mm thick, whose functions are described below, followed by two quartz plates (Q1 and Q2) required for DOG [17].The optical axis of the 250 µm first quartz plate (Q1) is oriented 45° to the input polarization.The slow axis of the 480 µm second quartz plate (Q2) is aligned parallel to the input polarization.The designed polarization gate width corresponds to ~2 fs in this experiment.Two mirrors are mounted on a piezo-electric translator (PZT) stage with position resolution of <1 nm to precisely control the delay between the signals from the two interferometer arms.
In the other arm, where the streak field is generated, the outer beam with 50 µJ pulse energy (15%) is reflected off the annular hole mirror (HM1) and travels through either a neutral density filter (NDF) to adjust the streak field intensity or a half-wave plate (HWP) to control laser polarization; both optics are 2 mm thick so that the dispersion from FS1 in the HHG arm is matched.The dispersion from the two quartz plates of the DOG optics is compensated by an extra quartz plate (QP3).The inner and outer beams are combined at the second annular hole mirror (HM2).The stability of the interferometer and the delay time between the two pulses are monitored and controlled by a co-propagated continuous wave laser (405 nm wavelength) [16].The stability of the interferometer with a 30 Hz feedback loop is 253 mrad RMS, corresponding to 54 as jitter, over 24 hours as shown in Fig. 1(c).The 30 Hz feedback frequency was limited by the fastest exposure time of the CCD camera used for continuous laser monitoring.Because the interferometer is located entirely outside of the experimental vacuum chamber, the effects of mechanical vibration from, for example, turbo pumps are reduced.

High-order harmonic generation
The beams from the interferometer are sent through a 150 µm BBO crystal, the final component of the DOG optics, which is located inside the vacuum chamber, and focused into a cell (2.5 mm long) filled with argon gas for HHG.In order to obtain isolated attosecond pulses, the interferometer must be configured so that only the beam from the HHG arm produces high harmonics.The 1 mm extra fused silica plate (FS2) in the HHG arm insures that this is the case via two mechanisms.First, the fused silica gives a group delay of ~5 ps for the HHG pulse (750 nm) relative to the streak pulse (750 nm or 400 nm).Since the delay time is significantly longer than the coherence time, the pulses from the HHG and streak arms do not temporally overlap in the gas cell nor do they interfere in the HHG process.Second, the fused silica plate (FS2) produces group delay dispersion (GDD) in the pulse from the HHG arm relative to the pulse from the streak arm.The spectral phase of the pulse sent into the interferometer is selected such that the pulse from the HHG arm is compressed at the cell.As a result, the streak arm pulse is temporally stretched because there is less dispersion in the streak arm.Therefore, the streak arm pulse has a lower peak intensity at the gas cell.The estimated peak intensities at the focus position of the 750 nm pulses from the HHG and streak arms were 1 × 10 15 W/cm 2 and <1 × 10 13 W/cm 2 , respectively.The peak intensity of the streak arm pulse gives <0.1% ionization probability of argon, as calculated from the ADK model [22], and no HHG.

Streak measurement with 750 nm field
The linearly polarized 750 nm pulse from the streak arm is aligned to the fast axis of the BBO, so that a 400 nm pulse is not generated from the streak pulse.The generated harmonic beam passes through either a 300 nm thick aluminum (band pass 17-72 eV), or a 200 nm thick tin (band pass 14 eV-24.5 eV) film mounted as the center portion of an annular filter that blocks the co-propagating HHG driver beam [23].The 750 nm streak beam passes through the outer portion of the annular filter, a 1 mm fused silica plate.In order to reduce energy loss from diffraction, the filter is constructed so that it is the optical image of the last hole mirror (HM2) in the interferometer.The thickness of the fused silica plate is chosen to match FS2 ensuring that the dispersion in the HHG and streak beam paths is now equal and that the harmonics and streak pulses are temporally overlapped.A Mo/Si coated spherical mirror focuses the beams to the neon (I p = 21.6 eV) or argon (I p = 15.8 eV) target gas.The generated photoelectrons are collected with a TOF spectrometer with energy resolution of 0.48 eV at 45 eV.In order to crosscheck the spatial and temporal overlap, the annular filter and 1 mm fused silica (FS2) in the HHG arm are removed and the 750 nm beams from both arms propagate to a CCD camera placed after the interaction region.

