fused silica
fused silica — kvarcinis stiklas statusas T sritis chemija apibrėžtis Iš gryno kvarcinio smėlio arba susmulkinto kalnų krištolo išlydytas stiklas. atitikmenys: angl. fused glass; fused silica; quartz glass; silica glass; vitreous silica rus. кварцевое стекло … Chemijos terminų aiškinamasis žodynas
fused silica crucible — lydytojo kvarco tiglis statusas T sritis radioelektronika atitikmenys: angl. fused silica crucible vok. Quarzglastiegel, m rus. тигель из плавленного кварца, m pranc. creuset en quartz fondu, m … Radioelektronikos terminų žodynas
fused silica. — See silica glass. * * * … Universalium
fused silica — /fjuzd ˈsɪləkə/ (say fyoohzd siluhkuh) noun → silica glass … Australian-English dictionary
fused silica. — See silica glass … Useful english dictionary
fused silica — noun see fused quartz … New Collegiate Dictionary
fused silica — noun see fused quartz … Useful english dictionary
Fused quartz — A sphere manufactured by NASA out of fused quartz for use in a gyroscope in the Gravity Probe B experiment. It is one of the most accurate spheres ever created by humans, differing in shape from a perfect sphere by no more than 40 atoms of… … Wikipedia
fused quartz — noun or fused silica : vitreous silica * * * glass made entirely from quartz: a form of silica glass. [1920 25] * * * fused quartz or fused silica «fyoozd», = quartz glass. (Cf. ↑quartz glass) … Useful english dictionary
Fused Silica
Deactivated fused silica capillaries are used throughout the interface systems to restrict the analyte flow to the required flow rates.
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Ceramic-Matrix Composites
8.8.2.1 Fused Silicon Dioxide as the Matrix
Fused silica (fused silicon dioxide) is silica glass (amorphous). In contrast to the presence of other ingredients in common glasses for reducing the melting temperature, fused silica has no other ingredient. As a result, fused silica has high-working and melting temperatures.
A fused silica-matrix composite with unidirectional carbon fibers (PAN-based, 6–7 μm diameter, 30 vol.%) and SiC particles (0.5 — μm average size, 20 wt.%) as a filler is prepared by SiC SI to the fiber perform, followed by hot pressing in nitrogen at 1350°C and 20 MPa. The slurry comprises fused silica particles (2.8 μm) and α-SiC particles. The SiC particle incorporation decreases the flexural strength in the fiber direction from 667 to 432 MPa, but increases the flexural strength in the transverse direction from 18 to 54 MPa. The increase of the transverse flexural strength is attributed to the SiC particles causing greater difficulty of sliding at the fiber–matrix interface ( Zhou et al., 2007 ).
In the absence of carbon fibers or SiC particles, the fused silica itself exhibits flexural strength 54.8 MPa and fracture toughness 1 MPa·m 1/2 . The carbon fiber composite without SiC particles exhibits longitudinal fracture toughness 20 MPa·m 1/2 , whereas the carbon fiber composite with SiC particles exhibits longitudinal fracture toughness 22 MPa·m 1/2 . Hence, the carbon fibers greatly enhance the flexural strength and fracture roughness in the longitudinal direction, but the transverse flexural strength of the carbon fiber composite without SiC particles is even lower than that of the fused silica itself. The further addition of SiC particles increases the longitudinal fracture toughness slightly, while decreasing the longitudinal flexural strength and increasing the transverse flexural strength ( Zhou et al., 2007 ).
Coupling of Capillary Electromigration Techniques to Mass Spectrometry
Christian Neusüß Jennifer Römer Oliver Höcker Kevin Jooß , in Capillary Electromigration Separation Methods , 2018
12.4.2 Capillary Coatings for CE-MS
Fused silica capillaries are commonly used due to their convenient handling and inertness. In addition, they are available in various diameters and are relatively inexpensive. However, measurements with a bare fused silica capillary are associated with some difficulties such as the adsorption of the analyte on the capillary wall. The use of a BGE of extreme pH can help avoid these interactions. At low pH, silanol groups are mostly protonated and uncharged. Thus, the ionic interactions with positively charged analytes are reduced. If a BGE of high pH is used, some analytes like proteins can have a negative net charge and are repulsed from the negatively charged silanol groups. Measuring negatively charged analytes by negative electrospray ionization can result in lower signal intensities compared with the same analytes in positive ESI mode. Depending on the noise, this can lead to higher limits of detection and loss of sensitivity. Furthermore, many analytes, such as proteins, still tend to adsorb on the wall of fused silica capillaries.
