Optical Photothermal Infrared (O-PTIR) Spectroscopy

Optical Photothermal infrared spectroscopy (O-PTIR) used on the Mirage IR microscope is a result of over a decade of expertise in photothermal physics after the initial development of AFM-based nanoscale IR spectroscopy.

Sub-micron IR spectroscopy and imaging

O-PTIR overcomes the IR diffraction limit by combining a mid-IR pulsed, tunable laser that heats the sample. When the IR laser is at a wavelength that excites a molecular vibration in the sample, absorption occurs, thereby creating photothermal effects including photothermal expansion. A visible probe laser, focused to 0.5 µm spot size, measures the photothermal response via the scattered light, as shown in the illustration above.

The component of the reflected visible laser signal that is modulated at the IR pump laser repetition rate is directly proportional to the absorption coefficient of the sample at that wavenumber. The IR pump laser can be tuned through the entire fingerprint region in one second or less, to obtain an IR spectrum.

By operating in reflection mode, O-PTIR eliminates several longstanding limitations and has substantial benefits for the IR community, including sub-micron resolution using a non-contact optical method. The sub-micron resolution is demonstrated (right), showing reflection mode spectra on a multi-layer packaging film measured 0.5 µm apart with highly differentiated chemical fingerprints indicating different materials.

Two spectra taken 0.5 µm apart, showing different polymer materials in a multilayer film

FTIR quality spectra in non-contact reflection mode

Measurements are collected quickly and easily without need for sample contact, unlike ATR spectroscopy. Additionally, O-PTIR provides spectra comparable to FTIR without the dispersive artifacts observed in ATR. By operating in reflection mode, the need for thin samples is also eliminated, leading to dramatically easier sample preparation and faster turnaround times.

O-PTIR was used to collect spectra off of samples over 20 µm thick in reflection mode (right). The resulting spectra show near-perfect correlation to polystyrene (PS), polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA) when compared to spectra from the KnowItAll® spectral database.

Even the strongest bands show no evidence of saturation. This is because the reflected signal is sampling only the top couple microns of the sample, making the depth of penetration comparable to what is achieved using ATR accessories, but without the optical band-shape distortions present in many ATR spectra.




Three different spectra collected on Mirage searched against the database with high matches for PS (top) PET (middle) and PMMA (bottom)

High resolution single wavelength imaging

Due to the wavelength tunability and high spectral resolution of Mirage, sub-micron chemical images can be created at specific wavelengths providing more accurate chemical composition of the surface at discrete wavenumbers (right).

Next generation infrared spectroscopy

O-PTIR eliminates several longstanding limitations for IR microscopy enabling sub-micron IR spectroscopy and minimizing sample preparation, making it a unique technique that provides a huge step forward for the IR spectroscopy community.

Single wavelength chemical imaging of a multilayer film taken at 1545 cm-1 with 0.5 µm spatial resolution, highlighting polyamide distribution.