The production of coherent radiation at ever-shorter wavelengths has been a long-standing challenge since the invention of lasers[1, 2] and subsequent demonstration of frequency doubling[3]. Modern x-ray free-electron lasers (FELs) use relativistic electrons to produce intense x-ray pulses at few-femtosecond timescales[4-6]. However, the shot noise that seeds the amplification produces pulses with a noisy spectrum and limited temporal coherence. To produce stable transform-limited pulses, a seeding scheme called echo-enabled harmonic generation (EEHG) has been proposed[7, 8] which harnesses the highly nonlinear phase mixing of the celebrated echo phenomenon[9] to generate coherent harmonic density modulations in the electron beam with conventional lasers. Here we report on a demonstration of EEHG up to the 75th harmonic, where 32 nm light is produced from a 2400 nm laser. We also demonstrate that individual harmonic amplitudes are controlled by simple adjustment of the phase mixing. Results show the potential of laser-based manipulations to achieve precise control over the coherent spectrum in future x-ray FELs for new science[10, 11].

Microacoustic resonators made on suspended continuous membranes of LiNbO3 were recently shown to have very strong coupling and low losses at ≥5 GHz, suitable for high‐performance filter design. Employing these simple resonator structures, the authors have designed, fabricated, and measured a 4.7 GHz bandpass ladder‐type filter having 1 dB mid‐band loss and 600 MHz bandwidth to address the 5G Band n79 requirements. The filter is fabricated on a monolithic substrate using standard i‐line optical lithography and standard semiconductor processing methods for membrane release, starting with commercially available ion‐sliced wafers having 400 nm thickness crystalline LiNbO3 layers. The filter is well‐matched to a 50 Ω network and does not require external matching elements. Through accurate resonator engineering using our finite element method software filter design environment, the passband is spurious‐free, and the filter provides better‐than 30 dB rejection to the adjacent WiFi frequencies. This filter demonstrates the performance and scalable technology required for high‐volume manufacturing of microacoustic filters >3.5 GHz.

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