Stabilizing the Carrier-Envelope Phase of the Kansas Light Source Eric Moon Zuoliang Duan 11-9-2005

Outline Theoretical Description of the CE phase Why do we care about the CE phase? Can we control it? Yes! Here’s how it’s done for the KLS and why it works. Single-Shot CE Phase Measurement Setup Results Future Plans

Why do we care about controlling the change of the carrier-envelope phase? Important for experiments utilizing few-cycle laser pulses, e.g. High Harmonic Generation Can use a stabilized frequency comb to perform spectroscopy. Related to this year’s Nobel prize! More applications to come!

Results from Others Fortier et al1, have reported phase coherence times of 326 s. Witte et al2, have observed coherence times of 500 s. Our group has observed coherence times of 85 s. The main goal is to achieve long term, on the order of hours, for running experiments. [1] Fortier et al, IEEE Journal Topics Quantum Electron, Vol. 9, 1002-1010, 2003 [2] Witte et al, App. Physics B, 78, 5-12, 2004

Theory1 Mode-locked lasers emit a regular train of pulses. For a single laser pulse: Envelope-function Carrier-frequency Carrier-envelope phase [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Theory1 Time-Domain Description of the Mode-Locked Pulse Train [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Theory1 Due to material dispersion inside the laser cavity, the CE phase changes. The laser cavity length: [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Theory1 Mode-Locked Pulse Train in the Time Domain: Mode-Locked Pulse Train in the Frequency Domain: [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Frequency Comb and Laser Spectrum1 [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Theory How? Can use a self-referencing technique! The regular spacing of the frequency comb allows access to the change of the carrier-envelope phase. How? Can use a self-referencing technique!

Theory1 The self-referencing technique requires an octave-spanning spectrum of the laser. Beating the second harmonic and fundamental frequency combs of the laser yields a frequency proportional to the change of the carrier-envelope phase. [1] Fortier et al, IEEE J. Select. Topics Quantum Electron., vol. 9, pp.1002-1010,2003.

Theory The CE phase change can be controlled by locking the offset frequency, f0, to a known frequency. In the case of the KLS, f0 is set equal to one-quarter of the repetition rate of the oscillator.

Experiment The KLS utilizes a Kerr-Lens Mode locked Ti:Sapphire Oscillator emitting a ~77 million pulses per second. The pulses are roughly 12 fs at the output of the laser and carry nJ energy per pulse. The oscillator is the starting point for the self-referencing technique.

Why not use the amplifier output? One reason: Spectrum too narrow!

KLS Oscillator Cavity Pump M5 Lens Ti:S A1 M1 ECDC-Module M0 M6 M7 CP OC M8 M2 M10 M4E Ultrashort Pulse Output M4

Stabilization Experimental Setup offset frequency photodiode APD collimating Lens f=30mm focusing Lens f=30mm focusing Lens f=30mm HR1064nm mirror HR532nm mirror BBO crystal λ/2 half wave plate 1064nm polarizing beam-splitter 532nm filter RG715 HR532nm mirror λ/2 half wave plate 532nm λ/2 HR532nm mirror half wave plate 532nm dichroic beam splitter HR 532nm,HT1064nm polarizing beam-splitter 532nm out-coupling objective f=8.55mm in-coupling objective f=7.5mm IR mirror Silver mirror PCF λ/2 half wave plate 800nm Chirped mirror Chirped mirror grating 900lines/mm From fs Laser

532 nm 1064 nm

~1064 nm, Doubled in BBO Crystal

Offset Frequency while Phase Locked

Observation of Beat Note and Frequency Comb frep-f0 f0=19.375MHz

CE Phase Stability After Pulse Amplification2 A second f-2f interferometer after the KLS amplifier provides a means for quantifying the CE phase stabilization stability. 10% of the KLS amplifier output is sent to the experimental setup. White-light is generated in a sapphire plate and a BBO crystal provides second-harmonic generation. [2] Baltuska et al.,IEEE J. Select. Topics Quantum Electron., vol. 9, pp. 972-989, 2003.

Theory2 Interference between the white light and second harmonic pulses: Phase of the Interference Signal: The shot-to-shot change of this phase can be monitored by the second f-2f setup. [2] Baltuska et al.,IEEE J. Select. Topics Quantum Electron., vol. 9, pp. 972-989, 2003.

Experiment compressor amplifier stretcher nonlinear interferometer Pump locking electronics AO modulator  M5 Lens Ti:S A1 M1 M0 M6 M7 HR IR mirror BS 50:50 CP M3 OC M8 M2 M4E M4 nonlinear interferometer spectral broadening HR IR mirror BS 9:1 compressor amplifier stretcher 1kHz fs laser HR IR mirror Single-shot phase measurement

f-2f Interferometer after KLS Amplifier 1kHz fs laser concave silver Mirrors: f=100mm half wave plate half wave plate spectrometer FCWL two silver mirrors SHG VNA VNA sapphire d=2.3mm FCWL: fundamental Continuum white light silver mirrors silver mirror f=70mm BBO polarizer SHG ∆T=0.265ps FCWL 532nm HR mirror 532nm HR mirror f=75mm

Spectrum of the Second Harmonic generated in the BBO Crystal

Single-Shot: Not Locked

Line-Out of the Interference Pattern

1 pulse Phase-Locked Not Phase-Locked

51 pulses Phase-Locked Not Phase-Locked

101 pulses Phase-Locked Not Phase-Locked

200 pulses Phase-Locked Not Phase-Locked

1000 pulses Phase-Locked 10000 pulses phase-locked 103000 pulses Phase-locked

Summary The change of the carrier-envelope phase of the KLS has been stabilized. A technique for observing the carrier-envelope phase change shot-to-shot has been utilized. CE phase coherence times of up to 85 seconds have been observed.

Future Send a slow CE phase drift signal from the second f-2f interferometer back to the locking electronics to achieve longer locking times.

Thanks! Dr. Zenghu Chang Al Rankin KLS Members: Mahendra Shakya, Shambhu Ghimire, Chris Nakamura, Chengquan Li, and Steve Gilbertson Zuoliang Duan for being a great partner on this project. Dr. Corwin and Dr. Washburn