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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 24 — Nov. 22, 2010
  • pp: 24619–24631
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Frequency modulation of high-order harmonic fields with synthesis of two-color laser fields

A. Amani Eilanlou, Yasuo Nabekawa, Kenichi L. Ishikawa, Hiroyuki Takahashi, Eiji J. Takahashi, and Katsumi Midorikawa  »View Author Affiliations


Optics Express, Vol. 18, Issue 24, pp. 24619-24631 (2010)
http://dx.doi.org/10.1364/OE.18.024619


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Abstract

We report periodical frequency modulation of high-order harmonic fields observed by changing the delay between the driving two-color laser fields consisting of the fundamental and its second harmonic (SH) field. The amplitude of modulation has been up to ∼0.4 eV, which is larger than the bandwidth of the fundamental field. Experimental results show that the intensity and chirp of the fundamental field can control this phenomenon. Numerical analysis by solving the time-dependent Schrödinger equation approves of these results and shows that anharmonic frequency components of the SH field have a crucial role in this phenomenon.

© 2010 Optical Society of America

1. Introduction

There has been growing interest to investigate high-order harmonic generation (HHG) by using two laser fields with different wavelengths, since it makes it possible to analyze or control HHG processes in situ by changing the shape of an instantaneous electric field with alternation of the delay (relative phase) between the two laser fields. HHG by a Ti:sapphire laser field superposed with its second harmonic (SH) field has been particularly interesting due to inversion symmetry breaking, resulting in generation of even-order harmonic fields in the plateau region.

Such dense harmonic spectra are on the way approaching generation of an isolated attosecond pulse (IAP). For example, Oishi et al. have demonstrated generation of a continuous spectrum in the extreme ultraviolet (XUV) spectral region when the pulse duration of the fundamental field approaches 9 fs [1

1. Y. Oishi, M. Kaku, A. Suda, F. Kannari, and K. Midorikawa, “Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field,” Opt. Express 14, 7230–7237 (2006). [CrossRef] [PubMed]

]. Feng et al. have succeeded in observing an IAP generated from a 28-fs multi-cycle laser pulse by adopting the polarization gating technique on a two-color laser field [2

2. X. Feng, S. Gilbertson, H. Mashiko, H. Wang, S. D. Khan, M. Chini, Y. Wu, K. Zhao, and Z. Chang, “Generation of isolated attosecond pulses with 20 to 28 femtosecond lasers,” Phys. Rev. Lett. 103, 183901 (2009). [CrossRef] [PubMed]

]. We also note that it is advantageous to generate the IAP by a two-color laser field synthesized from the fundamental field and its anharmonic field, rather than its exact SH field. Takahashi et al. have clearly observed generation of a continuous spectrum by extending the cut-off region. This result has been obtained from a 30-fs pulse of a Ti:sapphire laser superposed with an infrared (IR) laser pulse with a wavelength of 1300 nm [3

3. E. J. Takahashi, P. Lan, O. D. Mücke, Y. Nabekawa, and K. Midorikawa, “Infrared two-color multicycle laser field synthesis for generating an intense attosecond pulse,” Phys. Rev. Lett. 104, 233901 (2010). [CrossRef] [PubMed]

].

The theoretical work of Fleischer et al. predicts the position of the even-order harmonic fields when the frequency of the superposed field is detuned from that of the fundamental field [4

4. A. Fleischer and N. Moiseyev, “Attosecond laser pulse synthesis using bichromatic high-order harmonic generation,” Phys. Rev. A 74, 053806 (2006). [CrossRef]

]. This has been verified experimentally by Vozzi et al. [5

5. C. Vozzi, F. Calegari, F. Frassetto, L. Poletto, G. Sansone, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Coherent continuum generation above 100 eV driven by an ir parametric source in a two-color scheme,” Phys. Rev. A 79, 033842 (2009). [CrossRef]

] and Bandulet et al. [6

6. H. C. Bandulet, D. Comtois, E. Bisson, A. Fleischer, H. Pépin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, “Gating attosecond pulse train generation using multicolor laser fields,” Phys. Rev. A 81, 013803 (2010). [CrossRef]

] by using different IR frequencies in a two-color laser field. Although generation of new frequency components is observed, neither of these works has predicted or observed frequency modulation of high-order harmonic (HH) fields, which is the main concern of this paper. Generation of new spectral components using a two-color laser field synthesized from the fundamental field and its anharmonic field can result in generation of a quasicontinuum in the HH spectra, which can moderate the conditions of IAP generation. Therefore, frequency modulation of the HH fields simply by changing the delay between the two fields could add another degree of freedom to relaxing the conditions of IAP generation.

