Our Focus

Our current focus is precision atomic structure measurements of multi-electron atomic systems. Precise measurements of energy level structure (hyperfine splittings and isotope shifts) of complex atoms play an essential role in guiding the refinement and testing the accuracy of ab-initio atomic theory calculations which help to develop state-of-the-art atomic theories. Also, Group IA: Alkali atoms are used in many atomic physics based experiments including laser cooling & trapping, Bose-Einstein condensation and quantum computing. Our current candidates are Group IA: Rubidium, Cesium and Group IIIA: Gallium.

Direct Absorption Spectroscopy

Figure 1: Direct absorption spectroscopy

In direct absorption spectroscopy, a laser beam transmitted through an atomic vapor cell is directed towards a photodetector. When the laser frequency is tuned such a way that its energy matches the energy difference between two atomic energy levels, the atom will absorb the energy from the laser beam (i.e. the outer-most loosely bound valance electron will absorb energy and as a result it will jump into a higher energy level so that the atom is excited) and as a result the transmitted power of the beam is reduced. In other words, if we check the transmitted power of the beam while the laser is scanning (ramping frequency) it will show a power drop (an absorption dip) as shown in the figure 2. Since the atoms are in motion inside the vapor cell (similar to a gas) this signal is typically so broad (Doppler broadening).

This signal depends on atomic density N, length of the cell L and frequency dependent absorption cross-section \sigma. The product of these three quantities is called the optical depth (OD). For small optical depths, the signal may be modeled numerically using the Voigt line shape.

Figure 2: Typical Doppler broadned direct absorption spectra for small optical depth.

Saturated Absorption Spectroscopy

Figure 3: Saturated absorption spectroscopy setup

In saturation absorption spectroscopy, two low power laser beams transmitted through an atomic vapor cell is directed to a differential photodiode which produce an electric signal proportional to the power difference of the incident laser beams. When the laser frequency is tuned such a way that its energy matches the energy difference between two atomic energy levels, the atom will absorb the energy from the laser beam and as a result the transmitted power of the beam is reduced. In other words, if we check the transmitted power of one of the beams while the laser is scanning (ramping frequency) it will show a power drop (an absorption dip). Since the atoms are in motion inside the vapor cell (similar to a gas) this signal is typically so broad (Doppler broadening) such a way that we cannot resolve individual dips correspond to hyperfine transitions (see figure 3 -right for an example). All of these dips are embedded in one broad absorption dip as shown in the figure 4(a).

Figure 4: (a) direct absorption signal, (b) direct absorption signal with high power beam present, (c) Differential signal ((b) - (a))

However, when a strong (high power) counter propagating beam is present with one of the low power beam (see figure 3 for low-power, high-power overlapping beams) atom absorbs more energy from the strong beam than the weak low power beam when the laser frequency matches either of atomic hyperfine transition energies . In other words, low beam shows less absorption at those occasions and produce a signal as shown in the figure 4(b) at the photodetector. Use of differential photodiode allows us to create a difference signal (subtract 8(a) from 8(b)) and extract a signal which contain significantly narrow peaks corresponding to all hyperfine transitions described above (see figure 3; right for an example). This technique also helps us to reject common mode noise associated with individual signals. A typical signal may look like the one shown in figure 4(c). This signal may be fitted with sum of Lorentzian line shapes to determine the center positions of the peaks with high accuracy and hence provide the differences between peaks which correspond to hyperfine splittings measurement.

NOTE: Additional (cross-over) peaks may be observed.

Two-Step (two-photon) Absorption Spectroscopy

Figure 5: Two-step absorption spectroscopy setup

In two-setup excitation, the fist step laser is locked to the relevant 1st step transition (this locking-setup is not shown here for the clarity) and another 2nd step laser is scanned across the targeted 2nd excited state hyperfine levels. Both beams are overlapped in the vapor cell (interaction region). The direct 2nd step absorption signal is so week (~1% of 1st step absorption) such that hyperfine absorption dips are almost impossible to observe. This problem can be solved by employing a phase sensitive detection technique. We chop the 1st step laser beam using an optical chopper wheel and detect the 2nd step absorption signal oscillating at chopped frequency using a lock-in amplifier. The technique allows us to detect extremely small absorption dips embedded in the noise which is almost impossible to detect otherwise. The typical spectra corresponds to two 2nd excited state hyperfine levels is shown in the figure 6.

Figure 6: Typical 2-hyperfine level spectra observed using two-step phase sensitive detection. These Doppler-free spectra may be modeled by Lorentzian line shape. Such a fit is also shown here in red.