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Dipartimento di Fisica - Politecnico di Milano

Coherent Raman metrology of molecular hydrogen

Premise: the theses are connected the research activity of the CHROME laboratory on the subject of Coherent Raman metrology of molecular hydrogen.


Experiments vs ab-initio theory for the H2 rovibrational transition energies
The thesis focuses on the ultra-accurate experimental determination of the energies of several Q- and S-branch transitions of the fundamental rovibrational band of H2, which fall in the 4000-4600 cm-1 range. The molecular excitation is obtained in a Coherent Raman approach by means of two cw laser fields, the so-called pump (ωp) and Stokes (ωS) fields, whose frequency difference is made to match the vibrational frequency of the transition, according to the scheme depicted in the figure. For the measurements to be accurate, the frequencies of both lasers are calibrated against an atomic clock through interposition of an optical frequency comb (Nobel prize invention for Physics in 2005), which can be thought as an absolute ruler for optical frequencies. As measurements are performed at several pressures under the perturbation of collisions, multiple-fitting procedures are applied to the Coherent Raman spectra to extrapolate transition energies at zero-pressure and compare them with ab-initio calculations of the molecule at rest.


Frequency metrology of purely rotational lines through Coherent Raman comb-assisted spectroscopy
Using the same experimental approach described above for the fundamental rovibrational band of molecular hydrogen, also purely rotational lines (such as the S line depicted in Fig. 1) may be addressed. The Coherent Raman approach offers indeed the unique chance to populate rotational channels using near-infrared lasers instead of the far-infrared sources needed by conventional absorption spectroscopy. The calibration of pump and Stokes laser frequencies against an optical frequency comb adds the chance of performing unprecedented experiments of precision rotational spectroscopy with an absolute frequency axis. This approach will be applied to diatomic molecules such H2, D2 and N2 as well as to molecules such as CO2 and C6H6 whose symmetry prevents any electric-dipole allowed absorption transition.



Formative aspects of the theses
Besides an insight into several physics subjects, namely molecular spectroscopy, Coherent Raman spectroscopy, collisional physics and optical metrology with frequency combs, the thesis activity gives the chance of an intensive training over a large number of experimental areas: i) alignment of complex optical setups, ii) data acquisition and processing with FPGA boards; iii) PID (proportional integrative derivative) control loops, which are used for active stabilization of the comb repetition rate, of the laser intensity, of the pump laser frequency, of the cell pressure and temperature; iv) Gaussian beam profiling and optical fiber coupling, handling, splicing; v) modulation-transfer detection, which is used for the Stimulated Raman Scattering signal; vi) Labview programming for remote control of instruments and acquisition; vii) calculations and fitting of experimental data in a Matlab environment; vii) gas handling; viii) use of oscilloscopes, radiofrequency synthesizers, electrical spectrum analyzers, frequency counters, lockin amplifiers.


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