REPORT
on the activities during the year 2013
The activities in the INFN-ICTP lab during the year 2013 continued to develop in two main directions: the first one was studying the characteristics of the nonlinear system emitting mid-infrared (MIR) radiation at 6.78μm through direct difference frequency generation (DFG) in non-oxide crystals and the other one continuing the efforts to build an analytical system based on the Quantum Cascade Lasers (QCLs) laboratory and instrumentation grown in the last years.
From the big varietyofopticalschemessuitableforthegenerationofradiationwiththedesiredparameters for the mounic-hydrogen experiment, which is the final target of our development, wechosethe onethatinouropinion is the most promisingtoreachthegoal – directDifferenceFrequencyGeneration(DFG)innon-oxidecrystalswithpumpandsignalcomingfromonenarrowbandnanosecondfixedwavelengthandonetunablesolidstatelasersemittingatwavelengthsbelow2μm. The DFG schemes though having relatively smaller conversion efficiency have the advantage (compared to OPO, OPA and OPG) of much narrower linewidth which is of crucial importance for the muonic hydrogen experiment. The lasers that we have used to build the demonstrating system were Q-switchedsinglefrequencyNd:YAG(1064nm)andanarrowbandCr:Forsteritelaseroperatingat around 1260nm,pumpedby another Nd:YAGlaser. This test setup was first studied in 2012 in order to prove thepossibilityofbuildingalasersystemthatgeneratesalaseremission fulfillingtherequirementsofthe μh experiment [1] namely – a nanosecond mid-IR laser source tunable around 6785±3nm, with a linewidth smaller than 0.07nm and energy about 1 mJat a repetition rate of 50Hz (limited by the available muonic hydrogen sources), - several possible schemes were proposed.
Two among the known nonlinearcrystals which are suitable, duetotheirhighdamagethresholdandrelativelyhighnonlinearity, forthegenerationof tunableinfraredradiationaround6.8μm were studied – AgGaS2andLiInS2. Althoughthevalues, reportedintheliterature,forIRradiationfromLiInS2inthespectralrangearound6μm,arenotashighas those forAgGaS2,LiInS2 is apromisingcandidate since,whilehavingasimilarnonlinearcoefficientasAgGaS2,has amuchhigherdamagethreshold(almostanorderofmagnitude) andabout5timeslargerthermalconductivity.
In short the scheme we have tested is based on direct DFG using a fixed wavelength single-mode Nd:YAG laser and a tunable narrowband Cr:forsterite laser pumped by a second Nd:YAG synchronised to the first one,withthecorrespondingtimingbetweenthem,givingusthepossibilitytoreachtherelevantsynchronizationofthelaserpulses.
The layouts of the systems for the characterizationDFG(basedonthenonlinearcrystalschosen – AgGaS2andLiInS2) are shown on Fig.1 andFig.2.
A lamp pumped Q-switched Nd:YAG laser with output energy of 150 mJ is used to pump the Cr:forsterite laser. The latter contains a diffraction grating as an output coupler, allowing to obtain tunable light in the 1.200-1.280 μm range with an energy per pulse of up to 15 mJ and a line-width of about 8-10 pm. Its pulses are then combined with the pulses at 1.064 μm of a diode-pumped YAG (Nd:YAG2 on the figure) through a dichroic mirror and sent to the nonlinear crystal in a single pass or double pass geometry – everycrystalwastestedforbothschemes. The two lasers are triggered through a delay generator allowing to compensate the build-up time of the Cr:forsterite pulse and had a relative timing jitter below 1 ns. Theparametersstudied oftheIRradiationgeneratedbythetwoschemeswere:
- maximumoutputenergy/power,
- pulseduration,
- wavelength,
- tunability
- linewidth,
we also did anestimationoftheeffectiveconversionefficienciesanddamagethresholdsofthecommerciallyavailable nonlinear non-oxideNLcrystals – AgGaS2andLiInS2 – intheregimesofinterestfortheDFGschemes.
Five different crystals(two AgGaS2 crystals and three LiInS2 crystals)were tested.Theidea wastocomparethepropertieseach of the two types of the nonlinearcrystals and the properties crystalsfromdifferentsuppliers – this intermsofdamagethreshold,conversionefficiency,etc.
The damage thresholdsfor the radiation at 1.06μm are ~18MW/cm2 when the radius of the Nd:YAG laser beam is taken at 90% of peak energy and 56MW/cm2 in the case when the radius is taken at 50% of peak energy. As for the 1,26μm radiation no damage of the crystals was observed up to 150MW/cm2.
Thenonlinearcrystalsunderstudyarewithdimensions5x5x5mmand10x10x5mmwidth/height/lengthrespectively.Theoutputbeamsofbothlasers(Nd:YAGandCr-forsterite)were varied from 5mm (at 90% from peak energy)/3.2mm (at 50% from peak energy) to 3.8mm (at 90% from peak energy)/2.1mm (at 50% from peak energy) respectively. Thus varying the power densities from 10MW/cm2 to 25MW/cm2 (when radius is taken at 90% from peak energy) and from 18MW/cm2 to 56MW/cm2 (radius at 50% from peak energy) for the radiation at 1.06μm for the Nd:YAG laser. And varying the power densities from 5MW/cm2 to 12MW/cm2 (when radius is taken at 90% from peak energy) and from 8.5MW/cm2 to 28MW/cm2 (radius at 50% from peak energy) for the radiation at 1.26μm for the Cr:forsterite laser.
