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.

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

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.

Three different spectra collected off of samples of 20 µm thick searched against the KnowItAll® database with high matches for polystyrene (PS) (top) polyethylene terephthalate (PET) (middle) and polymethyl methacrylate (PMMA) (bottom)