Topas c

Transient absorption signal was recorded using the pump-probe setup described in our previous study35 (link). Laser pulses (800 nm, 80 fs pulse length, 1 kHz repetition rate) were generated by a regenerative amplifier (Spitfire XP) seeded by a femtosecond oscillator (Tsunami, both Spectra Physics).
Excitation pulses at the wavelength of 380, 390, 400, 420, 460, and 480 nm were acquired using an optical parametric amplifier (Topas C, Light Conversion). The used excitation photon flux at 480 nm was 1 × 10¹⁴ photons/cm²/pulse corresponds to <N> ~ 0.1 (the mean number of excited e-h pairs per QD). The intensity for all excitation wavelengths was adjusted to lead to approximately the same TA signal of the colloidal sample of 5–7.10⁻³ change in OD (measured in 1-mm static cell).
Probe pulses (broad supercontinuum spectrum) were generated from the 800-nm pulses in a sapphire plate and split by a beam splitter into probe pulse and a reference pulse. The probe pulse and the reference pulse were dispersed in a spectrograph and detected by two diode arrays (Pascher Instruments). The analysis of kinetics was done by averaging over 8 nm wide spectral bands of the probe spectra.
Thin film samples (typical OD of 0.02 at first exciton peak) were measured in a nitrogen atmosphere to avoid possible oxidation of QDs36 .

Excitation was performed with 100 fs pulses at 400 nm generated by frequency doubling part of the output of a standard 1 kHz Ti:Sapphire amplified system. The pump intensity on the sample was below 1 mJ cm⁻². The gate pulses were at 1340 nm and were produced by an optical parametric amplifier (TOPAS-C, Light Conversion). Detection of the up-converted spectra was performed with a CCD camera (Andor, DV420ABU). The FWHM of the cross correlation of the gate with the solvent Raman signal was ~170 fs. Corrected time-resolved emission spectra were obtained by calibration with secondary emissive standards. The temporal chirp was determined measuring the instantaneous response of BBOT (Radiant Dyes) in all the solvents used.

Transient absorption measurements were performed with a pump–probe differential spectrometer (Ultrafast Systems HELIOS-EOS), with both pump and probe wavelengths generated by a Ti:Sapphire regenerative amplifier (Coherent Libra-F-1K-HE-230), which delivers 100 fs long pulses at 800 nm with 1 KHz repetition rate. The main emission from the regenerative amplifier was split into two branches: one sent to an optical parametric amplifier (Light Conversion TOPAS C), in order to generate the pump wavelengths, and the other sent to the sapphire plate of the HELIOS spectrometer, where multi-color probe beam was generated by means of white light supercontinuum generation. The probe pulses were time delayed with respect to the pump pulses, by passing through a variable digitally controlled optical delay line. The pump and probe beams were then non-collinearly focused and overlapped on the sample surface, with the pump being chopped at 500 Hz, so that half of the transmission spectra were recorded with the pump on and half with pump off. The transmission spectra from the probe beam were recorded as a function of the relative delay time, by means of CCD spectrometers

A Libra F laser system (Coherent,
Inc.) produced 800 nm light pulses at a repetition rate of 1 kHz,
which was split for the excitation and probe pulse generation in a
roughly 90:10 ratio, respectively. The pulse width was ∼70
fs. The pump wavelength was tuned to 400 nm by a Topas C optical parametric
amplifier (Light Conversion Ltd.) followed by decreasing the excitation
energy density to 20 μJ cm^–2 using neutral
density filters. The white light continuum for the probe pulses was
obtained by directing ∼10% of the primary 800 nm pulse energy
to a water-filled cuvette. The measurement system (ExciPro, CDP, Inc.)
comprised a silicon CCD, using an optical chopper for the pump pulses
for reference measurements.

Time-resolved IR (TRIR) measurements were carried out with the same setup as described in [33 (link),37 (link)]. Excitation was achieved with 400 nm pulses generated by frequency doubling part of the output of a 1 kHz amplified Ti:Sapphire system (Solstice, Spectra-Physics). Probing was achieved with the output of an optical parametric amplifier (TOPAS-C, Light Conversion) connected to a difference-frequency mixing module (NDFG, Light Conversion) and polarised at magic angle relative to the pump pulse. Detection was performed with a 2 × 64 element MCT array (Infrared Systems Development) connected with a spectrograph (Triax190, 150 lines per mm, Horiba), resulting in a spectral resolution of 1.5 cm⁻¹. Sample handling and data acquisitions were the same as described in [33 (link)].

HD-SFG^{71 (link)} measurements were performed in a collinear beam geometry using a Ti:sapphire regenerative amplifier (centred at 800 nm, ∼40-fs pulse duration, 5-µJ pulse energy, 1-kHz repetition rate, Spitfire Ace, Spectra Physics). Part of the output was used to generate a broadband IR pulse in an OPA (TOPAS-C, Light Conversion) with a DFG crystal. The other part of the output was directed through a pulse shaper consisting of a grating-cylindrical mirror system to generate a narrowband visible pulse with a bandwidth of ∼10 cm⁻¹. The IR and visible beam were first focused into a 20-μm y-cut quartz plate as the local oscillator (LO). These beams were collinearly passed through a 2-mm SrTiO₂ plate for phase modulation and focused on the sample surface at an angle of incidence of 45°. The SFG signal from the sample interfered with the SFG signal from the LO, generating the SFG interferogram. The SFG interferogram was dispersed in a spectrometer (HRS-300, Princeton Instrument) and detected by a liquid nitrogen-cooled CCD camera (PyLoN, Princeton Instruments). The data were analysed using a previously described procedure^{79 (link)}. Briefly, the complex spectra of the second-order nonlinear susceptibility ${\chi }_{\mathrm{eff}}^{\left(2\right)}$ were obtained via Fourier analysis of the interferogram and normalization by a z-cut quartz crystal. All measurements were performed using the ssp polarization combination.