We report on utilizing the photon bunching effect in thermal light to enhance the efficiency of Two-Photon Excited Fluorescence (TPEF) under continuous wave illumination. This has potential applications in microscopy.

Two-photon excited fluorescence (TPEF) has become a standard technique in modern microscopy but it is still affected by photo-damage of the probe owed to high excitation intensities. This work reports on utilizing the photon bunching effect in thermal light to enhance the efficiency of TPEF under continuous wave excitation. A super-luminescence diode (SLD) was used as thermal light source and compared it to coherent excitation from a DFB diode laser emitting at the same wavelength. The degree of second order coherence g(2) of both sources was measured. The SLD showed photon bunching with g(2) = 1.90±0.2 as expected for thermal light, while the DFB diode laser emitted coherent light exhibiting no photon bunching at g(2) = 1.

Two-photon excited fluorescence (TPEF) is a standard technique in modern microscopy but still affected by photo-damage of the probe. It was proposed that TPEF can be enhanced by using entangled photons, 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, sub-wavelength lithography and metrology. 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 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.

Two-photon excited fluorescence (TPEF) is a standard technique in modern microscopy(1), but is still affected by photodamage to the probe. It has been proposed that TPEF can be enhanced using entangled photons(2,3), but this has proven challenging. Recently, it was shown that some features of entangled photons can be mimicked with thermal light, which finds application in ghost imaging(4), subwavelength lithography(5) and metrology(6). Here, we use true thermal light from a superluminescent diode to demonstrate TPEF that is enhanced compared to coherent light, using two common fluorophores and luminescent quantum dots, which suit applications in imaging and microscopy. We find that the TPEF rate is directly proportional to the measured(7) degree of second-order coherence, as predicted by theory. Our results show that photon bunching in thermal light can be exploited in two-photon microscopy, with the photon statistic providing a new degree of freedom.

The feasibility of ultrahigh resolution optical coherence tomography to image both healthy and pathological brain tissue morphology as well as the morphology and functional response of neuron cells is investigated.

The morphology of healthy and pathological human brain tissue, as well as the brain structural organization of various animal models has been imaged in-vitro using ultrahigh resolution optical coherence tomography (UHR OCT). Micrometer-scale OCT resolution (< 2 μm axial resolution) was achieved at different central wavelengths by interfacing three state-of-the-art broad bandwidth light sources (Ti:Al2O3, λc = 790 nm, Δλ = 260 nm and Pout = 50 mW; PCF based laser, λc = 1150 nm, Δλ = 350 nm and Pout = 2 W; Fiber laser based light source, λc = 1350 nm, Δλ = 470 nm and Pout = 4 mW) to a modular free-space OCT system, utilizing a dynamic focusing and designed for optimal performance in the appropriate wavelength regions. Images acquired from a fixed honeybee brain demonstrated the ability of UHR OCT to image the globular structure of the brain, some fine morphological details such as the nerve fiber bundles connecting the medulla (visual center) to the honeybee eyes, and the interfaces between different tissue layers in the medulla. Tomograms of various human neuropathologies demonstrated the feasibility of UHR OCT to visualize morphological details such as small (~20 μm) calcifications typical for fibrous meningioma, and enlarged nuclei of cancer cells (~10-15 μm) characteristic for many other neuropathologies. In addition UHR OCT was used to image cellular morphology in living ganglion cells.

The appropriate combination of a passively mode-locked Nd:YVO4 pump laser with 10 ps pump pulse duration at 1064 nm and a dispersion-adapted air–silica microstructured fiber generates efficiently supercontinuum radiation with an average power of 2.4 W in a spectral range from 700 to 1600 nm. The presented setup allows a very compact design of the white-light source. By means of experimental results will be shown, that newly generated frequency components mainly originate from self-phase modulation, parametric four-wave mixing as well as stimulated Raman amplification.