Femtosecond laser fabrication of black quartz for infrared photodetection applications

Quartz tuning forks have been recently employed as infrared photodetectors in tunable laser diodespectroscopy because of their high responsivities and fast response time. As for all sensitiveelementsemployed for photodetection, the main drawback is the limited bandwidth of their absorptionspectrum. For quartz crystals, the high absorptance for wavelengths above 5 µm guarantees excellent performanceinthe mid-infrared range, that cannot be easily extended in the visible/near-infrared range becauseof itstransparency from 0.2 to 5 µm. In this work, we report on the development of a laser surfacefunctionalization process to enhance the optical absorption of quartz crystals, named hereafter BlackQuartz, in the 1-5 µm spectral range. Black Quartz consists of surface modification of quartz crystal byultrafast-pulsed-laser-processing to create localized matrices-like patterns of craters on top. Thesurfacemodification decreases the transmittance of quartz in the 1-5 µm range from > 95% down to < 10%, whilethe transmittance above 5 µm remains unchanged. The Black Quartz process was applied on twoquartztuning-forks mounted in a tunable laser diode spectroscopy sensor for detecting two water vaporabsorption features, one in the near infrared and the other one in the mid-infrared. Acomparableresponsivity was estimated in detecting both absorption features, confirming the extension of theoperationin the near-infrared range. This works represents an important and promising step towards the realizationof quartz-based photodetector with high and flat responsivity in the whole infrared spectral range.