Recently, different experimental methods to investigate the entangled two-photon absorption (ETPA) process in a variety of materials have been reported. The present work reports on a different approach on which the ETPA process is detected based on the changes induced on the Hong-Ou-Mandel (HOM) interferogram of two photons produced by a spontaneous parametric down conversion (SPDC) Type-II process. Using an organic solution of the laser dye Rhodamine B as a model, it is demonstrated that ETPA at 800nm can be monitored by the changes in the temporal-width and visibility of the HOM interferogram produced by the photons at a beam splitter after interacting with the sample. Additionally, we present a detail model in which the sample is modeled as a filtering function which fullfills the energy conservation conditions required by ETPA, allowing to explain the experimental observations with excellent agreement. We believe that this work represents a new perspective to study the ETPA process, by using an ultra-sensitive quantum interference technique and a detailed mathematical model of the process.
Recently, different experimental methods to investigate the entangled two-photon absorption (ETPA) process in a variety of materials have been reported. The present work reports on a different approach on which the ETPA process is detected based on the changes induced on the Hong-Ou-Mandel (HOM) interferogram of two photons produced by a spontaneous parametric down conversion (SPDC) Type-II process. Using an organic solution of the laser dye Rhodamine B as a model, it is demonstrated that ETPA at 800nm can be monitored by the changes in the temporal-width and visibility of the HOM interferogram produced by the photons at a beam splitter after interacting with the sample. Additionally, we present a detail model in which the sample is modeled as a filtering function which fullfills the energy conservation conditions required by ETPA, allowing to explain the experimental observations with excellent agreement. We believe that this work represents a new perspective to study the ETPA process, by using an ultra-sensitive quantum interference technique and a detailed mathematical model of the process.
Recently, different experimental methods intended to detect the entangled two-photon absorption (ETPA) phenomenon in a variety of materials have been reported. The present work explores a different approach in which the ETPA process is studied based on the changes induced in the visibility of a Hong-Ou-Mandel (HOM) interferogram. By using an organic solution of Rhodamine B as a model of nonlinear material interacting with entangled photons at ∼800 nm region produced by spontaneous parametric down-conversion (SPDC) Type-II, the conditions that make possible to detect changes in the visibility of a HOM interferogram upon ETPA are investigated. We support the discussion of our results by presenting a model in which the sample is considered as a spectral filtering function which fulfills the energy conservation conditions required by ETPA, allowing us to explain the experimental observations with good agreement. We believe that this work represents a new perspective to studying the ETPA interaction, by using an ultrasensitive quantum interference technique and a detailed mathematical model of the process.
Recently different experimental schemes have been proposed to study the elusive phenomenon of entangled two-photon absorption (ETPA) in nonlinear materials. The attempts to detect ETPA using transmission-based schemes have led to results whose validity is currently under debate because the ETPA signal can be corrupted or emulated by artifacts associated with linear optical losses. The present work addresses the issue of linear losses and the corresponding artifacts in transmission-based ETPA experiments through a new approach that exploits the properties of a Hong-Ou-Mandel (HOM) interferogram. Here, we analyze solutions of rhodamine B (RhB), commonly used as a model of a nonlinear medium in ETPA studies. Then, by using the HOM interferometer as a sensing device, we first demonstrate the equivalence of the standard transmission vs pump power ETPA experiments, presented in many reports, with our novel approach of transmission vs two-photon temporal delay. Second, a detailed study of the effects of optical losses, unrelated to ETPA, over the HOM interferogram is carried out by: (1) characterizing RhB in solutions prepared with different solvents and (2) considering scattering losses introduced by silica nanoparticles used as a controlled linear loss mechanism. Our results clearly expose the deleterious effects of linear optical losses over the ETPA signal when standard transmission experiments are employed and show how, by using the HOM interferogram as a sensing device, it is possible to detect the presence of such losses. Finally, once we showed that the HOM interferogram discriminates properly linear losses, our study also reveals that under the specific experimental conditions considered here, which are the same as those employed in many reported works, the ETPA was not unequivocally detected.
The phenomenon of Entangled Two-Photon Absorption (ETPA) presents a persistent controversy in the literature, evidenced by a wide disparity in the reported values for the σE cross-sections. Much of this discrepancy is attributed to the difficulty in discriminating ETPA from various background processes that can mimic its signal, such as linear absorption or scattering. Given this need to certify the presence of ETPA unequivocally, this work introduces a key strategy to isolate the ETPA contribution through its spectral signature. This involves modeling the molecule as a two-photon notch filter and inducing a controlled asymmetric overlap with the joint spectral intensity (JSI) of the incident photons. This asymmetry is used to generate a measurable distortion in the transmitted JSI, and, complementarily, as a reduction in the visibility of the Hong-Ou-Mandel (HOM) dip. To ensure the experimental feasibility of this technique, a comprehensive analysis of the experimental conditions required for its detection is presented, establishing the absorption efficiency limits that must be overcome given the constraints of entangled-photon-pair flux and detector noise. A preliminary experiment addressing the ETPA detection limits using the standard RhB dye is presented.
