This paper reports a detection method of two-dimensional (2D) enhancement and three-dimensional (3D) reconstruction for subtle traces with reflectance transformation imaging, which can effectively locate the trace area of interest and extract the normal data of this area directly. In millimeter- and micron-scale traces, during 3D construction, we presented a method of data screening, conversion, and amplification, which can successfully suppress noise, improve surface and edge quality, and enhance 3D effect and contrast. The method not only captures 2D and 3D morphologies of traces clearly but also obtains the sizes of these traces.

Natural logarithm wavelength modulation spectroscopy (ln-WMS) is demonstrated in this Letter. Unlike the conventional wavelength modulation spectroscopy (WMS)-2f technique, it is a linear method even for large absorbance, which is the core advantage of ln-WMS. The treating method used in ln-WMS is to take the natural logarithm of the transmitted intensity. In order to determine the proper demodulation phase, the η-seeking algorithm is introduced, which minimizes the absolute value of the first harmonic within the non-absorbing region. Subsequently, the second harmonic of the absorption signal is extracted by setting the demodulating phase as 2η. To illustrate the validity of ln-WMS, it was applied to water vapor experimentally. The result shows that even if the absorbance (base-e) is between 1.60 and 6.26, the linearity between ln-WMS-2f and volume fraction is still established. For comparison, measurement with conventional WMS-2f was also done, whose response no longer kept linearity. The η values retrieved in continuous measurements and the residuals were shown so as to evaluate the performance of the η-seeking algorithm. Time consumed by this algorithm was roughly 0.28 s per measurement. As an alternative WMS strategy, ln-WMS has a wide range of potential applications, especially where the absorbance is large or varies over a wide area.

Two-dimensional (2D) Te nanosheets were successfully fabricated through the liquid-phase exfoliation (LPE) method. The nonlinear optical properties of 2D Te nanosheets were studied by the open-aperture Z-scan technique. Furthermore, the continuous wave mode-locked Nd:YVO₄ laser was successfully realized by using 2D Te as a saturable absorber (SA) for the first time, to the best of our knowledge. Ultrashort pulses as short as 5.8 ps were obtained at 1064.3 nm with an output power of 851 mW. This primary investigation indicates that the 2D Te SA is a promising photonic device in the fields of ultrafast solid-state lasers.