Ti:sapphire laser
A solid-state laser whose gain medium is sapphire (Al₂O₃) doped with Ti³⁺ ions, tunable from about 650 to 1100 nm with peak gain near 800 nm. Its broad gain supports mode-locked pulses of 10–100 fs, and it is usually pumped with 5–20 W of green light at 532 nm.
The titanium-sapphire laser uses a crystal of sapphire, Al₂O₃, in which a small fraction of the aluminium ions (typically about 0.1 % by weight of Ti₂O₃) are replaced by Ti³⁺. It is a solid-state laser with one of the broadest gain bandwidths of any laser crystal: continuous-wave versions tune from about 650 nm to 1100 nm, a span of 189 THz (most commercial systems cover about 700–1000 nm), and mode-locked oscillators routinely produce pulses of 10–100 fs near 800 nm at repetition rates of roughly 70–100 MHz. P. F. Moulton reported it in 1982, and since the early 1990s it has been the standard source for femtosecond optics.
Energy levels and pumping
Ti³⁺ has a single 3d electron, so the ion has only two electronic manifolds, ²T₂ and ²E, split by the crystal field of the sapphire host. Strong coupling to lattice vibrations broadens both absorption and emission into bands. Absorption spans roughly 400–600 nm, peaking near 490 nm, and emission extends from about 600 nm to beyond 1000 nm. The laser operates as a vibronic four-level system: the ion lases from the relaxed bottom of the upper manifold down to vibrationally excited levels of the ground manifold, which empty in picoseconds.
The upper-state lifetime is short, about 3.2 µs at room temperature, and the peak emission cross section is about 3–4 × 10⁻¹⁹ cm². The saturation intensity is therefore high: with cm² at 800 nm,
Reaching such intensities requires a pump focused to tens of micrometres, which rules out flashlamps for continuous operation. Early systems used argon-ion lasers at 488 and 514 nm; most now use a frequency-doubled Nd:YAG or Nd:YVO₄ laser at 532 nm. Pumping at 532 nm and lasing at 800 nm leaves 33.5 % of the absorbed pump energy as heat. Sapphire's thermal conductivity, about 33 W/(m·K) at room temperature, is two to three times that of YAG, which helps; high-average-power amplifiers are still often cooled to cryogenic temperatures, where the conductivity rises steeply and thermal lensing nearly disappears.
Femtosecond pulses
The gain bandwidth sets the shortest pulse the laser can support. For a Gaussian pulse the time-bandwidth product is 0.441, so a 10 fs pulse at 800 nm needs 44.1 THz, about 94 nm of spectrum, and a 100 fs pulse needs about 9.4 nm. Ti:sapphire has room for pulses of a few optical cycles: oscillators producing about 5 fs pulses, whose spectra span roughly 190 nm or more, have been built with careful dispersion control.
The usual mechanism is Kerr-lens mode locking, reported as self-mode-locking by Spence, Kean and Sibbett in 1991 and soon explained by the Kerr lens. The intensity-dependent refractive index of the crystal (the optical Kerr effect) focuses the intense pulse more tightly than continuous light, and a hard aperture or the pump-overlap region favours the tightly focused mode, acting as a fast saturable absorber. Prism pairs or chirped mirrors cancel the positive group-delay dispersion of the crystal. A typical oscillator with a 1.87 m cavity length runs at 80 MHz; at 1 W average power each pulse carries 12.5 nJ, and at 50 fs that is a peak power of about 235 kW.
For higher energies, chirped-pulse amplification (Strickland and Mourou, 1985) stretches the pulse, amplifies it in further Ti:sapphire stages, and recompresses it. The saturation fluence , about 0.6–0.8 J/cm² for the cross sections above, sets the energy an amplifier beam of given area can extract efficiently.
Where it is used
Ti:sapphire oscillators drive multiphoton microscopy, time-resolved spectroscopy, and the first self-referenced optical frequency combs. Amplified systems generate high harmonics and attosecond pulses, drive terahertz sources, and pump optical parametric amplifiers that extend the tuning range. Continuous-wave versions, often with an intracavity birefringent filter and etalon, serve as narrow-linewidth tunable lasers for atomic physics and spectroscopy. Ytterbium-doped fiber and thin-disk lasers have taken over applications that need high average power at 1 µm with pulses of a few hundred femtoseconds.
Pitfalls
Residual absorption in the near infrared, associated with Ti³⁺–Ti⁴⁺ pairs, lowers efficiency in poorly grown crystals; vendors quote a figure of merit, the ratio of pump absorption to residual absorption at the laser wavelength. Crystals are usually cut at Brewster's angle, so the cavity is polarization-sensitive. Short, intense pulses damage optics at lower fluences than nanosecond pulses, and group-delay dispersion from every window and lens broadens the pulse, so the pulse at the sample is longer than the pulse measured at the laser unless the dispersion is compensated.
Common questions
Why is Ti:sapphire used for femtosecond lasers?
Its gain bandwidth supports pulses down to a few femtoseconds, the crystal conducts heat well, and its Kerr nonlinearity allows self-mode-locking without a separate absorber.
Can a Ti:sapphire laser be pumped directly by diodes?
It has been demonstrated with blue and green laser diodes, but the short upper-state lifetime demands high pump brightness, and most commercial systems still use a frequency-doubled solid-state pump laser.
References: P. F. Moulton, Spectroscopic and laser characteristics of Ti:Al₂O₃, J. Opt. Soc. Am. B 3, 125 (1986); D. E. Spence, P. N. Kean, W. Sibbett, 60-fsec pulse generation from a self-mode-locked Ti:sapphire laser, Opt. Lett. 16, 42 (1991); D. Strickland, G. Mourou, Opt. Commun. 55, 447 (1985); A. E. Siegman, Lasers (University Science Books, 1986); W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006).