Shot-to-shot 2D IR spectroscopy at 100 kHz using a Yb laser and custom-designed electronics

Abstract

The majority of 2D IR spectrometers operate at 1-10 kHz using Ti:Sapphire laser technology. We report a 2D IR spectrometer designed around Yb:KGW laser technology that operates shot-to-shot at 100 kHz. It includes a home-built OPA, a mid-IR pulse shaper, and custom-designed electronics with optional on-chip processing. We report a direct comparison between Yb:KGW and Ti:Sapphire based 2D IR spectrometers. Even though the mid-IR pulse energy is much lower for the Yb:KGW driven system, there is an 8x improvement in signal-to-noise over the 1 kHz Ti:Sapphire driven spectrometer to which it is compared. Experimental data is shown for sub-millimolar concentrations of amides. Advantages and disadvantages of the design are discussed, including thermal background that arises at high repetition rates. This fundamental spectrometer design takes advantage of newly available Yb laser technology in a new way, providing a straightforward means of enhancing sensitivity.

Fig. 1.

Schematic and beam diagram for the 100 kHz mid-IR OPA. HR: 1032 nm High Reflector, λ/2: Half Wave plate, SM:Silver mirror, DM: Dichroic mirror, GM: Gold Mirror

Fig. 2.

Block diagram of the electronics designed to read out the MCT detector array at 100 kHz repetition rate; see text for details.

Fig. 3.

a) Spectrum of pump and probe pulses centered at 2050cm^-1 b) GVD and TOD parameter space mapped by scanning second and third order phase coefficients with the pulse shaper c) interferometric (black) and extracted intensity (red) autocorrelation of pump pulse pair after dispersion compensation.

Fig. 4.

Dark noise of measurement electronics and MCT detector. The inset (blue) shows measurement values over 500 laser shots (5 ms) together with a distribution function (right), revealing a standard deviation of ≈6 counts. The main panel (red) shows an auto-correlation function of the noise.

Fig. 5.

Linearity of the MCT detector measured with 50 ns pulses from a red LED (see text for details). The data (black squares) are fit by a 2^nd-order polynomial (red line), while the linear part of that fit is shown as a thin black line.

Fig. 6.

Sample of 100 mM NMA collected on a a) Yb:KGW spectrometer and a b) Ti:Sapphire spectrometer. Spectra were averaged for 10 minutes each. Lower panel show time domain data used to calculate S/N.

Fig. 7.

Change in signal decrease over one hour of averaging. 2D IR intensity obtained from a representative off-diagonal datapoint from Yb:KGW (ν_pu=1670 cm^-1, v_pr=1631 cm^-1) and Ti:Sapphire (ν_pu=1686 cm^-1, v_pr=1624 cm^-1) spectrometer spectra in Fig. 6(a) and Fig. 6(b).

Fig. 8.

2D IR spectra of 100 µM sodium azide obtained a) without referencing, b) with referencing using a correlation matrix estimated from 8,000 pump off shots before data collection, and c) with a gaussian fit background correction. Blue boxes in lower panels show region over which the standard deviation of the noise trace was evaluated.

Fig. 9.

2D IR spectra of (a) 1 mM CNPh, (b) 10 µM ReCO, and (c) 500 µM NMA. All spectra were averaged for 820 seconds. The CNPh spectrum was collected at a waiting time of 1 ps to mitigate solvent background at 4.5 µm excitation. Data in panels a and b were collected with t_1,max=4 ps.

Fig. 10.

a) Solvent background artifact observed in neat D₂O with 6 µm excitation measured with t_1,max = 4 ps, b) Simulation of solvent background according to the proposed transient grating mechanism. c) Transient absorption spectrum at reduced repetition rates. Transient absorption spectra were obtained from a separate pump-probe measurement with t₂ = 0 fs.

Fig. 11.

Feynman pathways responsible for diagonal artifact shown in Fig. 10(a). A pulse from laser shot N stimulates an echo from a coherent population grating excited by pulses from n laser shots prior. The subscript h indicates a ground state thermal population grating (h for “hot”). The subscript c indicates a ground state at room temperature (c for “cold”).