Two-photon absorption techniques

Characterising the two-photon absorption of a material includes studying both the absorption of the excitation light as it passes through a volume of the material, as well as measuring the properties of the two-photon induced fluorescence. These two properties are measured simultaneously to allow for a direct correlation between them based on a modified version of the two-photon excited fluorescence technique.[11]

A solution is made from the material and is placed in a quartz cuvette. The cuvette is placed in the path of the pump beam that is made to pass near the edge of the cuvette to avoid re-absorption of the two-photon induced fluorescence. Because two-photon absorption is a nonlinear process, its magnitude is much smaller than that of the linear absorption and therefore an excitation source with high peak power is required. The optical parametric systems described previously were used for these measurements, where nanosecond pulses were provided by the OPO system and femtosecond from the OPA. The pump beam is gently focused within the solution and its power is measured before and after the cuvette to determine the two-photon absorption magnitude. The induced fluorescence is collected from the side of the cuvette using an optical fibre with collection optics connected to a CCD spectrograph that allows the monitoring of both intensity and spectrum of the material’s fluorescence. The excitation wavelength is changed across the one-photon transparency region of the material to locate the wavelength for which absorption of the pump light and subsequent fluorescence are highest. The energy of the pump beam is also varied to examine the energy dependence of two-photon absorption and fluorescence, as it contains valuable clues about the nature of the observed photophysical processes.

500 525 550 575 600 625 650 675 700 725 0.00 0.05 0.10 0.15 0.20 A b s o rb a n c e Pump wavelength (nm) 0.00 0.25 0.50 0.75 1.00 P L in te n s ity (a .u .)

Figure 3.13 Two photon absorption (filled squares) and fluorescence (filled circles) measurements for a polyfluorene solution under two-photon excitation.

A picture of the setup used for the two-photon measurements can be seen in Figure 3.14, where the cuvette containing the sample is excited by 640 nm light from the nanosecond OPO. The power meter monitors the transmitted power of the pump beam and the collection fibre connects to a CCD spectrograph to monitor the material’s two-photon induced fluorescence.

Figure 3.14 Experimental setup of the two-photon absorption and fluorescence measurements. The cuvette contains a polyfluorene solution excited at 640 nm.

The absorption of the pump beam was measured in two different ways. In the case of nanosecond excitation, where the two-photon absorption coefficient was large, the amount of light absorbed was in the region of 40-70%, which meant that measuring the absorption of the pump beam was done by measuring the power of the pump beam before and after the cuvette. Under femtosecond

Power meter Collection

fibre Spectrograph

excitation though, the amount of light absorbed was much lower, as little as 2- 5% for the thin film measurements. Small fluctuations in the power of the pump laser were of the same order, making reliable measurements difficult. In order to perform a more accurate measurement, two identical photodiodes were used to pick off a small percent of the pump beam before and after the sample. The two photodiodes were connected to two lock-in amplifiers and the ratio of their outputs was recorded over a period of time to allow for averaging of the measured absorption, thus reducing the effect of small fluctuations of the pump beam intensity.

In both cases the quartz cuvette was filled with the solvent used for the solution and was placed in the path of the beam prior to the actual measurement. This helped determine any contribution coming either from the walls of the cuvette and the reflections they cause or from the solvent itself, as at such high excitation densities certain solvent display two photon absorption of their own that can interfere with the values measured for a material in solution. The baseline recorded this way was then subtracted from the two-photon absorption measurements, allowing for the net absorption of the organic material to be calculated in the final data sets.

Two-photon pumped laser measurements were done using the same experimental setup as for one-photon lasers (described in section 3.5.3), with the only difference being that the excitation wavelength is changed to match the peak of the two-photon absorption intensity as determined be the above measurement technique.

3.7.

References

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