Quasi-phasematching A versatile tool for coherent source development

1. Quasi-phasematchingA versatile tool for coherent source development LUMOS Group K-State University Kansas, U.S.A Karl Tillman Ultrafast Optics Group Heriot-Watt University Edinburgh, Scotland (U.K)

2. Outline • A Historical Perspective • The Basic Concepts • Energy, momentum and the phase condition • Conversion efficiency • Birefringent phasematching and Quasi-phasematching • The Modern QPM OPO • Interesting Adaptations • The Present • The Future

3. A Brief History • 1960 – Theodore Maiman builds the Ruby laser • Visible spectral region quickly populated by range of different gain mediums • Important problems - Atomic / Molecular spectroscopy • Infrared region is where all the good stuff happens! • Absorption region for common hydrocarbons (C-H, N-H, O-H) • Biological fingerprint region (amino acids, proteins, etc) • Nonlinear materials offered a solution • 2 – Parametric conversion process

4. The Motivation • Parametric frequency conversion offers advantages over normal laser action • Generated output wavelength determined by energy and momentum conservation rather than an atomic energy level structure • Level of tuneability not available from traditional laser sources (based on phasematching conditions) • Output profiles generally determined by pump pulse characteristics • Predetermined and predictable spectral and temporal characteristics

5. The Basics • Energy Conservation • Momentum conservation • Phase condition

6. The Basics • Parametric conversion efficiency:

7. The Basics • Efficient parametric conversion requires: • Appropriate nonlinear medium (i.e. birefringence) • Interacting waves maintain temporal overlap • Interacting waves remain phasematched (k ≈ 0) • Main problems: • Group velocity dispersion • Pulse walk-away causes loss of phase condition • Phase mismatch (Dk) increases with crystal length • Limited length  limited gain  high threshold

8. Birefringent Phasematching • Initial solution – Birefringence phasematching • Reduces effect of GVD, increasing interaction length • Relatively high parametric gain due to crystal length • Issues: • Gain medium must be sufficiently birefringent • Crystals require exact phase-matching angle • Relatively low damage thresholds (initially at least) • Rarely access highest nonlinear coefficient, deff

9. An Alternative • Birefringence not a perfect solution • Alternative originally suggested in 19621 • Concept of quasi-phasematching (QPM) is born • Removes need for a birefringent material • Phasematching condition becomes an issue of material engineering • Also not perfect but better in most situations • Solves some of the key drawbacks of BPM • Introduces a few different problems 1 J. A. Armstrong et al, Phys. Rev. A, 127 (6), p:1918-1939, 1962

10. Quasi-phasematching

11. More Basics • QPM allows wavelength selection • Crystal makes momentum contribution

12. QPM favours collinear phasematching More Basics • Periodic reversal of electric field • Regular domain structure with period: • k3, k2, k1co-propagate • Simple expt. set-ups

13. Summary • QPM enables phasematching to become a characteristic of the crystal structure • Can select phasematched wavelengths by design • Original concept used stacked single crystals • High boundary losses  poor conversion efficiency • Not technologically feasible initially • 1993 lithographic approach used to periodically reorient electric field in lithium niobate crystal1 • Introduction of the periodically poling technique 1 M. Yamada et al, Appl. Phys. Lett, 62 (5), p: 435-436, 1993

14. Summary • Periodic poling requires a ferroelectric medium • Larger choice of materials than BPM • Access to highest nonlinear coefficient (i.e. d33) • Tuning capabilities limited mainly by material transparency rather than Poynting vector walk-off • Common materials include: • Lithium Niobate (LN) • Potassium Titanyl Phosphate (KTP) • Associated isomorphs (KTA, RTA, CTA)

15. The Modern QPM OPO • The Optical Parametric Oscillator (OPO) • First successfully demonstrated in 19651 • Versatile parametric device • Very broad tuning range • Low operational threshold • Self-seeding • QPM allows larger choice of nonlinear materials • Choose material with highest nonlinearity • Design the phasematched wavelength • First successful QPM OPO built in 19952 1. J.A. Giordmaine et al, Phys. Rev. Lett. 14 (24) p. 973-976, 1965 2. L. E. Myers et al, Opt. Lett. 20 (1) p.52-54, 1995

16. The Modern QPM OPO • Q-Switched OPO • High repetition frequencies (kHz) • Fast pulse durations (ns) • Compact, robust devices • Very high pulse energies (~µJ) – damage risk!

