Ultrafast energy absorption and photoexcitation of bulk plasmon in
crystalline silicon subjected to intense near-infrared ultrashort laser pulses, Applied Surface Science 519 (2020) 146087

⁎Tzveta Apostolova a,b, , Boyan Obreshkov a , Iaroslav Gnilitskyi c,d,e
a Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Tsarigradsko chausse 72, 1784 Sofia, Bulgaria
Institute for Advanced Physical Studies, New Bulgarian University, 1618 Sofia, Bulgaria
c NoviNano Lab LLC, Pasternaka 5, 79015 Lviv, Ukraine
d Department of Photonics, Lviv Polytechnic National University, Stepana Bandery 14, 79000 Lviv, Ukraine
e University of Modena and Reggio Emilia (UNIMORE), Amendola 2, 42122 Reggio Emilia, Italy

 

A B S T R A C T
Keywords: Laser ablation, LIPSS, Silicon
Surface plasmon We investigate the non-linear response and energy absorption in bulk silicon irradiated by intense 12-fs near-infrared laser pulses. Depending on the laser intensity, we distinguish two regimes of non-linear absorption of the laser energy: for low intensities, energy deposition and photoionization involve perturbative three-photon transition through the direct bandgap of silicon. For laser intensities near and above 10 14 W/cm 2 , corresponding to photocarrier density of order 10 22 cm −3 , we find that absorption at near-infrared wavelengths is greatly enhanced due to excitation of bulk plasmon resonance. In this regime, the energy transfer to electrons exceeds a few times the thermal melting threshold of Si. The optical reflectivity of the photoexcited solid is found in good
qualitative agreement with existing experimental data. In particular, the model predicts that the main features of the reflectivity curve of photoexcited Si as a function of the laser fluence are determined by the competition between state and band filling associated with Pauli exclusion principle and Drude free-carrier response. The non-linear response of the photoexcited solid is also investigated for irradiation of silicon with a sequence of two strong and temporary non-overlapping pulses. The cumulative effect of the two pulses is non-additive in terms of deposited energy. Photoionization and energy absorption on the leading edge of the second pulse is greatly
enhanced due to free carrier absorption.

 

Femtosecond optical breakdown in silicon, Applied Surface Science, 2021

Tzveta Apostolova 1, 2 and Boyan Obreshkov
1 Institute for Nuclear Research and Nuclear Energy,
Bulgarian Academy of Sciences, Tsarigradsko chausse 72, 1784 Sofia, Bulgaria
2 Institute for Advanced Physical Studies,
New Bulgarian University, 1618 Sofia, Bulgaria

 

A B S T R A C T

We investigate photoinization, energy deposition, plasma formation and the ultrafast optical breakdown in crystalline silicon irradiated by intense near-infrared laser pulses with pulse duration τ ≤ 100 fs. The occurrence of high-intensity breakdown was established by the sudden increase of the absorbed laser energy inside the bulk, which corresponds to threshold energy fluence Φ th > 1 J/cm 2 . The optical breakdown is accompanied by severe spectral broadening of the transmitted pulse. For the studied irradiation conditions, we find that the threshold fluence increases linearly with the increase of the pulse duration, while the corresponding laser intensity threshold decreases. The effect of the high plasma density on the stability of diamond lattice is also examined. For near threshold fluences, when about 5 % of valence electrons are promoted into the conduction band, the Si-Si bonds are softened and large Fermi degeneracy pressure arises (with pressure up to 100 kbar). The mechanical instability of the diamond lattice suggests that the large number of electron-hole pairs leads directly to ultrafast melting of the crystal structure.

