Spcm aqr 13

All the fluorescence measurements are performed on a home-built confocal microscope setup. Au₁₈NC samples are excited at 557 nm using an iChrome-TVIS laser (Toptica GmbH, pulse duration ∼ 3 ps). The repetition rate of the laser is 40 MHz. A multiband dichroic mirror (ZT 405/488/561/640rpc, Chroma) reflects the laser towards the microscope, and a Zeiss C-Apochromat 63×, 1.2 NA water immersion objective lens is used to focus the excitation light. The same objective lens collects the PL signal in an epifluorescence configuration. The PL beam then passes through the same multiband dichroic mirror. To block the laser back reflection an emission filter (ZET405/488/565/640mv2, Chroma) is used. The fluorescence signal is focused onto an 80 μm pinhole. Two avalanche photodiode APDs (PerkinElmer SPCM-AQR-13) separated by a 50/50 beam-splitter in a Hanbury-Brown–Twiss configuration record the emitted photons in the 650–750 nm spectral range. The photodiode outputs are connected to a time-correlated single photon counting (TCSPC) module (HydraHarp 400, Picoquant). The integration time for each FCS experiment was set to 40 minutes for the confocal experiments and 5 minutes for the ZMWs.

The entangled two-photon spectroscopy technique has been previously described^{39 (link),46 (link),48 (link),49 (link)}. A sketch of the experimental setup used in this work is presented in Fig. 6. Orthogonally polarized entangled photon pairs were generated by the spontaneous parametric down-conversion (SPDC) process. A 0.5 mm BBO (β-Barium Borate) crystal (type II) is pumped with the second harmonic generation (SHG) beam, 400 nm, of a Ti:Sapphire pulsed laser emitting ~70 fs pulses (MaiTai, Spectra Physics). Entangled photon intensity is varied by changing the pump power on the SPDC crystal with a variable neutral density filter. Transmitted entangled photons are focused onto an avalanche photodiode (SPCM-AQR13, PerkinElmer). Fig. 6 shows the complete set-up, previously shown by Harpham et al.^{48 (link)}.Figure 6

Experimental setup used for entangled two-photon spectroscopy.

The measurements have been performed on a MicroTime 200 time-resolved confocal microscope (PicoQuant, Berlin, Germany) at the Physical Chemistry group of University of Potsdam. Samples were excited by a laser diode (LDH-P, PicoQuant) with wavelength of 635 nm, repetition rate of 20 MHz and pulse width of 70 ps. The excitation beam was focused by a 100x/NA1.4 oil immersion objective (Olympus, Hamburg, Germany). Fluorescence light emitted from dye passes through the dichroic mirror (z467/635rpc, AHF Analysentechnik, Tübingen, Germany), is guided through a 50 μm pinhole and is recorded by single photon avalanche diode (SPAD)(SPCM-AQR-13, Perkin-Elmer, USA). To calculate the autocorrelation functions and for analysis we used Matlab R2016a (9.0.341369). In order to obtain parameters r₀ and s entering the apparatus function, we performed a calibration experiment with Atto655 diffusion in water at 293.15 K, for which the diffusion is normal and the diffusion coefficient is known: D = 392 μm²/s²⁴ . Calibration leads to the values of parameters r₀ = 0.248 μm and s = 6.5.