'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 1 Frequency Comb Vernier spectroscopy C. Gohle, A. Renault, D.Z. Kandula, A.L. Wolf, W. Ubachs, K.S.E. Eikema Laser Centre, Vrije Universiteit Amsterdam, DeBoelelaan 1081, 1081 HV Amsterdam A. Ozawa, B. Bernhardt, B. Stein, A. Schliesser, Th. Udem, T.W. Hänsch Max-Planck-Institut für Quantenoptik, Hans-Kopfermannstraße 1, Garching

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 2 Outline Introduction: –Frequency combs and Optical resonators XUV comb generation Optical vernier spectroscopy Outlook

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 3 Frequency combs and optical resonators

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 4 Frequency Combs E(t)=A(t)e i  c t = ++ m=-m=-  A m e -im  r t-i  c t 1 n = n1 r + 1 CE, 1 CE < 1 r,  7=2  1 CE /1 r

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 5 Example: Hydrogen f(1S-2S) = (34) Hz

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 6 Fabry perot resonators light source

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 7 … provide stable references Narrow Markers in Frequency space –If high finesse High stability –~10 1 s –Hz 1 PHz –~ m length stability

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 8 … enhance nonlinear conversion P c =F/  –Output power grows with finesse 2 or higher! Example: –SHG 560nm->280nm –900mW driving power –20% conversion: 900mW->200mW

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 9 … enhance sensitivity Cavity absorbtion spectroscopy –Increased interaction length –Intrinsically narrow band Cavity ring down –Intrinsically robust –Can be broad band

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 10 Response function Can be matched to FC

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 11 Cavity enhanced HHG Obvious requirements –No dispersion Electric field in the pulse envelope has to look the same for both pulses -> equidistant modespacing –f rep = f FSR Timedelay between pulses = cavity roundtrip time –f CEO matches HHG inside the resonator

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 12 XUV Output C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, (2005) Circ. Power 40W, intensity in the focus 5 x W/cm 2

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 13 Coherence (of the 3rd harm.) C. Gohle et al., Nature, 436, 234 (2005) R. J. Jones et al., PRL, 94, (2005)

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 14 … coherence! (probably)

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 15 Possible Applications Direct frequency comb spectroscopy in the XUV –It is cw, so no transients Compact coherent XUV source for interferometry High repetition rate high intensity source for coincidence measurements BUT: power still low! And many technical problems Use an amplifier!

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 16 Frequency Comb Vernier Spectroscopy

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 17 Direct comb spectroscopy, the good 300 THz I(1)I(1) THz band width and 100 MHz mode spacing. 3,000,000 narrow band modes with 0.3  W power 1 Simultaneously tuneable and referencable Marian et al, PRL, 95, (2005)’ V.Gerginov et al. Optics Letters, 30, 1734 (2005)

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 18 … and the bad Large background –for absorbtion measurements –Causing stark shifts Aliasing –Spectra difficult to interpret Small power per mode –Small signal –Nonlinear (dopplerfree) spectroscopy difficult

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy the remedy

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 20 Data Single scan (10ms) Blue box: unique data Red boxes: identified features Gaussian PSF much larger than airy ! Brightness~Int egral of airy

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 21 Red:HITRAN data O 2 magnetic dipole intercombinationline (760nm) arXiv: v1arXiv: v1 [physics.optics]

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 22 Results* Absorbtion: Noisefloor < /cm (100 Hz) 1/2 = < /cm Hz 1/2 (shotnoise: <10 -8 ) > 4 THz bandwidth 1 GHz sampling (>4000 res. Datapoints in 10 ms) Quantitative agreement in Amplitude and Frequency to HITRAN** database Phase: - agrees with expectations (disp. features) -not optimized for good phase sensitivity -Still <0.1 mrad/Hz 1/2 * arXiv: v1 [physics.optics] arXiv: v1 ** Rothman, L. S. et al., J. Quant. Spect. Rad. Trans., 96, (2005) O 2 A-Band

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 23 … and the bad Large background –for absorbtion measurements –Causing stark shifts Aliasing –Spectra difficult to interpret Small power per mode –Small signal –Nonlinear (dopplerfree) spectroscopy difficult … may be possible … reduced

'Metrology with Frequency Comb Lasers', 2007 Frequency Comb Vernier Spectroscopy 24 Thanks Maximilian Herrmann (Ion Traps) Sebastian Knünz Valentin Batteiga Albert Schliesser Akira Ozawa (fs-Cavities) Birgitta Bernhardt Jens Rauschenberger Thomas Udem Theodor W. Hänsch Funding: (Hydrogen) Nikolai Kolachevski Janis Alnis Arthur Matveev Elisabeth Peters