Not only being useful in IAP generation, two-color laser fields have been also applied in in-situ measurement and control of the HHG processes by controlling the electron trajectory which happens at a sub-fs time scale. Mauritsson et al. have verified the effects of the delay on generation of attosecond pulse trains using a two-color laser field [7

7. J. Mauritsson, P. Johnsson, E. Gustafsson, A. L’Huillier, K. J. Schafer, and M. B. Gaarde, “Attosecond pulse trains generated using two color laser fields,” Phys. Rev. Lett. 97, 013001 (2006). [CrossRef] [PubMed]

], and Dudovich et al. have made an attempt to measure and control the birth of attosecond XUV pulses by changing the delay [8

8. N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, “Measuring and controlling the birth of attosecond XUV pulses,” Nat. Phys. 2, 781–786 (2006). [CrossRef]

]. Dahlström et al. have similarly used a two-color laser field for atomic measurement of attosecond pulse trains and have also compared the results to those obtained by macroscopic measurements [9

9. J. M. Dahlström, T. Fordell, E. Mansten, T. Ruchon, M. Swoboda, K. Klünder, M. Gisselbrecht, A. L’Huillier, and J. Mauritsson, “Atomic and macroscopic measurements of attosecond pulse trains,” Phys. Rev. A 80, 033836 (2009). [CrossRef]

]. Furthermore, quantum path selection in HHG by a two-color laser field has been investigated by Ishii et al. [10

10. N. Ishii, A. Kosuge, T. Hayashi, T. Kanai, J. Itatani, S. Adachi, and S. Watanabe, “Quantum path selection in high-harmonic generation by a phase-locked two-color field,” Opt. Express 16, 20876–20883 (2008). [CrossRef] [PubMed]

]. An orthogonally polarized two-color laser field has been even applied to increase the efficiency of the HHG process [11

11. I. J. Kim, C. M. Kim, H. T. Kim, G. H. Lee, Y. S. Lee, J. Y. Park, D. J. Cho, and C. H. Nam, “Highly efficient high-harmonic generation in an orthogonally polarized two-color laser field,” Phys. Rev. Lett. 94, 243901 (2005). [CrossRef]

]. All of these experiments are implemented with a pulse duration longer than 20 fs and we could not find any notable modulation of the frequency of the HH fields in the above results.

2. The experimental setup

We used a chirped pulse amplification (CPA) system of Ti:sapphire laser, which is capable of generating 9.9-fs pulses with a peak power of 1.1 TW without using a nonlinear spectral broadening method such as a hollow-core fiber filled with gaseous medium. One of the key optical elements for spectral broadening (pulse shortening) of the laser system is a gain narrowing compensator (GNC) inserted in the cavity of the regenerative amplifier. The details of the laser system was reported in Ref. [12

12. A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, and K. Midorikawa, “Direct amplification of terawatt sub-10-fs pulses in a CPA system of Ti:sapphire laser,” Opt. Express 16, 13431–13438 (2008). [CrossRef] [PubMed]

]. We have used a fundamental pulse with a pulse energy of 15 mJ and a pulse duration of ∼15 fs yielded by a brief modification in the GNC in the regenerative amplifier to realize a higher pulse energy and a more stable spectrum behind the multi-pass amplifier for this sensitive experiment.

The spectrum of the fundamental field is shown by the solid curve in Fig. 1(a) and has a foot-to-foot bandwidth of ∼0.33 eV (in photon energy units) at a central wavelength of ∼808 nm. The dashed curve in Fig. 1(a) shows the spectral phase after control by a liquid-crystal spatial light modulator (LC-SLM). The SH field has been generated by a self-standing Type I BBO crystal with a thickness of 100 μm to result in a Fourier-limit pulse duration of 20 fs (FWHM) at a central wavelength of 417 nm with pulse energy of 780 μJ, the spectrum of which is shown in Fig. 1(b) by the solid curve. Although the central wavelength was tunable in a range of ±7 nm, we could not get a strong SH field at a central wavelength of 400 nm. We will see that the anharmonic frequency components in the SH field have a crucial role in the observed novel phenomenon (Sec. 4). The dotted blue curve in the inset of Fig. 1(b) shows the Fourier-limit temporal profile of the SH field compared to the reconstructed temporal profile of the fundamental field with FWHM of 15.2 fs, shown by the solid red curve. The generated SH field and the residual fundamental field were sent into a two-color interferometer, the schematic of which is shown in Fig. 2. The SH field was reflected with a broadband dichroic mirror (BDM) at the entrance of the interferometer with an incident angle of 22.5°, while the fundamental field transmitted the BDM. This exceptional incident angle was needed to keep high reflectivity and low dispersion in the broad wavelength region of 355 nm up to 445 nm. The polarization of the SH field was rotated by 90° with a quartz rotator. The fundamental field passed through a delay line controlled by a translation stage with a piezo actuator, and combined spatio-temporally with the SH field on another BDM at the exit of the interferometer. Owing to the different paths of the two laser fields, we were able to independently control the focusing conditions of the two laser fields by adjusting the diameters of two iris diaphragms placed in the arms of the interferometer.

Fig. 1 Spectra and pulse shapes of the driving two-color laser fields. (a) Spectrum of the fundamental field (solid curve) together with the spectral phase after control by the LC-SLM (dashed curve). (b) The solid curve shows the spectrum of the SH field. The dotted blue curve in the inset shows the Fourier-limit temporal profile (FWHM 20 fs) of the SH field compared to the reconstructed temporal profile of the fundamental field with FWHM of 15.2 fs (solid red curve).
Fig. 2 Schematic of the two-color interferometer implemented to synthesize the broadband two-color laser field having an adjustable beam spot size at the focus.

