Development of a System for High Resolution Spectroscopy

Development of a System for High Resolution Spectroscopy with an Optical Frequency Comb Dept. of Applied Physics, Fukuoka Univ. , JST PRESTO, M. MISONO, M. OKUBO, K. DAIRIKI

AIM OF THE STUDY Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect High resolution spectroscopy molecular structure dynamics Precise measure of optical frequency Optical frequency

EXCITED STATES OF NAPHTHALENE Singlet 1 B 1 u 1 B 3 u 1 B Triplet 3 u(S 2) 4 b 1 u 1 B (S ) 1 u 1 Inter system crossing 3 B 3 u 3 B 1 u(T 2) 3 B 1 A g(S 0) 3 u(T 1) y (c) x (b) z (a)

EXPERIMENTAL SYSTEM Stabilization 1 octave optical frequency comb Nd 3+: YVO 4 laser Ti: Sapphire laser Single mode laser Stabilization High finesse resonator Photonic Crystal Fiber Photo Diode High resolution spectroscopy Frequency counter Beat measurement

HIGH RESOLUTION SPECTROSCOPY Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect High resolution spectroscopy molecular structure dynamics Precise measure of optical frequency Optical frequency

DOPPLER-FREE TWO-PHOTON ABSORPTION SPECTROSCOPY Mirror hν(1 + u /c) u ν ν(1 - u /c) ν(1 + u /c) 2 hν hν(1 - u /c) - Doppler free - Enhancement of light intensity for excitation - Selection rule is different from one photon absorption - Transition in UV region is excited by a visible laser - All molecules contribute to the signal independent of their velocities

EXPERIMENTAL SETUP Pump laser Dye laser λ/4 PD + － PZT PD Isolator Color filter Photomultiplier tube Photon counter Computer PD

OPTICAL RESONATOR AND SAMPLE CELL

PRECISE MEASUREMENT OF OPTICAL FREQUENCY Frequency shift, line broadening, line splitting, intensity anomaly, magnetic effect High resolution spectroscopy molecular structure dynamics Precise measure of optical frequency Optical frequency

1 OCTAVE OPTICAL FREQUENCY COMB Cs atomic clock in GPS satellites Stabilization Intensity f. CEO , frep ・Ultimate precision ・Low cost f. CEO frep （～ 100 MHz） Frequency optical frequency Atomic clock　　　　Precise measure of frequency

GENERATION OF 1 OCTAVE OPTICAL FREQUENCY COMB ～ 20 fs Ultra-short pulse laser Repetition rate = 100 MHz ～ 130 nm mode separation = 100 MHz Photonic Crystal Fiber > 1 Octave Frequency

DETECTION OF f. CEO Fundamental Intensity Second harmonics f. CEO High frequency end of fundamental Low frequency end of second harmonics Beat frequency Frequency f(2 n) = 2 nfrep + f. CEO 2 f(n) = 2 nfrep + 2 f. CEO (D. J. Jones, et al. , Science 288, 635 (2000))

CONTROL OF frep AND f. CEO Pumping intensity：f. CEO (, frep) Intensity modulator Output coupler End mirror (Cavity length)：frep, f. CEO RF synthesizer Cs atomic clock

BEAT NOTE OF REPETITION AND OFFSET Beat by offset 40 d. B Intensity [10 d. B/div] Beat by repetition f. CEO frep 0 Frequency [MHz] 120

STABILIZATION OF frep [MHz] 100. 000 012 100. 000 Gate time = 1 s 99. 999 988 0 Time [s] 600

STABILIZATION OF f. CEO Frequency [MHz] 26. 12 Gate time = 1 s 25. 00 0 Time [s] 700

SUMMARY • High resolution spectroscopy with a 1 octave optical frequency comb was studied to elucidate the structure and dynamics of polyatomic molecules. • An experimental system for Doppler-free two photon absorption spectroscopy was developed. • A stable mode locked Ti: Sapphire laser was developed and a 1 octave optical frequency comb was realized. • The repetition rate (mode separation) of the 1 octave optical frequency comb were stabilized.

HIGH PRECISION MEASUREMENT WITH OPTICAL FREQUENCY COMB Intensity flaser Frequency f. CEO frep fbeat flaser = nfrep + f. CEO+ fbeat

Optical frequency comb Intensity High resolution spectroscopy OPTICAL SYNTHESIZER flaser fn fsyn Single mode flaser RF Synthesizer fsyn fn+1 frep - fsyn flaser = fn + fsyn Frequency

OPTICAL SYNTHESIZER Frequency shift by AO modulator fsyn 0 fsyn frep - fsyn Frequency [MHz] 120 0 frep - fsyn Frequency [MHz] 120

STABILIZATION SYSTEM FOR OPTICAL FREQUENCY COMB Bandwidth 130 nm Pulse width 20 fs Repetition rate 100 MHz Intensity modulator Ti: Sapphire laser DM Photonic Crystal Fiber BBO Prescaler RF oscillator Tracking Oscillator RF synthesizer Cs atomic clock（GPS satellite） f. CEO HM PD frep DM PD PD: Photo diode DM: Dichroic mirror HM: Half mirror

frep Bandwidth: 300 k. Hz

Spectral Intensity [10 d. B/div] FREQUENCY REFERENCE FOR STABILIZATION OF f. CEO 25 MHz Frequency [25 k. Hz/div]

FREQUENCY REFERENCE FOR STABILIZATION OF f. CEO Frequency [MHz] 25. 00 24. 93 0 Time [s] 2000

DEVELOPMENT OF HIGH RESOLUTION SPECTROSCOPY Doppler effect precision ～ 10 -6 Doppler-free spectroscopy Measure of optical frequency precision ～ 10 -8 Optical frequency comb This study Free from limitation of experimental system precision ～ 10 -11

DOPPLER BROADENING ～ 4550 cm-1 Doppler-limited 235 　 　　　 Optical frequency 260 [nm] Doppler-free Laser light 0. 3 cm-1