Dual-Color PFIR Microscopy

One possible extension of the PFIR-type photothermal detection mechanism is to incorporate multiple laser radiations with different beam characteristics (i.e., frequency, timing etc.) Such integration of multiple frequency excitations enables new capabilities for peak force photothermal microscopy.

Dual-color PFIR microscopy

Dual-color PFIR microscopy is a variation of PFIR microscopy, in which two IR radiations of different frequencies are utilized to simultaneously deliver two PFIR images at different frequencies. The dual-color PFIR microscopy is good to image two chemical components of interest of non-sparse distributions, or with an open-loop AFM scanner, like the PeakForce Tapping enabled Multimode AFM of Bruker.

The dual-color PFIR microscopy is assembled with two separate mid infrared light sources (Quantum cascade laser QCL). The timing of the laser emission are synchronized to subsequent peak force tapping cycles. The photothermal responses are generated separately for each laser radiations and mechanically extracted with AFM cantilever oscillations. Two signals from two IR excitations are simultaneously registered, together with the AFM topography and mechanical property channels. The construction of dual-color PFIR microscope is described in our paper. Analytical Chemistry, 94, 2, 1425-1431 (2022)

Dual-color PFIR images of E Coil cell surface

PFIR spectra were collected from different locations of the E coli surface. These variations indicate heterogeneous chemical distribution on the cell surface.

Dual-frequency Peak Force Photothermal Microscopy

A variation of the dual-color PFIR microscopy is the dual-frequency peak force photothermal microscopy, in which both IR and visible excitation are used. In this development, both IR and visible photothermal responses are registered simultaneously. It allows multimodal characterizations, good for photovoltaic materials

Laser radiations from the IR and the visible sources arrive at the tip-sample region in consecutive peak force tapping cycles. Their photothermal responses are processed in parallel and registered simultaneously.

For more details, please see our paper The Journal of Physical Chemistry C, 126, 19, 8393–8399 (2022)

Frequency-domain Pump-probe PFIR microscopy

Another variation of the dual-color configuration is the pump-probe PFIR microscopy. As described in our paper Journal of Physical Chemistry C 125, 15, 8333-8338 (2021), the underlying principle is to measure the difference in photothermal responses for two excitation pulses when they temporally overlap or offset. The difference yields the indications of the joint spectroscopic pathways. Because the measurement is done with AFM, the limitation of spatial resolution is bypassed. This photothermal route provides a promising way to achieve spatial-temporal nano-imaging. Note that we later developed the time-domain PFIR-based 2DIR spectroscopy, which is much better than the frequency-domain pump-probe approach.

The pump and probe pulses are collimated and arrive at the tip-sample region within the same peak force tapping cycle.

The frequencies of the pump and the probe are scanned while registering the photothermal signals with AFM.

Pulse temporal overlap opens joint spectroscopic pathways that individual pulses cannot access--creating a difference in photothermal signals

Excited state absorptions and ground state depletions can be measured with pump-probe PFIR in 2D PFIR scan.

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