Polarization-Sensitive Nonlinear Optical Interaction of Ultrashort Laser Pulses with HPHT Diamond

Round 1

Reviewer 1 Report

This paper reports experimental results on the formation of luminous channels inside the bulk of (synthetic) diamond, irradiated by intense 300fs laser pulses with visible and near-infrared wavelengths. The laser polarization direction was varied relative to the (110) crystal lattice plane. 

The reported  pulse energy threshold for the formation of luminous channel inside the bulk is 400 nJ.  The channel length was measured as a function of the crystal azimuth angle (the angle between the polarization direction and the normal to a (110) plane) and the pulse energy.  For fixed pulse energy, the luminous channel length exhibits a modulation with the crystal azimuth angle, the modulation of the channel length decreases with the increase of the pulse energy. The dependence of the integral photo-luminescence  intensity on the laser pulse energy is also exhibited. Two regimes of non-linear laser-matter interaction were distinguished.  For laser pulse energy below 600 nJ, the luminescence signal exhibits sensitive laser-polarization dependence. Аbove this threshold, no dependence on the laser polarization direction was found.

The authors also report the dependence of the threshold power for filamentation as a function of the azimuth angle. The dependence of the filament channel length on the threshold power is found to be consistent with a semi-empirical Marburger formula. 

To explain the observed luminescence of the photo-excited diamond crystal, the authors assume that the signal is due to radiative re-commbination of photoexcited electron-hole pairs. They propose a theoretical scheme based on simplified rate equations, including multi-photon ionization, avalanche-ionization, Auger re-combination process. For the 1030nm laser wavelength, the model tends to reproduce the laser energy fluence scaling law found experimentally for zero azimuth angle.

In the discussion, it remains unclear to me how does the anisotropy of diamond's static band structure  affects the laser-intensity scaling law in the low-intensity regime (pulse energy below 600 nJ). For the 1030 nm wavelength, the minimum number of photons to cross the (indirect) bandgap is m=5, and 'm'  - the order of the multiphoton process  increases in any other direction in the Brillouin zone, thus the intensity slope should increase above 3.3.   The authors should comment or clarify the discussion on polarization-sensitive multi-photon ionization. For instance it may be helpful to include a plot or sketch the static band structure of diamond along specific directions in the bulk Brillouin zone.

I suggest to authors to include a reference (and comparison if possible)  to a paper of Kozak et al. PRB 99, 104305 (2019) on 'anisotropy and polarization dependence  of multi-photon ionization in diamond'.

  Best regards.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Review comments

Manuscript ID: photonics-2122179

 

In this manuscript, the authors describe a detailed study of the nonlinear optical interaction of ultrashort laser pulses with HPHT diamond. The dependence of the ultrashort laser filamentation process on the polarization azimuth angle was investigated at different intensities. The manuscript is well-organized. I recommend acceptance of the work after some minor modifications.

 

(1) The filamentation of ultrashort laser pulses involves many different physical processes. The authors have well analyzed the polarization-dependent regimes at relatively low intensities and found the polarization-independent phenomenon at high intensities. However, the physical mechanism of this transformation is still not very clear. The authors are suggested to discuss this in more detail.

 

(2) In Fig. 2b, the authors declare to use the green dashed line determined intensity level as the luminous channel level. What does this dash line mean? This process is suggested to be explained, and what's the reason for not using others, for example, FWHM?

 

(3) The font size of the insert figure legend in Fig. 1b is small and hard to read.

 

(4) In the part of "Introduction" talking about the recent development of femtosecond laser filaments and nonlinear optical effects, some recently reported references should be cited:

[1] Two-photon absorption and stimulated emission in poly-crystalline Zinc Selenide with femtosecond laser excitation. Opto-Electron Adv 5, 210036 (2022).

[2] Functional nonlinear optical nanoparticles synthesized by laser ablation. Opto-Electron Sci 1, 210007 (2022).

 

 

Author Response

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Author Response File: Author Response.docx