Sub-wavelength temperature probing in near-field laser heating by particles.

Department of Mechanical Engineering, Iowa State University, 2010 Black Engr. Bldg., Ames, Iowa 50011, USA.
Optics Express (Impact Factor: 3.55). 06/2012; 20(13):14152-67. DOI:10.1364/OE.20.014152
Source: PubMed

ABSTRACT This work reports on the first time experimental investigation of temperature field inside silicon substrates under particle-induced near-field focusing at a sub-wavelength resolution. The noncontact Raman thermometry technique employing both Raman shift and full width at half maximum (FWHM) methods is employed to investigate the temperature rise in silicon beneath silica particles. Silica particles of three diameters (400, 800 and 1210 nm), each under four laser energy fluxes of 2.5 × 10(8), 3.8 ×10(8), 6.9 ×10(8) and 8.6 ×10(8) W/m(2), are used to investigate the effects of particle size and laser energy flux. The experimental results indicate that as the particle size or the laser energy flux increases, the temperature rise inside the substrate goes higher. Maximum temperature rises of 55.8 K (based on Raman FWHM method) and 29.3K (based on Raman shift method) are observed inside the silicon under particles of 1210 nm diameter with an incident laser of 8.6 × 10(8) W/m(2). The difference is largely due to the stress inside the silicon caused by the laser heating. To explore the mechanism of heating at the sub-wavelength scale, high-fidelity simulations are conducted on the enhanced electric and temperature fields. Modeling results agree with experiment qualitatively, and discussions are provided about the reasons for their discrepancy.

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    ABSTRACT: Microparticle and microfiber induced near-field laser heating has been widely used in surface nanostructuring. Information about the temperature and stress fields in the nanoscale near-field heating region is imperative for process control and optimization. Probing of this nanoscale temperature, stress, and optical fields remains a great challenge since the heating area is very small (~100 nm or less) and not immediately accessible for sensing. In this work, thermal probing of a single microparticle and microfiber induced near-field focusing on a substrate with laser light is conducted experimentally and interpreted by high-fidelity simulations. The laser (λ = 532 nm) serves as both heating and Raman probing sources. It is very interesting to note that variation of the Raman intensity, wavenumber, and linewidth all can be used to precisely capture the size of the micro-size subject on the substrate. Nanoscale mapping of conjugated optical, thermal, and stress effects, and the de-conjugation of these effects are performed. The effect of the laser fluence on the temperature and stress in the nanoscale heating region is investigated. With laser fluence of 3.9 ×10<sup>9</sup> W/m<sup>2</sup> and for a 1.21 μm silica particle induced laser heating, the maximum temperature rise and local stress are 58.5 K and 160 MPa, respectively. For a 6.24 μm glass fiber, they are 33.0 K and 120 MPa, respectively. Experimental results are explained and consistent with three-dimensional high-fidelity optical, thermal and stress field simulation.
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    ABSTRACT: Micro/nanoparticle induced near-field laser ultra-focusing and heating has been widely used in laser-assisted nanopatterning and nanolithography to pattern nanoscale features on a large-area substrate. Knowledge of the temperature and stress in the nanoscale near-field heating region is critical for process control and optimization. At present, probing of the nanoscale temperature, stress, and optical fields remains a great challenge since the heating area is very small (∼100 nm or less) and not immediately accessible for sensing. In this work, we report the first experimental study on nanoscale mapping of particle-induced thermal, stress, and optical fields by using a single laser for both near-field excitation and Raman probing. The mapping results based on Raman intensity variation, wavenumber shift, and linewidth broadening all give consistent conjugated thermal, stress, and near-field focusing effects at a 20 nm resolution (<λ/26, λ = 32 nm). Nanoscale mapping of near-field effects of particles from 1210 down to 160 nm demonstrates the strong capacity of such a technique. By developing a new strategy for physical analysis, we have de-conjugated the effects of temperature, stress, and near-field focusing from the Raman mapping. The temperature rise and stress in the nanoscale heating region is evaluated at different energy levels. High-fidelity electromagnetic and temperature field simulation is conducted to accurately interpret the experimental results.
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Apr 8, 2013