The pulse duration of the SH field is slightly stretched by the dispersion of the rotator (Quartz, t=1.85 mm) and also the input window (UVFS, t=2 mm) of a focusing chamber and has an FWHM of ∼40 fs. Using a broadband 45° dichroic mirror, the two-color synthesized laser field is reflected into the focusing chamber consisting of an Al-coated off-axis parabolic mirror with a focal length of 500 mm to be focused into a gas cell with an interaction length of 12 mm. The HH spectra are measured with an XUV imaging spectrograph consisting of an entrance slit, a flat field grating, and an X-ray CCD camera. An aluminum foil with a thickness of 150 nm supported by a mesh has been placed between the slit and the grating in the XUV spectrograph to prevent saturation of the X-ray CCD camera by strong irradiation of the fundamental field. The X-ray CCD camera and the driver of the Piezo actuator are controlled by a personal computer to make the measurements of the XUV spectra full automatic. The schematic of the whole HH generation and measurement setup is shown in Fig. 3.

Fig. 3 Schematic of the HH generation and measurement setup.

3. HHG by the two-color laser field

For the first trial of HHG by the two-color synthesized laser field, the pulse energy of the fundamental laser field was 8.5 mJ and that of the SH field was 125 μJ both attenuated by using the iris diaphragms placed in the arms of the two-color interferometer. The two laser fields have almost the same beam spot at the focus, which is ∼100 μm to get a high HH yield. The target gas was argon (Ar) with a backing pressure of ∼20 torr. The HH spectrum consisting of only odd-order harmonic fields generated by the fundamental field alone is shown by the dashed curve in Fig. 4, and that generated by the two-color laser field at a fixed delay at which the peaks of the two fields overlap, is shown by the solid curve. The two-color laser field has resulted in generation of even-order harmonic fields as well and has decreased the total integrated HH yield to ∼65% of the yield obtained by the fundamental field alone. We suppose the lower HH yield originates from high degrees of ionization of the target Ar gas with high field amplitude of the two-color laser field, even though the increase of the pulse energy is only ∼1.5%. Spectral blue shifts of the odd-order harmonic fields (for example, ∼0.26 eV at the 27th order) also support this speculation.

Fig. 4 HH spectrum generated by the fundamental field alone (dashed curve). The solid curve shows the spectrum generated by the two-color laser field.

By changing the delay between the fundamental field and the SH field in 20 nm steps, we recorded the HH spectra for 100 laser shots per delay scan. The resultant spectrogram is shown in Fig. 5(a). After normalizing the delay with the optical period of the fundamental field (Tf), we can see the modulation of the intensity of each even-order harmonic field with a period of Tf/4, consistent with the results of Dahlström et al. [9

9. J. M. Dahlström, T. Fordell, E. Mansten, T. Ruchon, M. Swoboda, K. Klünder, M. Gisselbrecht, A. L’Huillier, and J. Mauritsson, “Atomic and macroscopic measurements of attosecond pulse trains,” Phys. Rev. A 80, 033836 (2009). [CrossRef]

], rather than those of Dudovich et al. [8

8. N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, “Measuring and controlling the birth of attosecond XUV pulses,” Nat. Phys. 2, 781–786 (2006). [CrossRef]

], who reported a modulation period of Tf/2. Numerical analysis also agrees with our experimental results and those of Dahlström et al., yielding a modulation period of Tf/4, as will be shown in Fig. 8.

Fig. 5 XUV spectra collection by scanning the delay between the two-color laser fields and the first observation of frequency modulation of HH fields. (a) Spectrogram of HH fields. The HH spectra are scaled with the optical frequency of the fundamental field. The delay is normalized with the optical period of the fundamental laser field (Tf). (b) Peak frequency of the 22nd harmonic field against the delay, which is extracted from panel-(a) by calculating the frequency of the HH field when its intensity reaches a maximum. All of the spectrograms in the following figures are depicted in the same manner.
Fig. 8 Numerical analysis results to verify the effects of the wavelength of the SH field and the intensity and chirp of the fundamental field on the frequency modulation of HH fields. (a) Calculated spectrogram by SH wavelength of 400 nm added to Fourier-limit fundamental field. (b) Calculated spectrogram by SH wavelength of 417 nm added to the same fundamental field. (c) Same conditions as panel-(b), but with a chirped fundamental field (GDD=−30 fs2). The introduction of the GDD results in 17.7 fs pulse duration, for which the peak intensity of the fundamental field is kept unchanged. (d) Same conditions as panel-(c), but with a higher fundamental pulse intensity (1.5 × 1014 W/cm2). (e) Same conditions as panel-(d), but with SH wavelength of 400 nm. (f) Effects of minus chirp on the frequency modulation of the 30th harmonic fields of panel-(c) (diamonds with solid curve) and panel-(b) shown by the diamonds with dashed curve. (g) Effects of intensity on the frequency modulation of the 18th harmonic fields of panel-(d) (diamonds with solid curve) and panel-(c) shown by the diamonds with dashed curve.

