Influence of surface cracks on laser-induced damage resistance of brittle KH_2PO_4 crystal

Abstract

Single point diamond turning (SPDT) currently is the leading finishing method for achieving ultra-smooth surface on brittle KH(2)PO(4) crystal. In this work, the light intensification modulated by surface cracks introduced by SPDT cutting is numerically simulated using finite-difference time-domain algorithm. The results indicate that the light intensification caused by surface cracks is wavelength, crack geometry and position dependent. Under the irradiation of 355 nm laser, lateral cracks on front surfaces and conical cracks on both front and rear surfaces can produce light intensification as high as hundreds of times, which is sufficient to trigger avalanche ionization and finally lower the laser damage resistance of crystal components. Furthermore, we experimentally tested the laser-induced damage thresholds (LIDTs) on both crack-free and flawed crystal surfaces. The results imply that brittle fracture with a series of surface cracks is the dominant source of laser damage initiation in crystal components. Due to the negative effect of surface cracks, the LIDT on KDP crystal surface could be sharply reduced from 7.85J/cm(2) to 2.33J/cm(2) (355 nm, 6.4 ns). In addition, the experiment of laser-induced damage growth is performed and the damage growth behavior agrees well with the simulation results of light intensification caused by surface cracks with increasing crack depths.

Determination of ultra-short laser induced damage threshold of KH2PO4 crystal: Numerical calculation and experimental verification

“…4, we modeled the scratch-induced light intensification using finite difference time domain (FDTD) method. 18 The light intensity distribution exhibited in Fig. 4(e) presents that the uniform incident laser field is highly distorted by the adjacent .…”

“…5,15 In the actual process to prepare high-quality KDP surfaces, it is inevitable to introduce undesirable superficial flaws like scratches, micro-cracks and dents with micro/nano dimensions on finished surfaces due to the delicate mechanical and physical properties, such as high water solubility, low fracture toughness and thermal sensitivity. 7,[16][17][18] Surface morphology with these isolated flaws, even not so large in number, would greatly modulate the incident laser and correspondingly produce light intensification up to hundreds of times. 18,19 It is well known that the electronic ionization rates, which determine the intrinsic laser damage mechanisms, are highly intensity-dependent.…”

“…7,[16][17][18] Surface morphology with these isolated flaws, even not so large in number, would greatly modulate the incident laser and correspondingly produce light intensification up to hundreds of times. 18,19 It is well known that the electronic ionization rates, which determine the intrinsic laser damage mechanisms, are highly intensity-dependent.…”

“…1,24,25 Therefore, the LIDTs in this work were measured following a deterministic R-on-1 protocol, in which the laser irradiates each test site with increasingly ramped energy until damage occurs. 7,18,34 The damage fluence is defined as the lowest laser fluence at which damage occurs. A total of 10 sites are involved in each test and the LIDT is determined as the average damage fluence.…”

Laser induced damage due to scratches in the surface of nonlinear optical crystals KH2PO4 (KDP)

“…Thus it is of great importance to carry out investigations into the damage characteristics, in particular, the effect of scratches on laser-induced damage of KDP. Chen et al have studied the potential effects of micro cracks on the laser induced damage of KDP [10,11]. Using finite-difference time-domain (FDTD) algorithm, they find that light intensity of incident laser can be intensified in the vicinity of microcracks and the intensification is related to many factors, including depth, width and shape and the angle of microcacks relative to crystal orientation as well as the wavelength of incident laser.…”