Optothermal response of a single silicon nanotip

Abstract

Its surprising efficiency could be explained by two distinct 83 scenarios associated with different time scales and physical 84 mechanisms, i.e., thermal effects and static field-induced 85 modification of nanotip properties. In the former scenario, 86 laser energy absorption generates hot carriers all along the 87 tip, whose relaxation and diffusion, as well as the diffusion 88 of the generated heat, lead to a delayed heating of the tip apex, 89 allowing evaporation to remain efficient on nanosecond time 90 scales [37]. In the latter, the application of an intense static 91 electric field leads to the accumulation of a high density of 92 positive charges at the apex, which considerably enhances its 93 optical absorption, inducing ultrafast heating followed by its 94 cooling on a picosecond time scale [11]. 95 A combination of these two experimental techniques on 96 the same type of nano-objects permits us to elucidate the 97 contribution of the induced thermal effects to the complex 98 laser assisted field evaporation process. In particular, this work 99 demonstrates that the nonuniform optical absorption along 100 the conical nanotip and the subsequent heat diffusion are at 101 the origin of the different measured kinetics of evaporation 102 at nanosecond scale. These can be quantitatively reproduced 103 using a complete model including the optical, thermal, and 104 electronic properties of the nanotip, clarifying the dependence 105 of the evaporation mechanism on illumination conditions and 106 nanotip shape. 107 II. EXPERIMENTAL RESULTS AND DISCUSSION 108 Silicon nanotips were prepared by focused ion beam (FIB) 109 annular milling from micrometer silicon posts (more details on 110 the sample preparation procedure can be found in Ref. [38]). 111 Their geometrical parameters (conical shape with apex radius 112 R and semi-angle β) were deduced from electron microscopy 113 observations. 114 A. Optical investigations on a single nanotip 115 The optical response of individual silicon nanotips was 116 quantitatively investigated using SMS, a technique relying on 117 the modulation of the position of a single nano-object inside 118 the focal spot of a tightly focused light beam, which results 119 in periodical variations of the transmitted (or reflected) light 120 power [20,29]. A tunable Ti:sapphire oscillator combined with 121 a visible optical parametric oscillator was used as the light 122 source, allowing nanotip optical properties to be investigated 123 in a broad spectral range (500-1000 nm wavelengths). Spatial 124 modulation was performed orthogonally to the tip axis at 125 f = 1.5 kHz frequency and lock-in detection was performed 126 at 2f. SMS optical transmission images with light polarization 127 parallel to the nanotip axis were acquired by scanning the 128 sample relatively to the light beam focused down to diffraction 129 limit (about 0.7λ full-width at half-maximum) by a 100× 130 microscope objective. 131 SMS optical cartographies of a single conical silicon nan-132 otip (R = 40 nm apex radius and β ...

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