Tutorials in vibrational sum frequency generation spectroscopy. II. Designing a broadband vibrational sum frequency generation spectrometer

1. IR laser line

Initially, we consider the IR laser pulses (shown in red in Fig. 2). To generate pulses in the mid-IR from 800 nm pulses requires use of some nonlinear optical techniques. Specifically, an optical parametric amplifier (OPA) is used to create tunable near-IR (1200–2400 nm) from the 800 nm input. In SurfLab, a Light Conversion TOPAS is used as this OPA, and similar OPAs are found in laboratories all over the world in many areas of science. The generated near-IR light is then sent into a difference-frequency generation (DFG) mixing stage, where it is turned into the mid-IR (2500–10 000 nm, or 1000–4000 cm−1) used in the VSFG experiment. The DFG stage is generally bought with the OPA, and they are operated synchronously to provide wavelength tuning over a very wide range.

As the OPA and DFG stages themselves rely on nonlinear optical processes, they require high input pulse intensities to be efficient. Creation of these pulses is really what demands the use of a powerful femtosecond laser system as the driving laser. Even with the high intensity pulses provided by such a laser, the overall efficiency of the system is rather low. The initial OPA is (at best) only around 40% efficient, and the DFG is generally less than 1% efficient. With around 2 W of 800 nm light entering the OPA, we can, therefore, expect around 5–10 mW of mid-IR light exiting our DFG stage. However, these pulses will be ultrashort (generally, around 200 fs duration in our case) and so after focusing can easily reach the intensities required for efficient SFG spectroscopy.

Having generated these pulses, we need to be able to steer them effectively through the rest of the setup. Immediately following the output of the DFG mixer are two steering mirrors which accomplish this. It is in the nature of the DFG mixing process that the pointing direction of the IR beam can change slightly as the wavelength is changed, and thus, it is critical to be able to compensate for this using steering mirrors between the DFG output and the first optic (A). Using two mirrors rather than just one is essential to get the best control.ii iiSee Fig. 8.2 and the subsequent discussion in Ref. 22. J. D. Pickering, Ultrafast Lasers and Optics for Experimentalists, 1st ed. (IOP, Bristol, UK, 2021)., for an explanation. Following this, we need to be able to control the polarization of the beam and focus it onto the sample surface. Optics (A) to (D) in Fig. 2 accomplish this. Optic (A) is a long-pass filter that is designed to block residual near-IR light from the OPA stage that leaks through the DFG, ensuring that only the desired mid-IR light is focused onto the sample. Optic (B) is a half-wave plate (HWP) that can rotate the polarization of the output DFG pulses.iii

iiiThe polarization state of all the pulses used here is linear, either S- or P-polarized.

HWPs generally do not work efficiently over a wide range of wavelengths, and thus, it is necessary to have several mid-IR HWPs in the lab so that VSFG across all mid-IR spectral regions is possible. It is also important to have a low-order or zero-order HWP to ensure that the HWP rotates light effectively across as broad a bandwidth as possible. Following the HWP, optic (C) is a wire-grid polarizer that will only transmit linearly polarized light and so acts to “clean” the beam by removing any light of the incorrect polarization that either leaks through the previous optics or is not properly rotated by the HWP. It is important that the substrate the wire-grid polarizer is built on is transparent to the mid-IR, and so, in general, CaF2, BaF2, or MgF2 is used.iv

ivChoice within this range is dictated by cost, durability, and GDD (group delay dispersion) considerations.

Wire-grid polarizers actually have a relatively “poor” extinction ratio,v

vThe ratio of the output power of the desired polarization to undesired polarizations—a measure of polarizer quality.

in absolute terms—normally only around 100:1. The effect of the polarizer quality can be put into context by considering what VSFG signal the 1% leakage through the 100:1 polarizer would produce. Simplistically, we may say that reducing the incident IR power to 1% of its initial value would reduce the VSFG signal so far that it is unmeasurable, and thus the 1% leakage will not affect measurements. However, there are situations where you may be measuring a weak VSFG signal using (for example) P-polarized IR light,vi

viP-polarized light is polarized parallel to the plane of incidence and S-polarized light is polarized perpendicular to the plane of incidence. The plane of incidence is defined as the plane that contains the incident beam and the normal vector to the surface of the optic, it is incident on—in our case, the sample surface.

but in your specific sample, there may be a strong VSFG signal that arises when using S-polarized IR light, with the same VIS and SF polarizations. In this case, 1% of the strong signal from the S-polarized IR (that arises due to polarizer leakage) may be comparable to or stronger than 100% of the weaker signal from the P-polarized IR. In this case, polarizer leakage is very problematic, and so a different solution must be sought.vii

viiFor example, using multiple wire-grid polarizers in series.

Consideration of your specific experiment and making sure that you have the most appropriate polarizers is the best course of action.When the beam is being steered after the polarizer by mirrors, it is important to note that reflecting the beam anywhere out of the horizontal or vertical planes (as could happen in a misaligned periscope, for example) will cause the polarization state to change. For this reason, it is especially important in a polarization-sensitive spectroscopy like VSFG that all periscopes are well aligned with only 90° angles, and that the beam is not reflected out of the horizontal plane except when in a periscope, and when reflecting toward the sample. Accessible introductions to how polarizing optics work can be found in Refs. 22. J. D. Pickering, Ultrafast Lasers and Optics for Experimentalists, 1st ed. (IOP, Bristol, UK, 2021)., 55. F. L. Pedrotti, L. M. Pedrotti, and L. S. Pedrotti, Introduction to Optics, 3rd ed. (Cambridge University Press, Cambridge, UK, 2017)., and 1111. D. Goldstein, Polarized Light, 2nd ed. (CRC Press, Boca Raton, Florida, 2003)., if you are unfamiliar.

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