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1. Photothermal behaviour of titanium nitride nanoparticles evaluated by transient X-ray diffraction

   https://doi.org/10.1039/D0NR08202C  

   Diroll, Benjamin T.; Brumberg, Alexandra; Leonard, Ariel A.; Panuganti, Shobhana; Watkins, Nicolas E.; Cuthriell, Shelby A.; Harvey, Samantha M.; Kinigstein, Eli D.; Yu, Jin; Zhang, Xiaoyi; et al (February 2021, Nanoscale)

   null (Ed.)

   The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface. 

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2. Visualizing ultrafast photothermal dynamics with decoupled optical force nanoscopy

   https://doi.org/10.1038/s41467-023-42666-9  

   Wang, Hanwei; Meyer, Sean M.; Murphy, Catherine J.; Chen, Yun-Sheng; Zhao, Yang (November 2023, Nature Communications)

   Abstract The photothermal effect in nanomaterials, resulting from resonant optical absorption, finds wide applications in biomedicine, cancer therapy, and microscopy. Despite its prevalence, the photothermal effect in light-absorbing nanoparticles has typically been assessed using bulk measurements, neglecting near-field effects. Beyond standard imaging and therapeutic uses, nanosecond-transient photothermal effects have been harnessed for bacterial inactivation, neural stimulation, drug delivery, and chemical synthesis. While scanning probe microscopy and electron microscopy offer single-particle imaging of photothermal fields, their slow speed limits observations to milliseconds or seconds, preventing nanoscale dynamic investigations. Here, we introduce decoupled optical force nanoscopy (Dofn), enabling nanometer-scale mapping of photothermal forces by exploiting unique phase responses to temporal modulation. We employ the photothermal effect’s back-action to distinguish various time frames within a modulation period. This allows us to capture the dynamic photothermal process of a single gold nanorod in the nanosecond range, providing insights into non-stationary thermal diffusion at the nanoscale. 

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3. Ultrafast temporal phase-resolved nonlinear optical spectroscopy in the molecular frame

   https://doi.org/10.1364/OPTICA.515959  

   Pandey, Siddhant; Tan, Liang_Z; Walz, Francis; Makhija, Varun; Shivaram, Niranjan (May 2024, Optica)

   In an ultrafast nonlinear optical interaction, the electric field of the emitted nonlinear signal provides direct access to the induced nonlinear transient polarization or transient currents and thus carries signatures of ultrafast dynamics in a medium. Measurement of the electric field of such signals offers sensitive observables to track ultrafast electron dynamics in various systems. In this work, we resolve the real-time phase of the electric field of a femtosecond third-order nonlinear optical signal in the molecular frame. The electric field emitted from impulsively pre-aligned gas-phase molecules at room temperature, in a degenerate four-wave mixing scheme, is measured using a spectral interferometry technique. The nonlinear signal is measured around a rotational revival to extract its molecular-frame angle dependence from pump-probe time-delay scans. By comparing these measurements for two linear molecules, carbon dioxide and nitrogen, we show that the measured second-order phase parameter (temporal chirp) of the signal is sensitive to the valence electronic symmetry of the molecules, whereas the amplitude of the signal does not show such sensitivity. We compare measurements to theoretical calculations of the chirp observable in the molecular frame. This work is an important step towards using electric field measurements in nonlinear optical spectroscopy to study ultrafast dynamics of electronically excited molecules in the molecular frame. 

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4. Photothermal mirror Z-scan spectrometry of opaque samples

   https://doi.org/10.1364/JOSAB.99.099999  

   Aristides Marcano Olaizola (September 2019, Journal of the Optical Society of America)

   Optica Publishing Group (Ed.)

   This contribution introduces a pump-probe photothermal mirror Z-scan method to measure the thermal quantum yield, the thermal diffusivity, and other photothermal parameters of an opaque solid. The focusing of a pump beam of light onto the sample generates thermoelastic surface distortions. The distorted surface acts as a mirror affecting the diffraction pattern of a reflected probe beam yielding the experimental signal. Scanning the focusing lens produces a single peak photothermal mirror Z-scan signature. The amplitude and time evolution of the signal determines the sample’s photothermal properties. The method is used to analyze gallium arsenide and silicon plates, obtaining good agreement with previous studies. 

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5. On-Chip Lock-In Detection for Ultrafast Spectroscopy of Single Particles

   https://doi.org/10.1021/acs.jpcc.4c01814  

   Adhikari, Subhasis; Gross, Niklas; Wilson, Kelly S; Verma, Ojasvi; Jia, Zhenyang; Landes, Christy F; Roberts, Sean T; Link, Stephan (May 2024, The Journal of Physical Chemistry C)

   Time-resolved spectroscopy of plasmonic nanoparticles is a vital technique for probing their ultrafast electron dynamics and subsequent acoustic and photothermal properties. Traditionally, these experiments are performed with spectrally broad probe beams on the ensemble level to achieve high signal amplitudes. However, the relaxation dynamics of plasmonic nanoparticles is highly dependent on their size, shape, and crystallinity. As such, the inherent heterogeneity of most nanoparticle samples can complicate efforts to build microscopic models for these dynamics solely on the basis of ensemble measurements. Although approaches for collecting time-resolved microscopy signals from individual nanoparticles at selected probe wavelengths have been demonstrated, acquiring time-resolved spectra from single objects remains challenging. Here, we demonstrate an alternate method that efficiently yields the time-resolved spectra of a single gold nanodisk in one measurement. By modulating the frequency-doubled output of a 96 MHz Ti:sapphire oscillator at 8 kHz, we are able to use a lock-in pixel-array camera to detect photoinduced changes in the transmission of a white light continuum probe derived from a photonic crystal fiber to produce broadband femtosecond transmission spectra of a single gold nanodisk. We also compare the performance of the lock-in camera for the same single nanoparticle to measurements with a single-element photodiode and find comparable sensitivities. The lock-in camera thus provides a major advantage due to its ability to multiplex spectral detection, which we utilize here to capture both the electronic dynamics and acoustic vibrations of a single gold nanodisk following ultrafast laser excitation. 

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