According to the theory of in-situ measurement of HHG process [8

8. N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, “Measuring and controlling the birth of attosecond XUV pulses,” Nat. Phys. 2, 781–786 (2006). [CrossRef]

], the phase of the intensity modulation of each even-order harmonic field could give us information on the emission time of the adjacent odd-order harmonic fields, if the SH field did not significantly perturb the motion of the electrons. In fact, the intensity of the SH field in our experiment is too high to execute the in-situ measurement of attosecond pulses, because even the intensity of the odd-order harmonic fields is modulated by alternation of the delay. Instead, we have found a novel feature in each frequency component of even-order harmonic fields. In Fig. 5(a), saw-toothed waveforms appear in the 18th up to the 28th even-order harmonic spectra. These waveforms are the consequences of the periodical peak frequency modulations of the even-order harmonic fields. This delay-dependent periodical frequency modulation has an amplitude of ∼0.4 eV for the 22nd harmonic field as shown in Fig. 5(b). This amplitude is larger than the foot-to-foot bandwidth of the fundamental field and accounts for ∼26% of the HH separation generated by the two-color laser field. This novel phenomenon has been also observed in krypton gas (Kr) and has almost the same characteristics compared to the case of Ar gas [13

13. A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, E. J. Takahashi, and K. Midorikawa, “Frequency modulation of high-order harmonics depending on the delay between two-color laser fields,” CLEO/QELS 2010, paper JThE121.

].

To verify the reason why the periodical frequency modulations appeared in this experiment, we changed the intensity of the fundamental field and kept the other parameters fixed and repeated the correlation experiment. In the later experiment, we faced a failure in the properties of two laser mirrors used in the delay line of the two-color interferometer to reflect the fundamental field and after replacing them, we could get a much better beam spot and therefore less amount of fundamental pulse energy was necessary for HHG. With fundamental pulse energy of 5.7 mJ and SH pulse energy of 85 μJ at the central wavelength of ∼417 nm, we could observe a large amplitude of frequency modulation for the even-order harmonic fields as shown in Fig. 6(a). Figure 6(b) shows the spectrogram obtained by a lower fundamental pulse energy of 4.2 mJ and SH pulse energy of 60 μJ to keep the relative intensity of the two laser fields almost constant. The other parameters such as the backing pressure of the Ar gas are the same.

Fig. 6 Spectrograms to investigate how the intensity of the fundamental field affects the frequency modulation of the HH fields. (a) Spectrogram obtained by a high-energy fundamental field (fundamental 5.7 mJ and SH 85 μJ). (b) Spectrogram obtained by a low-energy fundamental field (fundamental 4.2 mJ and SH 60 μJ). (c) Peak frequency of the 20th harmonic field extracted from panel-(a) shown by the diamonds with solid curve and that of panel-(b) (diamonds with dashed curve).

While frequency modulation could be observed in the lower-order harmonic fields generated by a higher fundamental pulse intensity, the frequency of the harmonic fields generated by a lower fundamental pulse intensity is not modulated to that extent. For example, Fig. 6(c) shows that the amplitude of frequency modulation of the 20th harmonic field (∼0.3 eV) is larger when the harmonics are generated by a pulse energy of 5.7 mJ (diamonds with solid curve) compared to that generated by the lower pulse energy of 4.2 mJ with an amplitude of ∼0.17 eV (diamonds with dashed curve). Other even-order harmonic fields are also in a similar relation. Therefore, the intensity of the fundamental field is a key parameter which can control this phenomenon. We also investigated the effects of the intensity of the SH field, but we noticed that its increase does not affect this phenomenon. Moreover, we also investigated the effects of phase matching on the amplitude of frequency modulation by changing the position of the gas cell. We found out that the amplitude gets larger when the gas cell is placed slightly before the focus rather than behind the focus, where total HH yield is higher.

We also investigated the effects of the chirp of the fundamental field on this phenomenon. After investigation, we found out that the position of the grating compressor being shifted to compensate for the dispersion of the BDMs used in the two-color interferometer and the input window of the vacuum chamber was slightly imposing a minus chirp on the fundamental field. The amount of the minus chirp considering the group delay dispersion (GDD) when the frequency modulation appeared strongly as in Fig. 6(a) was ∼−75 fs2. We imposed a higher amount of minus chirp of the order of ∼−125 fs2 on the fundamental field by changing the position of the grating compressor and repeated the correlation experiment with keeping the fundamental pulse enegy of 4.2 mJ and SH pulse energy of 70 μJ at the central wavelength of 417 nm.

The result is shown in Fig. 7(a), showing typical HH fields having frequency modulation in the lower-order harmonics. As we moved the compressor back to the position which is considered to be very close to get a Fourier-limit pulse, the amplitude of frequency modulation shrank as shown in Fig. 7(b) with the same experimental conditions but SH pulse energy of 80 μJ at a central wavelength of 417.8 nm, which is due to moving the grating compressor. To get a clearer view of the effects of the chirp of the fundamental field, the amplitudes of frequency modulation of the 24th harmonic field, which is selected for having the largest amplitude of frequency modulation, are compared in Fig. 7(c), which shows a larger amplitude of ∼0.26 eV when the fundamental field has minus chirp (diamonds with solid curve) compared to the case of unchirped fundamental field with an amplitude of ∼0.17 eV (diamonds with dashed curve). Therefore, the chirp of the fundamental field is also another key parameter which can control this phenomenon.

