1. Overview

Due to their broad absorption and emission spectra, titanium-doped sapphire (Ti: Sapphire) crystals have been widely used as a gain medium for high-power lasers, including solid-state and femtosecond lasers, with long lifetime and high thermal conductivity. The crystal structure of Ti Sapphire is hexagonal, with Ti ions occupying octahedral sites and sapphire (Al₂O₃) as the host lattice. The Ti ions are responsible for the broad absorption and emission spectra, making Ti Sapphire a versatile gain medium for a wide range of laser applications.

The quality of the Ti-Sapphire crystal is a critical factor that affects the performance of the laser. The crystal’s surface quality, including roughness, scratches, and subsurface damage, can significantly impact the laser’s output power, beam quality, and lifetime. Therefore, polishing is an essential step in producing high-quality Ti-Sapphire crystals.

This chapter provides an overview of Ti Sapphire crystals, including their history, crystal structure, and properties. We also discuss the various applications of Ti Sapphire crystals in lasers, microscopy, and medical devices. 

2. History 

The discovery of Ti-Sapphire can be traced back to 1963, when the first observation of laser action in ruby crystals was reported. In the early 1970s, researchers started looking for alternative materials to ruby for high-power laser applications. In 1975, Moulton and Birnbaum discovered the broad absorption and emission spectra of Ti Sapphire, which made it an ideal gain medium for high-power lasers.

Since then, Ti-Sapphire crystals have been extensively studied and developed, leading to their widespread use in various laser applications. The development of Ti Sapphire lasers has also led to significant advances in ultrafast and nonlinear optics. 

3. Crystal Structure

Ti Sapphire crystals have a hexagonal crystal structure with lattice parameters a = b = 4.758 Å and c = 12.957 Å. The crystal structure consists of alternating layers of Al2O3 and TiO2, where Ti ions occupy the octahedral sites in the Al₂O₃ lattice.

The Ti ions in Ti Sapphire can exist in different oxidation states, including Ti³⁺ and Ti⁴⁺. Ti³⁺ ions are responsible for the broad absorption spectrum in the visible and near-infrared regions, while Ti⁴⁺ ions contribute to the emission spectrum in the red region. 

4. Properties

The physical and optical properties of Ti Sapphire crystals are closely related to their surface quality, making the polishing process critical in the production of high-quality crystals. Some of the essential properties of Ti-Sapphire crystals are discussed below.

4.1 Optical Properties 

The optical properties of Ti Sapphire crystals are characterized by their broad absorption and emission spectra, long lifetime, and high quantum efficiency. The absorption spectrum of  Ti Sapphire extends from the ultraviolet to the near-infrared region, covering a wide range of wavelengths from 350 to 1100 nm.

The emission spectrum of Ti Sapphire has centered around 800 nm, which is the optimal wavelength for many laser applications, including femtosecond lasers. The long lifetime of Ti Sapphire (3.2 μs) allows for efficient energy storage and amplification, making it an ideal gain medium for high-power lasers.

4.2 Physical Properties

Ti-Sapphire crystals have excellent thermal conductivity (approximately 34 W/mK at 300 K), which helps to dissipate the heat generated during laser operation. The hardness of Ti Sapphire is 9 on the Mohs scale, making it resistant to scratching and abrasion.

The refractive index of Ti Sapphire is 1.76, and its birefringence is 0.008 at 800 nm, which is relatively low at 1.4.3 Surface Properties.

The surface properties of Ti Sapphire crystals play a crucial role in determining the crystal’s optical and physical properties. Surface defects, such as scratches, pits, and subsurface damage, can significantly impact the crystal’s performance, reducing output power, degrading beam quality, and decreasing lifetime.

Therefore, achieving optimal surface quality is critical in producing high-quality Ti Sapphire crystals. The polishing process involves the removal of surface defects and the creation of a smooth, flat surface. The quality of the polished surface is typically evaluated using parameters such as surface roughness, scratch and dig, and laser damage threshold.

5. Applications

Ti-Sapphire crystals are widely used in various laser applications, including solid-state lasers, femtosecond lasers, and ultrafast optics. They are also used in microscopy, medical devices, and spectroscopy.

5.1 Solid-State Lasers

Ti-Sapphire crystals are commonly used as gain media in solid-state lasers due to their broad absorption and emission spectra, long lifetime, and high thermal conductivity. They are used in a wide range of laser applications, including industrial cutting, drilling, welding, and medical and scientific research.

5.2 Femtosecond Lasers

Ti-Sapphire crystals are an ideal gain medium for femtosecond lasers, which are ultrafast lasers that generate pulses with durations in the femtosecond range (10^-15 seconds). Femtosecond lasers are used in various applications, including micro- and nano-machining, biophotonics, and ultrafast spectroscopy.

5.3 Microscopy

Ti Sapphire crystals are used in various microscopy techniques, including confocal and two-photon microscopy. These techniques allow for high-resolution imaging of biological tissues and structures.

5.4 Medical Devices

Ti-Sapphire crystals, including ophthalmic and skin resurfacing lasers, are used in medical devices. They are also used in diagnostic applications, such as fluorescence spectroscopy and imaging.

5.5 Spectroscopy

Ti Sapphire crystals are used in various spectroscopic techniques, including Raman spectroscopy, Fourier transforms infrared spectroscopy (FTIR), and fluorescence spectroscopy. These techniques are used in various applications, including material analysis, environmental monitoring, and chemical analysis.

In conclusion, Ti Sapphire crystals are a versatile material with many applications in lasers, microscopy, medical devices, and spectroscopy. The quality of Ti Sapphire crystals is critical for their performance, and the polishing process is an essential step in achieving optimal surface quality. In the following chapters, we will discuss the techniques used to polish Ti Sapphire crystals and the methods used to evaluate the surface quality and mitigate polishing-induced damage.