Laser-based precision spectroscopy and the optical frequency comb technique 1

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1 Laser-based precision spectroscopy and optical frequency comb technique 1 1 Alternatively: Why did Hänsch win Noble prize? Dr. Björn Hessmo Physikalisches Institut, Universität Heidelberg

2 The Nobel prize in physics for 2005 The Royal Swedish Academy of Sciences has decided to award Nobel Prize in Physics for 2005 with one half to Roy J. Glauber Harvard University, Cambridge, MA, USA "for his contribution to quantum ory of optical coherence and one half jointly to John L. Hall JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO, USA and Theodor W. Hänsch Max-Planck-Institut für Quantenoptik, Garching and Ludwig-Maximilians-Universität, Munich, Germany "for ir contributions to development of laser-based precision spectroscopy, including optical frequency comb techni To understand second half of prize we need to learn more about two things: What What is is laser-based precision spectroscopy? What What is is optical frequency comb comb technique?

3 Precision Spectroscopy -To measure a frequency accurately. Definition of second: Originally defined in 1889 as fraction 1/ of mean solar day. (not good because of irregularities in Earth s rotation) Redefined in 1956 to 1/ of length of tropical year for (difficult to measure, long measurement times) Redefined in 1967 to an atomic reference. Cs F=4 Some Some adavantages adavantages with with atomic atomic reference: reference: Fast Fast oscillation oscillation (compared (compared to to tropical tropical year). year) Identical Identical atoms atoms make make it it easy easy to to copy copy reference. reference Precise Precise spectroscopic spectroscopic methods methods can can be be used used to to obtain obtain time time F=3 f def = GHz One One second second is is defined defined as as duration duration of of cycles cycles of of microwave microwave light light absorbed absorbed or or emitted emitted by by hyperfine hyperfine transition transition of of cesium-133 cesium-133 atoms atoms in in ir ir ground ground state state undisturbed undisturbed by by external external fields. fields.

4 second Direct counting gives an uncertainty of ~10-10 Improved peak positioning gives ~10-12 Interferometric techniques and signal averaging gives a final uncertainty of ~10-15 How accurately can position of peak be determined? Estimate: ~1/100th of oscillation Corresponds to a 1 second drift in 30 million years

5 Fountain clocks Observatoire de Paris, NIST (Boulder), Observatoire Cantonal de Neuchâtel (Switzerland), The NIST-F1 clock

6 For increased accuracy a faster oscillator is needed! GHz GHz MICROWAVE Microwave electronics is highly developed technology! Optical Optical oscillators! Green light: 532 nm corresponds to an oscillating frequency o GHz! Clock improvement with a factor 10 5! Unfortunately, no electronics can handle this speed...

7 Laser-based precision spectroscopy Atoms can once more be used as references to stabilize frequency of laser light. Laser frequencies have been stabilized to within <10 mhz. Comparing this to frequency 500 THz one obtains a frequency uncertainty of < 10-17! This would be a nice clock... But problem remains: How to count tick-tacks of this clock? Solution: An optical frequency comb. Tune laser wavelength to an optical transition within an atom

8 Short laser pulses Laser pulses can be made extremely short, t < 10 femtoseconds. A short pulse has a broad spectrum. f rep A well-defined phase relation between frequency components builds up pulse 2f rep 3f rep The round-trip frequency is described by f rep

9 Temporal behavior of a pulse train T=1/f rep φ 2φ A phase phase shift shiftbetween each each pulse pulse may may appear appear due due to to a a difference difference in in group group velocity velocity and and phase phase velocity. velocity. This This depends depends on on properties properties of of optical optical media media etc. etc.

10 Spectrum of a laser pulse Pulse spectrum f n f 2n Frequencies supported by cavity: N f rep f n = n f rep +f 0, f 0 = φ f rep /2π f 2n = 2n f rep +f 0 2f n -f 2n = 2(n f rep +f 0 ) - 2n f rep -f 0 = f 0 Tune laser to remove f 0. This is done by self-reference. Removes all dependenices of laser, and generates a very uniform frequency comb. Need a full octave in frequency spectrum (non-trivial)

11 Beat note with frequency similar to f rep can be measured Tune f rep to satisfy: f laser = N f rep Reference laser N is a rar large number ~10 5 This provides a direct link between optical frequency and a radio frequency! Counting of a radio field is simple, and stability is inherited from optical clock...

12 Comparing spectrum of a reference laser with a frequency comb. Frequency up-conversion f laser 2f laser Convert two red photons into one blue photon. 2f red = f blue Count fringes! f laser =N f rep

13 Summary Precision laser spectroscopy allows us to define optical frequency standards superior to present Cesium time standard. The optical frequency comb allows us to transfer stability of an optical reference to or frequencies where we can use electronics to handle signals. Is it time to redefine second once more? Time will tell...