Snow penitentes form in sublimation conditions by differential ablation. Here we investigate the physical processes at the initial stage of penitente growth and perform the linear stability analysis of a flat surface submitted to the solar heat flux. We show that these patterns do not simply result from the self-illumination of the surface—a scale-free process—but are primarily controlled by vapor diffusion and heat conduction. The wavelength at which snow penitentes emerge is derived and discussed. We found that it is controlled by aerodynamic mixing of vapor above the ice surface.

Physical processes causing the formation of penitentes

Black silicon with order-disordered structures for enhanced light trapping and photothermic conversion

https://iopscience.iop.org/article/10.1088/1361-6439/ab208b/pdf

Nanosecond pulsed laser ablation of silicon—finite element simulation and experimental validation

Multi-scale micro-nano structures prepared by laser cleaning assisted laser ablation for broadband ultralow reflectivity silicon surfaces in ambient air

Ultrafast laser-induced black silicon, from micro-nanostructuring, infrared absorption mechanism, to high performance detecting devices

A black silicon structure with high-aspect-ratio surface spikes was designed and fabricated in vacuum, resulting in absorptance >90% over the range of 200–2500 nm. It is demonstrated that annealing, an essential step in the fabrication of semiconductor devices, has almost no effect on the infrared absorption of this material, while the infrared absorption of an identical structure fabricated in a SF6 drops dramatically after the annealing process. The characteristic of high infrared absorption and annealing-insensitivity is attributed to both the high-aspect-ratio structure and the phosphor-doped low impedance silicon. These results are important for the fabrication of highly efficient optoelectronic devices.

Annealing-insensitive “black silicon” with high infrared absorption

The invention relates to a device and a method for testing a nonlinear polarization coefficient and an absorption coefficient at a terahertz band. Laser is split into one path of pump light and one path of probe light by a beam splitter, wherein the probe light is reflected by a reflector group with an adjustable light path, is attenuated by a metal attenuation piece used for adjusting the beam intensity of incident laser and then is reflected to a high resistance silicon chip by the reflector; the pump light split by the beam splitter is reflected to enter a terahertz generation device so as to generate terahertz collimating light and the generated terahertz collimating light is focused on a sample rack at a focus position by a parabolic mirror by virtue of a terahertz polarizing plate, and the terahertz is converted into parallel light which is reflected to the high resistance silicon chip by another parabolic mirror; and the two paths of light are coincided at the high resistance silicon chip, are focused to a detection crystal by the parabolic mirrors, are focused to reach a Wollaston prism by virtue of a 1/4 wave plate after being scattered by using a convex lens and are respectively focused to two photoelectric detectors. A test sample only needs to be arranged at the position of the sample rack or the detection crystal in a light path, namely the nonlinear polarization coefficient and the absorption coefficient of the sample at the terahertz band can be tested.

Device and method for testing nonlinear polarization coefficient and absorption coefficient at terahertz band

We experimentally found that there is an approximately linear relation between the average height/interval distance of spikes and the number of injected femtosecond laser pulses for the pulse number below a threshold. The growth rate of the spike height as a function of the irradiated pulse number is dependent on the gas pressure. When the pulse number exceeds the threshold, a hole can be observed on the silicon surface. Furthermore, the depth of the hole increases with the increase of pulse number. These results are important for optimizing the surface microstructure for silicon-based solar cell fabrication.

http://iopscience.iop.org/article/10.7567/APEX.6.051303/pdf

Influence of Femtosecond Laser Pulse Number on Spike Geometry of Microstructured Silicon

The invention relates to a method for achieving terahertz wave center frequency continuous adjustability through pulse laser widening. Initial pulse lasers are divided into base frequency light and frequency multiplication light. Time domain widening is carried out on the base frequency light by adopting a pulse widening method to obtain the base frequency light after the time domain widening. After the frequency multiplication light passes through a reflective mirror and a light beam time delay system, space beam combination is carried out on the frequency multiplication light and the base frequency light after the time domain widening, light of different wavelengths in the frequency light and the base frequency light interacts to generate terahertz waves of different frequencies, and the terahertz waves are received by a terahertz wave detection system. The relative time coincident point between the two pulses generated by the base frequency light and the frequency multiplication light can be adjusted freely through the light beam time delay system. In the practical operating process, the relative time coincident point between the two pulses can be changed simply by controlling the light beam time delay system in a frequency multiplication light path, and thus the central frequency of the terahertz waves can be adjusted. The method is suitable for ultrashort pulse lasers of various wavelengths.

Method for achieving terahertz wave center frequency continuous adjustability through pulse laser widening

The invention relates to a gas detection and identification sorting system based on terahertz waves. By virtue of a terahertz wave gas detection device, the terahertz waves and gas molecules have a resonance effect in a sealed cavity; electrons at the peripheries of gas atoms are stimulated to jump, so that the molecules rotate and vibrate to obtain a characteristic absorption peak; an absorption spectral line of the gas atoms/molecules is detected; a heuristic algorithm is used for accurately identifying a detected gas spectral line; the system adopts the sealed cavity, a vacuum pump, a pressure monitoring device and the heuristic algorithm, so that the terahertz waves and the gas in the cavity are interacted under different pressure conditions and the function of predicating unknown gas by further sorting is realized; and the gas detection and identification sorting system has the advantages of simple device, easiness in operation and wide application range.

Chen Xiangqian

Papers

Annealing-insensitive “black silicon” with high infrared absorption

Device and method for testing nonlinear polarization coefficient and absorption coefficient at terahertz band

Influence of Femtosecond Laser Pulse Number on Spike Geometry of Microstructured Silicon

Method for achieving terahertz wave center frequency continuous adjustability through pulse laser widening

Gas detection and identification sorting system based on terahertz waves