Fig. 7 Spectrograms to investigate how the chirp of the fundamental field affects the frequency modulation of the HH fields. (a) Spectrogram obtained by a two-color laser field in which the fundamental field has minus chirp (GDD∼−125 fs2). (b) Spectrogram obtained by a two-color laser field in which the fundamental field is nearly Fourier limited. (c) Peak frequency of the 24th harmonic field extracted from panel-(a) shown by the diamonds with solid curve and that of panel-(b) (diamonds with dashed curve).

4. Numerical analysis and discussions

As a further step towards understanding this phenomenon, we numerically solved the time-dependent Schrödinger equation (TDSE) for Ar atom within the single-active-electron (SAE) approximation [14

14. K. L. Ishikawa, “High-harmonic generation,” in “Advances in Solid-State Lasers: Development and Applications,” ed. by M. Grishin, INTECH, 439–464 (2010).

]. One of the parameters we could not consider during the experiment was the wavelength of the SH field in a broad range. Hence, we investigate the effects of the SH wavelength as well as those of the intensity and chirp of the fundamental field on the frequency modulation of HH fields.

Figure 8(c) shows that adding a slight amount of minus chirp (Gaussian-profile-equivalent GDD=−30 fs2) to the fundamental field, while keeping the SH field (417 nm) unchanged, results in a larger amplitude of frequency modulation. For example, the frequency modulation of the 30th harmonic field resulted by the Fourier-limit fundamental field leading to an amplitude of ∼0.2 eV (diamonds with dashed curve) and that by the chirped fundamental field leading to an amplitude of ∼0.4 eV (diamonds with solid curve) are compared in Fig. 8(f). The 30th harmonic field has been selected for its relatively large amplitude of frequency modulation.

Keeping the other parameters fixed and slightly increasing the intensity of the fundamental field to 1.5 × 1014 W/cm2 and that of SH to 2.5 × 1012 W/cm2 to keep the relative intensity of the two laser fields constant, makes the amplitude of frequency modulation much larger [Fig. 8(d)]. Comparing the frequency modulation of the 18th harmonic field in Fig. 8(c) with a lower fundamental field intensity shown by the diamonds with dashed curve in Fig. 8(g) with an amplitude of ∼0.15 eV to that of Fig. 8(d) with an amplitude of ∼0.3 eV [diamonds with slolid curve in Fig. 8(g)], shows that a higher fundamental field intensity can increase the frequency modulation amplitude. While the 18th harmonic field has been selected for its visually noticeable difference in Figs. 8(c) and 8(d), other even-order harmonic fields are also in a similar relation.

All of these observations are consistent with the experimental results in the preceding section and show that the intensity and chirp of the fundamental field can control this phenomenon and lead to a larger amplitude of frequency modulation. To make sure that SH field with wavelength of 400 nm does not lead to the same results as those obtained by 417 nm, we have calculated the spectrogram with the same parameters used in Fig. 8(d), but with SH wavelength of 400 nm [Fig. 8(e)]. The result shows that a large amplitude of frequency modulation comparable to those obtained experimentally or numerically by SH wavelength of 417 nm does not exist for even-order harmonic fields and therefore emphasizes the crucial role of the anharmonic frequency components of the SH field in this phenomenon assisted by the high intensity and minus chirp of the fundamental field.

Although the above numerical analysis reveals the important laser parameters for observation of the frequency modulation of HH fields, it does not give us a physical insight into how the frequency modulation can be controlled by alternation of the delay between the two fields. To explain this point, let us begin with Eq. (1) of Dudovich at al. [8

8. N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, “Measuring and controlling the birth of attosecond XUV pulses,” Nat. Phys. 2, 781–786 (2006). [CrossRef]

] i.e.,
S2(t,ϕ)=S1(t)σ(t,ϕ).
(1)
S2 in Eq. (1) is the action in the two-color laser field and S1 is the unperturbed action, σ is the additional phase induced by the second field, ϕ is the relative phase between the two fields, controlled by the delay. If the second field is the exact SH field (ω2 = 2ω), then we obtain σ(t +π/ω, ϕ) = −σ(t, ϕ), which leads to generation of even-order harmonic fields.

If the central angular frequency of the second field is detuned from that of the exact SH field by −δ ωSH (ω2 = 2ωδ ωSH), as in the experiment, it satisfies
cos[ω2(t+π/ω)+ϕ]=cos[ω2t+{ϕ(π/ω)δωSH}].
(2)
Then using Eq. (2) we obtain
σ(t+π/ω,ϕ)=σ(t,ϕ(π/ω)δωSH(σ(t,ϕ)σϕ(πωδωSH)).
(3)

Provided that the fundamental pulse is sufficiently short so that δ ωSH · τ < 2π, with τ being its pulse duration, using Eq. (3) and similar dependence of σ(t, ϕ) on the frequency of HH fields, we would expect that the even-order harmonic fields appear not exactly at ωHH = 2 (n: integer), but at slightly modulated frequency of ωHH = 2δ ωHH with a modulation (δ ωHH) that satisfies
σϕ(πωδωSH)πωδωHH=0.
(4)
From Eq. (4) we obtain
δωHH=σϕδωSH.
(5)
Since σϕ depends on ϕ, the modulation of the frequency of the HH fields can be controlled by alternation of the delay as shown in Eq. (5).

