Broadband nano-focusing of high-order harmonics in soft X-ray region with ellipsoidal mirror

We demonstrate broadband focusing of multiple high-order harmonics of intense femtosecond laser pulses using an ellipsoidal mirror. The ellipsoidal mirror, with a high numerical aperture and a highly accurate surface, was fabricated using a replication process. The multiple high-order harmonics in the wavelength range between 10 and 20 nm were focused to the nearly diffraction-limited size of 350 × 380 nm². According to Rayleigh's quarter wavelength criteria, the wavefront aberration of the focused beam was estimated to be less than 5 nm, which corresponds to a temporal dispersion of 16 as. The developed focusing system is suited for producing an intense attosecond laser field with negligible wavefront aberration, with which nonlinear light-matter interactions in the attosecond time domain can be explored.