 

Ultrafast Laser Processing of Diamond Materials: A Review

Tzveta Apostolova 1,2 *, Vasyl Kurylo 3 and Iaroslav Gnilitskyi 3,4
1 Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Sofia, Bulgaria,

2 Institute for Advanced Physical Studies, New Bulgarian University, Sofia, Bulgaria,

3 NoviNano Lab LLC, Lviv, Ukraine,

4 Department of Photonics, Lviv Polytechnic National University, Lviv, Ukraine

Keywords: femtosecond laser, wide band gap semiconductor, photoionization, carrier scattering, opticalbreakdown, laser induced periodic surface structures, graphitization

 

A B S T R A C T

Diamond laser engineering is of great importance for designing devices, which find applications in radiation sensing and quantum technologies. A review of the present state of the art of experimental and theoretical studies on ultrashort laser irradiation of diamond is presented. For a wide range of laser parameters the optimization of laser-induced electronic, optical and structural modifications of the material requires quantitative understanding of the microscopic processes underlying the ultrashort strong field excitation in diamond as a wide band gap semiconductor.

 

Blueshifts of high-order harmonic generation in crystalline silicon subjected to intense femtosecond near-infrared laser pulse, Journal of Physics: Conference Series PAPER • OPEN ACCESS

Boyan Obreshkov and Tzveta Apostolova 2020 J. Phys.: Conf. Ser. 1571 012012 

Boyan Obreshkov 1 , Tzveta Apostolova 1,2
1 Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences,
Tsarigradsko chausse 72, 1784 Sofia, Bulgaria
2 Institute for Advanced Physical Studies, New Bulgarian University, 1618 Sofia, Bulgaria

 

A B S T R A C T
We present the generation of high order harmonics in crystalline silicon subjected to intense near-infrared 30fs laser pulses. The harmonic spectrum extends from the near infrared to the extreme ultraviolet spectral region. Depending on the pulsed laser intensity, we distinguish two regimes of harmonic generation: (i) perturbative regime: electron-hole pairs born during each half-cycle of the laser pulse via multiphoton and tunnel transitions are accelerated in the laser electric field and gain kinetic energy; the electron-hole pairs then recombine in the ground state by emitting a single high-energy photon. The resultant high harmonic spectrum consists of sharp peaks at odd harmonic orders. (ii) non-perturbative regime: the intensity of the harmonics increases, their spectral width broadens and the position of harmonics shifts to shorter wavelengths. The blueshifts of high harmonics in silicon are independent on the harmonic order which may be helpful in the design of continuously tunable XUV sources.

 

High harmonic generation in crystalline silicon irradiated by an intense ultrashort laser pulse

Eur. Phys. J. D (2021) 75:267 https://doi.org/10.1140/epjd/s10053-021-00265-7, THE EUROPEAN, PHYSICAL JOURNAL D Received 2 July 2021 / Accepted 6 September 2021©The Author(s) 2021

Tzveta Apostolova1,2,a and Boyan Obreshkov11

Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Tsarigradsko Chausse 72, 1784 Sofia, Bulgaria2 Institute for Advanced Physical Studies, New Bulgarian University, 1618 Sofia, Bulgaria


A B S T R A C T

We investigate the high harmonic generation in bulk silicon irradiated by intense near-infrared laser pulses with pulse duration ≤ 100 fs. For peak field strength of the applied laser is below 1 V/ ̊A, the spectral intensity of the emitted harmonics follows the prediction of perturbative nonlinear optics—the frequency comb consists of a series of discrete peaks at odd harmonic orders. For a pulse duration longer than 30 fs and peak laser field strength exceeding 1 V/ ̊A, non-perturbative effects and generation of even order harmonics occur. The appearance of even harmonics is due to optical rectification of the transmitted pulse, which includes weak quasi-DC component with electric field as low as 3 V/μm. In the strong coupling regime, when the peak field strength inside vacuum exceeds 1.5 V/ ̊A, the laser creates dense breakdown plasma of electron–hole pairs, which in turn results in severe spectral broadening of the transmitted pulse. The harmonic spectrum superimposes onto a continuous background, the spectral width of individual harmonics is substantially broadened, and their central wavelength undergoes a blue shift that covers the spacing between adjacent harmonic orders.