Photo-Thermal Spectroscopy with Plasmonic and Rare-Earth Doped (Nano)Materials

Study of matter and its properties dates back to ancient times. Study of interaction of light with the matter also has been the subject of many scientific discoveries over the past centuries. Because of the technological advances of the 20th century, the discovery of the properties of matter at the nano and microscale became possible. With new tools and resources in hand and benefiting from some of the fundamental theories of light-matter interactions already developed, the experimental and theoretical study of light and noble metal nanoparticles is now an attractive area of research.     

In this chapter we use various theories and equations to describe the photothermal properties of different plasmonic structures when being excited by an incident electromagnetic wave. These equations are based on Maxwell equations and the heat transfer equations. Although the solution to these equations are very challenging or impossible to obtain for most structures, this chapter provides an efficient first approximation that is suitable for many recurrent systems with various applications.

In this chapter, we present the relationship between the superheated liquid and bubble formation for a single optically excited nanostructure at the gold water interface using AlGaN:Er³⁺ nanothermometry, and contrast these properties with vapor formation in a colloidal solution of optically excited gold nanoparticles. Bubble formation for a single nanostructure at a solid water interface is an inherently a low probability event with superheating of the liquid to the spinodal decomposition temperature. This chapter is reprinted (adapted) with permission from ACS Nano 2014, 8 (2), 1439–1448. Copyright 2014 American Chemical Society.

Nanoscale temperature change and single particle absorption, scattering and emission measurements revealed presence of small concentration of ions in surrounding water. The presence of these ions attenuates the plasmon absorption and scattering properties of a nanostructure. This attenuation effect is found dependent upon the ionic strength of the solution and the attenuation is related to the amount of coverage of screen charge at the surface. In this chapter, we present the effects of changes on surrounding medium (solution) properties (by addition of small amount of ionic solute molecules) on fundamental light absorption and scattering properties of a plasmonic gold nanostructure(s). This chapter is reprinted (adapted) with permission from ACS Nano 2016, 10 (6), 6080–6089. Copyright 2016 American Chemical Society.

A new optical probe technique using a laser-trapped erbium oxide nanoparticle (~150 nm) can measure absolute temperature with a spatial resolution on the size of the nanoparticle. This technique (scanning optical probe thermometry) is used to collect the thermal image of an optically excited gold nanostructures. The thermal profile has a Gaussian line shape that is a convolution of the point spread function of the scanning optical probe thermometer and the true thermal profile. A convolution analysis reveals that the point spread function of our measurement is a Gaussian with a FWHM of 165 nm. We attribute the width of this function to clustering of Er₂O₃ nanoparticles in solution. Also, the scanning optical probe thermometer is used to measure the temperature where vapor nucleation occurs. Subsequently, the temperature inside the vapor bubble rises to the melting point of the gold nanostructure (~1300) where a temperature plateau is observed. The rise in temperature is attributed to inhibition of thermal transfer to the surrounding liquid by the thermal insulating vapor cocoon. This chapter is reprinted (adapted) with permission from Applied Physics A. (2016) 122: 340. Copyright 2016 Springer.

In this chapter, we study the potential of β-NaYF4:Yb³⁺,Er³⁺ nanocrystals and decorated β-NaYF4:Yb³⁺,Er³⁺ nanocrystals for temperature measurement at the nanoscale. We measure the temperature dependence in the temporal response of the green emission for both the H band (²H_11/2 → ⁴I_15/2 transition) and the S band (⁴S_3/2 → ⁴I_15/2 transition) for β-NaYF4:Yb³⁺,Er³⁺ nanocrystals and β-NaYF4:Yb³⁺,Er³⁺ nanocrystals decorated with 10 nm gold nanoparticles and found that the emission is quenched with temperature. Time-resolved measurements showed that the decay lifetime of UCNP/GNPs is bi-exponential with a dominant decay time an order of magnitude faster than the longer decay time around 300 μs. The UCNPs have a single exponential decay with a long decay time of ~175 μs. We measure the steady-state emission from the H and S band for UCNPs and UCNP/GNPs for temperatures between 300 and 450 K and obtain a linear relationship between the calculated and measured temperatures showing that quenching of the H and S bands does not affect the ability of UCNP/GNPs and UCNPs to be used as thermal sensors. This chapter is reprinted (adapted) with permission from ACS Photonics, 2017, 4(7), pp 1864–1869. Copyright 2017 American Chemical Society.

In this chapter we present a new optical temperature measurement and thermal imaging technique combining near-field microscopy and Er³⁺ photoluminescence spectroscopy. The technique is used with two different approaches towards local temperature measurement and thermal imaging. In the first approach, gold nanostructures on top of Al_0.94 Ga_0.06 N thin film embedded with Er³⁺ ions are optically excited through the SNOM tip with 532 nm CW laser to generate thermal images with the spatial resolution comparable to the true size of the nanostructures. In the second approach, nanostructures on top of thermal sensor film are excited with 532 nm CW laser through the substrate with a large spot size (FWHM ~ 10 µm). The Er³⁺ emission from the film is collected in transmission mode through the SNOM tip. In this chapter the steady state temperature change under optical illumination is measured for different sized clusters made from 40 nm diameter gold nanoparticles and it is found that the maximum temperature change and temperature decay length (r_1/2) into the surrounding medium increases linearly with cluster radius. Based upon this observation, we can conclude that if a large cell is embedded with optical heaters with a distance between heaters that allows for collective heating, then the thermal profile outside of the cell decays with a decay constant that is dependent upon the size of the cell. This chapter is reprinted (adapted) with permission from Nanoscale, 2017, 4(7), pp 1864–1869. Copyright 2017 Royal Society of Chemistry.