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Laser Physics Letters

Biphoton spectroscopy in a strongly nondegenerate regime of SPDC

А. А. Калачев, Dmitry A. Kalashnikov, Alexander A. Kalinkin, Tatiana G. Mitrofanova, A. V. Shkalikov, В. В. Самарцев

Published May 7, 2008• 26 citations
Publisher

Abstract

The absorption spectrum of Cr 3+ ions in Al 2 O 3 crystal in the range from 691 nm to 697 nm was measured at the room temperature using the biphoton light generated by nondegenerate spontaneous parametric down-conversion of the He-Cd laser radiation (325 nm) in the LiIO 3 crystal. The spectrum of the sample was measured by counting the coincidences when signal photons go through the impurity crystal, whilst the wavelength of the idler photons is resolved by a monochromator. A strongly nondegenerate regime of SPDC, when the difference in frequencies of correlated photons exceeds significantly their spectral width, was used in the biphoton spectroscopy for the first time. Such a regime allows one to obtain an absorption spectrum by measuring the wavelength in a completely different spectral range.

DOI
10.1002/lapl.200810044

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The absorption spectrum of Cr 3+ ions in Al 2 O 3 crystal in the range from 691 nm to 697 nm was measured at the room temperature using the biphoton light generated by nondegenerate spontaneous parametric down-conversion of the He-Cd laser radiation (325 nm) in the LiIO 3 crystal. The spectrum of the sample was measured by counting the coincidences when signal photons go through the impurity crystal, whilst the wavelength of the idler photons is resolved by a monochromator. A strongly nondegenerate regime of SPDC, when the difference in frequencies of correlated photons exceeds significantly their spectral width, was used in the biphoton spectroscopy for the first time. Such a regime allows one to obtain an absorption spectrum by measuring the wavelength in a completely different spectral range.
The absorption spectrum of Er3+ ions in YAG crystal in the range from 645 nm to 655 nm was measured with the resolution of 0.01 nm using the frequency-anticorrelated broadband photon pairs (biphotons) generated by spontaneous parametric down-conversion. The spectrum of the sample was measured by counting the coincidences when the impurity crystal was placed in front of the signal photon detector, whilst the wavelength of the idler photons was resolved by a monochromator which was placed in front of another photon detector. The results are in agreement with those obtained by the usual intensity measurements with a classical light source and a monochromator. It was shown experimentally that the biphoton spectroscopy, unlike the usual single photon spectroscopy, allows one to obtain the spectroscopic information in the presence of an external noise such as background thermal radiation.
Photon pairs from spontaneous parametric down-conversion (SPDC) are important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here we show the generic existence of particular phase-matching conditions in quasi-phase-matched KTP, MgO:LN, and SLT crystals that lead to ultra-broadband, widely nondegenerate photon pairs. It is based on the existence of group-velocity-matched, far apart wavelength pairs and, for 2 mm long crystals, results in SPDC bandwidths between 15 and 25 THz for photon pairs with the idler photon in the technologically relevant mid-IR range of 3-5 μm and the signal photon in the near-IR below 1100 nm. We experimentally demonstrate this type of broadband phase matching in ppKTP crystals for photon pairs centered at 800 and 3800 nm and measure a bandwidth of 15 THz. This novel method of generating broadband photon pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry, and optical coherence tomography with undetected mid-IR photons.
We report on experimental measurements of coincidence and single particle spectral shapes of biphoton signals when frequency entangled states are generated by SPDC crystals pumped by short pulses.It is shown evidence for biphoton coincidence spectrum narrower and single-particle one wider than the pump spectrum, with a large contrast between coincidence and singles distributions.The investigation of biphoton correlations has been performed by fixing one monochromator at the maximal transmission wavelength on down-converted signal gate and scanning the one placed in the idler branch in order to observe the spectral distribution of single counts and coincidences.The degree of entanglement is evaluated by the ratio of the FWHM of single particle and coincidence distributions: the greater the ratio, the greater is the entanglement.The degree of entanglement in frequency variables can be very high: it increases linearly with the length of the crystal, and decreases with the growing pump pulse.