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Abstract
Coherent imaging in the THz range promises to exploit the peculiar capabilities of these wavelengths to penetrate common materials like plastics, ceramics, paper or clothes with potential breakthroughs in non-destructive inspection and quality control, homeland security and biomedical applications. Up to now, however, THz coherent imaging has been limited by time-consuming raster scanning, point-like detection schemes and by the lack of adequate coherent sources. Here, we demonstrate real-time digital holography (DH) at THz frequencies exploiting the high spectral purity and the mW output power of a quantum cascade laser combined with the high sensitivity and resolution of a microbolometric array. We show that, in a one-shot exposure, phase and amplitude information of whole samples, either in reflection or in transmission, can be recorded. Furthermore, a 200 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view are observed, as compared to DH in the visible range. These properties of THz DH enable unprecedented holographic recording of real world dynamic scenes.
Highlights
Become too narrow to be correctly sampled
Infrared DH inherently benefits from wavelength scaling of both linear field of view and mechanical stability requirements[17,18]: i) the maximum lateral dimension D of a target at a certain distance d is equal to D = λ d/dp, where dp is the detector pixel pitch and λ is the wavelength; ii) the interferometric pattern containing the wavefront information is erased if the amplitude of environmental vibrations occurring during the acquisition is comparable to the wavelength; a holographic system is, 200 times more robust against vibrations in the THz range than in the visible range
The most relevant experimental works published to date are based on a frequency multiplied solid state oscillator[22], a gas laser[23] and, more recently, a multimode THz QCL24, and report proof of principle hologram acquisition of simple static transmission masks
Summary
Become too narrow to be correctly sampled. This is a specific drawback of DH with respect to classical holography, which limits the field of view. Infrared DH inherently benefits from wavelength scaling of both linear field of view and mechanical stability requirements[17,18]: i) the maximum lateral dimension D of a target at a certain distance d is equal to D = λ d/dp, where dp is the detector pixel pitch and λ is the wavelength; ii) the interferometric pattern containing the wavefront information is erased if the amplitude of environmental vibrations occurring during the acquisition is comparable to the wavelength; a holographic system is, 200 times more robust against vibrations in the THz range (at e.g. 100 microns) than in the visible range (at e.g. 0.5 microns) In this respect, the recent development of Mid-IR DH systems based on micro-bolometer arrays and CO2 lasers[19] or, more recently, QCLs20, allowed acquisition of holographic videos of human size dynamic scenes out of laboratory conditions. Preliminary results of real-time displacement measurements in the THz range were obtained by means of speckle metrology using the THz radiation of a Free Electron Laser[25]
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