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arXiv:2003.07483 · Optics Letters

Pump depletion in parametric down-conversion with low pump energies

Jefferson Flórez, Jeff S. Lundeen, Maria V. Chekhova

Published Jul 2, 2020• 30 citations
arXivHTMLPDF

Abstract

We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 µW (energies 0.1 µJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.

DOI
10.1364/ol.394925

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We report the efficient generation of high-gain parametric down-conversion, including pump depletion, with pump powers as low as 100 µW (energies 0.1 µJ/pulse) and conversion efficiencies up to 33%. In our simple configuration, the pump beam is tightly focused into a bulk periodically poled lithium niobate crystal placed in free space. We also observe a change in the photon number statistics for both the pump and down-converted beams as the pump power increases to reach the depleted pump regime. The experimental results are a clear signature of the interplay between the pump and the down-converted beams in highly efficient parametric down-conversion sources.
We present a theoretical and experimental study on the bandwidth of parametric down-converted photons generated from periodically poled lithium niobate (PPLN) crystal pumped by a pulsed laser. By comparison of crystals with different lengths and pump beams of different bandwidths, we demonstrate that the bandwidth of down-converted photons will increase for a broader bandwidth of pump pulse and decrease for a longer crystal, but the influence of crystal length will become weaker along with increase of both crystal length and pump pulse bandwidth. Especially under the conditions of our experiment, the bandwidth almost remains unchanged for longer crystals when pumped by a femtosecond laser. This may be helpful for schemes in which pulsed lasers are used to pump PPLN crystals.
In this paper we measure the efficiency of photon pair production from parametric downconversion at the photon counting level, generated in a laboratory prototype of PPLN waveguide at 1572 nm. The aim is to enhance the brightness of an heralded single photon source and to improve compactness for quantum communication applications, namely quantum key distribution. We observed a maximum of the conversion efficiency corresponding to 7.9 106 photon pairs/s for a pump power of 28 μW. This value is compared with the measured pairs production in a classical experiment of difference frequency generation, realized with the same sample by analogue classical detection. We highlight the advantages of using the waveguide versus a bulk PPLN for such applications and we foresee further possible improvements by adopting waveguide in a single photon level experimental setup.
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The high peak intensity achieved with ultrashort pump pulses drastically increases the conversion efficiency in spontaneous parametric down-conversion. However, when ultrashort pulses are used, dispersion limits the degree of squeezing. Here, we investigate the effects of dispersion on both quadrature squeezing and photon statistics by varying the interaction length when pumping a periodically poled lithium niobate crystal with femtosecond pulses. The down-converted light in the 1550 nm telecom band attains its maximum observed squeezing level of 3.1 dB for the shortest interaction length. Our photon-statistics measurements agree with the photon-pair distribution going from thermal to Poissonian as the interaction length is increased. This is explained by considering the dispersion-induced distinguishability of the generated photon pairs.
We present the experimental investigation of the coherence properties of the light produced by parametric down conversion in the macroscopic regime, also including pump depletion. In particular, we compare the results obtained in very similar geometric conditions by using two nonlinear crystals having different lengths. We observe that the number of generated photons, the size of spatio-spectral coherence areas, and the number of modes in the photon number statistics exhibit a similar behavior in the two crystals as a function of pump mean power, even if we notice that the absolute values are different. The available theory of parametric down conversion cannot account for these differences.
High-gain optical parametric amplification is an important nonlinear process used both as a source of coherent infrared light and as a source of nonclassical light. In this work, we experimentally demonstrate an approach to optical parametric amplification that enables extremely large parametric gains with low energy requirements. In conventional nonlinear media driven by femtosecond pulses, multiple dispersion orders limit the effective interaction length available for parametric amplification. Here, we use the dispersion engineering available in periodically poled thin-film lithium niobate nanowaveguides to eliminate several dispersion orders at once. The result is a quasi-static process; the large peak intensity associated with a short pump pulse can provide gain to signal photons without undergoing pulse distortion or temporal walk-off. We characterize the parametric gain available in these waveguides using optical parametric generation, where vacuum fluctuations are amplified to macroscopic intensities. In the unsaturated regime, we observe parametric gains as large as 71 dB (118 dB/cm) spanning 1700–2700 nm with pump energies of only 4 pJ. When driven with pulse energies > 10 p J , we observe saturated parametric gains as large as 88 dB ( > 146 d B / c m ). The devices shown here achieve saturated optical parametric generation with orders of magnitude less pulse energy than previous techniques.