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Advanced Physics Research

Photon Pair Source based on PPLN‐Waveguides for Entangled Two‐Photon Absorption (Adv. Phys. Res. 2/2024)

Tobias Bernd Gäbler, Patrick Hendra, Nitish Jain, Markus Gräfe

Published Feb 1, 2024• 0 citations
Publisher

Abstract

Entangled Two-Photon Absorption The front cover illustrates the fluorescence excitation of quantum dot molecules by absorption of entangled photon pairs. The so-called entangled two-photon absorption is highly affected by the generation of these photon pairs. In article number 2300037, Tobias Bernd Gäbler and co-workers describe the important properties of an ultra-bright photon pair source based on nonlinear waveguides and its usage to approach fluorescence excitation. (Image designed by Christian Süß).

DOI
10.1002/apxr.202470004

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Entangled Two-Photon Absorption The front cover illustrates the fluorescence excitation of quantum dot molecules by absorption of entangled photon pairs. The so-called entangled two-photon absorption is highly affected by the generation of these photon pairs. In article number 2300037, Tobias Bernd Gäbler and co-workers describe the important properties of an ultra-bright photon pair source based on nonlinear waveguides and its usage to approach fluorescence excitation. (Image designed by Christian Süß).
Abstract Fluorescence excitation by absorption of entangled photon pairs offers benefits compared to classical imaging techniques, such as the attainment of higher signal levels at low excitation power while simultaneously mitigating phototoxicity. However, current entangled photon pair sources are unreliable for fluorescence detection. In order to address this limitation, there is a need for ultra‐bright entangled photon pair sources. Among the potential solutions, sources utilizing nonlinear waveguides emerge as promising candidates to facilitate fluorescence excitation through entangled photons. In this paper, a source consisting of a periodically poled lithium niobate waveguide is developed and its key characteristics are analyzed. To demonstrate its suitability as key component for imaging experiments, the entangled two‐photon absorption behavior of Cadmium Selenide Zinc Sulfide quantum dot solutions is experimentally investigated.
Two-photon absorption (TPA) is a nonlinear optical process with wide applications in fluorescence microscopy and spectroscopy of organic molecules and compounds, as it enables atoms or molecules to transition into states that may otherwise be inaccessible by a single photon, either because of selection rules or energy conservation. Despite its common use, TPA is a rather inefficient process, often requiring very large photon numbers to produce significant results. It has been demonstrated and analytically described [1,2], that using frequency entangled pairs of photons can in some cases enhance the efficiency of TPA. For this to occur, the joint spectral amplitude (JSA) of the photon-pair state, which describes the spectral entanglement between the pair, must be engineered according to the spectral properties of the TPA response-function of the specific system in question. In this work, we consider different cases for the spectral functionality of TPA and propose that the control of the group indices of the modes involved in the pair-generation process is a key property in the design of photon-pair sources for optimized TPA.
We closely replicated and extended a recent experiment ("Spatial properties of entangled two-photon absorption," Phys. Rev. Lett. 129, 183601, 2022) that reportedly observed enhancement of two-photon absorption rates in molecular samples by using time-frequency-entangled photon pairs, and we found that in the low-flux regime, where such enhancement is theoretically predicted in-principle, the two-photon fluorescence signal is below detection threshold using current state-of-the-art methods. The results are important in the context of efforts to enable quantum-enhanced molecular spectroscopy and imaging at ultra-low optical flux. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observed two-photon-induced fluorescence in the high-gain regime but in the low-gain regime any fluorescence was below detection threshold. We supplemented the molecular fluorescence experiments with a study of nonlinear-optical sum-frequency generation, for which we are able to observe the low-to-high-gain crossover, thereby verifying our theoretical models and experimental techniques. The observed rates (or lack thereof) in both experiments are consistent with theoretical predictions and with our previous experiments, and indicate that time-frequency photon entanglement does not provide a practical means to enhance in-solution molecular two-photon fluorescence spectroscopy or imaging with current techniques.
Fluorescence microscopy has become integral to biological studies for the technique’s ability to elucidate structures of biomolecules for in-situ studies with high selectivity and specificity. Imaging of intrinsic indicators, such as fluorescent amino acids in proteins, provides important information, but can be challenging to accomplish. Current microscopy techniques that measure native fluorescence without the use of exogenous labels involve either direct UV excitation which is commonly non-localized and can be detrimental to the system, or multiphoton absorption which must be conducted at high intensities, therefore posing high risks of photodamage. As such, we seek to investigate an efficient way to gently excite native fluorescence in biological systems in a way that overcomes these limitations. Quantum entangled photon pairs generated via spontaneous parametric downconversion (SPDC), may be an alternative to conducting two-photon absorption (TPA) to excite fluorescence in amino acids without the high fluences currently used. These photon pairs are highly correlated in time. Thus, the arrival of one photon is simultaneously followed by the arrival of its sister photon. As a result, a molecule interacting with the photon pair should simultaneously absorb both photons, leading to a linear two-photon absorption rate, and the linearity of the two-photon process should dramatically reduce the light intensity necessary for TPA. Therefore, quantum entangled photon pairs offer the possibility of performing low intensity UV excitation using photons in the visible wavelength range. With this work, we generated and characterized entangled photons generated via SPDC, and investigated whether fluorescent amino acids can be excited, and the subsequent fluorescence induced with entangled two-photon absorption. Results show that much higher entangled two-photon rates than what are currently available are needed to measure significant signals with entangled two-photon excitation.