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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 8 — Apr. 16, 2007
  • pp: 4577–4584
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Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields

K. Y. Kim, J. H. Glownia, A. J. Taylor, and G. Rodriguez  »View Author Affiliations


Optics Express, Vol. 15, Issue 8, pp. 4577-4584 (2007)
http://dx.doi.org/10.1364/OE.15.004577


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Abstract

A transient photocurrent model is developed to explain coherent terahertz emission from air irradiated by a symmetry-broken laser field composed of the fundamental and its second harmonic laser pulses. When the total laser field is asymmetric across individual optical cycles, a non-vanishing electron current surge can arise during optical field ionization of air, emitting a terahertz electromagnetic pulse. Terahertz power scalability is also investigated, and with optical pump energy of tens of millijoules per pulse, peak terahertz field strengths in excess of 150 kV/cm are routinely produced.

© 2007 Optical Society of America

1. Introduction

Strong terahertz (THz) electromagnetic pulse generation is of great current interest due to many potential applications such as rapid THz imaging, intense THz excitation of semiconductors and their nanostructures, as well as nonlinear THz spectroscopy [1

1. M. S. Sherwin, C. A. Schmuttenmaer, and P. H. Bucksbaum, DOE-NSF-NIH Workshop on Opportunities in THz Science (http://www.er.doe.gov/bes/reports/files/THz_rpt.pdf).

]. While intense THz fields exceeding MV/cm can be obtained from large facility sources such as free electron lasers and synchrotron-based sources [1

1. M. S. Sherwin, C. A. Schmuttenmaer, and P. H. Bucksbaum, DOE-NSF-NIH Workshop on Opportunities in THz Science (http://www.er.doe.gov/bes/reports/files/THz_rpt.pdf).

], there is a growing demand for high-power tabletop-scale THz sources. In particular, laser-plasma-based schemes have drawn attention as potential powerful and scalable THz sources. For example, THz radiation by ponderomotive electron acceleration [2

2. H. Hamster, A. Sullivan, S. Gordon, W. White, and R. W. Falcone, “Subpicosecond, electromagnetic pulses from intense laser-plasma interaction,” Phys. Rev. Lett. 71, 2725–2728 (1993). [CrossRef] [PubMed]

] and transition radiation from laser-accelerated electrons crossing a plasma-vacuum boundary [3

3. W. P. Leemans, C. G. R. Geddes, J. Faure, Cs. Tóth, J. van Tilborg, C. B. Schroeder, E. Esarey, G. Fubiani, D. Auerbach, B. Marcelis, M. A. Carnahan, R. A. Kaindl, J. Byrd, and M. C. Martin, “Observation of terahertz emission from a laser-plasma accelerated electron bunch crossing a plasma-vacuum boundary,” Phys. Rev. Lett. 91, 074802/1-4 (2003). [CrossRef] [PubMed]

] have been demonstrated.

Recently, intense THz generation was observed upon mixing the fundamental and its second harmonic laser fields in air [4–7

4. D. J. Cook and R. M. Hochstrasser, “Intense terahertz pulses by four-wave rectification in air,” Opt. Lett. 25, 1210–1212 (2000). [CrossRef]

]. The underlying mechanism was initially interpreted as a four-wave difference frequency (FWDF) mixing parametric process in ionized air plasmas produced by the laser fields themselves [4–7

4. D. J. Cook and R. M. Hochstrasser, “Intense terahertz pulses by four-wave rectification in air,” Opt. Lett. 25, 1210–1212 (2000). [CrossRef]

]. However, the third order nonlinearity originating from either/both bound electrons of ions (χ (3) ions) or/and free electrons (χ (3) free-electrons) due to ponderomotive or thermal effects [8

8. J. F. Federici, “Review of four-wave mixing and phase conjugation in plasmas,” IEEE Trans. Plasma Sci. 19, 549–564 (1991). [CrossRef]

] is too small to explain the observed THz field strength [5

5. M. Kress, T. Löffler, S. Eden, M. Thomson, and H. G. Roskos, “Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves,” Opt. Lett. 29, 1120–1122 (2004). [CrossRef] [PubMed]

]. Nonetheless, a recent experiment performed by Xie et al. [7

7. X. Xie, J. Dai, and X.-C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96, 075005/1-4 (2006). [CrossRef] [PubMed]

] reported a THz amplitude scaling result (E THzχ (3) E 2ω Eω Eω supporting FWDF mixing as the principal mechanism for THz generation. In this study, however, the THz scaling was studied over a limited pump energy range, and the origin of χ (3) itself in the air plasma was not addressed.

