Spatio-temporally shaped deep UV laser source for ultrabright photocathodes using novel upconversion techniques

In this project, we seek to study a novel, potentially transformational approach to the implementation of high-brightness photocathode sources. Currently, the performance of many electron sources for applications such as the LCLS-II x-ray free electron laser (XFEL) as well as electron microscopes and ultrafast electron diffraction (UED) systems, is limited by the brightness of the electron beams. This is a particular issue in the implementation of very high repetition-rate ~MHz electron sources, since in this case, the use of a high quantum efficiency (QE) photocathode is necessary. KMLabs has recently developed and commercialized a new technology for generating ultrashort laser pulses in the deep-UV and VUV, at MHz repetition-rate. By using a new, highly-cascaded, four-wave mixing upconversion process - rather than the more-commonly used nonlinear crystals – it is possible to generate ultrafast pulses at wavelengths from 350 nm to 110 nm with extremely-high beam quality, and with unprecedented brightness and average power for these short wavelengths. The high beam quality results from the single-mode nature of this guided-wave highly-cascaded harmonic generation (HCHG) technique. Moreover, the nonresonant nature of the four-wave mixing interaction results in low spectral distortion during the upconversion process, making it possible to temporally shape pulses in the infrared and translate this shaping into the deep-UV with high fidelity. These capabilities make it an ideal candidate for next-generation photocathodes. In this project, we proposed to implement and characterize deep-UV sources using gas phase upconversion to ~250-270 nm, to drive Cs2Te photocathodes at repetition-rates from 100 kHz-1MHz. By shaping one or more of the infrared laser pulses that are used to drive the upconversion process, in Phase I we will show that we can generate UV pulses with a broad and precisely-controllable spectral shape. We have extensive experience with pulse shaping technology. Using spectrally-chirped 10’s of ps pulses required for XFEL photocathodes, it will be possible to generate precise temporally-shaped electron pulses for ultrahigh brightness sources, leveraging the US investment in XFEL technology to enable higher photon energy lasing of XFELs. In Phase II, we plan to scale our results to the MHz repetition rates and the >~µJ pulse energies required for full implementation, as well as to validate strategies for spatial profile shaping of the deep-UV light. In phase I, we expect to coordinate with both the photocathode group at LCLS-II (Sergio Carbajo) and the superconducting electron gun group at Argonne (Phillippe Piot, John Byrd), while in Phase II we anticipate experimental collaboration with these groups. We plan in Phase I to work primarily with an existing prototype MHz fiber laser system at KMLabs to characterize the beam quality of the output at 258 nm, and to demonstrate that this new process allows for flexible manipulation of the spectral shape of the UV output. Phase II will pursue a demonstrator system at higher power.