There are many advantages in using more than two photon for excitation of an atomic sample to an excited state, which may include the ability to replace UV lasers or expansive Second Harmonic Generation systems for one and two photon excitation process. When using more than two photons we can explore the high dipole coupling matrix between adjacent Quantum states, leading to laser frequencies among infra-red, which are much easier to build, control and are suitable for coupling to optical fibers. Our approach is to use the full extend of the proposal and observe Electromagnetic Induced Transparency (EIT) and Electromagnetic Induced Absorption (EIA) in a quantum system of five atomic states, coupled by four infra-red lasers. For this we choose to use a hot atomic vapor sample held in a quartz cell of 2 mm optical path. All lasers are overlapped at the center of the cell in a collinear configuration. The first laser couples that ground state of cesium (Cs) 6S1/2 to the first excited state 6P3/2 using the known D2 line of the cesium spectra, at the wavelength of 852 nm, this laser is also known as the probe laser, as is used in very low intensity (low power regime) and just probes the quantum states. The second Laser is a 1470 nm laser that couples the first excited state to 7S1/2 state, we called the dressing laser, the seconde dressing laser couples state 7S1/2 to the second intermediate state 8P1/2, and this state is couple to a Rydberg state using a 1770 nm laser. This laser is called the coupling laser since it saturates the transition on the last step of the excitation. All paths that leads to the Rydberg state can interfere mutually given rise to interference effects, this interference can be observed on the probe laser. An absorption signature is related to constructive interference pumping the atom to the final state, a transmission signature instead is due to destructive interference avoiding this excitation of the Rydberg state. All of this processes of quantum interference are known as Fano interferences.
More informations about the experimental setup can be found on the published paper:
Kondo, J.M., Šibalić, N., Guttridge, A., Wade, C.G., De Melo, N.R., Adams, C.S. & Weatherill, K.J. (2015).Observation of interference effects via four-photon excitation of highly excited Rydberg states in thermal cesium vapor. Optics Letters 40(23): 5570-5573.