Further remaining issue left from the above discussion is the reason why the amplitude of the frequency modulation can be larger than the bandwidth of the fundamental field. The spectrum of the top-hat fundamental field used in the above analysis in the frequency domain has a sinc2 function with broadly spanning frequency components. The main peak in this spectrum has a broad foot-to-foot bandwidth of ∼0.45 eV (in photon energy units), and the adjacent pedestal peaks are located at ±0.33 eV aside from the main peak. Thus, the amplitude of frequency modulation in the numerical analysis should not be directly compared to that in the experimental data obtained from a fundamental field with a finite bandwidth, shown in Fig. 1(a).

Instead, we consider multi-photon picture of HHG process contributed by many photons of the fundamental field and a single photon of the SH field to elucidate the large frequency modulation amplitude of an even-order harmonic field. The frequency of the 2nth harmonic field should correspond to two in-situ processes of (i) the sum of 2(n – 1) fundamental photons (ν2(n–1)) and the SH photon (νSH) and (ii) the sum of 2(n + 1) fundamental photons (ν2(n+1)) with the difference of the SH photon. Note that in both processes an odd number of total photons are involved in even-order harmonic generation [9

9. J. M. Dahlström, T. Fordell, E. Mansten, T. Ruchon, M. Swoboda, K. Klünder, M. Gisselbrecht, A. L’Huillier, and J. Mauritsson, “Atomic and macroscopic measurements of attosecond pulse trains,” Phys. Rev. A 80, 033836 (2009). [CrossRef]

]. In the experiment, νSH is slightly detuned from the exact SH frequency of 2ν by –δ νSH, resulting in νSH = 2νδ νSH. We have evaluated hδ νSH to be ∼0.125 eV. Here, ν is the frequency of the fundamental field and h is the Plank constant.

Furthermore, we have found that the peak frequency of each odd-order harmonic field is blue shifted from the exact odd-multiple frequency of the fundamental field, even without adding the SH field. For example, the energy shifts of the odd-order harmonic fields [under experimental conditions of Fig. 6(a)] are shown in Fig. 9(a). Indicating this blue shift by δ ν2n–1, the frequency of the (2n – 1)th harmonic field becomes ν2n–1 = (2n – 1) ν + δ ν2n–1. We can approximate that δ ν2n–1 linearly depends on the harmonic order. Thus, we express δ ν2n–1 = (2n – 1) δ νq, where q is a constant. Fitting the experimental data shown by the solid circles in Fig. 9(a) results in hδ ν of 0.055 eV (slope of the linear fit shown by the dashed line). This blue shift in the multi-photon picture of HHG process can be represented as a frequency difference in the fundamental frequency to result in ν2(n–1) = 2(n – 1)(ν+ δ ν) – q. Note that the spectral blue shift of the HH fields is commonly observed when the intensity of the fundamental field is reasonably high. This is due to the rapid change of macroscopic dipole phase and the suppression of the HH yield at the trailing edge of the fundamental field with crucial ionization of the target atoms [15

15. H. J. Shin, D. G. Lee, Y. H. Cha, K. H. Hong, and C. H. Nam, “Generation of nonadiabatic blueshift of high harmonics in an intense femtosecond laser field,” Phys. Rev. Lett. 83, 2544–2547 (1999). [CrossRef]

], which is not included in our numerical model of single atom response.

Fig. 9 Blue shift of the odd-order harmonic fields and energy level diagram showing generation of even-order harmonic fields by two in-situ processes. (a) Blue shift of the odd-order harmonic fields generated by a high-intensity fundamental field alone, expressed using photon energy shifts of the peak frequencies. Experimental data shown by the solid circles are those obtained under conditions of Fig. 6(a), showing an hδ ν of 0.055 eV (slope of the linear fit shown by the dashed line). (b) Photon energy of the even-order harmonic fields generated by the in-situ processes (i) and (ii), expressed in units of the photon energy of the fundamental field (). For clarity in the figure, δ νSH/ν is denoted by β, δ ν/ν by α, and q/ν by p. This results in even-order harmonic fields with energy of 2n+2(n – 1) αpβ and 2n + 2(n + 1) αp +β respectively, having energy difference of 4α+ 2β, which is equal to h(4δ ν+ 2δ νSH).

The frequency of the 2nth harmonic field (ν2n) generated with the in-situ process (i) is
ν2n=ν2(n1)+νSH=[2(n1)(ν+δν)q]+{2νδνSH}=2nν+2(n1)δνqδνSH,
(6)
while that with the in-situ process (ii) is
ν2n=ν2(n+1)νSH=[2(n+1)(ν+δν)q]{2νδνSH}=2nν+2(n+1)δνq+δνSH.
(7)
Thus, the difference between the two frequencies obtained in Eqs. (6) and (7) becomes
ν2nν2n=4δν+2δνSH.
(8)

This situation is schematically shown in Fig. 9(b). For more clarity, energy has been shown in units of the photon energy of the fundamental field. The energy difference calculated in Eq. (8), h(4δ ν + 2δ νSH ) is estimated to be 0.47 eV, which is sufficiently large to explain that the experimental maximum frequency modulation amplitude [∼0.4 eV in Fig. 5(b) or the 28th harmonic field in Fig. 6(a)] exceeds the bandwidth of the fundamental field. Frequency modulation can appear by changing the contributions of the two processes due to alternation of the delay between the two laser fields. Considering the resolution of the XUV spectrograph, a frequency modulation with an amplitude larger than ∼0.15 eV can be measurable. To get such an amplitude without considering the assist of the blueshift due to ionization, SH field at a central wavelength detuned from 400 nm by more than 10 nm could be enough to observe this phenomenon.

