LASER PHYSICS CENTRE

RESEARCH SCHOOL OF PHYSICAL SCIENCES AND ENGINEERING THE AUSTRALIAN NATIONAL UNIVERSITY CANBERRA ACT 0200 AUSTRALIA

The Laser Physics Centre undertakes research and training at the highest international level on a range of topics with a balance between fundamental, strategic and applied laser-based research. The work is currently divided into the following pursuits: Solid-state Laser Spectroscopy (SSP); Vacuum Ultraviolet Laser Spectroscopy (VUV); Laser Matter Interaction Physics (LMI); Non-linear Optics (NLO) and Atom Manipulation (AM). Within these pursuits generally fundamental questions have been addressed in studies of the interaction of intense laser light with matter; non-linear processes involving atoms; ultra-high resolution spectroscopy of solids; and atomic manipulation. Research of a more strategic nature includes work on atom optics; on photorefractive devices; on the properties of solitons and other non-linear waves; on non-linear optical materials for photonics; and aspects of solid-state physics concerned with processes such as energy transport in solids. Applied research includes the development of novel lasers; techniques for waveguide fabrication; and x-ray optics. Our research program spans many of the most exciting aspects of contemporary laser physics and quantum electronics.

Highlights from research in the past few years have included the development of concepts for photonic switching using dark spatial solitons and guiding light by light¹; the development of nonlinear organic optical materials with nonlinear properties superior to other known materials and with potential for use in all-optical switching²; a new method of using group velocity dispersion to compress optical pulses during frequency doubling³ - a technique which is now being applied to parametric amplification; the development and application of a unique ultra-high resolution laser system within the solid-state physics pursuit to studies of optical spectroscopy of solids with sub-kHz resolution⁴; and pioneering work in VUV laser spectroscopy in collaboration with the UV Physics group in the Atomic and Molecular Physics Laboratory (AMPL) in RSPhysSE⁵.

BACKGROUND

The Laser Physics Centre was established in 1987 by amalgamating groups from the Department of Solid-State Physics with the laser matter interaction work from the Laser Physics Group in the Department of Engineering Physics; and involved establishing a new activity in Vacuum Ultra-Violet Laser Spectroscopy.

By 1994 the research profile has evolved considerably. The Solid-State Laser Spectroscopy research has thrived and remains as a highly productive activity in the Centre. The initial goals of the VUV Laser Spectroscopy activity have been met and the Centre is now one of the few laboratories in the world able to make routine high resolution spectroscopic measurements in the VUV using coherent light. A decision has recently been made to wind down the research in Laser Matter Interaction Physics and this pursuit has largely been replaced by a major activity in Nonlinear Optics. Collaborative research in Atom Manipulation and Atom Optics is being established involving the Laser Physics Centre and both the Director's Unit in AMPL and The Physics Department in the Faculties. Both the Atom Manipulation work and a program in Nonlinear Optical Materials and Structures (NOMS) have received funding from the ANU Strategic Development fund in the past two years. These areas, therefore, represent new initiatives for both the School and the University. NOMS focuses on the development and use of third order nonlinear organic materials for photonic switching. It, therefore, contributes to the interdisciplinary materials research centre which is a major activity at the ANU, and also to photonics which has been identified, through the establishment of ANU Photonics, as a uniting theme within RSPhysSE. Collaboration with the Optical Sciences Centre in RSPhysSE on theory of nonlinear photonic devices, and with the Research School of Chemistry in the synthesis of organic nonlinear optical materials is central to NOMS. Support is also being provided by the Australian Photonics Collaborative Research Centre.

Yanjie Wang works on pulse compressed high power laser systems and paremetric amplifiers

The laboratory facilities available to the Centre improved substantially in 1994 with the opening of the new Carver Wing in RSPhysSE. The Department occupies two floors in the new building which offers a high stability environment because of the 1m thick concrete radiation shield for the particle accelerator in the Department of Nuclear Physics which forms the floor for the new laboratories.

