Doppler-free Saturated Absorption Spectroscopy By Priyanka Nandanwar.
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Transcript of Doppler-free Saturated Absorption Spectroscopy By Priyanka Nandanwar.
History
• Arthur Leonard Shawlow
• Nobel Prize 1981
• Beforehand:– Doppler widths of 500 MHz in optical spectra– Limits on resolution
Lasers
• Light Amplification by Stimulated Emission of Radiation
• Monochromatic• Directional• Coherent
Spectral broadening
• As the laser is finely tuned, atoms with a variety of velocities about 0msˉ¹ absorb radiation
• Absorption peak broadened
• Blurs transition energy
Doppler Shifts
• Random thermal motion of atoms and molecules creates shift in the absorbed radiation
• Blue or red-shifted• Laser frequency is increased (blue shifted)
from frame of atoms approaching it, so it needs to be less than ν 9 for transition to occur
•Doppler-broadened spectral lines shown beforehand obtained from second probe beam.
•←Doppler broadened spectral lines with hyperfine structure.
•Subtracting the two gives…
• Subtraction of the two spectral yields hyperfine structure
• Bottom waveform:– Piezo driver increases
and then decreases frequency
– Partial mirror image
First probe and pump beams
• Pump changes population of atomic states
• Probe detects changes
• Doppler effect casues atoms only with certain velocity Vz to be in resonance with left-moving probe beam
Laser frequency correctly tuned
• Atoms moving 0msˉ¹ (stationary) relative to beams absorb both probe and pump energy
• Pump depletes unexcited population• First probe reduced absorption• Second probe has no reduced absorption
•Frequency of laser<Resonance frequency •Pump photons absorbed if, in atoms’ frame, laser frequency shifted to resonance•Probe beam excites left-moving atoms with same speed as right-moving atoms•Subtract detected signals from each probe beam
SHG(Second Harmonic
Generator)LASER
HWP(Half-Wave Plate)
1560nm 780nm
Detector
99% p
PBSNPBS
Mirror
Polarising Beam SplitterMirror
Mirror
1% s
How we will use this…
• Rubidium vapour cell
• ~780 nm frequency standard
• At PBS, light split into opposite polarisations that progress to probe and pump beams
• NPBS and PBS cause two probes to pass through the cell
• Two detectors
Conclusion
• Doppler effect:– Broadens the atomic absorption spectrum
• Saturated absorption spectroscopy:– Doppler broadened peak is resolved– Exact energy of hyperfine transitions can be
found
Crossover peaks
• Appears midway between any two transitions that have the same lower level and two different excited levels.
• Laser tuned midway between two transitions
• Atoms moving relative to beams resonate with pump and probe