Nella regione spettrale compresa tra 3 e 11 micron si trova la maggior parte delle transizioni roto-vibrazionali fondamentali delle molecole. Tali transizioni presentano un’intensità di riga superiore di diversi ordini di grandezza rispetto a quella esibita nelle stesse specie molecolari dalle transizioni che cadono nella regione del visibile o del vicino infrarosso. Di conseguenza, l’eventuale disponibilità di sorgenti di radiazione coerente nel medio infrarosso accrescerebbe notevolmente la sensibilità di rivelazione per numerose molecole di interesse nella diagnostica ambientale e bio-medica. Di fatto, però, gli attuali limiti tecnologici offrono una scarsa e spesso inadeguata copertura della suddetta regione spettrale con sorgenti laser dirette. Per ovviare a tale lacuna, la radiazione della lunghezza d’onda desiderata viene prodotta mescolando opportunamente in un cristallo non lineare due fasci laser che emettono nel visibile o nel vicino infrarosso. L’interazione delle due onde, mediata dalla suscettività non lineare del mezzo, permette di generare in uscita al cristallo un nuovo fascio laser la cui frequenza è la differenza tra le frequenze dei fasci laser incidenti (fenomeno di generazione di frequenza differenza). La lunghezza d’onda λ così generata è data da:

λ=(1/λ₁-1/λ₂ )^-1(1)

L'analisi chimica in-situ ed in tempo reale di gas in traccia basandosi su tecniche di assorbimento rappresenta un problema rilevante in diverse aree di ricerca, dal monitoraggio ambientale alla diagnostica biomedica. Poichè molte specie di interesse sono naturalmente presenti in concentrazioni molto basse, da alcune parti per milione (ppmv) ad alcune parti per trilione in volume, le assorbanze risultanti possono essere molto basse.
Inoltre per applicazioni in campo di monitoraggio di aria- ambiente, un sistema basato su spettroscopia laser deve combinare un set-up compatto e robusto e insieme utilizzare tecniche ad elevata sensibilità. Nel nostro lavoro molti sforzi sono However, since most species of interest are naturally present in very low concentrations, ranging from parts-per-million (ppmv) to parts-per-trillion (pptv) by volume, the resulting absorbance can be extremely small. Hence for ambient-air monitoring field applications, a laser spectroscopic system must combine a compact and robust set-up with a high sensitivity technique. In our work, many efforts are focused on the use of laser sources operating at wavelenghts where the strongest transitions occurr, in conjunction to high-sensitivity spectroscopic techniques particularly suitable for field applications. For these purposes, the window between 2.5 and 3.5 μm is particularly attractive, because it exhibits several strong and well-isolated lines for a large number of molecules (C-H, N-H and O-H fundamental vibrations occur in this interval). However, this spectral region is not accessible at all to single, continuously tunable laser devices. Optical parametric oscillators (OPOs) and difference-frequency generators (DFGs) proved to be the only effective tools for high resolution and high sensitivity spectroscopy in this wavelength interval, thanks to their wide tunability, low noise and narrow linewidth. In this framework, the first step is represented by the realization of a compact and rugged portable spectrometer for field applications based on difference frequency generation (DFG) in a periodically-poled lithium-niobate crystal, emitting between 2.9 and 3.5 μm with a power of several milliwatts. The performance for real-time monitoring of trace-gases was evaluated with a direct-absorption scheme in a 13-m multiple-reflection cell, recording direct absorption profiles of natural-abundance methane at atmospheric pressure wiith a noise-equivalent absorption coefficient of 5.8·10^-7cm^-1·Hz^-1/2, (corresponding to a minimum detectable CH₄ concentration of 3 ppb·Hz^-1/2). To further improve the detectivity, preserving the possibility of performing field measurements, a frequency modulation and a long-path technique were demonstrated with the DFG spectrometer. In the first case two tone FM spectroscopy provided a significant noise reduction with a reasonable increase in technical demand. As a proof of sensitivity, CH₄ ro-vibrational transitions in ambient-air samples were detected, resulting in an enhancement of a factor 100 in the signal-to-noise ratio with respect to the direct absorption signal, and thus a minimum absorption coefficient of 5.3·10^-9cm^-1·Hz^-1/2. The other scheme used for high-sensitivity gas detection is off-axis integrated-cavity-output spectroscopy (OA-ICOS). This high finesse cavity-based technique can be realized with a very simple set-up, and, being intrinsically insensitive to small vibrations and misalignements, is particularly suitable for field applications. Its only disadvantage may rely in the reduced throughput of the cavity, that has so far prevented its use with DFG sources. However, thanks to the high output power of our DFG spectrometer, we could successfully use OA-ICOS to perform ambient air methane detection at atmospheric pressure. An 1.8 km equivalent cavity absorption pathlenght was demonstrated, yielding a minimum detectable concentration of 850 pptv·Hz^-1/2. Furthermore an analytical model, based on the off-axis cavity response function, enabled us to retrieve the absorber concentration directly from the area under the recorded absorption profile.

P. Maddaloni, G. Gagliardi, P. Malara, P. De Natale,
"A 3.5-mW continuous-wave difference-frequency source around 3 μm for sub-Doppler molecular spectroscopy",
Appl. Phys. B 80, 141 (2005).

P. Malara, P. Maddaloni, G. Gagliardi, and P. De Natale
Combining a difference-frequency source with an off-axis high-finesse cavity for trace-gas monitoring around 3 μm
Optics Express

P. Maddaloni, G. Gagliardi, P. Malara and P. De Natale
Off-axis integrated-cavity-output spectroscopy for trace-gas concentration measurements: modelling and performance
JOSA B, in press

P. Maddaloni, P. Malara, G. Gagliardi, and P. De Natale
Two-tone frequency modulation spectroscopy for ambient-air trace gas detection using a portable difference-frequency source around 3 µm
Appl. Phys. B, in press