Coherent field sensing of nitrogen dioxide.

We introduce a portable dual-comb spectrometer operating in the visible spectral region for atmospheric monitoring of NO2, a pollution gas of major importance. Dual-comb spectroscopy, combining key advantages of fast, broadband and accurate measurements, has been established in the infrared as a method for the investigation of atmospheric gases with kilometer-scale absorption path lengths. With the presented dual-comb spectrometer centered at 517 nm, we make use of the strong absorption cross section of NO2 in this spectral region. In combination with a multi-pass approach through the atmosphere, we achieve an interaction path length of almost a kilometer while achieving both advanced spatial resolution (90 m) and a detection sensitivity of 5 ppb. The demonstrated temporal resolution of one minute outperforms the standard chemiluminescence-based NO2 detector that is commercially available and used in this experiment, by a factor of three.

Inverse-Designed Narrowband THz Radiator for Ultrarelativistic Electrons.

THz radiation finds various applications in science and technology. Pump-probe experiments at free-electron lasers typically rely on THz radiation generated by optical rectification of ultrafast laser pulses in electro-optic crystals. A compact and cost-efficient alternative is offered by the Smith-Purcell effect: a charged particle beam passes a periodic structure and generates synchronous radiation. Here, we employ the technique of photonic inverse design to optimize a structure for Smith-Purcell radiation at a single wavelength from ultrarelativistic electrons. The resulting design is highly resonant and emits narrowbandly. Experiments with a 3D-printed model for a wavelength of 900 µm show coherent enhancement. The versatility of inverse design offers a simple adaption of the structure to other electron energies or radiation wavelengths. This approach could advance beam-based THz generation for a wide range of applications.