Peter Pokrowsky

High frequency wavelength modulation spectroscopy with diode lasers

Abstract High frequency wavelength modulation spectroscopy with diode lasers is accomplished by dithering the drive current at RF frequencies as high as 250 MHz. This technique is useful for fast and sensitive detection of absorption lines in the near-and mid-infrared spectral regions. Detection of 300 MHz wide spectral features corresponding to 1% changes in transmission is accomplished in time intervals as short as 500 ns. A potential application is for fast reading of information contained in frequency domain optical memories based upon photochemical hole burning.

Noncontact optoacoustic monitoring of flame temperature profiles

The first known totally noncontact optoacoustic measurement in a flame, using a pulsed laser to excite an acoustic pulse and observing the transient deflections of cw probe beams to detect the acoustic propagation, is described. All laser beams are coplanar, parallel, suitably displaced, and focused inside the flame. Such laser-based ultrasonic measurements provide a new way to map out temperature and flow profiles in flames or other hostile environments.

Improved laser technique for high sensitivity atomic absorption spectroscopy in flames

Abstract Frequency modulation laser spectroscopy is utilized for high sensitivity detection of trace species aspirated into a flame of the type commonly used for atomic absorption spectroscopy (AAS). The method is applied to the detection of sodium atoms in an air-acetylene flame and data are obtained on the distribution of ground and excited state populations as a function of height above the burner. Absorptions as small as 1.5 × 10 -4 may be detected with a signal-to-noise of one. With multi-passing, a detection limit of 3.3 × 10 6 sodium atoms/cm 3 was obtained.

Hole burning spectroscopy of R′ aggregate color centers in polycrystalline LiF thin films using a GaAlAs diode laser

The first spectroscopic measurements of R′ aggregate color centers contained in polycrystalline LiF hosts are reported. The inhomogeneous and homogeneous widths of the 830‐nm zero phonon line are measured by conventional grating spectroscopy and by photochemical hole burning spectroscopy using a current tuned GaAlAs diode laser with 3‐mW output power. An extensive photochemical hole burning comparison between single and polycrystalline samples is presented.

Optical methods for thickness measurements on thin metal films

The thickness and the dielectric constants of thin metal films on glass substrates are determined by two different methods. The first method is a combination of transmission and ellipsometer measurements (TELL method) and the second is based on attenuated total reflection (ATR method in the Kretschmann arrangement). For comparison, both methods are applied to gold films within a thickness range of 20-80 nm. Furthermore, the TELL method was applied to chromium films of thicknesses up to 150 nm. All experiments are done with a He-Ne laser at 633-nm wavelength.

Measurement of time dependent optical gain using frequency modulation spectroscopy

Abstract Frequency modulation (FM) spectroscopy is demonstrated to be an attractive tool for gain analysis. It offers the three major features: a) potential high sensitivity in absolute gain measurement, b) high spectral resolution allowing the determination of the spectral gain profile, c) high temporal resolution capable of monitoring gain in short-pulsed systems. As an example, results of a gain analysis of the 6328 a line in a He-Ne discharge are presented.

Absorption of H 2 S at DF laser wavelengths

H2S absorption in the 3.6–4-μm wavelength region is studied with a tunable diode laser spectrometer. Wavelengths, line strengths, and pressure broadening coefficients of the strongest lines and of the lines adjacent to deuterium fluoride (DF) lines are determined. With these data the absorption cross sections of H2S at 1P DF wavelengths are calculated for atmospheric conditions and compared with the absorption cross sections measured directly with a DF laser.

Determination of offset between a fixed wavelength laser and an absorption line using frequency modulation spectroscopy

Abstract Frequency modulation spectroscopy is used as a new method to determine the detuning or optical frequency offset between a fixed laser line and the center of an adjacent absorption line. Because the offset is measured directly using high precision RF techniques, the accuracy is limited only by the laser linewidth. The method was demonstrated using (a) a dye laser set at a fixed frequency and an etalon resonance and (b) an infrared He-Ne laser and a methane absorption line.

Ellipsometrische Charakterisierung von C-Schutzschichten für die SpeichertechnologieEllipsometric characterization of protective carbon coatings for storage technology

Die spektroskopische Ellipsometrie ist eine schnelle, zerstorungsfreie und zuverlassige Methode zur Charakterisierung von Dunnschichten, die auf der Wechselwirkung von Licht mit einem Schichtsystem beruht. Bei den hier durchgefuhrten Untersuchungen wurde dieses Verfahren im sichtbaren Spektralbereich zur Charakterisierung von Kohlenstoff-Schutzschichten (kurz: C-Schutzschichten) von 2-7nm Dicke auf Magnetspeicherplatten eingesetzt. Der spezifische Schichtaufbau einer Magnetspeicherplatte konnte mit einem angepassten optischen Einschicht-Modell beschrieben werden. Im Hinblick auf eine genaue Analyse der bedeckenden C-Schutzschicht wurde ein zuverlassig reproduzierbares Verfahren zur Bestimmung des unterliegenden metallischen Materials entwickelt. Das verwendete Modell zeigt sich bis zu Kohlenstoff-Dicken von 2 nm geeignet, was sich in einer linearen Abhangigkeit der ellipsometrischen Parameter (ψ, Δ) von der Schichtdicke ausert. Uber Simulationsberechnungen konnten Kriterien zur Selektion von optimierten Wellenlangen zur Schichtcharakterisierung gefunden werden. Weiterhin wurde ein Anstieg des Extinktionskoeffizienten κ fur einen zunehmenden Stickstoff-Gehalt in der Schicht festgestellt. Auftretende zeitliche Instabilitaten bei der Bestimmung der Schichtdicke d und des Extinktionskoeffizienten κ der C-Schutzschicht liesen sich durch Adsorptionsprozesse an der Oberflache erklaren. Spectroscopic Ellipsometry is a fast, non-destructive and reliable method for characterizing thin films, based on interaction between incident light and a multilayer system. For our investigations, light in the visible spectral range has been used to characterize protective carbon coatings with thicknesses of 2-7nm on magnetic hard disks. The specific disk layer stack has been described with an adequate one-layer model. With regard to an accurate analysis of the covering carbon coating a reproducible procedure for determining the underlying metallic material has been developed. The measured ellipsometric parameters (ψ, Δ) display a linear dependence on carbon film thickness which shows an appropriate application of the used model down to a thickness of 2 nm. By means of simulation calculations, criteria for the selection of optimized wavelengths with respect to film characterization has been established. Furthermore, an increasing extinction coefficient κ with rising nitrogen content in the carbon coating could be stated. Apparent time instabilities in the determination of layer thickness d and extinction coefficient κ of the carbon film could be explained as due to adsorption processes on the surface.

Flame Diagnostic Using Optoacoustic Laserbeam Deflection And Frequency Modulation Spectroscopy

Two laser-based techniques, Optoacoustic Laserbeam Deflection (OLD) and Frequency Modu-lation (FM) spectroscopy, were introduced for flame diagnostics. Flame temperature was measured by OLD with 1.5% precision and flame temperature profiles were taken. Ground state and excited state sodium within an aspirated flame were detected by FM-spectroscopy. Present limits of sensitivity are absorptions of 1.5 . 10-4 and ground state sodium densi-ties of 3.3 • 106 cm -3.