Two-photon excitation spectroscopy of 1,5–Diphenyl-1,3,5-hexatriene using phase modulation

Abstract

We have used two-photon Fourier transform spectroscopy to investigate the first singlet excited state (S 1 ) of a prototypical polyene molecule 1,5-Diphenyl-1,3,5-hexatriene. As the S 1 state in the polyenes is a one-photon forbidden transition, the structure of its vibrational levels cannot be studied using resonant linear excitation. Although this level is accessible with two-photon excitation, previous studies done by using wavelength tunable pulsed lasers did not have enough resolution to investigate the details of the vibrational levels. In Fourier transform spectroscopy, one uses a pair of laser beams to excite the sample. The measurements are done by varying the time delay between the pulses. The spectral resolution is given by the inverse of the maximum time delay rather than the spectral width of the pulses. We have used the method to investigate the vibrational levels of the S 1 state. In our implementation, we have used phase modulation to carry out the measurements in the rotating frame, which requires less data points along the time delay thereby significantly reducing the measurement time.

Nature of the different emissive states and strong exciton–phonon couplings in quasi-two-dimensional perovskites derived from phase-modulated two-photon micro-photoluminescence spectroscopy

“…In these measurements, one observes a ratio of 4:1 for the signal amplitudes at a and 2 a . 37,55 Similarly, in the emission spectrum from the recombination of the free carriers, an amplitude ratio of 56:28:8:1 for the contributions at a , 2 a , 3 a , and 4 a is expected. When the processes in a system results in both emissions, the ratio of the higher harmonics obtained from the Fourier analysis of the signal differs from the ratios expected for either type of recombination.…”

Enhanced Radiative Recombination of Excitons and Free Charges Due to Local Deformations in the Band Structure of MAPbBr3 Perovskite Crystals

“…The schematic diagram of the two-photon phase-modulated PL is presented in Figure a. Details are given elsewhere. − A mode-locked Ti: sapphire oscillator (Synergy, Femtolasers) with a repetition rate of 70 MHz, the center wavelength at 790 nm, and the pulse duration of about 20 fs was used as a light source (spectrum shown in Figure d). A Mach–Zehnder interferometer was used to produce two identical replicas of the beams, and an acousto-optic modulator (AOM) placed on each of the arms of the interferometer was used to shift the carrier frequency ω of the beams.…”

“…The emission from excitons formed directly after the excitation modulates at two frequencies, ϕ 21 and 2ϕ 21 , , whereas the emission from free charges has additional modulations at 3ϕ 21 and 4ϕ 21 (see Figure a). Previous investigations of molecular systems have exhibited a pure exciton recombination where the amplitudes at ϕ 21 and 2ϕ 21 are in a ratio of 4:1. , As an example, Figure b shows the Fourier transform of the PL from a 100 μM solution of 1,3,5-diphenyl-hexatriene dissolved in toluene. The difference in the phase modulation frequency is 50 kHz, and the PL is recorded for 0.1 s. The signals at 50 and 100 kHz are in the ratio of 4:1.…”

“…When two beams, whose phases are modulated at frequencies ϕ 1 and ϕ 2 , excite a fluorescent particle by a two-photon absorption process, the emissions modulate at frequencies m ϕ 21 , where m is an integer and ϕ 21 = ϕ 2 – ϕ 1 . The emission from excitons formed directly after the excitation modulates at two frequencies, ϕ 21 and 2ϕ 21 , , whereas the emission from free charges has additional modulations at 3ϕ 21 and 4ϕ 21 (see Figure a). Previous investigations of molecular systems have exhibited a pure exciton recombination where the amplitudes at ϕ 21 and 2ϕ 21 are in a ratio of 4:1. , As an example, Figure b shows the Fourier transform of the PL from a 100 μM solution of 1,3,5-diphenyl-hexatriene dissolved in toluene.…”

“…Previous investigations of molecular systems have exhibited a pure exciton recombination where the amplitudes at ϕ 21 and 2ϕ 21 are in a ratio of 4:1. 42,43 As an example, Figure 1b shows the Fourier transform of the PL from a 100 μM solution of 1,3,5-diphenyl-hexatriene dissolved in toluene. The difference in the phase modulation frequency is 50 kHz, and the PL is recorded for 0.1 s. The signals at 50 and 100 kHz are in the ratio of 4:1.…”