Subrelativistic Alternating Phase Focusing Dielectric Laser Accelerators

We demonstrate a silicon-based electron accelerator that uses laser optical near fields to both accelerate and confine electrons over extended distances. Two dielectric laser accelerator (DLA) designs were tested, each consisting of two arrays of silicon pillars pumped symmetrically by pulse front tilted laser beams, designed for average acceleration gradients 35 and $50\mathrm{MeV}/\mathrm{m}$, respectively. The DLAs are designed to act as alternating phase focusing (APF) lattices, where electrons, depending on the electron-laser interaction phase, will alternate between opposing longitudinal and transverse focusing and defocusing forces. By incorporating fractional period drift sections that alter the synchronous phase between $\pm 60°$ off crest, electrons captured in the designed acceleration bucket experience half the peak gradient as average gradient while also experiencing strong confinement forces that enable long interaction lengths. We demonstrate APF accelerators with interaction lengths up to $708\mathrm{\mu }\mathrm{m}$ and energy gains up to $23.7\pm 1.07\mathrm{keV}$ FWHM, a 25% increase from starting energy, demonstrating the ability to achieve substantial energy gains with subrelativistic DLA.

Figure 1

Left: DLA period with Lorentz force vectors overlaid on electric field $-E_z$ from accelerating mode. Right: $E_z$ and Lorentz forces acting on a reference particle versus interaction phase. Two bunches are drawn at ${\varphi }_s=\pm 60°$, each experiencing half maximum accelerating force $F_z$ and confinement forces of opposite sign in $\widehat{y}$ and $\widehat{z}$.

Figure 2

Synchronous phase and energy (top) and Courant-Snyder ${\widehat{\beta }}_y$ function (bottom) for infinitesimal emittance versus travel distance extracted from DLAtrack6D simulation. The nominal electric field is $106\mathrm{MV}/\mathrm{m}$ for DLA70 and $250\mathrm{MV}/\mathrm{m}$ for DLA100, respectively.

Figure 3

Top: accelerator system overview. Bottom left: DLA100 $476\mathrm{\mu }\mathrm{m}$ on a $500\mathrm{\mu }\mathrm{m}$ long mesa. Cells labeled as black are longitudinally focusing and transversely defocusing, while white are longitudinally defocusing and transversely focusing. Bottom right: DLA100 $46\mathrm{\mu }\mathrm{m}$ powered by two $60.5\pm 0.7°$ incident PFT laser pulses. The orange line shows the electron-laser overlap on the laser pulse as it travels through the structure.

Figure 4

Simulated and measured laser-on and laser-off spectra for DLA70 and DLA100. (a) Integrated MCP camera image of laser on DLA70 $480\mathrm{\mu }\mathrm{m}$. Plots in (b)–(f) are obtained by integrating horizontal slices near peak transverse pixel. (b),(c) Simulated and measured DLA100 spectra for laser on and laser off. (d)–(f) Simulated and measured DLA70 spectra for laser on and laser off. A semitransparent overlay on the measured spectra covers the 25%–75% quantile. Experimental parameters in Table 1.

Figure 5

Measured laser-on spectra for DLA70 and DLA100 with different peak electric fields. DLA70 and DLA100 operate optimally at peak electric field of 106 and $250\mathrm{MV}/\mathrm{m}$, respectively. A semitransparent overlay on the measured spectra covers the 25%–75% quantile.