I know as least for nanosecond, or even picosecond, laser in which there is enough time dumping out energy that will be absorbed by Si. However, as for femtosecond laser, the times seems to be too short for Si to absorb making electrons jumping into higher energy band level. I have seen some papers in which femtosecond laser is used to treat Si material surface. I fail to understand why it works and what is the detailed explanation towards scenario.
Thanks in advance
Chenjin
It depends on laser fluence. For example, for laser fluence more than 60 mJ per square centimeter some phase trandformations can take place - crystallization e.t.c. See for example - V. A. Volodin, M. D. Efremov, G. A. Kachurin, A. G. Cherkov, M. Deutschmann and N. Baersch. Phase Transitions in a-Si:H Films on a Glass Irradiated by High-Power Femtosecond Pulses: Manifestation of Nonlinear and Nonthermal Effects. JETP Letters, 2007, Vol. 86, No. 2, pp. 119–122.
Dear Chenjin, the femtosecond lasers enables non linear interactions with the materials on which interact, like the multiphoton absorption, since the electron recombination from the conduction band to the valence band or the thermal relaxing from higher levels of the conduction band to the bottom of the latter are phenomena that happen in picoseconds. Regarding the absorption, instead, under certain conditions well described in the previous answer to your question, there are no problems using femtosecond pulses.
Hi Alessandro,
Thanks for you answer. There is one question that I fail to understand is why could not the thermal relaxing or recombination takes place after the end of femtosecond laser pulse. Then there is no difference compared to nanosecond laser pulse. When I go over papers, it seems to me that thermal relaxing happens only during pulse duration. Is that due to the phonton interaction with electrons and atom that results in the electron from higher conduction band to lower erngy level?
Dear Chenjin, what you say is true if you send a single pulse on the material, therefore the excited electron has the time to relax or recombine. Nevertheless, even in that case you can see different interaction respect to a picosecond pulse, e.g. thermal effects. But if you send a train of pulses the things change because the electrons don't have the time to relax or recombine, obviously depending on the repetition rate of the pulses.
I'm afraid that in physics "immediately" has not much sense. At temperatures higher than 0 K the electrons thermal relaxation is dominated by electrons-phonons collisions that have a timing of tens of fs. Since many of these are required for the electrons Energy to be effectively exchanged with phonons some picoseconds are required for the excited electrons to relax at the bottom of the conduction band (https://books.google.it/books?id=8WvWV7237UAC&pg=PA255&lpg=PA255&dq=thermal+relaxation+electrons+time+picoseconds&source=bl&ots=pn-qZWMgwn&sig=tSe0XSSiKeBiBrb3AuV6qzzjspg&hl=it&sa=X&ved=0ahUKEwjnqv_jhrzOAhUBchQKHTfmC6oQ6AEIKDAC)
Let me add another - more "helicopter view" - aspect:
If energy hits a target, 3 things may happen:
- tramsmittance
- reflection
- absorption
The sum of all three is the power that hits. Transmittance should not affect the target, as well as reflection.
So the critical part is the absorption one: whatever energy is absorbed will alter the target if it is not conveyed some way: by (stimulated) emission (I'd include the 'generation' of electrical current in PV Systems here) or heat transfer. Any absorption not compensated by these 2 mechanisms will heat the target locally where the beam hits.
Regarding the apparent difference between fs and ns pulses:
you have to look at the pulse energy, the pulse duration and the repetition rate: pulse energy divided by pulse duration gives some 'intensity' figure while repetition rate multiplied by pulse energy give the longer-term energy delivered.
Regarding the short-term effect of absorbed energy on the target, heat Transfer is very important: it takes time.
I can easily imagine that the intensity of fs pulses in General is so not much lower than that of ns pulses. Thus the intensity of ns pulses will be lower, eventually not exceeding what the target can cope with. While the intensity of a fs pulse may easily exceed the target's capabilities - leading to vaporization. Even if the repetition rates may be low, each pulse is 'doing harm'.
"I have seen some papers in which femtosecond laser is used to treat Si material surface." -- With respect to material machining (e.g., creating pits in the surface, or for laser-fired contacts of solar cells), it makes a difference with respect to the "cleanliness" whether a picosecond or a femtosecond laser is used: The picosecond laser does not lead to complete evaporation of the ablated material, therefore unwanted droplets, originating from molten silicon, will form somewhere on the surface. The femtosecond laser, however, transfers the energy so fast that all material is evaporated, so the surface stays clean.
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