5. Summary

By performing a high-resolution correlation experiment using a two-color synthesized laser field consisting of the fundamental field of a terawatt 15-fs laser system and its SH field, we have been able to observe delay-dependent periodical frequency modulation of HH fields, for the first time. The amplitude of the frequency modulation depends on the intensity and chirp of the fundamental field as demonstrated by the experimental results. The numerical analysis revealed the crucial role of the anharmonic frequency components of the SH field in this phenomenon and the obtained results with SH wavelength of 417 nm highly approve of the experimental results and show that a fundamental field with a slight amount of minus chirp and a higher intensity can result in a larger amplitude of frequency modulation. Also, the frequency modulation amplitude has been estimated by considering the frequency difference of even-order harmonic fields generated by two in-situ processes when the SH field has anharmonic frequency components and the HH fields are blue shifted due to the high intensity of the fundamental field. The estimated amplitude agrees with the experimentally observed one. This novel phenomenon could be applied in in-situ precise control of the intensity and pulse duration of attosecond pulse trains.

Acknowledgments

This work has been supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through a grant for Extreme Photonics Research and has been also partially supported by the Advanced Photon Science Alliance (APSA) project of Japan. The numerical analysis was performed by using the RIKEN Integrated Cluster of Clusters (RICC) facility. A. A. E. is grateful to Junior Research Associate (JRA) program of RIKEN.

References and links

1.

Y. Oishi, M. Kaku, A. Suda, F. Kannari, and K. Midorikawa, “Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field,” Opt. Express 14, 7230–7237 (2006). [CrossRef] [PubMed]

2.

X. Feng, S. Gilbertson, H. Mashiko, H. Wang, S. D. Khan, M. Chini, Y. Wu, K. Zhao, and Z. Chang, “Generation of isolated attosecond pulses with 20 to 28 femtosecond lasers,” Phys. Rev. Lett. 103, 183901 (2009). [CrossRef] [PubMed]

3.

E. J. Takahashi, P. Lan, O. D. Mücke, Y. Nabekawa, and K. Midorikawa, “Infrared two-color multicycle laser field synthesis for generating an intense attosecond pulse,” Phys. Rev. Lett. 104, 233901 (2010). [CrossRef] [PubMed]

4.

A. Fleischer and N. Moiseyev, “Attosecond laser pulse synthesis using bichromatic high-order harmonic generation,” Phys. Rev. A 74, 053806 (2006). [CrossRef]

5.

C. Vozzi, F. Calegari, F. Frassetto, L. Poletto, G. Sansone, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, “Coherent continuum generation above 100 eV driven by an ir parametric source in a two-color scheme,” Phys. Rev. A 79, 033842 (2009). [CrossRef]

6.

H. C. Bandulet, D. Comtois, E. Bisson, A. Fleischer, H. Pépin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, “Gating attosecond pulse train generation using multicolor laser fields,” Phys. Rev. A 81, 013803 (2010). [CrossRef]

7.

J. Mauritsson, P. Johnsson, E. Gustafsson, A. L’Huillier, K. J. Schafer, and M. B. Gaarde, “Attosecond pulse trains generated using two color laser fields,” Phys. Rev. Lett. 97, 013001 (2006). [CrossRef] [PubMed]

8.

N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, “Measuring and controlling the birth of attosecond XUV pulses,” Nat. Phys. 2, 781–786 (2006). [CrossRef]

9.

J. M. Dahlström, T. Fordell, E. Mansten, T. Ruchon, M. Swoboda, K. Klünder, M. Gisselbrecht, A. L’Huillier, and J. Mauritsson, “Atomic and macroscopic measurements of attosecond pulse trains,” Phys. Rev. A 80, 033836 (2009). [CrossRef]

10.

N. Ishii, A. Kosuge, T. Hayashi, T. Kanai, J. Itatani, S. Adachi, and S. Watanabe, “Quantum path selection in high-harmonic generation by a phase-locked two-color field,” Opt. Express 16, 20876–20883 (2008). [CrossRef] [PubMed]

11.

I. J. Kim, C. M. Kim, H. T. Kim, G. H. Lee, Y. S. Lee, J. Y. Park, D. J. Cho, and C. H. Nam, “Highly efficient high-harmonic generation in an orthogonally polarized two-color laser field,” Phys. Rev. Lett. 94, 243901 (2005). [CrossRef]

12.

A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, and K. Midorikawa, “Direct amplification of terawatt sub-10-fs pulses in a CPA system of Ti:sapphire laser,” Opt. Express 16, 13431–13438 (2008). [CrossRef] [PubMed]

13.

A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, E. J. Takahashi, and K. Midorikawa, “Frequency modulation of high-order harmonics depending on the delay between two-color laser fields,” CLEO/QELS 2010, paper JThE121.

14.

K. L. Ishikawa, “High-harmonic generation,” in “Advances in Solid-State Lasers: Development and Applications,” ed. by M. Grishin, INTECH, 439–464 (2010).