Therefore, it is common to choose a suitable capillary coating to reduce analyte adsorption and to enhance the stability and performance of the separation. This can be achieved by using a neutral or a charged coating. Neutral coatings prevent analyte adsorption via shielding the silanol groups of the capillary wall. Charged capillary coatings, mainly positively charged, can prevent analyte adsorption via electrostatic repulsion.
Apart from analyte wall interactions, capillary coatings influence the electroosmotic flow (EOF). The direction and magnitude of the EOF are crucial for a stable and successful separation. If the EOF is directed to the capillary inlet, sheath liquid and even air can enter the capillary. This leads to an unstable current or even a total current breakdown. For this reason, the EOF needs to be directed toward the mass spectrometer. In case of a weak EOF, it is possible that sheath liquid counterions enter the capillary changing the separation conditions, for example, pH, with a loss of resolution. To avoid this problem, either a strong EOF is needed or the sheath liquid counterions should have the same pKa values and electrophoretic mobility. If sheathless liquid ionization is applied, the Taylor cone and following electrospray is only generated by the BGE. Therefore, the EOF needs to be directed to the MS to obtain a stable spray. Thus, charged capillary coatings are more common with this kind of interface.
Coating techniques can be classified as either dynamic or static. Dynamic coating agents are incorporated into the BGE. The interactions between analytes and silanol groups of the column wall are reduced by competition with the dynamic coating compounds. The coating compounds enter the MS, which can lead to ionization suppression, additional background signals, and contamination of the ion source.
In contrast, static coatings are anchored to the capillary wall prior to separation. This can be performed by either chemical bonding (static-covalent) or by adsorption (static-adsorbed). These coatings offer the advantage that almost no coating material enters the MS during the separation. Statically adsorbed coatings such as the cationic polybrene (PB) or neutral acrylamide-based Ultratrol (LN) coatings have the advantage that the coating is performed using simple rinsing steps. After a few runs, these capillary coatings need to be renewed. In general, these adsorbed precoatings are not suitable for use at extreme pH. A comparison of the separation of erythropoietin (EPO) measured with a neutral and a cationic coating is shown in Fig. 12.7 .
Fig. 12.7 . CE-MS of human recombinant erythropoietin using a neutral coating (A) and a cationic coating (B).
Static-covalent coatings are used to permanently modify the capillary wall and are stable over a wide pH range. Poly(vinylalcohol) (PVA) can be permanently fixed on the capillary wall by thermal immobilization and for higher stability crosslinked with glutaraldehyde. The coating procedure, including the polymerization by heating, is more extensive than generating a statically adsorbed coating by flushing. The resulting coating suppresses the EOF and is stable over a wide pH range.
In conclusion, dynamic coatings are rarely applied, whereas both types of static coatings are used in CE-MS. Static-covalent coatings are more common and more stable compared with statically adsorbed coatings. Further details for capillary coatings in CE-MS can be found in the review of Huhn et al. [34] .
Forensic Science, Applications of Mass Spectrometry
Rodger L. Foltz , . David M. Andrenyak , in Encyclopedia of Spectroscopy and Spectrometry , 1999
Workplace drug testing
The analysis of urine specimens from employees to determine whether they are inappropriately using drugs that can affect job performance falls under the broad category of forensic toxicology because of the legal consequences that can arise. In the United States, workplace drug testing is widely practised, and can be divided into ‘regulated’ and ‘nonregulated’ testing. ‘Regulated drug testing’ refers to programmes that fall under federal regulations, such as the ‘Mandatory Guidelines for Federal Workplace Drug Testing Programs’ issued in 1988. These ‘mandatory guidelines’ define how workplace drug testing must be performed, and include the requirement that the testing be performed by laboratories certified by the National Laboratory Certification Program. Another key component of the guidelines requires a two-tier testing process consisting of an initial series of immunoassays followed by confirmatory tests based on gas chromatography–mass spectrometry. For a sample to be reported as positive for a drug, the immunoassay screen must show the presence of a drug class at a concentration above the cutoff values shown in Table 1 and the GC-MS confirmation assay must show a drug, or metabolite within the drug class, to have a concentration above the cutoff values listed in Table 2 .