A photoionization-induced polarization model has also been proposed to explain such THz emission, considering the generation of free elections via tunneling ionization and the resulting polarization transient in the laser field, especially for the few-cycle laser-air interaction [9

9. M. Kreβ, T. Löffler, M. D. Thomson, R. Dörner, H. Gimpel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, J. Ullrich, and H. G. Roskos, “Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy,” Nature Phys. 2, 327–331 (2006). [CrossRef]

]. However, in this report, the electron motion is considered only to within a fraction of a laser cycle, as the electron scattering time was extremely underestimated (t sc ≈ 1 fs) for atmospheric-pressure air [9

9. M. Kreβ, T. Löffler, M. D. Thomson, R. Dörner, H. Gimpel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, J. Ullrich, and H. G. Roskos, “Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy,” Nature Phys. 2, 327–331 (2006). [CrossRef]

]. If the plasma polarization changes on the sub-femtosecond timescale, the expected radiation falls mostly in the ultraviolet, and not in the THz range. As a consequence, this model fails to provide a proper mechanism for the THz emission, even for the few-cycle laser-air interaction, making extrapolation to the more complex two-color field case impossible.

In this paper, we develop a photocurrent model to explain the observed THz emission from air plasmas in two-color laser fields, and provide experimental evidence to question the χ (3) ions and χ (3) free-electrons based FWDF THz generation mechanisms. In our model, a nonvanishing transverse plasma current J = eNeυe, where e is the electron charge, Ne is the electron density, and υe is the electron velocity, can be produced when the bound electrons are stripped off by an asymmetric laser field, such as a mixed two-color field with the proper relative phase. This photocurrent surge, occurring on the timescale of the photoionization, can produce an electromagnetic pulse at THz frequencies. We note that asymmetric photocurrents were previously observed in gases [10

10. D. W. Schumacher and P. H. Bucksbaum, “Phase dependence of intense-field ionization,” Phys. Rev. A 54, 4271–4278 (1996). [CrossRef] [PubMed]

], semiconductors [11

11. A. Haché, J. E. Sipe, and H. M. van Driel, “Quantum interference control of electrical currents in GaAs,” IEEE J. Quantum Electron. 34, 1144–1154 (1998). [CrossRef]

], and semiconductor quantum well structures [12

12. E. Dupont, P. B. Corkum, H. C. Liu, M. Buchanan, and Z. R. Wasilewski, “Phase-controlled currents in semiconductors,” Phys. Rev. Lett. 74, 3596–3599 (1995). [CrossRef] [PubMed]

], irradiated by mixed two-color laser fields.

2. Theoretical background and simulations

We first show that a nonzero photocurrent can be produced in an alternating laser electric field even if the average field vanishes <EL> = 0. The laser field containing the fundamental and its second harmonic with the same linear polarization can be expressed as

EL(t)=E1cos(ωt+ϕ)+E2cos[2(ωt+ϕ)+θ],
(1)

where E 1 and E 2 are the amplitudes of the fundamental (ω) and the second harmonic (2ω) fields, and θ is the relative phase between the ω and 2ω fields. We assume that the bound electrons of atoms under study are librated at a phase ϕ via photoionization. Figure 1(a) shows a combined electric field of ω (λ = 800 nm) and 2ω (λ = 400 nm) lasers with intensities of Iω= 1015 W/cm2 and I 2ω = 2 × 1014 W/cm2 (assuming 20% efficiency of frequency doubling), respectively, for a relative phase of θ = 0 and π/2. Figure 1(b) shows the trajectories of electrons born at various phases of ϕ = -9π/10, -π/10, π/10, and 9π/10 with respect to the fundamental laser field (for both θ = 0 and π/2). Once bound electrons are liberated via ionization, the electron trajectories and velocities are calculated by classical mechanics. This method is analogous to the model developed to explain high harmonic generation (HHG) and nonsequential ionization in a semi-classical manner [13