17. The Modern QPM OPO • Synchronously pumped (SPOPO) • Ultrafast devices (fs/ps) • Very high repetition frequencies (~MHz) • High peak powers (~kW), modest average powers (~W) • Operate well below crystal damage thresholds

18. The Modern QPM OPO Limitations still exist: • Temporal overlap still an issue • GVD more pronounced in QPM OPOs • Limited useful crystal length, restricted gain • Limitation on appropriate materials • Crystal dimensions restricted by fabrication capabilities • Only pole Lithium Niobate if wafer ≤500µm • Restricts aperture size (limits power scaling) • Could use other materials but fabrication technology not as mature • Increases device costs

19. Adaptations Ability to control phasematching condition by engineerable methods allows limitations to be addressed and other ideas tested • Increase tuning capabilities • Compression techniques • Improve efficiency • Designer pulses • Non-ferroelectric materials – e.g. semiconductors

20. Adaptations • Increasing tuning capabilities Lithography allows the design of several gratings to be ‘written’ into the structure of a single crystal. • Different grating different PM  • Grating tuning • Temperature tuning • Successfully demonstrated in 19951 1. L. E. Myers et al, Opt. Lett. 20 (1) p.52-54, 1995

21. Adaptations • Increasing tuning capabilities Lithium niobate is the most common material used to date in QPM devices. Suffers from photorefractive effects below Tc~80°C so requires heating. • Impurities reduce onset of photorefractive damage • MgO doping allows lithium niobate to operate at room temperature • Increased temperature tuning range

22. Adaptations • Compression techniques • Lithography able to produce aperiodic structure • Chirped structures used to generate a compressed SHG output1 1. M. A. Arbore et al, Opt. Lett. 22 (12) p.865-867, 1997

23. Adaptations • Compression techniques • Spatial localisation of conversion uses GVD as a temporal control • Shorter wavelengths generated earlier • Travel faster than longer wavelengths generated later in crystal resulting in a pulse compression 1. T. Beddard et al, Opt. Lett. 25 (14), p.1052-1054, 2000; 2. P. Alverez et al, JOSA B, 16 (9), p. 1553-1560, 1999

24. Adaptations • Improving Efficiency Conversion efficiency dependant on available gain determined by crystal length • Longer crystals have a narrower conversion bandwidth • GVD  pump/signal pulse walk-away increased • Both reduce conversion efficiency Chirped grating could maintain conversion bandwidth as crystal length is increased

25. Adaptations • Improving Efficiency • Allows the use of longer crystals

26. Adaptations • Improving Efficiency • Longer crystals  More parametric gain  Better conversion efficiency  Lower threshold[1,2]  Smaller OPOs possible as less pump power needed • Longer pump pulses reduce effect of GVD • K. A. Tillman et al, Opt. Lett. 28 (7) p.543-545, 2003 • K. A. Tillman et al, J. Opt. Soc Am B. 20 (6) P.1309-1316, 2003

27. Adaptations • Improving Efficiency • Photon recycling • Generate two identical pulses using one pump • Cascaded process • 3 tuneable outputs • Improves quantum efficiency 1. K. A. Tillman et al, J. Opt. Soc.Am. B. 21 (8) p.1551-1558, 2004

28. Adaptations • Designer Pulses • Each grating has well defined phase response • Possible to design arbitrary aperiodic grating structures based on the overall phase response • Careful design can lead to generation of pulses with desirable temporal profile • Double and triple pulses • Triangle pulses • square pulses • stepped profiles 1. U. K. Sapaev et al, Opt. Exp. 13 p.3264-3276, 2005

29. Adaptations • New Materials Semiconductors can have very high nonlinearities in comparison to current QPM materials e.g. • LN: d33 = 27pm/V • KTP: d33 = 18pm/V • GaAs: d14=170pm/V • InSb: d14=307pm/V Organic molecules can be even higher (~105)

30. The Present • New Materials Semiconductors not ferroelectric so alternative poling method needed • Growth techniques (MBE & HPVE)1 Stanford group (M. Fejer & co) OP - GaAs • Ion implantation2 St Andrews group (W. Sibbett & co) PNS - GaAs • L. Eyres et al, Appl. Phys. Lett. 79 (7) p. 904-906, 2001 • D. Artigas et al, IEEE J. Quan. Opt. 40 (8) p.1122-1130, 2004

31. The Future • Directly diode pumped QPM OPOs • Dual colour OPOs for phase stabilized operation 1 • Heriot-Watt, Edinburgh • Compact semiconductor QPM devices for operation at GHz repetition rates 2 • St Andrews • QPM devices for THz frequency generation 3 • Stanford • Organic QPM OPOs ?? • J. H. Sun et al, Opt. Lett. 31 (13) p.2021-2023, 2006 • T. C. Brown et al, New Journal of Physics, 6 Art.175, 2004 • K. L. Vodopyanov et al, Appl. Phys. Lett. 89 (14) 141119, 2006

32. PPLN CEO phase-locking of Cr:Forsterite • Joined LUMOS group 9/25 • Project: CEO phase stabilization of Cr:Forsterite laser • In the process of locking frep and fceo • Next task – Lock laser to GPS signal then use it to make absolute spectral measurement of acetylene P(11) – P(16) absorption lines at ~ 1.53mm

33. The End