Table 1 . Initial test cutoffs
| Drug class | Cutoff (ng mL −1 ) |
|---|---|
| Marijuana metabolites | 50 |
| Cocaine metabolites | 300 |
| Opiate metabolites | 300 |
| Phencyclidine | 25 |
| Amphetamine | 1000 |
Table 2 . Confirmation test cutoffs
| Drug | Cutoff (ng mL −1 ) |
|---|---|
| 9-Carboxy-Δ 9 -THC a | 15 |
| Benzoylecgonine b | 150 |
| Morphine | 300 |
| Codeine | 300 |
| Phencyclidine | 25 |
| Amphetamine | 500 |
| Methamphetamine | 500 |
‘Nonregulated’ workplace drug testing refers to programmes that do not fall under federal regulations. These programmes may include testing for other psychoactive drugs such as benzodiazepines, barbiturates and lysergic acid diethylamide (LSD). Also, nonregulated drug testing programmes sometimes include analysis of other types of specimens, such as hair or sweat.
Federal regulations do not identify specific methods for performing GC-MS confirmations. However, certain mass spectrometric procedures have become widely adopted by laboratories that perform workplace drug testing. A laboratory must first demonstrate and document that each confirmation assay is specific for the target drug and that it is sufficiently sensitive to permit accurate measurement of the drug at its established confirmation cutoff.
Typically, an analytical batch of urine samples will include each of the following: (1) calibration standards with at least one standard at the analyte's cutoff concentration; (2) quality-control samples containing known concentrations of the analyte both above and below the analyte's cutoff; (3) a sample blank containing none of the analyte; and (4) the test samples submitted for confirmation. A specific amount of internal standard is added to each of these samples. Deuterium-labelled analogues, now commercially available for all common drugs of abuse, are generally the first choice as internal standards. After addition of the internal standard, each sample is extracted to separate the analyte from other components of the urine. This step may involve a simple liquid–liquid extraction with a water-immiscible organic solvent, extraction onto a solid adsorbent and elution with an organic solvent, or a combination of these procedures.
Most drugs and metabolites are chemically converted to more volatile and stable derivatives before GC-MS analysis. Table 3 gives examples of derivatizations commonly used for GC-MS confirmation of drugs of abuse.
Table 3 . Examples of derivatization procedures for GC-MS analysis of common drugs of abuse
| Drug group | Derivatization procedure |
|---|---|
| Amphetamines | Acylation with acid anhydrides such as trifluoroacetic anhydride (TFAA), pentafluoropropionic (PFPA) or heptafluorobutyric anhydride (HFBA) |
| Opiates | Trimethylsilylation with bis(trimethylsilyl)trifluoroacetamide (BSTFA) or acylation with a fluorinated anhydride |
| 9-Carboxyl-Δ 9 -THC | Trimethylsilylation with BSTFA or alkylation with methyl iodide in dimethyl sulfoxide |
| Benzoylecgonine | Trimethylsilylation with BSTFA |
| Benzodiazepines | Trimethylsilylation with BSTFA |
| Barbiturates | Alkylation with trimethylanilinium hydroxide (TMAH) |
Before any test samples are analysed, the mass spectrometer's mass calibration is checked by analysis of a reference material such as perfluorotributylamine (PFTBA), and the measured mass-to-charge (m/z) values for the major PFTBA ions are compared with the corresponding theoretical m/z values. A reference solution containing a known concentration of the analyte or analytes of interest is then injected into the GC-MS system to evaluate the instrument's performance. If the analyte's peak intensity, retention time and chromatographic peak shape are acceptable, analysis of a batch of samples can be initiated.
Fused-silica capillary gas chromatographic columns are universally used for analysis of drugs in forensic laboratories. Typically the columns are coated to a thickness of approximately 0.3 μm with relatively nonpolar methylsilicone or phenylmethylsilicone adsorptive phases and have internal diameters of 0.2–0.3 mm and are 10–30 meters in length. Extracted and derivatized samples injected into the GC-MS are most often detected by electron ionization and selected-ion monitoring (SIM) of three prominent analyte ions and two prominent internal standard ions. The analyte concentration is determined by reference to a calibration curve based on a plot of the ratio of the area of the analyte's quantitating ion to the area of the internal standard's quantitating ion versus analyte concentration. For a urine sample to be confirmed as positive for a drug, the analytical results must meet all of the following criteria:
The drug's measured concentration must be equal to, or greater than, the established cutoff concentration for the drug (see Table 2 ).