13. P. B. Corkum, “Plasma perspective on strong-field multiphoton ionization,” Phys. Rev. Lett. 71, 1994–1997 (1993). [CrossRef] [PubMed]

]. We further simplify our model by including a non-relativistic treatment, thereby ignoring the laser magnetic field, justifiable for our laser intensity regime. Assuming that the initial electron velocity is zero, the drift velocity υd of electrons born at ϕ laser phase is given by υd = eE 1 sinϕ/(meω) + eE 2 sin(2ϕ + θ)/(2meω), where me is the electron mass.

Fig. 1. (a). Laser fields with the fundamental and second harmonic with a relative phase θ = 0 and π/2. (b) Electron trajectories born at various phases of ϕ = -9π/10, -π/10, π/10, and 9π/10. (c) Drift electron velocity versus ϕ(solid line), overlaid with the laser field (dashed line).

As shown in Fig. 1(b), with even-function symmetry (θ = 0), the drift velocity cancels for electrons born at opposite laser field slopes (for example, at ϕ = -π/10 and π/10, ϕ = -9π/10 and 9π/10, correspondingly). With θ = π/2, however, there is a non-vanishing drift velocity in the positive direction. The drift velocity for all ϕ is plotted in Fig. 1(c) for θ = 0 and π/2, along with the corresponding laser field (dotted line). Since the ionization occurs near the peak of the laser field, only the shadowed area of the electron drift velocity contributes to the overall electron current. In contrast to the θ = 0 case, there is a net electron current with θ = π/2, and this, with a time-varying electron density over the entire laser field envelope, produces a current surge which emits THz radiation.

To quantify the resulting THz yield and spectrum, we performed simulations taking into account photoionization and subsequent electron motion using a temporal Gaussian two-color laser field. For the intensity regime of interest (> 1015 W/cm2), the Keldysh parameter [14

14. L. V. Keldysh, “Ionization in the field of a strong electromagnetic wave,” Soc. Phys. JETP 20, 1307–1314 (1965).

] is γ=Ui(2UP)=0.36<1, where Ui = 15.576 eV is the ionization potential energy of N2 (the primary constituent gas of atmospheric air), and Up = 59.7 eV is the laser ponderomotive potential energy at I = 1015 W/cm2. The modified Keldysh parameter γ for small diatomic molecules such as N2 can be even smaller [15

15. M. J. Dewitt and R. J. Levis, “Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules,” J. Chem. Phys. 108, 7739–7742 (1998). [CrossRef]

, 16

16. G. Rodriguez, C. W. Siders, C. Guo, and A. J. Taylor, “Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H2, N2, and O2,” IEEE J. Sel. Top. Quantum Electron. 7, 579–591 (2001). [CrossRef]

]. For γ < 1, tunneling ionization becomes the dominant ionization route and we therefore use the Ammosov–Delone–Krainov (ADK) tunneling ionization rate [17

17. M. V. Ammosov, N. B. Delone, and V. P. Krainov, “Tunnel ionization of complex atoms and of atomic ions in an alternating electromagnetic field,” Sov. Phys. JETP 64, 1191–1194 (1986).

