Intrinsic spatial chirp of subcycle terahertz pulsed beams

G. A. Hine and M. Doleans

Phys. Rev. A 104, 032229 – Published 30 September 2021

Abstract

We present an analysis of spatial chirp in subcycle pulsed beams corroborated by spatiotemporal electro-optic sampling measurements of terahertz radiation. Modeling the subcycle pulsed beam as a superposition of monochromatic Gaussian beams, free-space propagation is shown to directly lead to lateral spatial chirp with the spectrum on-axis substantially bluer than the overall energy spectrum. The two-transverse $+$ one-temporal dimensional profile of terahertz subcycle pulsed beams from organic crystals DSTMS and OH1 are measured and compared with observed spatiospectral correlations consistent with the model.

Received 25 May 2021
Accepted 8 September 2021

DOI:https://doi.org/10.1103/PhysRevA.104.032229

Physics Subject Headings (PhySH)

Classical optics Nonlinear optics Optics & lasers Plasma acceleration & new acceleration techniques Ultrafast optics

Optical techniques

Accelerators & BeamsGeneral PhysicsAtomic, Molecular & OpticalInterdisciplinary Physics

Authors & Affiliations

G. A. Hine ^* and M. Doleans

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*hinega@ornl.gov

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Vol. 104, Iss. 3 — September 2021

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Images

Figure 1
Spectral redistribution upon focusing. (Top) The amplitude spatiospectrum of a Gaussian SCPB focused with constant NA. Inverse proportionality of beam waist to frequency leads to a horn-shaped spatiospectrum. (Bottom) When compared to $A_{0} (f)$ (black dashed curve), the local spectrum is shifted blue on-axis [cyan (light gray) solid curve] and red off-axis [red (dark gray) solid curve].
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Figure 2
Theoretical spatiotemporal profile of a SCPB. The superposition of Gaussian beams according to the spectrum in Fig. 1 produces (a) sine- and (b) cosinelike SCPBs for absolute phases $π$ and 0, respectively. Black contours at 5% and $- 5 %$ of the maximum field magnitude are overlaid to visualize the change in period with transverse coordinate $x$ . The period is shortest on axis at the peak of the pulse and increases in the transverse periphery where it both precedes the on-axis pulse and persists after it has passed.
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Figure 3
Experimental setup. A 1400-nm (1300-nm) pump generates a THz SCPB by optical rectification in DSTMS (OH1). The THz is relayed through a polyethylene filter (PE) and focused into the electro-optic medium (GaP). A 800-nm 30-fs probe passes through a variable delay line (d) and is focused through a 3-mm drilled hole in the final focusing mirror, expanding to over twice the transverse size of the THz SCPB as they copropagate in the GaP. The probe is imaged to a CCD through a quarter-wave plate (WP) and a polarizer (P), with local changes in polarization causing changes in fluence on the CCD.
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Figure 4
Pump profile. (a) Pump profile incident on organic crystals OH1 and DSTMS with a clear aperture of the crystal mount indicated with a 3-mm diameter red pupil. The color bar ranges from 0 to the maximum fluence. (b) A horizontal line out of the pump profile at the vertical location indicated by the dotted line in (a), overlaid on a super-Gaussian profile of order 1.5 with good agreement. Vertical dashed red lines indicate the clear aperture of the crystal mount.
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Figure 5
Spatiotemporal measurement of THz SCPBs. THz electric fields from (a) and (b) OH1 and (c) and (d) DSTMS as seen in (b) and (d) $x − t$ and (a) and (c) $y − t$ slices through the center. The full temporal window in (a) shows the noise floor before the THz arrival followed by trailing oscillations from resonances in air and the crystals. Black dashed lines indicate the temporal window used for panels (b)–(g). The wave fronts can be seen to flare out towards the pulse periphery as seen in Fig. 2. The pulse on axis, indicated by the cyan (light gray) dashed line in (b), is shown in panel (e) in cyan (light gray). In contrast, at 0.8-mm off axis [indicated by the red (dark gray) dashed line in (b)], the pulse exhibits a longer period and duration and arrives sooner [red (dark gray) in (e)]. Spatiospectra in (f) and (g) were computed from the SCPBs in (c) and (b), respectively, showing horn-shaped spatiospectral correlations as in Fig. 1. The two spatiospectra fit within the same $1 / f$ horn indicated by white dotted curves, owing to the shared focusing geometry.
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