Entangled two-photon absorption (ETPA) has recently become a topic of lively debate, mainly due to the apparent inconsistencies in the experimentally-reported ETPA cross sections of organic molecules. In this work, we provide a thorough experimental study of ETPA in the organic molecules Rhodamine B (RhB) and Zinc Tetraphenylporphirin (ZnTPP). The goal of this contribution is twofold: on one hand, it seeks to reproduce the results of previous experimental reports and, on the other, it aims to determine the effects of different temporal correlations -- introduced as a controllable time-delay between the photons to be absorbed -- on the strength of the ETPA signal. In our experiment, the samples are excited by entangled pairs produced by type-I SPDC, with a spectral distribution centered at 810 nm. Surprisingly, and contrary to what was expected, the time delay did not produce in our experiments any systematic change in the cross-sections when monitoring the ETPA signal using a transmission measurement scheme. As a plausible cause of this unexpected result, we argue that the photon-pair flux, typically-used in these experiments, is not sufficient to promote the two-photon absorption process in these molecules. This suggests that the actual absorption cross-section values are lower than those previously reported, and therefore do not lead to a measurable ETPA effect for the transmission method.
In recent years, the process of absorbing two photons using a non-classical light sources has been studied thoroughly. In particular, it has been experimentally demonstrated that the absorption cross section of two entangled photons is much greater than the absorption cross section of two uncorrelated photons. In this dissertation a source of entangled photons produced through spontaneous parametric down conversion or SPDC was constructed and characterized. It was found that for a pump laser of 411.32 nm and a BBO crystal of 1 mm the produced photons followed the same optical path as the laser, it was also found that the joint spectrum is symmetric at the degenerate wavelength of 822 ± 0.5 nm. This work conclude by showing a first approximation to execute a measurement of the Hong-Ou-Mandel effect. The objective of the measurements of joint spectrum and the Hong-Ou-Mandel effect are to understand the correlation properties of the photon source, with this information a virtual state spectroscopy technique can be implemented on the experiment constructed
By controlling quantum interactions of molecules and light, new technology in quantum information science promises to outperform classical-based technology in both function and efficiency. In particular, chemists stand to gain much advantage from quantum sensing techniques, allowing them to measure new spectroscopic signals that classical techniques cannot do, control photochemical reactions in new ways, and even measure nonlinear interactions with a simple push of a button. Much of the current work in using entangled photons for spectroscopy has focused on the unique physics of the photons. There is still much to be learned about how the molecule’s structure influences quantum spectroscopic signals and leaves its signature. In this dissertation, the unique ways that organic chromophores interact with entangled photons from spontaneous parametric down-conversion (SPDC) has been analyzed in detail. In Chapter 3, I calculate entangled two-photon absorption (ETPA) cross-sections for a few diatomic molecules to show how the varying dipole properties of the molecules affect the ETPA cross-section oscillation vs the entanglement time, Te. In Chapter 4, new theory shows that the excited state linewidth from ETPA excitation is extremely narrow compared to classical TPA excitation. I show that the reason for the ETPA linewidth being so narrow is due to the large arrival time uncertainty of the entangled photon pair, which makes the first photons’ absorption time in the molecule largely uncertain and effectively narrows the excited state linewidth. In Chapter 5, I used Type-II SPDC to prove, for the first time, that two halogenated anesthetic ethers, sevoflurane and isoflurane, have the ability to target and interact with quantum particles by showing that they interact with 800 nm entangled photons but not 800 nm classical photons. In Chapter 6, I build an ETPA experimental setup that uses CW laser pumping of Type-I SPDC, which is shown to significantly improve the accuracy, signal-to-noise ratio (SNR), and limit of detection of ETPA while also making the experiment more compact and cost-effective. I also show how to quantify the degree of frequency entanglement using the Schmidt decomposition of the joint frequency spectrum and propose how the Schmidt modes can be tuned to control photochemical reactions. Contrary to prior belief in the literature, I show experimentally in Chapter 7 that ETPA with Type-I SPDC increases linearly with increasing Te, allowing one to increase the ETPA cross-section by an order of magnitude by increasing Te from a few fs to ~10 ps. This enhancement is due to the entanglement area, Ae, decreasing as the frequency bandwidth decreases, which is how Te is increased. This spatial-spectral coupling has previously been overlooked in the context of ETPA, causing previous theoretical models to underestimate the ETPA cross-section at ps Te by 3-4 orders of magnitude. Using ps Te Type-I SPDC, ps-scale excited state dynamics can be studied using ETPA, and photochemical reactions with intermediate steps, such as isomerizations or solvent reorganization, can be controlled. The experimental techniques and theoretical work here has provided a much needed improvement to make ETPA a more quantitatively robust analytical technique that is useful and accessible to the researchers who can benefit most from its applications: chemists, biologists, and anyone who studies the quantum principles underlying all aspects of chemistry.