Doppler-free Saturated Absorption Spectroscopy By Priyanka Nandanwar

What I’m going to talk about Theory Doppler-free spectrum Role in the Laser Stabilisation

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

Interaction with atomic vapour

Spectral broadening

Maxwell Boltzmann-distribution

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 ̥ for transition to occur

Doppler broadened spectrum of rubidium sample Δν½

The Solution

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 Vzto 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) 1560nm780nm Detector 99% p PBS NPBS Mirror Polarising Beam Splitter 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