Laser beam intensity at the focal point as a function of the focal length of the. The precipitation of these phases made the material inhomogeneous, leading to a significant change in the optical and dielectric properties due to the composite nature of the material. Ultrashort time scale laser pulses are one of the more recent developments in the field of stimulated laser emission.
The wavelength of the pump beam was chosen because 532 nm corresponds to the highest absorption bandwidth for Ti:sapphire. The final interaction of the laser beam with the oscillator cavity takes place at the output coupler. At the beginning of the pulse interactions, energy is transferred to the material within the first 10 to 100 fs by nonlinear photoionization mechanisms.
TDS uses pulses of radiation in the far-infrared region of the electromagnetic spectrum to probe and analyze the dielectric properties of materials. A fs pulse synchronized with the THz emission gates the photoconductive gap in the detector, creating a current proportional to the electric field. Generally, the amplitude of the waveform decreases after passing through the sample and the pulse is delayed in the time domain.
![Figure 1. Crystal structure of YAG viewed along the [001] direction. Yttrium is represented by green balls, aluminum by blue, and oxygen by red](https://thumb-ap.123doks.com/thumbv2/123dok/10518310.0/15.918.203.753.139.669/figure-crystal-structure-viewed-direction-yttrium-represented-aluminum.webp)
Sample Preparation
These pellets were sintered at 1750°C in a tungsten grid heated vacuum furnace at below 10-6 torr and then hot isostatically pressed at 1700°C for 2 hours. In a final preparation step, the samples were thinned to 1 mm thickness and polished using abrasive SiC media, followed by polishing with a diamond-impregnated polymer film to achieve optical surface treatment. These ceramic samples were translucent to transparent in the visible spectrum with optical attenuations better than 2 cm-.
To determine the amount of metal nitrate precursor required to achieve the target atomic ratio in a 10 g sample, the calculations were worked backwards from the fully oxidized stoichiometry. Pellets were pressed into a 12.2 mm diameter die, with isopropanol added to the powder to help bind it during pressing. Since three pellets were made for each of the ten compositions, three sintering runs were performed, each using one pellet from each of the ten compositions.
The pellets are arranged numerically in a rectangular high purity alumina boat, covered with a dense high purity alumina plate and fired in ambient air in a box furnace.
Laser System
In addition, opaque YAG ceramics of the same chemical composition as transparent ceramics were irradiated to study the effect of different microstructures on the interaction between the laser pulse and the material. Instead, the average beam power is measured with a thermopile detector and divided by the pulse repetition rate to calculate the pulse energy. Assuming that the energy, E, contained in each pulse is constant, and based on the definition of power as the rate of change of energy flow (energy per unit time), we can consider two types of power.
To calculate the flow and intensity of power density of the laser beam incident on the sample surface, it was first necessary to calculate the beam divergence from the beam quality factor, or M2 factor specified for the laser system.49 Figure 8 shows the case where the laser beam is focused on a small spot. Laser beam intensity at the focal point as a function of the focal length of the lens used. Note the 532 nm pump beam located on the right side of the image, compared to the 800 nm beam invisible to the naked eye passing through the prism compressor on the left.
The short arm of the oscillator cavity consists of the path from MO1 to SBR, while the long arm of the cavity consists of all the mirrors from MO5 to the output coupler (OC). The multipass Herriott cell greatly increases the effective path length of the long arm. Several reflective neutral density filters were used to reduce the intensity below the camera's saturation limit, multiple reflections between the different filters resulted in the edges seen in the image.
The sample was placed with its surface concentrically aligned with the laser beam path, and was moved longitudinally in and out of the beam center at the focal point of the lens. The sample was moved lengthwise back to the focal point in increments of 0.25 cm, while the output spectrum of the laser was monitored with a spectrometer. When the sample was moved to within about 0.30 cm of the focal point, the laser spectrum suddenly changed to CW mode, similarly, with the sample located within the focal point of the lens and moving outwards into the focal point, at a distance of 0.30 cm from the focal length the spectrum changed to CW.