The ratios of peak areas for the monitored ions of the analyte and internal standard must fall within 20% of the corresponding peak area ratios for the calibration standard at the cutoff concentration.
The retention times for the analyte and the internal standard must be within 2% of those for the cutoff calibrator.
The chromatographic peaks for the analyte and the internal standard must be clearly separate from co-eluting compounds.
The analytical methods most widely practised in laboratories for workplace drug testing have been well validated and are highly reliable. However, some problems continue to be of concern. For example, ingestion of certain legitimate food products can result in a urine sample testing positive for a controlled drug. Poppy seeds, widely used in bakery products, contain low concentrations of morphine, and hemp oil contains low concentrations of Δ 9 -tetrahydrocannabinol, the psychoactive component of marijuana. Efforts to develop analytical methods for testing urine that would permit distinction between ingestion of these food products and nonmedical use of regulated drugs have so far been unsuccessful. However, methods are now available for determining whether urine found to be positive for methamphetamine reflects nonmedical use of methamphetamine or legitimate use of ‘Vick's Inhaler’, an over-the-counter medication which in the United States contains the l-enantiomer of methamphetamine. To make this distinction, the urine must be analysed by a method that can separate and identify each of the optical isomers of methamphetamine. This can be accomplished by derivatizing the extracted methamphetamine with a chiral derivatizing reagent such as N-trifluoroacetyl- l -prolyl chloride. This reaction forms separate diastereoisomers of d— and l-methamphetamine that are easily distinguished by GC-MS analysis.
FILLERS – ORIGIN, CHEMICAL COMPOSITION, PROPERTIES, AND MORPHOLOGY
2.1.79.2 Fused silica
Denka Kagaku Kogyo Co., Ltd., Ibaraki, Japan
FS, FR–crushed type fused amorphous silica
FB, FBX–spherical fused amorphous silica
SFP, UFP–submicron size type fused amorphous silica
Denka fused silica filler–coarse particle type fused amorphous silica
Quarzwerke GmbH, Frechen, Germany
Fused silica flours of different particle sizes. Available with aminosilane (AST grade) and epoxysilane (EST); extremely low thermal expansion
Fused silica flour is produced from electrically fused SiO2 by iron free grinding, followed by air separation. As an option, it may be coated with silane. Quarzwerke GmbH treats flour with amino and epoxysilanes. Denki Kagaku Kogyo Co., Ltd. manufactures spherical grades of fused amorphous silica.
The properties of this filler can be appreciated when compared with silica sand discussed below in a separate section. The comparison shows a very low linear thermal expansion coefficient, thermal conductivity, and very high specific electrical conductivity. These unusual properties, similar to those of the pure quartz crystal, are exploited in applications in electronics and dental prosthetics.
Thermal writing of photonic devices in glass and polymers by femtosecond lasers
11.3.1 Low repetition rate fabrication of optical waveguides in glasses
At low repetition rates, the time between pulses is long enough so that thermal diffusion has carried the heat away from the focus before the next pulse arrives. The threshold repetition rate for heat accumulation depends on several factors including the glass properties (heat capacity, thermal diffusivity, absorption), the laser pulse energy, the focusing NA and the scanning speed, which will be discussed in further detail in the next section. In the low repetition rate waveguide writing regime, the ensuing pulses may add to the overall modification, but still act independently of one another. Most results in the field of femtosecond laser microfabrication have been carried out at low repetition rates, and usually at 1 kHz, due to the common availability of 800-nm regeneratively amplified Ti:Sapphire femtosecond lasers at this repetition rate. One limitation of waveguide writing in the single-pulse interaction regime is that waveguide cross-sections take on a shape similar to the asymmetric focal volume since the depth of focus is larger than the transverse spot size. The resulting waveguides written with the transverse writing scheme, where the sample is translated transversely relative to the incident laser, are elliptical-like, giving modes that couple poorly to optical fiber. Several methods have been proposed to produce a more symmetric focal volume including astigmatic focusing with a cylindrical lens telescope ( Osellame et al., 2003 ), slit reshaping ( Ams et al., 2005 ; Cheng et al., 2003 ), multiscan writing ( Nasu et al., 2005 ) and two-dimensional deformable mirrors ( Thomson et al., 2008 ).