] in our model. Although, the ADK model has been used primarily for noble gases, it also works well for structureless atomic-like molecules such as neutral N2 [18

18. C. Guo, M. Li, J. P. Nibarger, and G. N. Gibson, “Single and double ionization of diatomic molecules in strong laser fields,” Phys. Rev. A 58, R4271–R4274 (1998). [CrossRef]

]. A field-ionized gas can undergo further ionization through electron-ion and electron-neutral collisional process. For an initially singly ionized plasma of atmospheric gas density, the electron-ion collision rate is νei ~ 1012 s-1 under our condition [16

16. G. Rodriguez, C. W. Siders, C. Guo, and A. J. Taylor, “Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H2, N2, and O2,” IEEE J. Sel. Top. Quantum Electron. 7, 579–591 (2001). [CrossRef]

], which corresponds to a time between collisions of ~1 ps—a time much longer than the pulse length of ~200 fs used in the experiment. The electron-neutral impact ionization rate is also similarly small compared to the ADK rate [16

16. G. Rodriguez, C. W. Siders, C. Guo, and A. J. Taylor, “Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H2, N2, and O2,” IEEE J. Sel. Top. Quantum Electron. 7, 579–591 (2001). [CrossRef]

]. Hence, collisional processes during the laser pulse can be ignored for atmospheric pressure gases, but in general they tend to wash out the coherent electron motion on a picosecond time scale, ultimately terminating the photocurrent.

Results of our simulations are shown in Fig. 2 for Gaussian-enveloped ω and 2ω fields with 50 fs (FWHM) pulsewidths, at peak intensities of Iω = 1015 W/cm2 and I 2ω = 2×1014 W/cm2, and with θ = π/2.

Fig. 2. (a). Electron density of atmospheric nitrogen gas irradiated by a combined ω and 2ω laser field with 50 fs pulsewidths and peak intensities of Iω = 1015 W/cm2 and I 2ω = 2×1014 W/cm2, with a relative phase θ = π/2. (b) Time-varying electron current. (c) Radiation spectrum obtained from the transient electron current. The inset shows an expected THz waveform obtained with 10 THz low-pass filtering.

Figure 2(a) shows a time-varying electron density, overlaid with the combined laser field. Here, N2 gas is ionized up to N2 2+ with the characteristic stepwise ionization twice per laser cycle. The resulting transverse electron current, J (t) = ∫t t0 e (t,t′)Ne(t′)dt′, where υe(t, t′) is the velocity of electrons born at t=t′ with the electron density of Ne(t′), was computed and is plotted in Fig. 2(b). In this computation, the contribution from electron displacements is ignored because the maximal displacement during the interaction is much smaller than the THz radiation wavelength, and thus is considered a point source. Moreover, transport and space charge effects are excluded for simplicity. In Figure 2(b), it is seen that in addition to its oscillatory feature, a non-zero DC electron current appears after the laser pulse is gone, giving rise to THz emission in the far field. The calculated power spectrum of radiation induced by the transient of current, dJ (t)/dt, is shown in Fig. 2(c), with θ = π/2, two-color (solid line) and single-color ω (dashed line) fields. Also shown is THz emission with a >20 THz bandwidth (FWHM) (indicated in the box), which is enhanced as compared to the single-color case. The corresponding THz waveform with low frequency filtering (<10 THz) is shown in the inset of Fig. 2(c).

3. Experiment

Fig. 3. (a). Typical experimental THz waveform and the corresponding THz spectrum in the inset. (b) THz yield vs. BBO-to-plasma distance d with two different BBO angles φ = 28° and 34°.