NONLINEAR OPTICS

Spatial Solitons

We are pioneering the use of spatial solitons for all-optical switching in photonics. This research has generated considerable international recognition and has evolved as a collaborative program with the Optical Sciences Centre. We have particularly focused on the process of guiding light by light which uses the fact that all spatial solitons are self-guided waves: this means that they "write" their own waveguide into the nonlinear material in which they are created. Information carrying probe beams can propagate as bound modes of these soliton induced waveguides, and can be switched by varying the properties of the solitons (for example, their propagation direction). Theoretical work and numerical simulations of the concept have been compared with the results of experiments using two dimensional dark spatial solitons in various media. Highlights of the research include the demonstration that technologically important structured waveguides can be created using dark spatial solitons and used in guiding and manipulating light. Recently the ideas have been extended to three dimensional systems containing optical vortex solitons. We are also developing novel concepts for controlling spatial solitons in nonlinear waveguide arrays.

Nonlinear Optical Materials and Devices

Generally experimental studies of spatial solitons and all-optical switching have been slow to progress experimentally due to the lack of suitable nonlinear optical materials. With funding from the Harry Triguboff AM syndicate we have established a major program involving a broad study of the chemical and physical aspects of organic materials for nonlinear optics. The work has been extremely successful and we have now available materials with nonlinear optical properties amongst the most favourable ever observed. Most of our interest has focused on the well known polymer poly(p-phenylenevinylene) [PPV]. Our best PPV has a nonresonant ultrafast nonlinearity >10^-11cm²/W - about two orders of magnitude greater than the best nonresonant nonlinearity in an inorganic semiconductor. The materials also has sufficiently low two-photon absorption for device applications. If these numbers are translated into device performance, an all-optical switch with pJ switching energy should be achievable.

The nonlinearity of PPV can be affected by chaning the conjugation length of the polymer chain. This also shifts the positionof the maximum absorption in the UV visible region as shown above.

The project involves not only the development of novel nonlinear optical materials; but also problems of the fabrication of nonlinear waveguides; the characterisation of nonlinear materials; and eventually development of devices. The research involves collaboration with groups in the Research School of Chemistry and other members of ANU Photonics.

Photorefractivity

We are studying the use of photorefractive oscillators as incoherent to coherent optical converters; photorefractive effects in waveguides; and the nonlinear dynamics of photorefractive optical systems. Recently we have created photorefractive coupling gratings in planar slab waveguides (including chirped gratings); observed amplification of guided beam by free space waves via two wave mixing; and observed optical phase conjugation in a waveguide.

Second order nonlinearities

We have concentrated on some of the novel effects which occur during frequency conversion of ultra-short optical pulses. A major result has been the development of techniques for using group velocity dispersion to compress optical pulses during frequency doubling. Using this technique, which we call group velocity mismatched frequency doubling, it is possible to obtain an order of magnitude reduction in the pulse duration of the second harmonic output and simultaneously very high energy conversion efficiency (>60%). Further pulse compression by parametric down conversion can be obtained using a two step process. The major collaborator in the early stages of this work was Dr David Meyerhofer's group at the University of Rochester's Laboratory for Laser Energetics.

Anita Smith prepares a polishing machine in the optical preparation laboratory

Waveguide lasers

We are developing unusual designs of miniature mode-locked lasers as sources for optical communications or metrology. One concept involves fabrication of nonlinear waveguide structures which act as passive modelocking elements. Our main interest has been in the use of nonlinear directional couplers as mode-lockers, and we are now attempting to fabricate prototype lasers in the fibre geometry (fibre fabrication is carried out at the University of New South Wales or the University of Sydney).

In other programs we are investigating the use of waveguide lasers to create low cost sources for consumer applications. Parts of this program have been carried out with the support of local industry. Novel laser technology has also been developed for a local company, Electro-Optic Systems Pty Ltd, including an advanced diode pumped solid state laser as part of a NASA contract obtained by EOS and partners Aerospatiale and McDonnell Douglas.

HIGH RESOLUTION VUV LASER SPECTROSCOPY

This work involves the development of nonlinear optical techniques to provide sources of coherent vacuum ultraviolet (VUV) radiation for application to high resolution spectroscopy of atmospheric molecules. The research is supported by a sophisticated facility comprising two excimer pumped, high intensity, narrowband, independently tunable dye lasers with output wavelengths ranging from the infrared to the near UV. This facility has recently been reproduced under a large equipment grant in the UV Physics Unit (AMPL) with an extended scanning range laser system for intensive spectroscopic measurements, while the VUV Laser Laboratory concentrates increasingly on technique development.

A series of nonlinear optical techniques are used to convert the laser radiation into the VUV (100 - 200 nm). These include anti-Stokes stimulated Raman scattering (ASRS), third harmonic generation (THG), and four-wave difference frequency mixing. The last method provides the greatest tuneability.