Silicate and phosphate glasses
Although pure fused silica is the most common glass for photonic applications due to its high transmission, excellent temperature resistance and compatibility with biomaterials, silicate and phosphate glasses offer similar characteristics at a reduced cost. Further, silicate and phosphates may be doped with active ions for amplification or lasing applications.
The most common silicate glasses are boro-aluminosilicate glasses, which in addition to silica (SiO2) contain significant concentrations of boron trioxide (B2O3), aluminum oxide (Al2O3) and sodium oxide (Na2O). Waveguides have been successfully fabricated by low repetition rate femtosecond lasers in several borosilicates including Schott Duran ( Ehrt et al., 2004 ), Corning 7890 ( Streltsov and Borrelli, 2002 ) and Corning 1737 ( Low et al., 2005 ). In Corning EAGLE2000, a common glass used primarily in displays owing to its low density, Zhang et al. explored a wide range of processing conditions with a 1-kHz Ti:Sapphire femtosecond laser. By tuning the compressor alignment, pulse durations of 50 fs–2 ps were studied, revealing promising windows for waveguide writing at both 100 fs and 1 ps ( Zhang et al., 2007 ), disproving the widely held belief that sub-200 fs pulses were needed to form optical waveguides in glass. The most important consequence of this work was a serendipitous discovery from scanning the sample at 0.5 mm/s so that successive pulses only partially overlapped, resulting in a periodic refractive index modulation along the waveguide. The partially overlapped refractive index voxels resulted in segmented waveguides that showed strong and narrowband Bragg reflection while maintaining high optical confinement and low-loss waveguiding at 1550-nm wavelength. For more discussion of waveguide Bragg gratings and related sensing and lasing devices, the reader is referred to Chapter 10 .
In Schott BK7, the most common borosilicate glass used in commercial optics, initial reports of waveguide writing with 1-kHz Ti:Sapphire lasers suggested that only negative refractive index modification was possible ( Bhardwaj et al., 2005 ; Ehrt et al., 2004 ; Mermillod-Blondin et al., 2006 ). However, it was later shown that by using temporally shaped pulses of
1 ps duration, positive index changes are indeed possible in BK7. It was also recently demonstrated that optical waveguides may be formed at low repetition rates in BK7 using a slit beam reshaping technique ( Dharmadhikari et al., 2011 ). Waveguides may be more easily formed in BK7 without correction techniques by applying higher repetition rates, with demonstrations of low-loss (
0.2 dB/cm) waveguides at both 2 MHz ( Eaton et al., 2008a ) and 11 MHz ( Allsop et al., 2010 ).
Good quality optical waveguides have been demonstrated in phosphate glass, which is easily doped with Er and Yb ions for active waveguide applications. Using the astigmatic writing method with a cylindrical lens telescope, waveguides with low damping loss (0.25 dB/cm) at 1550-nm wavelength were written at 20 μm/s with a 1-kHz, 150-fs Ti:Sapphire laser with a 0.3-NA microscope objective and 5-μJ pulse energy ( Osellame et al., 2003 ). This work is significant since it was the first time a beam-shaping method was applied to correct for the asymmetric intensity distribution in the transverse waveguide writing. The slit shaping method was later applied by Withford and coworkers to produce symmetric waveguides ( Fig. 11.4 ) with similar propagation loss in the same active glass ( Ams et al., 2005 ).
11.4 . Microscope images of waveguides fabricated in phosphate glass (a) without and (b) with a 500 μm slit ( Ams et al., 2005 ).
Pure fused silica glass
Many groups have applied the femtosecond laser-writing method to pure fused silica glass, but few have shown good quality waveguides with operation at both visible and near-infrared wavelengths, for use in biophotonics and telecom devices, respectively. The best result in fused silica to date was obtained with a multiscan writing procedure to form waveguides with nearly square cross-sections (7.4 μm × 8.2 μm) with a refractive index change of 4 × 10 −3 . suitable for low-loss coupling to single mode fiber (SMF) ( Nasu et al., 2005 ). The refractive index profile obtained by refracted near field (RNF) profilometry for a waveguide written in Ge-doped silica glass for planar lightwave circuits (PLCs) is shown in Fig. 11.5 . The waveguides written in PLC glass were nearly identical to the waveguides written in pure silica. A 775-nm, 150-fs Ti:Sapphire laser with 1-kHz was applied in the transverse writing geometry with a 0.4-NA objective, 182-nJ pulse energy and 10-μm/s writing speed. The waveguides were fabricated with 20 scans separated transversely by 0.4 μm. A propagation loss of 0.12 dB/cm at 1550 nm was reported, which is the lowest reported in the field, and is attributed to the gentle refractive index modification enabled by the novel low-fluence, multiscan fabrication method.
