Enhanced two-photon absorption using true thermal light

Two-photon excited fluorescence (TPEF) is a standard technique in modern microscopy [1] but still affected by photo-damage of the probe. It was proposed that TPEF can be enhanced by using entangled photons [2, 3], but has proven to be challenging. Recently it was shown that some features of entangled photons can be mimicked with thermal light, which finds application in ghost imaging [4], sub-wavelength lithography [5] and metrology [6]. Here, we utilize true thermal light from a super-luminescence diode to demonstrate enhanced TPEF compared to coherent light using two common fluorophores and luminescent quantum dots. We find that the two-photon absorption rate is directly proportional to the measured [7] degree of second-order coherence, as predicted by theory. Our results show that photon bunching can be exploited in two-photon microscopy with the photon statistic providing a new degree of freedom. ∗email:ajechow@uni-potsdam.de 1 ar X iv :1 31 0. 42 97 v1 [ qu an tph ] 1 6 O ct 2 01 3 Introduction Photon bunching was discovered by Hanbury Brown and Twiss (HBT) in their seminal paper in 1956 [8]. This peculiar effect was one of the cornerstones of early quantum optics and Glaubers quantum theory of optical coherence [9]. Today, HBT interferometry is still widely used in many modern photon counting experiments. Simultaneous two-photon absorption (TPA) was theoretically predicted even earlier by Goeppert-Mayer in 1931 [10]. Due to the very small TPA cross sections (σTPA ≈ 10−50cm4s) it took three decades until this rare process was observed experimentally [11], shortly after the invention of the laser and HBTs discovery. Subsequently, the influence of photon statistics of the light used to excite the TPA process was investigated theoretically [12, 13]. The calculations revealed that there is a linear proportionality between the TPA rate and the degree of second-order coherence (DSOC) g(0) of the exciting light. Thus, the TPA rate should actually be twice as high for thermal (chaotic) light sources than for coherent light [14] (see methods). However, thermal light sources delivering sufficient intensity to trigger TPA processes were not available and mainly laser light sources were utilized for TPA experiments. An early experimental attempt to prove the theoretical predictions relied on a pulsed laser and diffusing glass plates to mimic a thermal light source [15] but was hampered by a strong one photon absorption background of the Cs3Sb photocathode and the residual coherence of the light source. New interest in thermal light sources has arisen from the rapidly developing field of ghost imaging, which was first discovered with entangled photons [16] but later proposed [17] and demonstrated with (pseudo-) thermal light [4]. This has stimulated a vast number of experimental work mainly based on pseudo-thermal light sources in the last decade and there exist an ongoing theoretical debate about when to treat ghost imaging as a quantum or a purely classical effect [18]. Recently, even sunlight was used to demonstrate ghost imaging [19] and thermal light has been used for other applications e.g. sub-wavelength lithography [5] and metrology [6]. However, to our knowledge two photon absorption with true thermal light and an experimental evidence of the early theoretical predictions was not demonstrated, yet. Several ways to artificially produce thermal like or pseudo-thermal light exist but mainly rely on distorting highly coherent laser radiation [4–6]. Another way to attain thermal light