These techniques have been successfully applied to high resolution studies of the oxygen Schumann-Runge band (175-180nm), the Tanaka long bands (110-130nm) and various other band regions. Both absorption and predissociation fluorescence measurements have been performed on oxygen and other species such as CO, NO and the rare gases for wavelength calibration purposes. Information on line positions, strengths and widths has been obtained, as well as novel features such as shape resonances, rotational absorption edges and variations in the isotopic molecular spectra observed here for the first time. The program also involves a continuing collaboration with the US National Institute of Standards and Technology to measure the Lamb shift in helium, which has relevance to the development of wavelength standards for the XUV.

A second excimer laser available in 1995 will open up a range of entirely new experiments based on photofragment spectroscopy. Here one laser is used to photodissociate a molecule, and the other laser system is used to perform spectroscopic measurements on the product states. A plan has been formulated to undertake photofragment spectroscopy of a number of molecular systems, including ozone.

Marek Samoc analyses data from four wave mixing experiments used to determine the optical nonlinearity of organic thin films

NONLINEAR OPTICS OF ATOMIC SYSTEMS

In parallel with the development of the "conventional" nonlinear optical techniques for coherent VUV generation outlined above, the VUV Laser Laboratory has investigated nonlinear optical processes in atoms that result in the generation of multiple excitation pathways. The presence of multiple pathways produces quantum mechanical interference which can yield both constructive or destructive effects on the output processes (ionisation, four wave mixing etc.)

Recent experiments have concentrated on understanding one of these interference processes - Laser Induced Continuum Structure (LICS), which has been reported in a number of experiments but whose observations are currently at variance with theory. Other processes such as enhancement of four wave mixing and the suppression of parametric emission have also been studied. Further work on the four-wave mixing interferences will include state-dependent studies of the different interfering pathways, and parametric studies of the effects of saturation and competing processes at high laser intensities.

ATOMIC MANIPULATION PROGRAM

Relatively recently it has been demonstrated that intense beams of laser light can be used to manipulate, cool and trap neutral atoms and this has led to the rapid development of a new field - atom optics. Our recent research carried out in collaboration with the Physics Department, The Faculties, at the ANU has included the reflection of atoms from a moving evanescent light grating in a project designed to produce a wide angle beamsplitter for atom interferometry; and the development of a "zoom lens" for atoms using a standing laser light field.

Very recently a new atom manipulation project has been established in the School. This is a joint program with the Atomic and Molecular Physics Laboratory and has two main aims:

* to use laser light fields to cool, collimate and trap atomic beams to provide a controlled, dense and well characterised source of atoms prepared in well defined states for the measurement of fundamental atomic parameters.

* to manipulate the trajectories of atoms using both free space and evanescent light fields, for the purpose of performing massively parallel, as well as customised (single beam) direct-write lithography.

The former process will use laser light fields in free space to control the motion of atoms from the intense, collimated metastable source. The latter process would utilise hollow core optical fibres to create atomic waveguides, which would also have other atom optics applications, such as the demonstration of a "laser" cavity for atoms.

Phil Dooley works on the laser system in the VUV Laser Spectroscopy Laboratory

SOLID-STATE LASER SPECTROSCOPY

The focus of attention in the Solid State Group within the Laser Physics Centre is the study and utilisation of nonlinear optical spectroscopy to probe optical transitions in solids in ways that avoid the broadening effects that arise from crystal strain. Several order of magnitude improvements in spectral resolution are regularly achieved compared with conventional techniques and this is high lighted by the Group having the record for the observation of the narrowest spectral line of 3kHz in a solid. The field of study has technical implications for the development of new laser materials, for high performance optical storage, for laser stabilisation schemes and for new scheme for modulating light, but the major motivation is the understanding of the underlying physical processes. The Solid State Group will continue to develop its involvement and capabilities in this area, loosely termed optical holeburning .

OPTICAL HOLEBURNING

Ultra-High Resolution Laser

The homogeneous line width of the optical transitions in rare earth systems are frequently narrower than the 1MHz linewidth of commercial high resolution lasers and as a result there has been an ongoing program to develop an ultra high resolution laser. This program which is carried out in collaboration with the Standards Group (Dr P T H Fisk), at Applied Physics, CSIRO, Lindfield, NSW and the Spectroscopy Group (Dr P Hannaford) in Materials Science and Technology, CSIRO, Clayton, Victoria, has been very successful and we currently have a laser with a 1kHz stability. As well as using this laser for the intended spectroscopic study of rare earth systems several new opportunities have arisen. The hole burning is found to have advantages for the characterisation of laser stability and the technique itself can be used to stabilize lasers. Some of the ideals have commercial applications and these will be pursued. The stabilizing of laser frequency in association with optical hole burning has now emerged as a major effort of the Solid State Group and will continue in the next three year period.

