Enhancing two-photon excited fluorescence by using thermal light

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.