25582
2006, 110, 25582-25585 Published on Web 12/07/2006
Entangled Photon Absorption in an Organic Porphyrin Dendrimer Dong-Ik Lee and Theodore Goodson III* Department of Chemistry and Applied Physics, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: October 14, 2006; In Final Form: NoVember 16, 2006
Two-photon absorption spectroscopy is an intensity dependent nonlinear effect related to the excitation of virtual intermediate states. The classical two-photon absorption has an extremely low efficiency which is quantified by its cross-section (δ ∼ 10-48 cm4 s at 800 nm). To overcome this limitation, we demonstrate a novel effect of the two-photon absorption method utilizing the high degree of quantum optical correlation between photon pairs created by the process of spontaneous parametric downconversion. A large entangled two-photon absorption cross-section (σe ∼ 10-17 cm2 at 800 nm) was measured in an organic porphyrin dendrimer. We also discuss the nonmonotonic behavior of variation of the entangled two-photon absorption cross-section by controlling the entanglement time. This novel effect may open new avenues for ultrasensitive detection in chemical and biological systems. TPA spectroscopy has been considered as a powerful tool in physics, chemistry, and biology. The inherent nonlinear process of the classical TPA is distinguishable from the single photon absorption (SPA) linear process. Although the benefits of greater penetration depth and better control and reduction of scattering, the TPA spectroscopy has been restricted by the necessity of a high power optical source due to the low efficiency of the TPA effect. The use of entangled photons from a correlated source for the purpose of the two-photon effect is promising in this regard as one may obtain two-photon effects with very small numbers of photons.
The nonclassical property of the correlated photon pairs, which is created by the spontaneous parametric down conversion (SPDC) process,1,2 has been broadly studied and applied in quantum systems. A theoretical investigation of the entangled two-photon absorption (ETPA) process was achieved with a simple hydrogen atom model.3 However, the experimental achievement has not been reported due to the difficulty of increasing the low efficiency of the SPDC process to create the high degree of correlated photon pairs as well as the control of the entanglement area and entanglement time. Using the correlated photon flux, the sum frequency generation4 (SFG) process and cascaded two-step SPA effects5 with trapped cesium (Cs) atom were announced as a linear process due to the resonant single photon absorption related to a real intermediate state of Cs atom was reported. Unlikely the cascade two-step SPA process, we report the ETPA effect experimentally in an organic material without the involvement of real intermediate states. These findings are novel not only because of the large entangled photon absorption crosssection obtained with a very small number of photons (compared to the classical TPA method) but also because of the observation of the non-monotonic behavior of the ETPA cross-section of the organic dendrimer. This nonmonotonic absorption behavior as a function of the entanglement time gives strong evidence of the quantum optical property of the correlated photon pairs. This behavior is impossible to observe in the classical optics. This effect may also make virtual state spectroscopy3,6-8 possible in organic materials for the discovery of the energy levels of virtual states. * Corresponding author. E-mail:
[email protected].
10.1021/jp066767g CCC: $33.50
For this ETPA experiment a third generation porphyrin dendrimer (tetraphenylporphyrin, H2TPP) was used. To increase the interaction between correlated photon pairs and the molecules of the sample, the porphyrin dendrimer was fabricated as a thin film by spin coating a mixture of the material with an inert (no absorption beyond ∼300 nm) poly(vinylbutyral) (PVB) polymer. For the ETPA process, the porphyrin dendrimer has several advantages including its high TPA cross-section9,10 and the relevance to other biological systems. Also, the structure and spectroscopy11,12 of porphyrins are relatively well studied compared to other complex molecules, as shown in Figure 1. The H2TPP dendrimer has a primary absorption B (soret) band at ∼430 nm and another at 520 nm (Q-band). The single photon absorption cross-section at 800 nm is negligibly small (
