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1 February 2008 Sub-diffraction optical imaging by high-spatial-resolution photodetectors and Fourier signal processing
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With the advance of nano-lithography and nano-fabrication, individual sizes of electronic, photonic, and mechanical components, as well as their integration densities, have progressed steadily towards the sub-100 nm regime. Therefore, being able to image such feature sizes becomes imperative. Many conventional high-resolution imaging tools such as SEM, STM, AFM, and NSOM either require operation under high vacuum or slow scanning across the sample. A far-field optical imaging instrument would thus be highly desirable. Optical imaging, however, is subject to the diffraction limit, which limits the size of the smallest resolvable feature to be ~ λ/2, where λ is the wavelength of the imaging light. Recently, negative-index materials and super lens have been proposed to overcome this limit and achieve high-resolution optical imaging [1-4]. In this paper, we propose a different approach to achieve sub-diffraction optical imaging with far-field microscopy. The technology builds on a high-spatial resolution quantum-dot (QD) photodetector with high sensitivity that we have demonstrated [5]. The photodetector consists of several nanocrystal QDs between a pair of electrodes with 50-nm width spaced ~ 25 nm apart. An optically effective area of 13515 nm2 was determined by modeling the electric field distribution in-between and around the electrodes using FEMLab. High-sensitivity photodetection has been demonstrated by measuring the tunneling photocurrent through the QDs, with a detection limit of 62 pW of the input optical power. The proposed sub-diffraction optical imaging system consists of an array of such photodetectors. We performed theoretical simulations assuming a two slit source and then pixilated the far-field diffraction pattern to simulate the photodetector array. A Fourier transform of the detector signal is then performed to determine how much of the original aperture information remains. Using a wavelength of 500 nm and a screen distance of 10 cm, we found that, as expected, the quality of the resultant image generally degraded with larger pixilation size. With 50-nm one-dimensional spatial resolution at the detection plane, it appears that the original slit image with 100-nm width and 300-nm spacing can still be restored.
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Milad Hashemi, Michael Hegg, Babak A. Parviz, and Lih Y. Lin "Sub-diffraction optical imaging by high-spatial-resolution photodetectors and Fourier signal processing", Proc. SPIE 6900, Quantum Sensing and Nanophotonic Devices V, 690017 (1 February 2008);

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