To verify the validity of our photocurrent model, we studied the phase dependence θ of THz radiation yield. Figure 3(b) shows the THz intensity versus the BBO-plasma distance d (see the inset) for two different BBO angles φ = 28° and 34° with respect to the BBO ordinary axis of ω. The least squares fits of data (solid lines) are also shown in Fig. 3(b) and extrapolated toward d = 0, where near d = 0 the direct measurement is impossible because of laser-induced damage to the BBO crystal. It is clear that the THz yield exhibits an oscillatory behavior due to air dispersion for ω and 2ω. Here the phase at the plasma front can be expressed as θ = ω(nω-n 2ω)d/c + θ 0, where c is the speed of light in vacuum, nω and n 2ω is the refractive index of air at ω and 2ω frequency, respectively, and θ 0 is the ω-2ω phase difference right after the BBO crystal. With careful angular tuning (±0.2° around the phase-matching angle), the dephasing length can be as large as 1 mm. Hence, with our 100-μm crystal thickness, the ω and 2ω fields can be considered as phase-matched (θ 0 ≈ 0) directly after the crystal and, even for nonzero φ cases, there are always in-phase ω and 2ω components projected along the incoming p-polarized ω field. We note that the relative phase slippage in the plasma is given by Δθ = (3π/4)(l/λ)(Ne/Nc), where l ≈ 7 mm is the plasma length and Nc = meω/(4πe 2) = 1.7 × 1021 cm-3 is the critical density at λ = 815 nm. With Ne ~ 1.24 × 1016 cm-3, as the upper-limit electron density obtained from the critical density at 1 THz, the estimated relative phase slippage is Δθ ≈ 0.3 rad, which still produces a phase-matching condition in the plasma. With θ ≈ 0 (or dθ 0 ≈ 0), the extrapolated THz yield approaches zero, consistent with the photocurrent model. However, this is in sharp contrast with polarization-based FWDF mixing which predicts the maximum THz yield at θ = 0, as the rectified term from the third order polarization is proportional to E 2 ω(t)E 2ω(t)cosθ [4–7

4. D. J. Cook and R. M. Hochstrasser, “Intense terahertz pulses by four-wave rectification in air,” Opt. Lett. 25, 1210–1212 (2000). [CrossRef]

]. We note that sinθ dependence can be obtained from FWDF mixing with an assumption of Eω(t) = sinωt and E 2θ(t) = sin(2ωt + θ) [5

5. M. Kress, T. Löffler, S. Eden, M. Thomson, and H. G. Roskos, “Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves,” Opt. Lett. 29, 1120–1122 (2004). [CrossRef] [PubMed]

]. In either case, THz radiation from polarization-based FWDF mixing becomes maximal when the peaks of ω and 2ω fields overlap in time. In contrast, the photocurrent model predicts that the maximal THz yield occurs with a θ = π/2 phase slippage (or sinθ dependence) between the ω and 2ω fields.

As a complementary experiment looking for THz contribution from χ (3) ions and χ (3) free-electrons, we examined the response in a preformed plasma. The preformed plasma was produced prior to the main ω and 2ω pulse arrival by reprogramming the regenerative amplifier Pockels cell switch-out timing, conveniently generating a contrast-controllable prepulse (with respect to the main pulse) of 9.6 ns. Figure 4 shows the THz emission yield for five cases (A, B, C, D, and E) of first to second pulse intensity contrast ratio.

Fig. 4. THz emission yield from a preformed air plasma with a varying double-pump-pulse contrast

Regarding the long time scale (9.8 ns) between the pre- and main pulse, we note that the preformed plasma can cause a significant plasma defocusing effect to the main pulse for subpicosecond time intervals. This strongly affects the main pulse propagation and the resulting THz generation. In contrast, nanosecond delays can provide a sufficient time for the preplasma to evolve and provide a plasma channel suitable for waveguiding of the main laser pulses [20

20. C. G. Durfee III, J. Lynch, and H. M Milchberg, “Development of a plasma waveguide for high-intensity laser pulses,” Phys. Rev. E 51, 2368–2389 (1995). [CrossRef]

].

Fig. 5. Peak THz field strength as a function of the incoming laser energy with ±5% error sizes.

4. Conclusion

In conclusion, we have developed a transient photocurrent model in which optical field ionization and subsequent electron motion in a symmetry-broken laser field are key mechanisms for producing a quasi-DC electron current and simultaneous THz pulse radiation. In general, the directional current and resulting THz generation becomes most efficient when the fundamental and its properly phase-locked second harmonic fields are mixed. With any other two-color wavelength combination, our simulation shows that the resulting quasi-DC current becomes extremely weak, compatible to that obtained with a single-color field. This is due to the fact that the current magnitude and direction varies at each combined laser cycle and the net current summed over an entire set of cycles vanishes unless one of the laser frequencies is an integer multiple of the other one. Our theoretical and experimental investigation provides insight into the THz generation mechanism and will speed further optimization and scalability of tabletop, ultrafast THz sources.

