Control over the Crystalline State of Sapphire

Abstract

Phase transitions of solid-state materials can be controlled optically by electronic excitation. In semiconductors, the semiconductor-to-metal transition occurs upon excitation of approximately 10 % of the valence electrons.[1] This bandgap collapse occurs nonthermally, faster than the thermal-energy transfer from electrons to atoms when ultrashort pulses are utilized. In dielectrics, a metallic plasma state can be created at the focus of a typical tightly focused 100 nJ/200 fs pulse, creating a ∼ 10 14 W cm -2 intensity within a volume of subwavelength cross section. Such a pulse ionizes the focal volume almost instantaneously within one or few optical cycles by multiphoton absorption and launches a shock wave with subsequent fast (< 100 ns) thermal quenching. Transitions leading to different materials phases [2][3][4] with altered chemical properties [5,6] are expected to be formed.Here, we demonstrate control over the crystallinity and chemical reactivity of sapphire (Al 2 O 3 ) using femtosecondpulse exposure. Crystalline-to-amorphous and amorphous-topolycrystalline transitions were induced inside a sapphire sample using single-and multi-pulse irradiation, respectively. Wet etching of the amorphized sapphire in an aqueous solution of hydrofluoric acid showed extremely high selectivity (> 10 4 , calculated as the ratio of the channel's lengthening to its widening) relative to that of the crystalline and polycrystalline phases. This method allowed us to fabricate a 3D network of channels without apparent constraints on their length. The processing of sapphire, the most chemically inert and the hardest oxide, opens new opportunities in different industries; for example, sapphire substrates can be patterned for the growth of defect-free GaN in high-luminosity light-emitting diodes (LEDs) and laser diodes (LDs).The femtosecond pulses used in our experiments possess a peak power of tens of killowatts, which is much lower than the threshold for self-focusing (several megawatts), allowing the unique conditions of energy delivery to the focal volume to be exploited by keeping the nonlinear effects of light propagation negligible. The benchmark feature size for 3D nanostructures, usually set at 100 nm, can be surpassed by using tightly focused femtosecond pulses. [7] In crystalline dielectrics and glasses, the dielectric breakdown, a full ionization of the focal volume, causes extensive crack formation when pulses longer than 1 ps are utilized.[8] However, we show that in the case of femtosecond pulses, the photomodified region can be elastically sustained without crack formation inside the crystalline phase as long as it is confined within small sub-micrometer dimensions. Figure 1 shows a typical transmission electron microscopy (TEM) image of the cross section of the photomodified region made by a single femtosecond pulse inside sapphire. The di- COMMUNICATIONS

In-bulk and surface structuring of sapphire by femtosecond pulses

“…It was possible to empty the multi-pulse irradiated sites by etching (Fig. 2); in contrary to the etching of channels formed at the same irradiation [2]. This is most probably due to the large nano-cracks density at the edge of photo-modified region and undamaged sapphire.…”

“…This reduces chemical reactivity [2] of photo-modified regions and the channels recorded by continuous scanning become not wet etchable inside sapphire. Here, we have inspected by SEM voids recorded by exposure to increasing number of pulses without laser beam nor sample scanning.…”

“…3). Such pattern of cracks should be present in the case of channel recording, however, the poly-crystalline phase formed in the central region of the channel hampered solubility and removal of the shock-affected sapphire from the channel; as a result no channel was formed [2]. Here, sample with the voids was split in two halves and immersed into etching solution for better removal of material.…”

“…For example, a 100 nJ laser pulse can form a void under tight focusing conditions even in sapphire (Young modulus of 400 GPa). This opens new material processing routes for inert dielectrics [2]. The structural, physical, and chemical properties of nano-materials formed inside the shock-affected volume are expected to have novel properties not present in the bulk counterparts; e.g., the negative surface energy favors nano-crack and void growth in sapphire [3] or an annealing causes hardening of nano-phases of metal [4] (the opposite tendency to the macro-properties of the same metal).…”

Three-dimensional microstructuring of yttrium aluminum garnet crystals for laser active optofluidic applications

“…In the case of crystalline dielectrics, it has been found to be much more difficult to apply the technique with the same level of results. Although there are various experimental reports of wet etching laser processed lm-size features in crystals such as crystalline quartz and sapphire, 15,16 and of even sub-lm features in lithium niobate, 17 the possibility of fabricating l-fluidic devices with functional 3D l-structures inside optical materials such as solid state laser crystals is still to be demonstrated. The fabrication of complex structures in laser materials, for example, combining hollow l-channels with optically active waveguides, will constitute an advanced generation of self-activated optofluidic devices in which light sources are integrated in the device.…”