Sapphires

Description. Sapphire occurs in a wide range of colors such as blue, pink, padparadscha orange, yellow, green, purple, black. Color is due to trace impurities of FeG Fe , Ti, and yellow color centers. The most expensive color is an intense cornflower blue these are sometimes referred to as Kashmir sapphires having a highly saturated, slightly milky, violet blue color. Padparadscha is next in value, followed by pink, then orange, purple and yellow, respectively. Nowadays 400-500 tonnes of synthetic sapphire are produced by the Verneuil process each year. 

Thermal treatment. Thermally treated sapphires are widespread while gamma irradiated stones are less common. Usually, pale yellow or colorless sapphires are heat treated in air in the temperature range 1500-1900°C to yield a dark yellow, golden, golden-brown, orange, or reddish-brown color due to the oxidation of Fe into to Fe . Pink sapphire containing traces of chromium can be heat treated to yield a padparadscha orange-pink color, while dark blue 

A local epitaxy of diamond crystallites has been reported in Ref. [200], where Si(lOO) substrate was pretreated in microwave plasma under conditions of c —0.1%CH4/H2, P= 100-120Torr, and 7 = 1040-1050°C for lOh, and successively, diamond growth was done under conditions of c= 1%CH4/H2, 7 = 50-60 Torr, and 7s = 850-880 °C for 40 h. On the Si substrate, there were two mutually oriented large diamond crystals of 120 x 150 pm in size and a number of small oriented 

In Ref. [201], it was claimed that diamond films would be heteroepitaxially grown on mirror-polished (not scratched) Si(lOO) by thermal CVD at 1150-1250°C 

At this stage, it may be relevant to briefly describe the interface between diamond and Si substrate. The major issue is whether an interface layer is present, and what are the interface material and its atomic structure. 

In Ref. [202], it was reported by XPS measurements that when Si was used as a substrate for diamond growth by HFCVD, the native oxide, Si02, was removed, and a SiC layer was formed before diamond started to grow. 

The atomic images at diamond/substrate interfaces is shown later. The presence or absence and the structures of the interface layers depend on the method of substrate pretreatment, the diamond CVD method and the growth conditions used. The investigation of the atomic interface structures will give us useful insight into the nucleation and heteroepitaxial growth of diamond. 

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Diamond behaves somewhat differently in that n is low in air, about 0.1. It is dependent, however, on which crystal face is involved, and rises severalfold in vacuum (after heating) [1,2,25]. The behavior of sapphire is similar [24]. Diamond surfaces, incidentally, can have an oxide layer. Naturally occurring ones may be hydrophilic or hydrophobic, depending on whether they are found in formations exposed to air and water. The relation between surface wettability and friction seems not to have been studied. 

Infrared pulses of 200 fs duration with 150 of bandwidth centred at 2000 were used in this study. They were generated in a two-step procedure [46]. First, a p-BaB204 (BBO) OPO was used to convert the 800 mn photons from the Ti sapphire amplifier system into signal and idler beams at 1379 and 1905 mn, respectively. These two pulses were sent tlirough a difference frequency crystal (AgGaS2) to yield pulses... 

Ti sapphire laser excitation Chem. Phys. Lett. 233 519-24... 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

Other frequently used resonators are dielectric cavities and loop-gap resonators (also called split-ring resonators) [12]. A dielectric cavity contains a diamagnetic material that serves as a dielectric to raise the effective filling factor by concentratmg the B field over the volume of the sample. Hollow cylinders machmed from Ilised quartz or sapphire that host the sample along the cylindrical axis are conunonly used. 

Xu Z H, Ducker W and Israelachvili J N 1996 Forces between crystalline alumina (sapphire) surfaces in aqueous sodium dodecyl sulfate surfactant solutions Langmuir 12 2263-70... 

Horn R G, Clarke D R and Clarkson M T 1988 Direct measurement of surface forces between sapphire crystals in aqueous solutions J. Mater. Res. 3 413-6... 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

The most connnon commercially prepared amplifier systems are pumped by frequency-doubled Nd-YAG or Nd-YLF lasers at a 1-5 kHz repetition rate a continuously pumped amplifier that operates typically in the 250 kHz regime has been described and implemented connnercially [40]. The average power of all of the connnonly used types of Ti-sapphire amplifier systems approaches 1 W, so the energy per pulse required for an experiment effectively detennines the repetition rate. 