Streak measurement with 400 nm field
In order to perform streaking at 400 nm, the polarization of the 750 nm pulse in the streak arm of the interferometer is rotated by 90 degrees by an achromatic half-wave plate.The orthogonally polarized 750 nm pulse produces a second harmonic pulse from the BBO crystal that is used for DOG.A pulse duration of ~20 fs after the BBO is estimated from Ref [24].The central wavelength of the second harmonic pulse is 400 nm, as measured by a photon spectrometer.In this case, the polarization direction of the 400 nm pulse and the TOF geometry allow efficient detection of the induced photoelectron momentum shift [15].The fused silica part of the annular filter stretches the 400 nm pulse from 20 fs to ~90 fs at the gas target.The 400 nm streak pulse has ~65 fs group delay relative to the co-propagating 750 nm pulse as a result of the BBO crystal and the fused silica plate.This long group delay ensures that the 750 nm and 400 nm pulses are not temporally overlapped in the gas target.versus time delay using an aluminum filter and neon as the target gas in the photoelectron spectrometer.The CEP dependence has 2π periodicity from DOG as seen in Fig. 2(a).This is caused by a shift of the driving laser field inside the polarization gate as the CEP is varied.The 2π periodicity is evidence of DOG as discussed in Ref [11].Figure 2(b) shows the measured streak trace [25,26] using a 750 nm streak field.Based on the streak momentum shift, the estimated peak intensity of the 750 nm pulse is 5 × 10 12 W/cm 2 .The spectrogram reconstruction shown in Fig. 2(c) used a blind iterative algorithm, the Principal Component Generalized Projections Algorithm (PCGPA) [27].The reconstructed temporal profile and phase (with linear phase subtracted) indicates a 114 ± 4 as pulse duration, as shown in Fig. 2(e).The Fourier transform limited (FTL) pulse duration would be 96 as, estimated from the harmonic spectrum without the 750 nm field.The pre-and post-pulses are suppressed with less than a 1% contribution at 750 nm half ( ± 1.25 fs) and full ( ± 2.5 fs) cycles in the reconstructed temporal profile, indicating a well isolated pulse.In addition, the measured and the reconstructed harmonic spectra agree well as shown in Fig. 2(f).frequency of the isolated attosecond pulses.Of course, future experiments are not limited to the frequencies selected in this study; indeed, various XUV and VUV thin filters and mirror coatings can be used in combination to specifically select photon energy ranges for experiments.With these measurements and our previous result of Ref [8], we have demonstrated the ability to generate and characterize isolated attosecond pulses covering the energy range of 16 to 95 eV.

3.3
Pulse characterization in 26-67 eV energy range using 400 nm streak field Finally, using the experimental configuration in Sec.2.5, we measured a streak spectrogram with the argon-produced harmonics using the aluminum filter and neon as the target gas in the photoelectron spectrometer and retrieved the amplitude and the phase of the photoelectron wave packet using a 400 nm streak field.Figures 4(a The measured and the reconstructed harmonic spectra agree well as shown in Fig. 4(e).As mentioned before, although the fused silica on the annular filter stretches the pulse from 20 fs to 90 fs, it is possible to reduce this effect with a thinner fused silica plate or a low dispersive material.This has the advantage of increasing the photoelectron momentum shift because the peak intensity will be higher.We note that either the 750 nm or 400 nm pulse could easily be blocked by using coated optics, or a two-color synthesized field can be created using birefringent optics.The compact interferometer design allows the streak field to be modified independently of the driving HHG field, while keeping mechanical stability high enough for attosecond experiments.

Conclusions
We have demonstrated an important step for future attosecond dynamics studies by generating isolated attosecond pulses with variable center frequency.XUV and VUV pulses with 114 as duration (26-67 eV) and 395 as duration (16-25 eV), respectively, were characterized.The tunability of isolated attosecond pulses will allow a greater variety of dynamics in atoms and molecules to be studied.In addition, the characterization of an isolated attosecond pulse (118 as duration, 26-67 eV) with a streaking measurement using a 400 nm streak field was demonstrated for the first time.The implementation of a compact MZ type interferometer allows either 750 nm or 400 nm streak fields to be generated while producing a stability of 54 as time jitter over 24 hours.In future experiments, the compact interferometer located outside of the vacuum chamber will easily allow the use of birefringent optics to produce elliptically polarized fields, the use of sum/differential frequency generation, the introduction of optical parametric amplifier pulses [28], the use of an adaptive spatial light modulator [29], etc.The increased flexibility of both the isolated attosecond field and the probe field will greatly extend the capabilities of attosecond applications.

