arXiv subjects

Xiaoji G. Xu

Publications and source records attributed to Xiaoji G. Xu.

4 recordsLinked to original sources

arXiv2026

Photo-thermal 2D spectroscopy: a different type of action

Advances in multidimensional spectroscopy have seen the rise of action detection, where coherent response is encoded into incoherent signals. These are typically proportional to the excited-state population, an example in 2D electronic spectroscopy (2DES) is fluorescence-detected 2DES (F-2DES), and in 2D infrared (2DIR) tag-loss 2DIR (TL-2DIR). Very recently, a new type of photo-thermal action signal has been introduced in 2DIR, detecting the generated heat by atomic force microscopy-based 2DIR (AFM-2DIR). We present a unified theoretical framework for population- and heat-based 2D spectroscopy, highlighting their complementary features. In the infrared, TL-2DIR reflects the system linear response, whereas AFM-2DIR produces spectra resembling conventional 2DIR, with sensitivity to anharmonicity and mode coupling. Our model reproduces key experimental AFM-2DIR features, confirming measurement in a highly nonlinear regime. Extending photothermal detection to electronic spectroscopy, we compare F-2DES with proposed photo-thermal 2DES (PT-2DES). PT-2DES closely resembles conventional 2DES, while enjoying the advantages of action detection.

physics.chem-ph↗

arXiv2020

Probing Mid-Infrared Phonon Polaritons in the Aqueous Phase

Phonon polaritons (PhPs), the collective phonon oscillations with hybridized electromagnetic fields, concentrate optical fields in the mid-infrared frequency range that matches the vibrational modes of molecules. The utilization of PhPs holds the promise for chemical sensing tools and polariton-enhanced nanospectroscopy. However, investigations and innovations on PhPs in the aqueous phase remains stagnant, because of the lack of in situ mid-infrared nano-imaging methods in water. Strong infrared absorption from water prohibits optical delivery and detection in the mid-infrared for scattering-type near-field microscopy. Here, we present our solution: the detection of photothermal responses caused by the excitation of PhPs by liquid phase peak force infrared (LiPFIR) microscopy. Characteristic interference fringes of PhPs in 10B isotope-enriched h-BN were measured in the aqueous phase and their dispersion relationship extracted. LiPFIR enables the measurement of mid-infrared PhPs in the fluid phase, opening possibilities, and facilitating the development of mid-IR phonon polaritonics in water.

physics.optics↗

arXiv2019

Three-dimensional Near-field Analysis Through Peak Force Scattering-type Near-field Optical Microscopy

Scattering-type scanning near-field optical microscopy (s-SNOM) is instrumental in exploring polaritonic behaviors of two-dimensional (2D) materials at the nanoscale. A sharp s-SNOM tip couples momenta into 2D materials through phase matching to excite phonon polaritons, which manifest as nanoscale interference fringes in raster images. However, s-SNOM lacks the ability to detect the progression of near-field property along the perpendicular axis to the surface. Here, we perform near-field analysis of a micro-disk and a reflective edge made of isotopically pure hexagonal boron nitride (h-11BN), by using three-dimensional near-field response cubes obtained by peak force scattering-type near-field optical microscopy (PF-SNOM). Momentum quantization of polaritons from the confinement of the circular structure is revealed in situ. Moreover, tip-sample distance is found to be capable of fine-tuning the momentum of polaritons and modifying the superposition of quantized polaritonic modes. The PF-SNOM-based three-dimensional near-field analysis provides detailed characterization capability with a high spatial resolution to fully map three-dimensional near-fields of nano-photonics and polaritonic structures.

physics.optics↗

arXiv2008

Narrowband spectroscopy by all-optical correlation of broadband pulses

High peak power ultrafast lasers are widely used in nonlinear spectroscopy but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multi-photon interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach, in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into the traditional spectroscopic setup, enables direct, robust and simultaneous detection of multiple narrow resonances with a single femtosecond pulse.

physics.optics↗