The OPA should not be confiised with an optical parametric oscillator (OPO), a resonant-cavity parametric device that is syncln-onously pumped by a femtosecond, mode-locked oscillator. 14 fs pulses, tunable over much of the visible regime, have been obtained by Hache and co-workers [49, with a BBO OPO pumped by a self-mode-locked Ti-sapphire oscillator. 

Figure B2.1.3 Output of a self-mode-locked titanium-sapphire oscillator (a) non-collinear intensity autocorrelation signal, obtained with a 100 pm p-barium borate nonlinear crystal (b) intensity spectrum.

Figure B2.1.6 Femtosecond spectrometer for transient hole-burning spectroscopy with a continuum probe. Symbols used bs, 10% reflecting beamsplitter p, polarizer. The continuum generator consists of a focusing lens, a cell containing flowing water or ethylene glycol or, alternatively, a sapphire crystal and a recollimating lens.

Spence D E, Kean P N and Sibbett W 1991 60 fs pulse generation from a self-mode-locked Ti sapphire laser Qpt. Lett. 16 42—4... 

Pshenichnikov M S, de Boeij W P and Wiersma D A 1994 Generation of 13 fs, 5 MW pulses from a cavity-dumped Ti sapphire laser Opt. Lett. 19 572-4... 

Norris T B 1992 Femtosecond pulse amplification at 250 kHz with a Ti sapphire regenerative amplifier and application to continuum generation Opt. Lett. 17 1009-11... 

Joo T, Jia Y and Fleming G R 1995 Ti sapphire regenerative amplifier for ultrashort high-power multikilohertz pulses without an external stretcher Opt. Lett. 20 389-91... 

Le Blanc C, Grillon G, Chambaret J P, Migus A and Antonetti A 1993 Compact and efficient multipass Ti sapphire system for femtosecond chirped-pulse amplification at the terawatt level Opt. Lett. 18 140-... 

Backus S, Peatross J, Huang C P, Murnane M M and Kapteyn H C 1995 Ti sapphire amplifier producing millijoule-level, 21 fs pulses at 1 kHz Opt. Lett. 20 2000-2... 

Fundamentals of teclniiqne and theory for nltrafast experiments in chemistry, written before the titaninm-sapphire revolntion bnt still indispensable. 

Tilsch M and Tschudi T 1997 Self-starting 6.5-fs pulses from a Ti sapphire laser Opt. Lett. 22 1009-11... 

Brakenhoff G J, Squier J, Norris T, Bliton A C, Wade M FI and Athey B 1996 Real-time two-photon confocal microscopy using a femtosecond, amplified Ti sapphire system J. Microscopy 181 253-9... 

Rothberg L, Higashi G S, Allara D L and Garoff S 1987 Thermal disordering of Langmuir-Blodgett-films of cadmium stearate on sapphire Chem. Phys. Lett. 133 67-72... 

Figure C2.16.2 shows tire gap-lattice constant plots for tire III-V nitrides. These compounds can have eitlier tire WTirtzite or zincblende stmctures, witli tire wurtzite polytype having tire most interesting device applications. The large gaps of tliese materials make tliem particularly useful in tire preparation of LEDs and diode lasers emitting in tire blue part of tire visible spectmm. Unlike tire smaller-gap III-V compounds illustrated in figure C2.16.3 single crystals of tire nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growtli of device stmctures. Substrate materials such as sapphire and SiC are used instead.

Figure C3.1.13. Experimentai configuration for far-UV nanosecond CD measurements using a frequency-upconverted Ti sapphire iaser as a probe source. Pj and P2 are Mgp2 Rochon poiarizers at cross orientations. SP is a strained transparent piate with about i ° of iinear birefringence for quasi-nuii eiiipsometric CD detection. Prism PMj and the iris Ij seiect the far-UV fourth hannonic of the argon iaser-pumped Ti-sapphire iaser s near-IR fundamentai output to probe the eiiipticity of the sampie. A second iaser beam at 532 nm is used to pump CD...

The most powerful teclmique for studying VER in polyatomic molecules is the IR-Raman method. Initial IR-Raman studies of a few systems appeared more than 20 years ago [16], but recently the teclmique has taken on new life with newer ultrafast lasers such as Ti sapphire [39]. With more sensitive IR-Raman systems based on these lasers, it has become possible to monitor VER by probing virtually every vibration of a polyatomic molecule, as illustrated by recent studies of chlorofonn [40], acetonitrile [41, 42] (see example C3.5.6.6 below) and nitromethane [39, 43]. 

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