Dear Md Abu Taher , thanks for this interesting question. In principle LIPSS spatial period can be close to the laser wavelength for Low Spatial Frequency LIPSS (LSFL) or it could be smaller than the laser wavelength in High Spatial Frequency LIPSS (HSFL), where the period would be close to the laser wavelength divided by the refractive index of the material.

LIPSS is caused by the interference between the laser and the surface waves (be them surface plasmon polaritons or other sort of surface waves related to surface roughness or defects).

Hope this helps. Good luck with your research.

Let me add:

- Some people consider an additional surface morphology called "grooves" as LIPSS as they are periodic and laser-induced and exhibit periods larger than the wavelength. For a good review on supra-wavelength grooves, see

Article Generation of Supra-Wavelength Grooves in Femtosecond Laser ...

- Even for LSFL under specific experimental conditions (such as processing of galss samples at elevated temperatures) supra-wavelgth LIPSS were reported:

Article Femtosecond Laser-Induced Periodic Surface Structures on Fus...

- If you tilt the irradiated sample (non-normal incidence), you can easily obtain supra-wavelength LSFL, see

Article Maxwell Meets Marangoni—A Review of Theories on Laser‐Induce...

and the historic literature cited therein. But I guess you referred to normal incident radiation...

- For some hydrodynamic aspects see

Article Convection roll-driven generation of supra-wavelength period...

Hope it helps...

Jörn Bonse

PS: Let me clarify: HSFL are defined as having periods < lambda/2 at normal incidence and there are two different types of them. On dielectrics HSFL (type I) often have periods of lambda/(2n), with n being the refractive index; note that LSFL-type II on dielectrics often feature the mentioned lambda/n dependence. For an explanation and the cross-link between HSFL-I and LSFL-II on dielectrics see

Article Spontaneous periodic ordering on the surface and in the bulk...

Dear Manuel Gómez and Jörn Bonse Sir, thank you so much for your respective answers. While analyzing my experimental data, I have observed that the grating periods are very close to the laser wavelength. In a few cases, the grating periods are slightly higher than the laser wavelength (which may be relevant to the ' grooves' idea). Your ideas and recommendation are highly appreciable. I will go through the recommended articles.

Jörn Bonse Sir, I have read the recommended articles, and these are helpful and informative.

There are few doubts developed after reading these articles.

1. It may be possible that at high fluence and more number pulses, there could be the formation of supra-wavelength even in metallic surfaces without using an external CO2 continuous laser source. Is it correct, Sir?

2. If the convective theory can explain the supra-wavelength formation, is there any upper limit of spatial periodicity?

Thank you, Sir.

Dear Md Abu Taher,

regarding your questions:

ad (1): I assume that you refer to LSFL-I structures on metals being irradiated by linearly polarized laser radiation at normal incidence, right? I cannot exclude supra-wavelength LSFL-I although, currently, I am not aware of any publication (might exist, but I do not recall it); However, one must be careful in the analysis here since the laser-processing (pulses) itself can change the local angle of incidence at the irradiated surface for the following laser pulses: Then you may have the situation of intense linearly polarized radiation that hits the surface under an elevated (non-normal) angle of incidence (e.g. in the crater/groove edges) and then supra-wavelenth LSFL-I can easily manifest (see my first answer). Thus, just looking to a top-view SEM micrograph may not be a sufficient criterion.

ad (2): For me the weak part of the hyothesis of hydrodynamics being responsible for LIPSS comes from the fact that LIPSS are usually oriented perpendicularor parallel with regard to the linear laser beam polarization. This cannot be explained by hydrodynamics alone, there must be a coupling mechanism acting...

But if we assume that the initial spatial characteristics of the LIPSS is seeded by electromagnetic scattering/interference/resonances/absorption effects then - provided the laser-irradiated surface stays long enough in a molten state - certainly hydrodynamic effects may act and can affect the LIPSS pattern characteristics. Certainly the lifetime of the melt will limit the process. But I am not able to predict a universal upper limit for the supra-wavelength LIPSS spatial periods here from that, sorry.

Dear Jörn Bonse Sir,

Referring to your answers,

(1) Yes sir, I am referring LSFL-1. It's an excellent explanation. That's mean if someone analyzes the spatial periodicity of LIPSS deep inside the grooves or slant surface, it might not be the actual results.

(2) Thank you Sir for providing the basic and relevant concepts. I learned a lot.

You can find a report on the LIPSS with the lambda/25 created by THz Free Electron Laser on Si substrate in APL.

Article Laser-induced fine structures on silicon exposed to THz-FEL