15.

H. J. Shin, D. G. Lee, Y. H. Cha, K. H. Hong, and C. H. Nam, “Generation of nonadiabatic blueshift of high harmonics in an intense femtosecond laser field,” Phys. Rev. Lett. 83, 2544–2547 (1999). [CrossRef]

OCIS Codes
(190.4160) Nonlinear optics : Multiharmonic generation
(320.7090) Ultrafast optics : Ultrafast lasers

ToC Category:
Ultrafast Optics

History
Original Manuscript: September 3, 2010
Revised Manuscript: October 15, 2010
Manuscript Accepted: October 18, 2010
Published: November 10, 2010

Citation
A. Amani Eilanlou, Yasuo Nabekawa, Kenichi L. Ishikawa, Hiroyuki Takahashi, Eiji J. Takahashi, and Katsumi Midorikawa, "Frequency modulation of high-order harmonic fields with synthesis of two-color laser fields," Opt. Express 18, 24619-24631 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-24-24619


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References

  1. Y. Oishi, M. Kaku, A. Suda, F. Kannari, and K. Midorikawa, "Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field," Opt. Express 14, 7230-7237 (2006). [CrossRef] [PubMed]
  2. X. Feng, S. Gilbertson, H. Mashiko, H. Wang, S. D. Khan, M. Chini, Y. Wu, K. Zhao, and Z. Chang, "Generation of isolated attosecond pulses with 20 to 28 femtosecond lasers," Phys. Rev. Lett. 103, 183901 (2009). [CrossRef] [PubMed]
  3. E. J. Takahashi, P. Lan, O. D. Mücke, Y. Nabekawa, and K. Midorikawa, "Infrared two-color multicycle laser field synthesis for generating an intense attosecond pulse," Phys. Rev. Lett. 104, 233901 (2010). [CrossRef] [PubMed]
  4. A. Fleischer, and N. Moiseyev, "Attosecond laser pulse synthesis using bichromatic high-order harmonic generation," Phys. Rev. A 74, 053806 (2006). [CrossRef]
  5. C. Vozzi, F. Calegari, F. Frassetto, L. Poletto, G. Sansone, P. Villoresi, M. Nisoli, S. De Silvestri, and S. Stagira, "Coherent continuum generation above 100 eV driven by an ir parametric source in a two-color scheme," Phys. Rev. A 79, 033842 (2009). [CrossRef]
  6. H. C. Bandulet, D. Comtois, E. Bisson, A. Fleischer, H. Pépin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, "Gating attosecond pulse train generation using multicolor laser fields," Phys. Rev. A 81, 013803 (2010). [CrossRef]
  7. J. Mauritsson, P. Johnsson, E. Gustafsson, A. L’Huillier, K. J. Schafer, and M. B. Gaarde, "Attosecond pulse trains generated using two color laser fields," Phys. Rev. Lett. 97, 013001 (2006). [CrossRef] [PubMed]
  8. N. Dudovich, O. Smirnova, J. Leveseque, Y. Mairesse, M. Yu. Ivanov, D. M. Villeneuve, and P. B. Corkum, "Measuring and controlling the birth of attosecond XUV pulses," Nat. Phys. 2, 781-786 (2006). [CrossRef]
  9. J. M. Dahlström, T. Fordell, E. Mansten, T. Ruchon, M. Swoboda, K. Klünder, M. Gisselbrecht, A. L’Huillier, and J. Mauritsson, "Atomic and macroscopic measurements of attosecond pulse trains," Phys. Rev. A 80, 033836 (2009). [CrossRef]
  10. N. Ishii, A. Kosuge, T. Hayashi, T. Kanai, J. Itatani, S. Adachi, and S. Watanabe, "Quantum path selection in high-harmonic generation by a phase-locked two-color field," Opt. Express 16, 20876-20883 (2008). [CrossRef] [PubMed]
  11. I. J. Kim, C. M. Kim, H. T. Kim, G. H. Lee, Y. S. Lee, J. Y. Park, D. J. Cho, and C. H. Nam, "Highly efficient high-harmonic generation in an orthogonally polarized two-color laser field," Phys. Rev. Lett. 94, 243901 (2005). [CrossRef]
  12. A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, and K. Midorikawa, "Direct amplification of terawatt sub-10-fs pulses in a CPA system of Ti:sapphire laser," Opt. Express 16, 13431-13438 (2008). [CrossRef] [PubMed]
  13. A. Amani Eilanlou, Y. Nabekawa, K. L. Ishikawa, H. Takahashi, E. J. Takahashi, and K. Midorikawa, "Frequency modulation of high-order harmonics depending on the delay between two-color laser fields," CLEO/QELS 2010, paper JThE121.
  14. K. L. Ishikawa, "High-harmonic generation," in "Advances in Solid-State Lasers: Development and Applications," ed. by M. Grishin, INTECH, 439-464 (2010).
  15. H. J. Shin, D. G. Lee, Y. H. Cha, K. H. Hong, and C. H. Nam, "Generation of nonadiabatic blueshift of high harmonics in an intense femtosecond laser field," Phys. Rev. Lett. 83, 2544-2547 (1999). [CrossRef]

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