Momentum Signatures for Schwinger Pair Production in Short Laser Pulses with a Subcycle Structure

We investigate electron-positron pair production from vacuum for short laser pulses with a subcycle structure, in the nonperturbative regime (Schwinger pair production). We use the nonequilibrium quantum kinetic approach and show that the momentum spectrum of the created electron-positron pairs is extremely sensitive to the subcycle dynamics—depending on the laser frequency $\omega$, the pulse length $\tau$, and the carrier phase $\varphi$—and shows several distinctive new signatures. This observation could not only help in the design of laser pulses to optimize the experimental signature of Schwinger pair production but also ultimately lead to new probes of light pulses at extremely short time scales.

Figure 1

Shape of the electric field Eq. (1), for carrier phase $\varphi =0$, when passing from $\sigma =3$ (dotted line) to $\sigma =4$ (dashed line) to $\sigma =5$ (solid line). The pure Gaussian field (dashed-dotted line) is given as a reference.

Figure 2

Asymptotic distribution function $f(\overrightarrow{k},\infty )$ for ${\overrightarrow{k}}_{\perp }=0$, $E_0=0.1E_{\mathrm{cr}}$, and $\varphi =0$ when passing from $\sigma =3$ (dotted line) to $\sigma =4$ (dashed line) to $\sigma =5$ (solid line). Note that for $\sigma =3$ the canonical momentum value $k_{\parallel }=0$ corresponds to a kinetic momentum $p_{\parallel }(\infty )\approx 75\mathrm{keV}$, whereas for $\sigma =5$ the value $k_{\parallel }=0$ is equivalent to $p_{\parallel }(\infty )\approx 0$.

Figure 3

Asymptotic distribution function $f(\overrightarrow{k},\infty )$ for ${\overrightarrow{k}}_{\perp }=0$ for $\sigma =5$, $E_0=0.1E_{\mathrm{cr}}$, and $\varphi =-\pi /4$. The center of the distribution is shifted to $p_{\parallel }(\infty )\approx 102\mathrm{keV}$.

Figure 4

Asymptotic distribution function $f(\overrightarrow{k},\infty )$ for ${\overrightarrow{k}}_{\perp }=0$ for $\sigma =5$, $E_0=0.1E_{\mathrm{cr}}$, and $\varphi =-\pi /2$. The center of the distribution is shifted to $p_{\parallel }(\infty )\approx 137\mathrm{keV}$.

Figure 5

Comparison of the asymptotic distribution function $f(\overrightarrow{k},\infty )$ for ${\overrightarrow{k}}_{\perp }=0$ (oscillating solid line) with the prediction of Eq. (4) (dotted line) and the improved WKB approximation based on an expansion of Eq. (5) (dashed line) for $\sigma =5$, $E_0=0.1E_{\mathrm{cr}}$, and $\varphi =0$.