The highest achieved energy at 6.8μm was 72μJ, that gives a conversion efficiency of 0.25%, the results are for radii of the beams 3.8mm (at 90% from peak energy)/2.1mm (at 50% from peak energy) in the single pass geometry (Fig.1) using two identical LiInS2 nonlinear crystals (each 5mm long) put in sequence. Theresultsobtainedshowthat for a laser system with theappropriate parameters (output energies),evenkeepingthepowerdensitiesatthepresentlevelsbelowthedamagethresholdsoftheARcoatings and the nonlinear crystals,itispossibletoobtain infrared radiation at 6.8μm of about 2,5mJfrom commercially available LiInS2crystalwithdimension 20x10x10mm. Previous results have shown that in the double pass geometry the generated energy at 6.8μm is more than 60% higher than in the single pass geometry. This means that from a single LiInS2 with dimensions 20x10x10mm it possible to generate more than 4 mJ infrared radiation at 6.8μm exceeding the 0.25mJ required for the experiment. It has to be remembered that there is always a possibility to put more than one nonlinear crystal in sequence.
The tunability range of the infrared radiation generated with the present setup was studied. The results for two of the LiInS2 crystals are plotted on Fig.3.
A wavelengthmeter (HighFinesse-Angstrom WS6-200) for a precise analysis of the emission spectrum of the infrared radiation with resolution 0.001nm is used. This device also allows to measure the linewidth of the infrared emission with resolution of 200MHz. Initial test of the wavelengthmeter have been performed. For a cross check, the linewidths of the DFG radiation along with the linewidths of the radiations of the available QCL's were measured. The obtained values are: 1.5GHZ for the DFG radiation at 6.8μm, 450MHz for the pulsed QCLs at 6.8μm and 150MHz for the cw QCLs at 9.1μm, respectively.
New proposal (on the amount of ~1 million Euro) for the realization the mounic-hydrogen experiment was submitted to the financing board of the Group III (nuclear physics), of the INFN. The proposal was approved for the duration of 4 years and will allow the scientific team, which includes scientists from 7 research organizations, to measure hyperfine splitting of the 1S state of the muonic hydrogen contributing this way to throw some light to the theme of the socalled proton radius puzzle and opens a new line of precision experiments using intense pulsed muon beams. The first tests will be performed at Ratherford Laboratory (GB) mid 2014.
A visit to the Laser physics group at theDepartment of Applied Physics,School of Engineering Sciences,atKTH -(The Royal Institute of Technology, Stockholm, Sweden), was done and an agreement for a future collaboration was achieved. In the scheme which has been elaborated and will be proposed quantum cascade lasers, QCL's, emitting in the spectral range 6.8μm will be used to seed a laser system – developed at KTH, based on optical parametric oscillation (OPO) in ZnGeP2 nonlinear crystal in order to decrease the linewidth of the inherently wide broadband of the OPO emission.
For the next 2014 year the activities on improving and studying of the existing at the moment nonlinear optical system will continue together with the work required by the above mentioned collaboration. We consider also to test a new optical scheme with the LiInS2 nonlinear crystal put in the resonator of the Cr:forsterite laser.
The work on the QCLs was split in two main directions; the use of an array/matrix of QC amplifier stages with a single seed CW laser was considered as very promising to reach high power monochromatic light. If proved this approach will give us the possibility to predefine the emission parameters of an array/matrix of QCLs by only varying the parameters of the seeding QCL. Thus avoiding the more complicate case of driving independently all QCLs in an array/matrix.To prove this approach a setup was build in order to study the possibility to increase the monochromatic output from the QCLs by injecting a beam from Fabry-Perot QCLs with narrow-bandwidth distributed feedback (DFB) QC laser. To produce strong pulses it was decided to use one of the possible approaches – namely to use a cw QCL as a pump and power the gain media (a broadband QCL) at a low duty cycle, also this option should provide clean pulses as the gain media is opaque to the pump when not biased. This approach was chosen based on the QCLs already available at the laboratory. For this purpose home-made housings for the room-temperature operating FP QCLs were designed and build. The temperature variation (in the interval from -30˚C up to +25˚C) is achieved by Peltier coolers and regulated by temperature controllers with accuracy 0.1˚C. The dependence of the output power of the four available FP QCLs from the applied voltage at different operating temperatures was studied.
A home-made power supply for driving the cw QCL was design and build. The dependence of the output power and the wavelength tunability of the cw QC laser from the operating temperatures and the applied current was studied. Also the linewidth of the cw QCL was measured by the Angstrom-HighFinesse wavelengthmeter WS6-200.
The other implementation of the QCLs was to build a setup for detection of gases by using cavity ring-down spectroscopy (CRDS) this activity was conducted mainly by Dr. B. Andreson, (ICTP associate – Gana) and Dr. K. Gadedjisso-Tossou (TRIL/ICTP – Togo). The setup was build but no final results were obtained since the existing available pre-amplifier of the Judson detector is designed to work in cw mode while the laser used for the cavity ring-down spectroscopy is a pulsed QCL (a condition predefined from the lab equipment: the mirrors of the ring-downcavity are coated for 6.8μm and the available QCLs emitting at 6.8μm are working in pulsed mode). To resolve the problem a suitable pre-amplifier was bought, which will be delivered in February.
The broadband emission spectrum of the FP QCLs will be studied with the Angstrom-HighFinesse wavelengthmeter WS6-200.
References:
[1]D. Bakalov, E. Milotti, C. Rizzo, A. Vacchi, and E. Zavattini, “Experimental method to measure the hyperfine splitting of muomic hydrogen (μ-p)1S”,Phys. Lett. A 172, 277 (1993).
Andrzej Adamczak , Dimitar Bakalov, Lyubomir Stoychev, Andrea Vacchi “Hyperfine spectroscopy of muonic hydrogen and the PSI Lamb shift experiment” Nuclear Instruments and Methods in Physics Research B 281 (2012) 72–76