Ultrasharp nonlinear photothermal and photoacoustic resonances and holes beyond the spectral limit

Abstract

High-resolution nonlinear laser spectroscopy based on absorption saturation, Lamb-dip and spectral hole-burning phenomena have contributed much to basic and applied photonics. Here, a laser spectroscopy based on nonlinear photothermal and photoacoustic phenomena is presented. It shows ultrasharp resonances and dips up to a few nanometres wide in broad plasmonic spectra of nanoparticles. It also demonstrates narrowing of absorption spectra of dyes and chromophores, as well as an increase in the sensitivity and resolution of the spectral hole-burning technique. This approach can permit the study of laser-nanoparticle interactions at a level of resolution beyond the spectral limits, identification of weakly absorbing spectral holes, spectral optimization of photothermal nanotherapy, measurements of tiny red and blue plasmon resonance shifts, multispectral imaging and multicolour cytometry.

Figure 1. Phenomenologic model of nonlinear PA/PT spectroscopy

(a) General diagram. (b) Typical signal amplitude as a function of laser energy. (c) Ultrasharp PA/PT resonances and dips in a homogenous absorption profile. (d) PA guidance of spectral-hole burning in a mixture of gold nanoparticles with overlapping spectra. (e) Double PA/PT resonances with energy-dependent red and blue shifts and a central broad PA/PT hole.

Fig. 2. Nonlinear spectra of gold nanorods

(a) PT resonance in 10–25-nm gold nanorods at 550 nm. (b) PT dips obtained with the pump–probe technique in 10–35-nm gold nanorods with a plasmon resonance at 650 nm. (c) Nonlinear resonances obtained with a one-beam schematic in 15–45-nm gold nanorods with plasmon resonances at 860 nm. (d) Nonlinear PT spectra of a mixture of 30-nm gold nanospheres and six gold nanorods with resonances at 525 , 585, 650, 700, 753, 870, and 1064 nm. (e) PT monitoring of spectral hole-burning in a mixture of six gold nanorods with plasmon resonances at 585, 650, 717, 775, 808, and 870 nm at energy laser fluence of 0.5 J/cm². Black line shows narrowing of the spectral hole at 2.5 J/cm². PA/PT signal amplitudes were normalized to the maximum linear absorption spectrum. The error bars represent the standard error of the mean.

Figure 3. Ultrasharp resonances in gold-based nanoparticles and quantum dots

(a) PA resonance (dashed red line) of 11.8–98-nm golden carbon nanotubes, normalized on the optical absorption spectra (solid black curve) at 800 nm. (b) Absorption spectra, emission spectra, and PA resonances of quantum dot-folate conjugates (0.5- hr incubation at 22°C) in MB-MDA-231 breast cancer cells, normalized on the maximum absorption of conventional spectra at the wavelength of 560 nm. (c) Spectra of 160-nm gold nanoshells. The error bars represent the standard error of the mean.

Figure 4. Sharpening of PT spectra of cells

(a) PT spectra of mitochondrion marked by a dashed red line in the PT image of cancer cell (the inset). (b) PT spectral identification of rare melanoma cells in mouse blood. The inset shows a single melanoma cell in blood. (c) PT and absorption spectra of S. aureus (the inset). Arrows indicate the positions of the maximum spectral absorption of carotenoids. The error bars represent the standard error of the mean.

Figure 5. PA and conventional absorption spectra of dyes

(a) Absorption spectra of an 11.5-μL solution of indocyanine green and its PA spectra at different laser fluences. PT spectra were normalized on absorption spectra. (b) Absorption spectra of FITC alone (green). Absorption (blue) and nonlinear PA (red) spectra of FITC-labeled mouse erythrocytes. The error bars represent the standard error of the mean.