Acknowledgments

This work was supported through the Los Alamos National Laboratory Directed Research and Development Program Project No. 20040969PRD2 by Los Alamos National Security, LLC under auspices of the Department of Energy Contract Number DE-AC52-06NA25396.

References and links

1.

M. S. Sherwin, C. A. Schmuttenmaer, and P. H. Bucksbaum, DOE-NSF-NIH Workshop on Opportunities in THz Science (http://www.er.doe.gov/bes/reports/files/THz_rpt.pdf).

2.

H. Hamster, A. Sullivan, S. Gordon, W. White, and R. W. Falcone, “Subpicosecond, electromagnetic pulses from intense laser-plasma interaction,” Phys. Rev. Lett. 71, 2725–2728 (1993). [CrossRef] [PubMed]

3.

W. P. Leemans, C. G. R. Geddes, J. Faure, Cs. Tóth, J. van Tilborg, C. B. Schroeder, E. Esarey, G. Fubiani, D. Auerbach, B. Marcelis, M. A. Carnahan, R. A. Kaindl, J. Byrd, and M. C. Martin, “Observation of terahertz emission from a laser-plasma accelerated electron bunch crossing a plasma-vacuum boundary,” Phys. Rev. Lett. 91, 074802/1-4 (2003). [CrossRef] [PubMed]

4.

D. J. Cook and R. M. Hochstrasser, “Intense terahertz pulses by four-wave rectification in air,” Opt. Lett. 25, 1210–1212 (2000). [CrossRef]

5.

M. Kress, T. Löffler, S. Eden, M. Thomson, and H. G. Roskos, “Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves,” Opt. Lett. 29, 1120–1122 (2004). [CrossRef] [PubMed]

6.

T. Bartel, P. Gaal, K. Reimann, M. Woerner, and T. Elsaesser, “Generation of single-cycle THz transients with high electric-field amplitudes,” Opt. Lett. 30, 2805–2807 (2005). [CrossRef] [PubMed]

7.

X. Xie, J. Dai, and X.-C. Zhang, “Coherent control of THz wave generation in ambient air,” Phys. Rev. Lett. 96, 075005/1-4 (2006). [CrossRef] [PubMed]

8.

J. F. Federici, “Review of four-wave mixing and phase conjugation in plasmas,” IEEE Trans. Plasma Sci. 19, 549–564 (1991). [CrossRef]

9.

M. Kreβ, T. Löffler, M. D. Thomson, R. Dörner, H. Gimpel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, J. Ullrich, and H. G. Roskos, “Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy,” Nature Phys. 2, 327–331 (2006). [CrossRef]

10.

D. W. Schumacher and P. H. Bucksbaum, “Phase dependence of intense-field ionization,” Phys. Rev. A 54, 4271–4278 (1996). [CrossRef] [PubMed]

11.

A. Haché, J. E. Sipe, and H. M. van Driel, “Quantum interference control of electrical currents in GaAs,” IEEE J. Quantum Electron. 34, 1144–1154 (1998). [CrossRef]

12.

E. Dupont, P. B. Corkum, H. C. Liu, M. Buchanan, and Z. R. Wasilewski, “Phase-controlled currents in semiconductors,” Phys. Rev. Lett. 74, 3596–3599 (1995). [CrossRef] [PubMed]

13.

P. B. Corkum, “Plasma perspective on strong-field multiphoton ionization,” Phys. Rev. Lett. 71, 1994–1997 (1993). [CrossRef] [PubMed]

14.

L. V. Keldysh, “Ionization in the field of a strong electromagnetic wave,” Soc. Phys. JETP 20, 1307–1314 (1965).

15.

M. J. Dewitt and R. J. Levis, “Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules,” J. Chem. Phys. 108, 7739–7742 (1998). [CrossRef]

16.

