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1. Photothermal Mirror Z-Scan

   Marcano, A. (May 2019, Conference on Lasers and Electro-Optics, OSA Technical Digest)

   This work describes a pump-probe photothermal mirror Z-scan experiment aimed at the determination of thermal diffusivity, thermoelastic coefficient, and quantum yield of thermal heating of the surface of transparent and non- transparent solid samples including films. 

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2. White-Light Photothermal Mirror Spectrophotometer

   Marcano, A.; Hlaing, M. (May 2019, Conference on Lasers and Electro-Optics, OSA Technical Digest)

   We describe an arc-lamp based pump-probe photothermal mirror spectrophotometer to measure the spectrum of the thermal quantum yield of the surface of solid samples. We discuss the advantages of the method to characterize solid nontransparent materials. 

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3. Influence of thermal diffusion on the spatial resolution of photothermal microscopy

   https://doi.org/10.1117/12.2607795  

   Brown, Brendan S.; Hartland, Gregory V. (March 2022, Proc. SPIE 11990, Nanoscale and Quantum Materials: From Synthesis and Laser Processing to Applications)

   Kabashin, Andrei V.; Farsari, Maria; Mahjouri-Samani, Masoud (Ed.)

   Photothermal microscopy is a powerful method for investigating biological systems and solid state materials. Using a modulated pump to excite the sample, a continuous probe beam monitors the change in the refractive index of the sample due to the modulated heating. These experiments are typically performed at high frequencies to reduce the 1/f noise, achieving a higher signal to noise ratio. In this paper, we explore how the resolution and sensitivity of the photothermal experiments change when the modulation frequency is brought down below 100kHz. In the instance that the pump and probe are cofocused at the sample, the resolution is determined by the size of the pump beam. On the other hand, when a widefield pump is used, significant broadening occurs for frequencies under 20kHz. This broadening is attributed to thermal diffusion. However, the amount of broadening is less than that expected from the thermal diffusion length, which is about 1.7μm at 10kHz for nanoparticles in glycerol. We also explore the situation where the point spread functions of the pump and probe beams are smaller than the particle size as well as how the penetration depth depends on the properties of the pump and probe beams. 

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4. Sample exchange by beam scanning with applications to noncollinear pump–probe spectroscopy at kilohertz repetition rates

   https://doi.org/10.1063/1.4986628  

   Spencer, Austin_P; Hill, Robert_J; Peters, William_K; Baranov, Dmitry; Cho, Byungmoon; Huerta-Viga, Adriana; Carollo, Alexa_R; Curtis, Anna_C; Jonas, David_M (June 2017, Review of Scientific Instruments)

   In laser spectroscopy, high photon flux can perturb the sample away from thermal equilibrium, altering its spectroscopic properties. Here, we describe an optical beam scanning apparatus that minimizes repetitive sample excitation while providing shot-to-shot sample exchange for samples such as cryostats, films, and air-tight cuvettes. In this apparatus, the beam crossing point is moved within the focal plane inside the sample by scanning both tilt angles of a flat mirror. A space-filling spiral scan pattern was designed that efficiently utilizes the sample area and mirror scanning bandwidth. Scanning beams along a spiral path is shown to increase the average number of laser shots that can be sampled before a spot on the sample cell is resampled by the laser to ∼1700 (out of the maximum possible 2500 for the sample area and laser spot size) while ensuring minimal shot-to-shot spatial overlap. Both an all-refractive version and an all-reflective version of the apparatus are demonstrated. The beam scanning apparatus does not measurably alter the time delay (less than the 0.4 fs measurement uncertainty), the laser focal spot size (less than the 2 μm measurement uncertainty), or the beam overlap (less than the 3.3% measurement uncertainty), leading to pump–probe and autocorrelation signal transients that accurately characterize the equilibrium sample. 

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5. Broadband photothermal spectroscopy with a mid-infrared quantum cascade laser frequency comb

   https://doi.org/10.1364/OE.544229  

   Huang, Baichuan; Gomółka, Grzegorz; Mikkonen, Tommi; Wysocki, Gerard (January 2025, Optics Express)

   We demonstrate a broadband photothermal spectroscopy in the mid-infrared region using a quantum cascade laser frequency comb operating between ∼7.7 and ∼8.2 µm covering a frequency range of ∼70 cm^-1. The photothermal spectroscopy technique employs a Mach-Zehnder interferometer operating in a pump-probe configuration, where the mid-infrared pump beam is modulated by a Fourier transform spectrometer. A 76-m Herriott-type multipass cell is used for signal enhancement. As a proof-of-concept, we have measured the photothermal spectra of nitrous oxide that show good agreement with the HITRAN database. A minimum detection limit of 83 ppb of nitrous oxide in nitrogen is estimated from a broadband photothermal spectrum with 9.9 GHz spectral point spacing and acquired over 78 minutes. This detection scheme also provides over three orders of magnitude of photothermal signal linearity with gas concentration. This spectroscopic method combines the functionality of high sensitivity and background-free detection of photothermal spectroscopy as well as broadband mid-infrared operation of quantum cascade laser frequency comb, which could find applications in trace gas sensing systems that benefit from these features. 

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