LAUSR

The letter by Liao and Wynne (1) criticizes the established knowledge in the field of optical trapping prior to our work. In this reply, we highlight some key literature along with the experiments we performed to reconfirm the preestablished facts. We estimated the temperature increase as 20 mK (2) based on Ito et al. (3). Mao et al. (4) presented a model to calculate the temperature increase by laser and experimentally verified it. Their model (ref. 4, equation 4a) predicts a 70-mK increase, at most, in our experiments. Two different methods estimate a similarly small temperature increase. The letter by Liao and Wynne (1) lacks a description of key parameters used in their “back-of-the-envelope calculation.” It is hard to comment whether they simply made an error in calculation or the model may not be suited for this case (e.g., dimensionality of the heat equation) to obtain a 163-K increase. Liao and Wynne used Raman thermometry to support their claim and estimated an even higher temperature increase (400 K). Neither raw Raman spectra nor the wavenumber used for calculating temperature are shown, which makes it hard to point out what they did wrong. Raman thermometry requires careful measurement and analysis (5). Also, as a basic principle, one should avoid extrapolating data for 1.2 W using data from 10 to 50 mW. Fig. 1 shows Raman spectra of SnCl2 aqueous solution measured on our setup. The laser power was varied up to Fig. 1. (A) Low-frequency Raman spectra measured on the setup described in Urquidi et al. (2). The detection part was modified using SureBlock XLF Notch filters (Coherent) to be able to measure the low-frequency region of Raman spectrum. The sample is SnCl2 dissolved in water (1 mol L ). This sample is suited for this purpose because it has well-defined peaks at the region where we can measure both Stokes and anti-Stokes Raman spectrum. No change of the spectra was observed from 150 mW up to 1.2 W. If the temperature were elevated by the 532-nm laser, the relative peak intensity of Stokes Raman (IS) to anti-Stokes Raman (IAS) would have decreased. (B) The expected ratio of IS to IAS at 111 cm 1 peak as a function of temperature calculated using the model in ref. 5. As expected, the IS=IAS decreases as temperature increases. (C) The IS=IAS calculated from the data shown in A. The ratio remains constant around room temperature even at 1.2-W laser power.