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Register a new userExploiting k-space - frequency duality in Fourier optics towards real-time compression-less terahertz imaging
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Added by
Maksim Skorobogatiy
Publication date
2017
fields
Physics
and research's language is
English
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We present theoretical formulation and experimental demonstration of a novel technique for the fast compression-less terahertz imaging based on the broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics which allows using a single-pixel detector to perform angular scan along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compression-less image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with the image linear size, thus promising real-time acquisition of high-resolution THz images. Additionally, our imaging technique handles equally well and on the equal theoretical footing the amplitude contrast and the phase contrast images, which makes this technique useful for many practical applications. A detailed analysis of the novel technique advantages and limitations is presented, as well as its place among other existing THz imaging techniques is clearly identified
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The concepts of Fourier optics were established in France in the 1940s by Pierre-Michel Duffieux, and laid the foundations of an extensive series of activities in the French research community that have touched on nearly every aspect of contemporary optics and photonics. In this paper, we review a selection of results where applications of the Fourier transform and transfer functions in optics have been applied to yield significant advances in unexpected areas of optics, including the spatial shaping of complex laser beams in amplitude and in phase, real-time ultrafast measurements, novel ghost imaging techniques, and the development of parallel processing methodologies for photonic artificial intelligence.
Terahertz (THz) waves have been significantly developed in the last fifteen years because of their great potential for applications in industrial and scientific communities1,2. The unique properties of THz waves as transparency for numerous materials and strong absorption for water-based materials are expected to broadly impact biosensing3 such as medical imaging4, chemical identifications5, and DNA recognition6. In particular, for accessing information within a scale comparable to the cell size where interactions between cell membrane and other organelle structures occur, micron size spatial resolution is required. Due to the large wavelength, however, the joint capability of THz near-field imaging with real-time acquisition, which is a highly desirable ability for observing real-time changes of in vivo sample, remains undone. Here, we report a real-time THz near-field microscope with a high dynamic range that can capture images of a 370 x 740 {mu}m2 area at 35 frames per second. We achieve high spatial resolution on a large area by combining two novel techniques: THz pulse generation by tilted-pulse-front excitation7 and electro-optic (EO) balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a THz pulse. Our results are the first demonstration of a direct quantification of a 2-dimensional subwavelength THz electric field taken in real-time.
Due to its fast and high resolution characteristics, dual-comb spectroscopy has attracted an increasing amount of interest since its first demonstration. In the terahertz frequency range where abundant absorption lines (finger prints) of molecules are located, multiheterodyne spectroscopy that employs the dual-comb technique shows an advantage in real-time spectral detection over the traditional Fourier transform infrared or time domain spectroscopies. Here, we demonstrate compact terahertz dual-comb spectroscopy based on quantum cascade lasers (QCLs). In our experiment, two free-running QCLs generate approximately 120 GHz wide combs centered at 4.2 THz, with slightly different repetition frequencies. We observe that $sim$490 nW terahertz power coupling of one laser into the other suffices for laser-self-detecting the dual-comb spectrum that is registered by a microwave spectrum analyzer. Furthermore, we demonstrate practical terahertz transmission dual-comb spectroscopy with our device, by implementing a short air path at room temperature. Spectra are shown of semiconductor samples and of moist air, the latter allowing rapid monitoring of the relative humidity. Our devices should be readily extendable to perform imaging, microscopy and near-field microscopy in the terahertz regime.
Time and frequency transfer lies at the heart of the field of metrology. Compared to current microwave dissemination such as GPS, optical domain dissemination can provide more than one order of magnitude in terms of higher accuracy, which allows for many applications such as the redefinition of the second, tests of general relativity and fundamental quantum physics, precision navigation and quantum communication. Although optical frequency transfer has been demonstrated over thousand kilometers fiber lines, intercontinental time comparison and synchronization still requires satellite free space optical time and frequency transfer. Quite a few pioneering free space optical time and frequency experiments have been implemented at the distance of tens kilometers at ground level. However, there exists no detailed analysis or ground test to prove the feasibility of satellite-based optical time-frequency transfer. Here, we analyze the possibility of this system and then provide the first-step ground test with high channel loss. We demonstrate the optical frequency transfer with an instability of $10^{-18}$ level in 8,000 seconds across a 16-km free space channel with a loss of up to 70~dB, which is comparable with the loss of a satellite-ground link at medium earth orbit (MEO) and geostationary earth orbit (GEO).
Synthetic dimensions in photonic structures provide unique opportunities for actively manipulating light in multiple degrees of freedom. Here, we theoretically explore a dispersive waveguide under the dynamic phase modulation that supports single pulse manipulations in the synthetic (2+1) dimensions. Compared with the counterpart of the conventional (2+1) space-time, we explore temporal diffraction and frequency conversion in a synthetic time-frequency space while the pulse evolves along the spatial dimension. By introducing the effective gauge potential well for photons in the synthetic time-frequency space with the control of the modulation phase, we show that a rich set of pulse propagation behaviors can be achieved, including confined pulse propagation, fast/slow light, and pulse compression. With the additional nonlinear oscillation subject to the effective force along the frequency axis of light, we provide an exotic approach for actively manipulating the single pulse in both temporal and spectral domains, which shows the great promise for applications of the pulse processing and optical communications in integrated photonics.
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