Photodetectors

As for the excitation sources a flexible modular concept was realised for the detection path, which is shown schematically in Fig. 3.3.

flip mirrors

Spectro-graph CCD

APD

Streak-Camera

from microscope

Figure 3.3: Schematic illustration of the detection light path and the photodetectors.

Avalanche Photodiode

For the acquisition of fluorescence excitation spectra (chapter 4) and time-correlated single photon counting measurements (TCSPC, see Ref. [160]) an avalanche photodiode (APD, SPCM-AQR-14, EG&G) with a dark count rate of less than 100 s⁻¹ has been utilised. The quantum efficiency of the APD exceeds 70 % at 700 nm and is 40 – 60 % in the spectral re-gion of the MeLPPP emission (450 – 550 nm, see Fig. 3.1b). The active element of the APD is cooled and temperature stabilised by a Peltier element and has a diameter of 180µm.

This chip is a silicon p⁺pn-structure biased above the breakdown voltage and therefore every impinging photon generates an electron avalanche, which is then actively quenched.

Each detected photon generates a TTL pulse (2.5 V amplitude and 30 ns duration in a 50 Ω load). The single photon arrival time can be measured with an accuracy of about 350 ps.

Further digital signal processing of the TTL pulses is performed by a computer-based mul-tifunctional counter board (NI 6024E, National Instruments) and home-made programmes

3.2 Experimental Setup

in the graphical language LabView (National Instruments).

CCD Camera and Spectrographs

Widefield images and fluorescence spectra (see chapters 4 and 6) have been recorded with the combination of an imaging spectrograph (SpectraPro-150, Acton Research Corporation or 250 IS, Bruker) and a sensitive charge coupled device (CCD) camera (SensiCam QE, PCO).

The SpectraPro-150 spectrograph has a focal length of 150 mm and is equipped with a turret containing a 600 lines/mm grating for the acquisition of emission spectra and a mirror for imaging. The 250 IS spectrograph has a focal length of 250 mm and is equipped with three gratings (150, 300, and 1800 lines/mm) mounted on a turret. For this device imaging can be performed in 0^th order diffraction by the gratings. The blaze wavelength of all gratings is 500 nm and the diffraction efficiencies are about 70 % in the spectral range of the MeLPPP emission. The signals in both spectroscopy and imaging mode are acquired with the CCD camera which is connected to the spectrographs by a standard C-mount. This ensures that the CCD chip is precisely positioned in the focal plane of the spectrographs. Depending on the gratings and the widths of the entrance slits of the spectrographs spectral resolutions of 3.5 cm⁻¹ (SpectraPro-150) and better than 1 cm⁻¹ (250 IS), respectively, can be achieved.

The CCD chip has a size of 1376 × 1040 pixels each with an area of 6.45×6.45µm². In order to reduce the dark counts, the chip is cooled to -12^◦C by a two-stage Peltier cooler. The dark current at -12^◦C is 0.1 electrons per second per pixel and the A/D (analogue/digital) conversion factor is 2 electrons per count. The peak quantum efficiency of the CCD camera is 65 % at 520 nm, and 55 – 65 % in the spectral region of the MeLPPP emission. The CCD camera is controlled by a software provided by PCO (Camware) and the images are saved in a binary format. Further data processing was done by home-made LabView and Perl programmes.

Streak Camera System

The streak camera system, that was utilised for the time-resolved measurements (chap-ter 5), is a combination of a 250 mm imaging spectrograph (250 IS, Bruker) and a streak-scope (C5680-24C Synchroscan, Hamamatsu Photonics). The basic idea behind the streak system is to translate both the temporal and the wavelength information into spatial infor-mation that can be detected with high precision and allows to acquire both the temporal and the spectral information of a fluorescence decay simultaneously. To this end, the

fluorescence light from the sample is focused onto the entrance slit of the spectrograph where the light is spectrally dispersed by a grating (50 lines/mm, blaze wavelength 600 nm;

150 lines/mm, blaze wavelength 500 nm; 1200 lines/mm, blaze wavelength 500 nm), i. e. the spectral information is converted into a spatial (horizontal) information. The spectrally resolved light is imaged onto the horizontal entrance slit of the streak scope and hits a photocathode where the light is converted into photoelectrons. These are accelerated and pass a pair of horizontally arranged deflection plates where a high-speed sweep voltage is applied with a frequency that is synchronised with the repetition rate of the Ti:Sa-laser system (80.75 MHz). Thus the photoelectrons are the stronger deflected the later they pass the deflection plates, i. e. the later they are generated at the photocathode by the impinging photons. In this way the temporal information is converted into a spatial (ver-tical) information. Finally, the photoelectrons are multiplied by a factor of about 10⁴ in a microchannel plate (MCP) and hit a phosphor screen. The resulting image on this screen is referred to as ”streak image” where the horizontal axis corresponds to wavelength and the vertical axis corresponds to time. This image is read out by a CCD camera (C4742-92 Orca-ER, Hamamatsu Photonics) and integrated in computer memory, which allows mea-surements with a high signal-to-noise ratio. The streak system and the CCD camera is controlled by a commercial software (HPD-TA, Hamamatsu Photonics). The integrated streak images are stored in a binary file format and are further processed by home-made LabView programmes.

The temporal resolution of the streak system depends on several factors, such as the time range (i. e. the length of the time axis of the streak images), the trigger method for the synchroscan unit (optical triggering by a pin-diode or electronic triggering by a TTL-signal output from the AOM-electronics of the Ti:Sa-laser), and the chosen grating in the spectrograph. The best temporal resolution that can be achieved with this streak system is about 2 ps (optical triggering, 50 lines/mm grating). In the configuration used in the experiments described in chapter 5 (electronic trigger, 1200 lines/mm grating) a temporal resolution of about 5 ps was obtained. The overall detection efficiency of the streak system can be estimated to about 3 %. The losses arise mainly from the diffraction efficiencies of the gratings of about 70 %, the quantum efficiency of the photocathode of 15 % (both in the spectral region of the MeLPPP emission), and from the electron optics in the streak tube with about 70 % loss.

The spectral resolution of the streak system depends on the width of the entrance slit of the spectrograph and the grating, and a spectral resolution of about 3 cm⁻¹ was achieved.

3.2 Experimental Setup