Design and characterization of a compact complementary metal-oxide-semiconductor (CMOS) based soft x-ray transmission grating spectrometer with application to laser-produced plasmas

Spectroscopy is a fundamental tool in characterizing laser-produced plasmas. In this article, the design and deployment of a centimeter-scale soft x-ray transmission grating spectrometer that utilizes a commercial Complementary Metal-Oxide-Semiconductor (CMOS) detector are discussed in detail. This spectrometer was designed to facilitate the spectral characterization of micrometer-scale soft x-ray emitting laser-produced plasmas with a short working path length. A free-standing silicon nitride transmission grating with a pitch of 100 nm, a working width of 5 mu m, and an effective length of 3 mm is utilized in conjunction with precision scale machining and 3D printing techniques to construct the device. With the spectrometer, spectral images are produced, showing emission features in the 1-3 nm region from multiple atomic targets. We present these results compared to spectra recorded with a higher-resolution spectrometer and also investigate the utility of photon counting based spectroscopy, whereby the energy dependent response of the CMOS detector itself is utilized. The spectrometer impulse response function is modeled using the Fresnel diffraction integral, with a fixed grating aperture. Simulated spectra are produced using FLYCHK, a dimensionless collisional radiative model for the laser-produced plasma, which serves as a model input to test the veracity of the optical modeling. In addition to developing a compact and affordable spectrometer, a primary motivation behind this device was to record spectral data from laser plasma emission in high pressure helium environments, and data are presented on this.