This allowed the beam reflected from the sample surface to be directed to a beam dump instead of back into the laser oscillator cavity. Using the software motion control of the XYZ sample stage, the sample was moved perpendicular to the laser beam at different speeds so that linear patterns were scanned across the sample surface.
Characterization of material properties
In cases where the row is blank, there was insufficient definition of the grain boundaries in the SEM micrograph to make an accurate grain size analysis. All samples were processed as identically as possible so that the cause of the variation in microstructures is not caused by the process variations. The exposure of the 2 mm thick transparent 1 atomic % Nd:YAG single crystal at NPL did not show any ablation or other structural changes in the surface, as shown in Figure 19.
Outside the focal length of the beam, only a dull red dot was visible on the sample surface and the transmitted red beam was visible on the screen behind the sample. Changing the distance of the sample from the lens changed the color, as did adjusting the prism compressor to change the pulse duration. Each step of the trial step motion was done manually, so at the left and right edges of the pattern, there was a short time before the beam was moved to the next row.
This retention explains the ablation craters that formed on the left and right edges of the image. In the center of the sample, the beam moved continuously, and it is believed that the craters formed there may be due to subsurface porosity or surface topography. The loose grain structure of the material appears to have been melted by the laser beam and solidified upon cooling.
Upon further magnification and examination, the direction of movement of the pattern can be seen alternating from row to row in Figure 23. As the direction of sample movement changed, the beam stayed at this point long enough to remove a deep crater. By measuring the diameter of the affected area surrounding the ablation craters, it appears that the diameter of the incident laser beam was approx. 120 µm.
The interior of the ablation crater, out of focus and appearing as a dark void, is approximately 80 µm. In the area surrounding the rim of the crater, the affected region narrows with increasing depth in the sample. The densification caused by the laser exposure penetrated approximately 33 µm into the sample surface and shrinkage due to densification caused the channel to crack away from the bulk material.
However, there is no corresponding reduction in concentration around the high concentration region that would indicate diffusion of yttrium from the bulk region to the densified region.
All calculations were performed using data between 0.003 and 2.5 THz because above 2.5 THz the time domain signal fell below the measurement noise level. The absorption at νo = 3.5 THz is independent of temperature while the mainband contribution increases with temperature. The first is the matrix element of the coupling term between the phonons involved in the process.
The second describes the temperature dependence of the phonon occupancy number of the phonon states involved in the process, and the third concerns the phonon state density. Since νo = 3.5 THz corresponds to the lowest polar mode of the YAG lattice, here only a high-order two phonon difference (2PD) process, involving the mechanism (i) of Figure 40, can reduce the absorption below 3 Declare .5 THz. Before analyzing the refractive index and absorption data, the cutoff frequency at which the measured spectrum intensity fell below the noise floor of the measurement was determined for each sample62.
Frequency spectrum of an opaque YAG bead illustrating the threshold frequency where the intensity of the spectrum drops to the noise floor of the measurement. Due to the porous nature of the sintered materials, the true properties of the material could not be directly measured. In Figure 49, the absorption coefficient is plotted as a function of bulk density, and the best-fit linear relationship is calculated with an R-squared value of 0.7363.
Absorption coefficient of the opaque YAG ceramic as a function of the average bulk density of each sample. The focal volume of the laser beam was scanned over the surface of the samples at different coherent speeds, and the results were observed by SEM. In areas where melting occurred, the fine grains of the original microstructure were consolidated into long, narrow grains running perpendicular to the direction of beam travel.
The circular shape of the beam was also visible on the surface of the sample in the form of successive wavy semicircles visible along the beam path. Selective movement of the dopant (yttrium) from the matrix towards the focal point. In future work, it will be necessary to characterize the beam/sample interaction in order to achieve a better control of the process variables.
It would also be useful to characterize the exact area of the sample surface that will be exposed to the laser beam so that the results can be correlated with the surface before exposure.
