The SEM image of these nanorods shown in Figure indicates the dimensions. The experimental UVvis spectra of these nanorods was compared with a Au nanorod of aspect ratio, A = 3 and diameter, D = 50 nm to show good agreement between the two. Since the UV-vis spectra of the nanorod colloid is a sum of the optical cross-sections of a large number of nanorods randomly oriented in the solution, the numerical model is an average of three orientation angles, 0 o , 45 o and 90 o . 3.1 Absorption cross-section and steady state temperatures. The absorption cross-section is the equivalent surface area that covers the total amount of EM wave absorbed by the nanoparticle. In Figure (a), the absorption cross-sections for sphere diameter from 40 nm to 180 nm in the given wavelength range is shown. The LSPR wavelengths for all the diameters lie in the range 500-550 nm. As the nanoparticle volume increases, naturally, the amount of absorbed light increases. Thus, the overall increase in the absorption cross-section is observed. In Figure (c), the corresponding steady state temperatures of unconfined nanoparticles under laser intensity of 1 mW/µm 2 are also shown in the plot below. The temperature plots have the maximum at the LSPR wavelength as expected. However, these temperatures do not follow an increasing trend with the increasing diameter, although the absorption cross-section increases with diameter. This is due to the fact that the steady state temperature depends on both energy generated per unit volume and the surface area to volume ratio. As the surface area to volume ratio decreases with the increasing diameter, the steady state temperature will naturally increase if the energy generated per unit volume is constant. However, while going from 80 to 100 or 100 to 120, although the surface area to volume ratio decreases, the drop in the energy generated per unit volume is also significant. As a result, the steady state temperature drops. The highest temperature rise of 68 o C above the initial temperature 25 o C was achieved for the 180 nm nanosphere. It is also important to note that the steady state temperature drops rapidly beyond the LSPR wavelength. Thus, one should apply strictly monochromatic laser with the correct wavelength for the photothermal application of these nanoparticles to be effective. It is important to note that for the nanosphere diameter 100 nm, the temperature rise is approximately 55 o C which agrees well with the previous work by Baffou et al. . Honda et al. reported an experimentally measured a steady state temperature rise of ~ 10 o C for pNIPAM-coated Aunanospheres (D = 40 nm, coating thickness = 20 nm) in water under ~ 1 mW continuous wave laser (532 nm), which is comparable to the present predictions for idealized conditions . In Figure (b), the absorption cross-sections for increasing aspect ratios of Au nanorods is shown for the case where the angle of orientation of the nanorod with respect to the incident laser is θ = 0 o . Apparently, there is a red-shift of the LSPR wavelength from visible to infrared as well as an increase in the absorption cross-section as the aspect ratio i.e. the length of the rod increases for constant diameter, 50 nm. In the corresponding steady state temperature plots for a laser intensity 0.5 mW/µm 2 in Figure (d), the increase of the steady state temperature with aspect ratio is apparent. The reason for considering a lower intensity of 0.5 mW/µm 2 is to avoid vapour bubble formation which will render the initial assumptions invalid. It is interesting that the temperature rise for A = 5 by infrared laser is 81, while the same for A = 1 i.e. nanosphere by visible laser is only 18 when the diameter is kept constant. Importantly, the volume of the 180 nm diameter sphere is 3.05 x 10 -3 µm 2 which is much greater than the volume 0.46 x 10 -3 μm 2 of the nanorod of D = 50 nm and A = 5, but the absorption cross-section for the nanorod is significantly higher than the sphere. Thus, the use of nanorods of high aspect ratio provides much higher temperature rise as compared to nanospheres for the same intensity of light. Although nanorods have high absorption in the infrared region, the absorption characteristics are very much dependent on the orientation of the rod with respect to the incident laser. Thus, the effect of orientation naturally becomes an important aspect to study. In Figure The steady state temperatures evidently decrease as the nanorod is oriented from θ = 0 o to 90 o . Also for the same θ, the effect of ϕ is not significant. One important observation here is that the temperatures for A = 2 and A = 3 overlap. Also in Figure (b), the temperatures of A = 2 and 3 at corresponding LSPR wavelengths are very close. The reason for this is similar to that for the spheres. Although the surface area to volume ratio decreases with A increasing from 2 to 3, the heat generation per unit volume also drops and the overall effect is that the steady state temperatures are similar. For A = 1, orientation does not hold any significance. From this analysis, it is clear that the orientation of the nanorods with respect to the incident laser is an important aspect to consider. For many applications, the tolerance for the orientation angle θ may be up to 30 o , while ϕ does not have any significant effect.