Scott Holmstrom and Neil Manson from the Solid-State Laser Spectroscopy Group prepare an experiment with one of the high resolution lasers

High Resolution Spectroscopy of Rare Earth Solids

The Solid State Group has a outstanding reputation for the high resolution study of rare earth doped crystals and this will continue to be the basic strength of the Group. It has been responsible for utilising a enormous range of modern high resolution techniques for the study of a diversity of materials many prepared within the Laser Physics Centre. A specific area targeted recently has been holeburning in rare earth systems exhibiting hyperfine splittings. As a result we have published significantly more on this topic than any other group. In the future the emphasis will change to explore coherent pulse sequences, both RF and optical, to identify weak interactions masked presently by overlap of the spectral features in the frequency domain. In this way the Group will maintain its prominent position in high resolution spectroscopic studies of rare earth materials.

Raman Heterodyne Detected Magnetic Resonances

Raman heterodyne is a coherent optical/RF double resonance technique capable of detecting magnetic resonances in solids and the Solid State Group have developed this technique more than any other group in the world. The technique has been used to observe both NMR and EPR in many systems and our capabilities continue to be expanded.

The Group has identified a hyperfine transition which allows a study of the fundamental properties of 2-level systems. The study has already provided dramatic illustration of the effects of 2-level system interacting with one, two and three radiation fields - many observations being made for the first time. The aim of the study is making measurements on one centre only to provide the most extensive review of strongly driven 2- and 3-level systems ever achieved.

One of the narrowest spectral features ever observed in a solid is this 3.5kHz wide line from Eu³⁺:Y2O3 recorded using the ultra-high resolution laser in the Solid-State Laser Spectroscopy Laboraory

SUPPORT FACILITIES

The Centre supports its research with a wide range of laser and ancillary equipment. Laser facilities include a fs mode-locked Ti:sapphire; a sub-ps Nd laser system; and sub-ps OPO/OPA - all for nonlinear optics work; a suite of high resolution tunable CW lasers including both dye and Ti:sapphire systems; and a number of excimer-pumped pulsed high resolution dye lasers. This film coating facilities; crystal growth facilities; and optical polishing are available. The Centre operates its own mechanical and electronic engineering workshops and accesses the main facilities in RSPhysSE for more complex engineering tasks. Extensive computing facilities are available including the ANU's VP2200 and CM-5 supercomputers.

STAFF

Professor and Head of Centre: Barry Luther-Davies.

Senior Fellows: Neil Manson

Fellows: Ken Baldwin ; Wieslaw Krolikowski; Eugene Gamaly; Marek Samoc.

Research Fellows: John Martin; Anna Samoc.

Postdoctoral Fellows:

QEII Fellow: Andrei Rode.

ARC Postdoctoral Fellow: Changjiang Wei.

Visiting Fellows: Yanjie Wang; Yue Gao; Vladimir Tikhonenko.

Technical Staff: Ian McRae; John Bottega; Craig Macleod; Maryla Krolikowska; Mike Pennington; Anita Smith; Warren Baker.

Administrator: Katrina Anne Blight .

Students: Subho Banerjee; Doug Body; Max Bott; Jason Christou; Phil Dooley; Tim Dyke; Hugo Giordano; Andrew Greentree; Scott Holmstrom; Weijian Lu; Geoff Pryde; Kylie Waring; Wenqian Yu.

REFERENCES

¹B. Luther-Davies, X. Yang, Optics Letters 17, 496-498 (1992).

²B. Luther-Davies, A. Samoc, M. Samoc, M. Woodruff, Nonlinear Optics, in press.

³Y. Wang, B. Luther-Davies, Optics Letters 17, 1459-1461 (1992).

⁴M. Sellars, R.S. Metzler, P.T.R. Fisk, N.B. Manson, J. Opt. Soc. Am. B11, 1468-1473 (1994).

⁵B.R. Lewis, S.T. Gibson, K.G.H. Baldwin, J.H. Carver, J. Opt. Soc. Am. B6, 1200-1208 (1989).