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

EXCITED STATES OF NAPHTHALENE 3B3u(T1)3B3u(T1) 3B1u(T2)3B1u(T2) 3B1u3B1u 3B3u3B3u 1B1u(S1)1B1u(S1) 1B3u(S2)1B3u(S2) 1B3u1B3u SingletTriplet Inter system crossing 1Ag(S0)1Ag(S0) 4b1u4b1u 1B1u1B1u z(a) y(c) (b) x

Photonic Crystal Fiber Ti:Sapphire laser Nd 3+ :YVO4 laser Stabilization EXPERIMENTAL SYSTEM Single mode laser High finesse resonator Stabilization Frequency counter Photo Diode Beat measurement High resolution spectroscopy 1 octave optical frequency comb

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

DOPPLER-FREE TWO-PHOTON ABSORPTION SPECTROSCOPY ν(1 - u /c)ν(1 + u /c) Mirror ν u hν(1 - u /c) hν(1 + u /c) 2 hν - 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

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

OPTICAL RESONATOR AND SAMPLE CELL

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

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

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

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

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

BEAT NOTE OF REPETITION AND OFFSET Intensity [10 dB/div] Frequency [MHz] Beat by offset Beat by repetition f CEO f rep 40 dB

STABILIZATION OF f rep f rep [MHz] Time [s] Gate time = 1 s

STABILIZATION OF f CEO Frequency [MHz] Time [s] Gate time = 1 s

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 f laser = nf rep + f CEO + f beat Frequency Intensity f rep f CEO f beat f laser

OPTICAL SYNTHESIZER Single mode laser RF Synthesizer Optical frequency comb High resolution spectroscopy f laser f syn f laser = f n + f syn Frequency Intensity fnfn f syn f laser f n+1 f rep - f syn

OPTICAL SYNTHESIZER Frequency [MHz] f syn f rep - f syn Frequency [MHz] f syn f rep - f syn Frequency shift by AO modulator

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

f rep Bandwidth: 300 kHz

FREQUENCY REFERENCE FOR STABILIZATION OF f CEO 25 MHz Frequency Spectral Intensity [10 dB/div] [25 kHz/div]

FREQUENCY REFERENCE FOR STABILIZATION OF f CEO Frequency [MHz] Time [s] 25.00

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

～ 4550 cm [nm] 0.3 cm -1 Optical frequency Laser light Doppler-limited Doppler-free DOPPLER BROADENING