Optical Frequency Comb Referenced Sub-Doppler Resolution Difference-Frequency-Generation Infrared Spectroscopy K. Iwakuni, S. Okubo, H. Nakayama, and H. Sasada Department of Physics, Faculty of Science and Technology, Keio University, Japan H. Inaba National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology 1 International Symposium on Molecular Spectroscopy Ohio State University June, 18, 2012

Outline 1.Optical Frequency Comb Referenced Infrared Spectroscopy Absolute frequency measurements of CH 4 (the ν 3 band) and CH 3 I (the ν 1 band) 2. Modulation Spectroscopy Determination of an electric quadrupole interaction constant 2

Spectrometer in the Last Talk 3 ECAC

Optical-Comb-Referenced Difference-Frequency-Generation Spectrometer 4 current PZT ECAC

ν ECLD Nd:YAG laser f beat2 f beat1 Frequency Control with Optical Frequency Comb f rep

ν ECLD Nd:YAG laser ν DFG = Δn f rep + ( f beat1 – f beat2 ) Δn = n 1 – n 2 ν Nd:YAG = f ceo + n 1 f rep + f beat1 ν ECLD = f ceo + n 2 f rep + f beat2 f beat2 f beat1 ν DFG Frequency Control with Optical Frequency Comb mode number difference Δn

Absolute Frequency Measurement of CH 4 Linewidth (HWHM) 220 kHz Frequency (49) kHz CIPM * (20) kHz CH 4 ν 3 P(7) F 2 (2)  DFG sweep 13.2 kHz/step ( f rep ; 0.01 Hz/step), 10 ms/step  10 times average  pressure 0.4 Pa *) Comité international des poids et mesures

Setup for Determination of Mode Number beat detection 8 InSb PD amplifier f rep 10.7 MHz×2 3.4 μm

Spectra of CH 4 and CH 3 I 9 A1A1 F 1 (1) F 2 (2) CH 4 ν 3 P(6) CH 3 I ν 1 Doppler limited spectra around 2958 cm -1 3 GHz frequency marker signal A2A2 F 2 (1) E frequency marker signal f rep frequency 21.4 MHz

Spectra of CH 4 and CH 3 I 10 A1A1 F 1 (1) F 2 (2) CH 4 ν 3 P(6) CH 3 I ν 1 Doppler limited spectra around 2958 cm -1 3 GHz A2A2 F 2 (1) E P (J=23, K=5) P (J=22, K=6) frequency marker signal frequency frequency marker signal f rep 21.4 MHz

Absolute Frequency Measurement of CH 3 I Hyperfine Structure CH 3 I Hyperfine Structure  DFG sweep 13.2 kHz/step ( f rep ; 0.01 Hz/step), 20 ms/step  40 times average  pressure 0.4 Pa P(J = 22, K=6) frequency – / MHz  linewidth kHz frequency – / MHz CH 3 I ν 1 P(J = 23, K=5) F’’ = 49/2 F’’ = 51/2 39/2 47/2 41/2 45/2 43/2 41/2 49/2 43/2 47/2 45/2

Modulation Spectroscopy 12 ECAC

frequency – / MHz single scan Long-Time Averaging P(J = 22, K=6) CH 3 I ν 1  pressure 0.4 Pa  DFG sweep 13.2 kHz/step ( f rep ; 0.01 Hz/step), 20 ms/step

14 10 times average P(J = 22, K=6) CH 3 I ν 1 Long-Time Averaging  DFG sweep 13.2 kHz/step ( f rep ; 0.01 Hz/step), 20 ms/step  pressure 0.4 Pa frequency – / MHz

1f Signals frequency – / MHz P(J = 22, K=6) P(J = 23, K=5) frequency – / MHz  pressure 0.4 Pa  linewidth kHz CH 3 I ν times average 30 minutes for measurement  DFG sweep 13.2 kHz/step ( f rep ; 0.01 Hz/step), 20 ms/step

Electric Quadrupole Interaction Constant S. Carocci, et al., J. Mol. Spectrosc. 191, 368 (1998) signal / arb. unit frequency – / MHz frequency – / MHz eqQ ‘ / MHz χ J ’ / kHzχ K ’ / kHzχ D ’ / kHz (11) (78)22.45 (32) P (J = 22, K=6) P (J = 23, K=5) ground state :

eqQ / MHz rotation independent χ J ” = χ K ” = χ D ” = 0 eqQ / MHz rotation dependent χ J ” = χ J ’, χ K ” = χ K ’, χ D ” = χ D ’ Previous work * / MHz P ( 22, 6 ) (15) (15) P ( 23, 5 ) (13) (13) Center frequency ** Calculation - Observation / MHz F’’Calculation / MHz Observation / MHz 49/ / / / / / Results of Fitting Center frequency *** Calculation - Observation / MHz F’’Calculation / MHz Observation / MHz 51/ / / / / / (3) *) E. Arimondo, et al., Phys. Rev. A, 17, 1375 (1978) **) MHz P ( J=22, K=6 ) P ( J=23, K=5 ) ***) MHzF’ = F” - 1

Conclusion The sensitivity of the spectrometer is improved by averaging the signal for a long time without any frequency drift or spectral broadening. The absolute frequency of the hyperfine components of the ν 1 band of CH 3 I has been measured for the first time. The absolute frequency of weak transitions can be measured using this spectrometer. 18

19 Thank you for your attention.