11.5 . Refractive index profile of a waveguide written in Ge-doped silica PLC glass by multiple scans ( Nasu et al., 2005 ).
The effect of writing speed on waveguide properties was evidenced in a study in pure silica with a 120-fs 1-kHz Ti:Sapphire laser ( Will et al., 2002 ). With 3-μJ energy pulses focused 0.5 mm below the sample surface with a 0.25-NA lens, the initially single mode waveguide at 1 mm/s scan speed ( Fig. 11.6a ) showed higher confinement at a decreased scan speed of 0.5 mm/s ( Fig. 11.6b ). As the scan speed was further reduced, increasing the net fluence, the waveguide became multimode as the effective index was further increased with the V-number exceeding 2.4, the single-mode cut-off value ( Agrawal, 1997 ).
11.6 . Influence of the writing speed on waveguide properties at a wavelength of 514 nm. Only the highest-order guided modes are shown for a writing speed of (a) 1 mm/s, (b) 0.5 mm/s, (c) 80 μm/s and (d) 25 μm/s ( Will et al., 2002 ).
The effect of writing laser polarization on waveguide transmission properties was studied using a 1-kHz, 120-fs Ti:Sapphire laser with a 0.5-mm slit placed before the 0.46-NA focusing objective ( Little et al., 2008 ) to obtain symmetric waveguide cross-sections. With 3-μJ pulse energy measured after the slit and 25-μm/s writing speed, a refractive index change of 2.3 × 10 −3 was measured for circular polarization, which was about twice as high as that obtained with linear polarizations. This enhancement was attributed to the higher photo-ionization rate for circular polarization compared to linear polarization in the range of intensities studied (42–50 TW/cm 2 ). It is probable that nanogratings also influence the transmission properties of femtosecond laser-written waveguides in fused silica. For more details on nanogratings and their application to post-etching of buried microchannels, the reader is referred to a review by Taylor et al. (2008) .
Exotic glasses
Waveguides were written in a highly nonlinear heavy-metal oxide (HMO) glass with a 1-kHz, 800-nm, 100-fs Ti:Sapphire laser ( Siegel et al., 2005 ). HMO glasses are attractive due to their high optical nonlinearity (n2 ≈ 10 −18 m 2 /W), but this presents significant challenges in femtosecond laser writing because of strong self-focusing, resulting in a delocalized spatial distribution of the laser energy which is difficult to control. By focusing 1.8-μJ femtosecond laser pulses with a 0.42-NA objective and scanning the sample transversely at 60 μm/s, elongated damage structures of 65-μm vertical length were observed when the sample was viewed from the end facets. These elongated structures were the result of filamentation when self-focusing balances against plasma defocusing. The waveguiding regions were found to be adjacent to the filament-induced damage zone, with propagation losses below 0.7 dB/cm demonstrated at 633-nm wavelength. The regions of refractive index increase adjacent to the filament were attributed to compressive stress induced outside the laser-damaged zone, similar to observations during waveguide writing in crystalline materials.
Femtosecond-laser-induced refractive index modifications for photonic device processing
10.3.6 Nanostructure in fused silica
Focusing femtosecond pulses into fused silica can result in a quasi-periodic modification where the orientation of periods depends on the polarization of the femtosecond laser pulses. These types of modifications are termed nanostructures, or nanogratings, as the period of the modulation can be as small as 20 nm. It is not yet understood why this modification appears to be unique to fused silica and the mechanism by which these structures appear is still under debate. Pattathil et al. (2005) proposed that the quasi-periodic modulation arises as a result of the field enhancement around an initial ‘seed’ defect which in turn causes nanogratings to grow in a self-organized fashion. Shimotsuma et al. (2005) proposed an alternative explanation where the quasi-periodic modulation arises as a result of interference between waves excited within the electron plasma and the laser pulse.
Nanostructure formation is thought to be the underlying reason why modified glass regions experience an enhanced etch rate when exposed to hydrofluoric (HF) acid, and why the etch rate can be increased by a factor of 100 by varying the polarization of the femtosecond pulses ( Hnatovsky et al., 2005 ). Modifying fused silica in this fashion is advantageous for fabricating microfluidic channels.
