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arXiv:1706.01110 · Optics Letters

Interferometric autocorrelation in the ultraviolet utilizing spontaneous parametric down-conversion inside an enhancement cavity

Patrick Michelberger, Roland Krischek, Witlef Wieczorek, Akira Ozawa, Harald Weinfurter

Published Mar 26, 2012• 2 citations
arXivHTMLPDF

Abstract

Autocorrelation is a common method to estimate the duration of ultrashort laser pulses. In the ultraviolet (UV) regime it is challenging to employ the process of second-harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal in the infrared (IR) for UV pulses. Our method utilizes the nth-order emission of the SPDC process, which occurs for low pumping powers proportional to the nth power of the UV intensity. Thus, counting 2n down-converted photons directly yields the nth-order autocorrelation. The method, now with detection of near-IR photons, is applied to the first direct measurement of ultrashort UV pulses circulating inside a UV enhancement cavity.

DOI
10.1364/ol.37.001223

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Autocorrelation is a common method to estimate the duration of ultrashort laser pulses. In the ultraviolet (UV) regime it is challenging to employ the process of second-harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal in the infrared (IR) for UV pulses. Our method utilizes the nth-order emission of the SPDC process, which occurs for low pumping powers proportional to the nth power of the UV intensity. Thus, counting 2n down-converted photons directly yields the nth-order autocorrelation. The method, now with detection of near-IR photons, is applied to the first direct measurement of ultrashort UV pulses circulating inside a UV enhancement cavity.
Summary form only given. Pairs of entangled photons display a complex quantum state that cannot be represented through a product of single-particle states. The spontaneous parametric down-conversion (SPDC) process, that takes place when a laser pulse pumps a second-order nonlinear crystal has become recently a convenient source of such nonclassical states of light in quantum optics experiments. Nonstationary effects are mainly observed when the pump pulse temporal duration is significantly shorter than the characteristic interaction time in the nonlinear crystal. We introduce a nonstationary theory of SPDC to describe the results of fourth-order quantum-interference experiments when no spectral filtering is applied to the down-converted fields. Experiments involving nonlinear SPDC crystals with three different lengths were performed. The ultraviolet laser pulses were obtained from a Ti:sapphire femtosecond laser with a second-harmonic generator. These pulses were used to pump nonlinear crystals to initiate the process of nonstationary spdc and generate orthogonally-polarized photon pairs. These pairs then went into a polarization interferometer, and the coincidence counts of the detectors at the two output ports of the interferometer were recorded.
We present a frequency-correlated non-degenerate photon-pair source consisting of a second-order nonlinear crystal that generates ultraviolet UV-A and infrared light via spontaneous parametric down-conversion (SPDC). Quantum imaging and sensing techniques like quantum imaging with undetected light and quantum ghost imaging leverage wavelength correlations between down-converted photon pairs to decouple sensing and detection wavelengths, thereby exploiting established camera technology within the visible spectrum. Our results open up quantum sensing application scenarios in the ultraviolet domain, with potential implications for advancements in biomedical and non-destructive testing fields.
Recent years have seen the development of quantum sensing concepts utilizing nonlinear interferometers based on correlated photon pairs generated by spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency degeneracy allows a “division of labor” between the mid-infrared photon for the strongest sample interaction and the correlated near-infrared photon for low-noise detection. The small number of photons provided by SPDC and the resulting inferior signal-to-noise ratio are, however, a limiting factor preventing the wide applicability of the novel sensing concept. Here, we demonstrate a nonlinear interferometer based on pump-enhanced SPDC with strongly improved emission rates while maintaining broadband spontaneous emission. For validation of the concept, we demonstrate high-resolution mid-infrared spectroscopy with near-infrared detection, showcasing improved accuracy. Although the number of mid-infrared photons is about five orders of magnitude smaller than in classical spectrometers, the sensitivity of the quantum spectrometer becomes comparable, marking an essential step toward real-world applications.
Abstract We report the generation of highly non-degenerate spontaneous parametric down conversion (SPDC) in a novel mid-infrared nonlinear crystal BaGa 2 GeS 6 , and provide a direct experimental comparison with the widely used AgGaS 2 crystal. Using a continuous-wave near-infrared (near-IR) pump laser, we generate correlated photon pairs with signal wavelengths in the near-IR ∼815–900 nm and idler wavelengths extending deep into the mid-infrared (mid-IR) up to 22–24 µ m. Wavelength-angular SPDC spectra are measured for both crystals under identical experimental conditions and analyzed using available dispersion data. Our results demonstrate, for the first time, the efficient generation of correlated photon pairs in BaGa 2 GeS 6 via SPDC. A quantitative comparison shows that BaGa 2 GeS 6 exhibits approximately twice the nonlinear conversion efficiency of AgGaS 2 , which is in good agreement with values reported in the literature. In addition, BaGa 2 GeS 6 produces a narrower external SPDC emission cone due to its lower refractive indices, which is advantageous for photon collection and nonlinear interferometric applications. Due to its large effective second-order nonlinearity and broad mid-IR transparency BaGa 2 GeS 6 emerges as a highly promising platform for bright and stable sources of correlated photon pairs. These results establish BaGa 2 GeS 6 as a strong candidate for mid-IR quantum spectroscopy, imaging, and gas-sensing applications.
Time-correlated photons can be used to demonstrate emerging technical applications such as ghost imaging, and they can demonstrate quantum-based phenomena in order to interest undergraduate students in engineering applications involving optics and quantum mechanics. In this work, we use spontaneous parametric down-conversion to generate time-correlated photons. To achieve this, we simulated laboratory parameters with MATLAB and designed optical and electrical setups. Specifically, we used a 405 nm diode laser, various common optical components, and a nonlinear crystal designed to generate time-correlated photons, and we used single-photon detectors, time-to-amplitude converters, custom flash-conversion analog-to-digital converters, a Teensy 4.1 microcontroller, and the Arduino IDE to detect the time-correlated photons. We present two significant results. First, we measured >100,000 counts per second 5 ± 1° on either side of our input laser which was within our theoretical expectations of 4–9° and much greater than our < 2000 dark counts per second. Our second major result was finding 15.8 ± 0.7 time-correlated counts per second with an SNR of 2.7. We also outline future extensions of our work including ghost imaging and generating polarization-entangled photons.