G. Rodriguez, C. W. Siders, C. Guo, and A. J. Taylor, “Coherent ultrafast MI-FROG spectroscopy of optical field ionization in molecular H2, N2, and O2,” IEEE J. Sel. Top. Quantum Electron. 7, 579–591 (2001). [CrossRef]

17.

M. V. Ammosov, N. B. Delone, and V. P. Krainov, “Tunnel ionization of complex atoms and of atomic ions in an alternating electromagnetic field,” Sov. Phys. JETP 64, 1191–1194 (1986).

18.

C. Guo, M. Li, J. P. Nibarger, and G. N. Gibson, “Single and double ionization of diatomic molecules in strong laser fields,” Phys. Rev. A 58, R4271–R4274 (1998). [CrossRef]

19.

Q. Wu and X.-C. Zhang, “Free-space electro-optic sampling of terahertz beams,” Appl. Phys. Lett. 67, 3523–3525 (1995). [CrossRef]

20.

C. G. Durfee III, J. Lynch, and H. M Milchberg, “Development of a plasma waveguide for high-intensity laser pulses,” Phys. Rev. E 51, 2368–2389 (1995). [CrossRef]

OCIS Codes
(260.5210) Physical optics : Photoionization
(300.6270) Spectroscopy : Spectroscopy, far infrared
(320.7120) Ultrafast optics : Ultrafast phenomena
(350.5400) Other areas of optics : Plasmas

ToC Category:
Ultrafast Optics

History
Original Manuscript: December 22, 2006
Revised Manuscript: March 16, 2007
Manuscript Accepted: March 16, 2007
Published: April 3, 2007

Citation
Ki-Yong Kim, James H. Glownia, Antoinette J. Taylor, and George Rodriguez, "Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields," Opt. Express 15, 4577-4584 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-8-4577


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References

  1. M. S. Sherwin, C. A. Schmuttenmaer, and P. H. Bucksbaum, DOE-NSF-NIH Workshop on Opportunities in THz Science (http://www.er.doe.gov/bes/reports/files/THz_rpt.pdf).
  2. H. Hamster, A. Sullivan, S. Gordon, W. White, and R. W. Falcone, "Subpicosecond, electromagnetic pulses from intense laser-plasma interaction," Phys. Rev. Lett. 71, 2725-2728 (1993). [CrossRef] [PubMed]
  3. W. P. Leemans, C. G. R. Geddes, J. Faure, Cs. Tóth, J. van Tilborg, C. B. Schroeder, E. Esarey, G. Fubiani, D. Auerbach, B. Marcelis, M. A. Carnahan, R. A. Kaindl, J. Byrd, and M. C. Martin, "Observation of terahertz emission from a laser-plasma accelerated electron bunch crossing a plasma-vacuum boundary," Phys. Rev. Lett. 91, 074802/1-4 (2003). [CrossRef] [PubMed]
  4. D. J. Cook and R. M. Hochstrasser, "Intense terahertz pulses by four-wave rectification in air," Opt. Lett. 25, 1210-1212 (2000). [CrossRef]
  5. M. Kress, T. Löffler, S. Eden, M. Thomson, and H. G. Roskos, "Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves," Opt. Lett. 29, 1120-1122 (2004). [CrossRef] [PubMed]
  6. T. Bartel, P. Gaal, K. Reimann, M. Woerner, and T. Elsaesser, "Generation of single-cycle THz transients with high electric-field amplitudes," Opt. Lett. 30, 2805-2807 (2005). [CrossRef] [PubMed]
  7. X. Xie, J. Dai, and X.-C. Zhang, "Coherent control of THz wave generation in ambient air," Phys. Rev. Lett. 96, 075005/1-4 (2006). [CrossRef] [PubMed]
  8. J. F. Federici, "Review of four-wave mixing and phase conjugation in plasmas," IEEE Trans. Plasma Sci. 19, 549-564 (1991). [CrossRef]
  9. M. Kreß, T. Löffler, M. D. Thomson, R. Dörner, H. Gimpel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, J. Ullrich, and H. G. Roskos, "Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy," Nature Phys. 2, 327-331 (2006). [CrossRef]
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