Basic technologies for microsystems
2.4.2 Glass Wet Etching
Amorphous glasses, including fused silica , can be etched in a similar way to SC quartz. Examples are the isotropic etching of borosilicate glass in HF acid, in which a strip-opening in the masking layer will result in a hemispherical channel structure if sufficient etch time is allowed. Fig. 2.15 demonstrates the application of this technique to the fabrication of microfluidic channels [ 55 ]. Today, the research field of microfluidics already offers a broad range of industrial opportunities.
Fig. 2.15 . Scanning electron micrograph of cleaved edge of microfluidic channel etched in glass by HF.
Reproduced from L.B. Koutny, D. Schmalzing, T.A. Taylor, M. Fuchs, Microchip electrophoretic immunoassay for serum cortisol, Anal. Chem. 68 (1) (1996) 18–22. Copyright © 1996 American Chemical Society.
Various chip manufacturers and high-tech suppliers, such as Philips and Agilent, have already intensively investigated microfluidics in Lab-on-a-Chip technology to develop their own applications in this field.
Poling of Glasses and Optical Fibers
12.8.3 Quasi-Phase Matching
Thermal poling of a fused silica QPM waveguide with periodic electrodes made by photolithography has been demonstrated [ 231 ]. Phase matching over
2 mm at 1064 nm was accomplished, and a nonlinearity equivalent to d11/200 of that of quartz was estimated from the measurements. An alternative technique reported to create a QPM structure makes use of a thermally poled planar fused silica that is exposed to spatially periodic illumination with UV, erasing the second-order nonlinearity (i.e., the recorded field) at the correct period (
48 μm) to guarantee phase-matched SHG [ 232 ]. A similar technique has been exploited for frequency doubling in fibers, as described in Section 12.9.2 .
A considerable amount of work was carried out trying to exploit the surface plasmon resonance from in-diffused silver for frequency doubling, in which a nonlinear coefficient
25 pm/V was reported [ 109, 183, 233 ]. Other noble metals were also investigated [ 234 ]. The main difficulty encountered in exploiting the resonance was avoiding the loss incurred by the silver clusters [ 234, 235 ]. Quasi-phase matched structures were fabricated [ 236 ], but no efficient conversion has been demonstrated. It is possible that the presence of silver reduces the recorded electric field in the glass, thus reducing the benefit of increasing the third-order nonlinearity.
Basic Technologies for Microsystems
2.4.2 Glass Wet Etching
Amorphous glasses, including fused silica , can be etched in a similar way to single-crystal quartz. Examples are the isotropic etching of borosilicate glass in hydrofluoric acid (HF), in which a strip-opening in the masking layer will result in a hemispherical channel structure if sufficient etch time is allowed. Figure 2.15 demonstrates the application of this technique to the fabrication of microfluidic channels [54] .
Figure 2.15 . Scanning electron micrograph of cleaved edge of microfluidic channel etched in glass by HF.
Reproduced from reference [54] . Copyright © 1996 American Chemical Society
This process is very important for the fabrication of microfluidic-assisted biochips, which are a successful emerging market. Various chip manufacturers and high-tech suppliers, such as Philips and Agilent, have invested in Lab-on-a-Chip technology to develop their own applications in this field.
CdxHg1−xTe/CdTe Heteroepitaxy in a Microgravity Environment
2 Experimental Conditions, Procedures, and Characterization
The growth container consisted of a fused silica double ampoule of ∼ 10 cm outer length and 15 mm inner diameter, with the source material at one end and the CdTe substrate at the other flat end. Details of source material and HgI2 preparation, substrate properties and pretreatment, ampoule preparation, loading, and sealing procedures were the same for the ground test and space experiments ( Wiedemeier et al. 1995 ). The growth ampoules were enclosed in metal cartridges with thermocouples located inside the cartridge in contact with the ampoule. The ground test experiments were performed in the same type Crystal Growth Furnace (CGF) and according to the same procedures as those in the microgravity environment. The nominal source and growth temperatures were 595°C and 545°C (±1°C), respectively, and the growth times for the flight (and ground test) experiments were 1.5 h, 2.5 h, 6.4 h, and 8.1 h. The integrity of the overall ampoule–cartridge assemblies (thermocouple locations) was confirmed to be the same for ground and flight (before and after) experiments by X-ray diffraction photographs.
For the comparative analysis, the best results observed for the ground control experiments are compared with those of the epitaxial layers grown under microgravity conditions.