The use of ultrashort pulses for fundamental studies and applications has been increasing rapidly in the past decades. Along with the development of ultrashort lasers, exploring new pulse diagnositic approaches with higher signal-to-noise ratio have attracted great scientific and technological interests. In this work, we demonstrate a simple technique of ultrashort pulses characterization with a single semiconductor nanowire. By performing a frequency-resolved optical gating method with a ZnO nanowire coupled to tapered optical microfibers, the phase and amplitude of a pulse series are extracted. The generated signals from the transverse frequency conversion process can be spatially distinguished from the input, so the signal-to-noise ratio is improved and permits lower energy pulses to be identified. Besides, since the nanometer scale of the nonlinear medium provides relaxed phase-matching constraints, a measurement of 300-nm-wide supercontinuum pulses is achieved. This system is highly compatible with standard optical fiber systems, and shows a great potential for applications such as on-chip optical communication.
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Ultrashort pulse lasers have made great progress since its emergence. As an important research field, they have been motivated by the rapid advance of various applications ranging from optical imaging and spectroscopy, to laser processing and optical communication1,2. Their dramatic development and extensive applications urge us to fully and intactly characterize an ultrashort pulse. Among all the techniques, frequency-resolved optical gating (FROG) is a common and effective approach. Conventional FROG system requires a nonlinear optical (NLO) material in bulk in order to gain abundant signal photons for detection, and as a trade-off, the setup is always elaborate and presents a considerable complexity, especially for pulses like super-continuum3,4. Meanwhile, modern spectro/microscopy like coherent Raman spectroscopy, tip-enhanced spectroscopy, and multi-photon microscopy have got emerging interests towards higher resolution in temporal, spectral and spatial domains. Because of the sophisticated system, however, it is impossible to implement an in-situ measurement, and thus detailed information of the target site is still missing5,6,7.
Nano-scale materials have the inherent advantage on fine structure imaging and spectral analysis, since they can be employed as tracking material without perturbing the local environment. The usage of nanomaterial based FROG (nano-FROG) would relieve the phase-matching condition, and provide an avenue to monitor the nano-domain distortions in near field8,9,10,11. Exsiting nano-FROG methods, either via randomly dispersed nanoparticles or via nanoparticles attached to fiber taper, their FROG signal collections still rely on free-space optics. The loss by a series optical elements largely limits the sensitivity and reliability of the apparatus. Moreover, the NLO signal of these methods is mixed with fundamental wave in collection, so it requires optical filters to differentiate them. The irradiation detection further imposes a restriction in measuring broadband pulse when fundamental wave partly overlaps with signal in spectrum, and thus filters are not able to separate them. Besides, the variation and irregularity of the nanomaterials make them improbable to reproduce a consistent result among different measurements. More recently, transverse frequency conversion in semiconductor nanowires has been attracted lots of attentions because of its high conversion efficiency and low divergence angle8,12,13,14; although the feasibility of pulse measurement has been discussed, however, using this scheme to perform FROG measurement and retrieval of ultrashort and broadband pulses have not been reported yet. In this paper, we demonstrate a waveguide based FROG method with single nanowire (NW) as NLO material, through which the phase and amplitude of pulses are measured.
(a) Scanning electron microscope image of a ZnO NW. (b) Enlarged SEM image of ZnO NW after fracture. (c) Bright-field optical microscope image of ZnO NWs deposited on a silicon substrate. (d) Light coupling approach in to a 180-μm-long ZnO NW using a fiber taper. (e) Optical microscope image of guiding a 1064-nm-wavelength laser.
The development of low loss optical fibers, compact and efficient semiconductor lasers operating at room temperature, optical detectors and optical amplifiers have truly revolutionized the field of telecommunication and have provided us with a communication system capable of carrying enormous amount of information over intercontinental distances. When information carrying light pulses propagate through an optical fiber, they suffer from attenuation, temporal broadening and they even interact with each other through nonlinear effects in the fiber. These effects which tend to distort the signals need to be overcome to achieve high speed communication over long distances. In this chapter, we will give a brief outline of the various linear and nonlinear propagation effects in optical fibers and their impact on optical fiber communication systems. Important components such as optical fiber amplifiers and dispersion compensators, which are playing a very important role in the fiber optic revolution, will also be discussed. 2ff7e9595c
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