Fig. 1 .
Fig. 1.Experimental setup and the stability with MZ interferometer.(a) Schematic of the entire setup.(b) Schematic of the interferometer.HM1 and HM2: hole drilled mirrors.FS1 and FS2: fused silica plates with 1 mm and 2 mm thickness, respectively.QP1 and QP2: quartz plates with 250 µm and 480 µm thickness, respectively.PZT: piezo-electronic translation stage.NDF: neutral density filter.: HWP: achromatic half wave plate with 2 mm fused silica substrate.(c) The interferometer stability (filled circles) measured by CW laser of 405 nm wavelength and the PZT displacement (solid line) over 24 hours.Right upper figure corresponds to a short time scale of 1 minute.

Figure 2
Figure 2 shows the CEP dependence of the argon-produced harmonic spectrum without the streak field [Fig.2(a)] and the measured and reconstructed streak traces [Figs.2(b) and 2(c)]

Fig. 2 .
Fig. 2. Pulse characterization in 26-67 eV energy range using 750 nm streak field.(a) CEP dependence of harmonic spectra without the streak field at an argon gas pressure of 10 mbar.(b) The measured streak trace with the streak field.(c) The reconstructed streak trace.The reconstruction error in the PCGPA is 5%.(d) Line profile at 45 eV from (b).(e) The reconstructed harmonic temporal profile (solid line) and phase (dotted line).(f) The reconstructed harmonic spectrum (solid line) and phase (dotted line).Also shown is the measured harmonic spectrum (dashed line) without the streak field for comparison.

Figure 2 (
d) shows the line profile of the trace shown in Fig. 2(b) at 42 eV photoelectron energy showing the 750 nm cycle periodicity.

Figure 3 (
Figure 3(a) shows the CEP dependence of the argon-produced harmonic spectrum with 2π periodicity obtained with the tin filter and argon as the target gas in the photoelectron spectrometer.Figures 3(b) and 3(c) show the measured and reconstructed streak traces.The estimated intensity of the 750 nm streak pulse is 5 × 10 10 W/cm 2 .Figure 3(d) shows a line profile taken at 6 eV photoelectron energy from Fig. 3(b), again showing the 750 nm cycle periodicity.The reconstructed temporal profile and phase indicate a 395 ± 6 as pulse duration as shown in Fig. 3(e).The pulse is stretched from the expected FTL pulse duration of 300 as.The pre-and post-pulses were suppressed to less than a 1% contribution at the 750 nm half and full cycle regions.The measured and the reconstructed harmonic spectra agree well as shown in Fig. 3(f).This result indicates the flexibility of selecting the XUV or VUV

Fig. 3 .
Fig. 3. Pulse characterization in 16-25 eV energy range using 750 nm streak field.(a) CEP dependence of harmonic spectra without the streak field at an argon gas pressure of 10 mbar.(b) The measured streak trace with the streak field.(c) The reconstructed streak trace.The reconstruction error in the PCGPA is 2%.(d) Line profile at 6 eV from (b).(e) The reconstructed harmonic temporal profile (solid line) and phase (dotted line).(f) The reconstructed harmonic spectrum (solid line) and phase (dotted line).Also shown is the measured harmonic spectrum (dashed line) without the streak field for comparison.
) and 4(b) show the measured and reconstructed streak traces.The estimated peak intensity of the 400 nm pulse is 5 × 10 10 W/cm 2 .Figure 4(c) shows line profiles at 40 eV photoelectron energy taken from the spectrogram in Fig. 4(a).The 400 nm cycle periodicity is nearly half of the 750 nm periodicity.The reconstructed temporal profile and phase indicate a 118 ± 10 as pulse duration as shown in Fig. 4(d).

Fig. 4 .
Fig. 4. Pulse characterization in 26-67 eV energy range using 400 nm streak field.(a) The measured streak trace with the streak field.The harmonics are generated at an argon gas pressure of 8.5 mbar.(b) The reconstructed streak trace.The reconstruction error in the PCGPA is 9%.(c) Line profile at 40 eV from (a).(d) The reconstructed harmonic temporal profile (solid line) and phase (dotted line).(e) The reconstructed harmonic spectrum (solid line) and phase (dotted line).Also shown is the measured harmonic spectrum (dashed line) without the streak field for comparison.