Wavelength Frequency Measurements

Transcription

1 Wavelength Frequency Measurements Frequency: - unit to be measured most accurately in physics - frequency counters + frequency combs (gear wheels) - clocks for time-frequency Wavelength: - no longer fashionable - unit [m] no longer directly defined - always problem of the medium- index of refraction Units: exercise convert - Ångstrom -> nm - ev, J, cm -1, Hz

2 Notes Laser Spectroscopy, Vol 1 W. Demtröder Chapter 4 Spectroscopic instrumentation ( ) Spectrographs, Monochromators, Prisms and Gratings Interferometers Fabry-Perot (etalon), Michelson, Mach-Zehnder Wave meters Michelson, Sigma, Fizeau, Fabry-Perot Chapter 9 Frequency measurement/frequency comb (9.7)

3 (Wave)Length standard Until 1960 Krypton (Kr): International Standard of Length Now, since 1983 L = c t m c = s The picture shows a device holding a tube of krypton gas. The isotope Kr-86 contained in the tube can be excited so that it emits light. The international standard of length is one meter, which is 1,650, wavelengths of radiation emitted by Kr-86. A practical realisation of the metre is usually delineated (not defined) today in labs as 1,579, (39) wavelengths of helium-neon laser light in a vacuum.

4 Time standard (and realization) the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom accuracy Cs fountain clock

5 Classical Spectrometers Spectral resolution limited by the diffraction determined by total aperture (size of prism or grating) study this Prisms: Grating: how does the dispersion (resolution) depend on geometry/material of prism study the avantages of the Echelle grating

6 Spectroscopy and Calibration Absolute measurements (wavelength or frequency) Relative measurements (line separations) Spectral referencing (use of atlases) I 2, Te 2, Hollow-cathode lamps, Th-Ar Scanning vs Multiplex spectroscopy

7 Example: The Ultraviolet-Visibile Echelle grating spectrometer at VLT

8 8 m mirror at VLT

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11 Detection of all orders on the CCD

12 Instant calibration of all orders by illuminating with ThAr lamp

13 Science CCD s Calibration CCD s

14 Spectral referencing in laser spectroscopy H 2 O absorption

15 Frequency conversion and calibration

16 Referencing against tellurium 130Te2

17 Precision Doppler Free Spectroscopy 1. Saturation spectroscopy Lamb dips 2. Polarization spectroscopy 3. Two-photon spectroscopy 4. Molecular beam spectroscopy Bennet peak Bennet hole Burn a fraction out of the velocity distribution

18 Lamb Dips Saturation holes Lamb dip in scanning Willis E Lamb Nobel Prize in Physics 1955

19 Lamb Dips Saturation in Homogeneous broadening Saturation in Heterogeneous broadening Standing wave field Saturation in Heterogeneous case Weak field probing

20 Lamb dip spectroscopy unraveling overlapping lines

21 Saturated absorption for referencing + Phase sensitive detection lock-in principle

22 Doppler-free two-photon absorption (excitation) Doppler shift: k v ω' = ω Resonance condition: ( E E )/ = ( ω ' + ω ) = ω + ω v ( k + k ) f i 1 2' All molecules, independent of their velocities, absorb at the sum frequency ω + ω 2ω 1 2 =

23 Sub-Doppler spectroscopy in a beam ω' ω0 k v = ω kv x = 0 Reduction of the Doppler width: ωd ' = ωd sin ε ω D = 2ω 0 ln 2 c v p

24 Molecular beam spectroscopy

25 Laser-based calibration techniques Harmonics + saturation

26 Calibration of H 2 spectral lines (in XUV) P(3) C-X (1,0) R(0) B-X (9,0) line