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ULTRAFAST OPTICS Chip Scale Light Deflector Enables Solid-State Streak Camera John E. Heebner, Chris H. Sarantos and Susan M. Haynes Swept spatial representation of temporal waveform Short pulse pump beam Serrated gold mask Pump activated prism array Temporal waveform C Gu d la id S Input coupling lens di ing s ub Focusing lens ng er lay tr a te High dynamic range camera Output coupling lens Single-shot recording 10,000 Intensity [counts] S cientists have used the rapid deflection of beams to provide a record of events at many time scales. Conventional, electron-based streak cameras represent the fastest embodiment of this concept, but they are limited by spacecharge effects that blur the focused beam when high signal amplitudes are present. This forces a tradeoff between temporal resolution and dynamic range. A streak camera that deflects a beam of photons would eliminate this tradeoff. Unlike electron beams that are readily manipulated via electromagnetic forces, the sustained deflection of an optical beam through many picosecond-scale resolvable spots has been historically difficult to achieve. For each resolvable spot of deflection in the far-field, the near-field wavefront must be rapidly tilted by one wave. Nonlinear optical mechanisms based on the Kerr effect are ultrafast1 but also ultraweak, making them impractical. Optically excited carriers have a much stronger influence on the refractive index of a semiconductor. Due to a long-lived (nanosecond-scale) electron-hole recombination time, they have often been overlooked as a means for devising ultrafast optical switches. We demonstrate a deflector concept that achieves picosecond response, exploiting these strong refractive index changes and actually benefitting from long recombination times. The device concept is illustrated in the figure. A signal beam carrying a temporal waveform is coupled into a planar waveguide. When the temporal region of interest is fully contained, a normally incident pump beam patterned by a serrated mask imprints a one-dimensional array of prisms in the waveguide core. The prisms are generated via optical nonlinearities (plasma loading, band filling and bandgap shrinkage),2 which turn on rapidly and remain latched for the sweep. Signal input beam 1,000 100 10 0 0 10 20 3040 50 Time [ps] The SLIDER concept is based on the optically induced deflection of an optical signal injected into a planar slab waveguide. The deflection is caused by a sequential array of prisms that are initially nonexistent, then simultaneously created by a sub-ps pump pulse while the signal is in transit. (Inset) A single-shot recorded trace of a ring-down test signal consisting of 1-ps impulses separated by 10 ps. The signal then experiences a distributed deflection that is finely discretized over a large number of prisms. Because the prism array is created while the signal is in transit through it, later portions of the signal propagate through more prisms. The signal thus deflects in linear proportion to its time delay. The swept beam is focused onto a conventional camera for recording. We term this concept serrated light illumination for deflectionencoded recording (SLIDER).3 To test this concept, we fabricated a planar waveguide with a GaAs guiding layer, surrounded by AlGaAs claddings. A Ti:sapphire regenerative amplifier provided an above-bandgap (800 nm) 150-fs pump pulse that was spatially formatted to a uniform fluence of 65 μJ/cm2. An optical parametric amplifier was used to generate a below-bandgap (950 nm) signal that was spectrally filtered to 1.4 nm (1 ps transform-limited). A ring-down test pattern was then generated by a Gires–Tournois cavity with a round-trip time of 10 ps. SLIDER enabled a singleshot recording of 1-ps impulses resolved at 2.5 ps across a record of 50 ps. The dynamic range of the measurement was 3,000:1, limited by the camera. The SLIDER technique is potentially scalable to a high dynamic range (104) across hundreds of picoseconds, making it a credible replacement technology for conventional streak cameras. t John Heebner ([email protected]) and Susan Haynes are with the Science and Technology Directorate of the Lawrence Livermore National Laboratory in Livermore, Calif., U.S.A. Chris Sarantos is now with Phoebus Optoelectronics. This work was performed under the auspices of the U.S. Department of Energy under contract DE-AC52-07NA27344. References 1. J. Hübner et al. Opt. Lett. 30, 3168 (2005). 2. B R. Bennett et al. IEEE J. Quantum Electron. 26, 113 (1990). 3. C.H. Sarantos and J.E. Heebner. Opt. Lett. 35, 1389 (2010). December 2010 | 51 ULTRAFAST OPTICS Versatile Airy–Bessel Light Bullets Andy Chong, William H. Renninger, Demetrios N. Christodoulides and Frank W. Wise A 52 | OPN Optics & Photonics News Axicon ug x Position [mm] y -500 XC signal [a.u.] ny localized wave packet tends to broaden in space and time under the combined actions of two universal physical processes: diffraction and dispersion. Researchers have explored strategies to overcome the often undesirable consequences of these two effects since the early days of lasers. In the early 1990s, nonlinear schemes based on three-dimensional (3-D) soliton solutions (or soliton bullets) have been suggested as a means of overcoming this problem.1 However, 3-D soliton bullets are by nature highly unstable and thus tend to disintegrate during propagation. In addition, they require high power levels that are on many occasions detrimental, especially in biomedical applications. Researchers have also reported 3-D linear wave packets that can propagate in the form of “O,” “X,” and Bessel-X waves. Apart from the fact that such solutions involve complex spectra and often require diffraction/dispersion equalization, their utility is severely limited by the need to tailor the properties of the wave packet precisely to material properties. Ideally, such light bullets should be versatile and able to withstand any level of diffraction and dispersion, irrespective of whether the latter is normal or anomalous. The only way to circumvent this problem is to disengage space and time. To overcome these limitations, we have recently explored a new approach based on the unique properties of onedimensional Airy wave packets.3 Earlier this year, we reported the first observation of a new class of versatile three-dimensional linear light bullets,4 which combine Bessel beams5 in the transverse plane with temporal Airy pulses. These Airy–Bessel waves are impervious to both dispersion and diffraction effects. Thus, their evolution does not depend critically on the mate- 1.0 0.5 0 -0.5 z 500 0 -500 0 500 Position [mm] -500 0 500 1,000 Delay [fs] (Top) Schematic to generate Airy-Bessel wave packets. (Bottom) Propagation of an AiryBessel wave packet A initial spatial and temporal profile, B profiles after propagation through 3.3 LR and 1.8 Ld, C 5.4 LR and 3.6 Ld, D 7.5 LR and 5.4 Ld. (LR =diffraction length of a beam with a diameter of 180mm, Ld =dispersion length of a 100 fs pulse). rial in which they propagate. Temporal self-healing and free acceleration, which are signatures of Airy packets, were also demonstrated. These spatiotemporal waves are possible for either sign of dispersion and do not require any diffraction/dispersion equalization. This family of robust and versatile waves can be used in ultrafast probing or imaging in media with poorly known, or even time-varying, properties. The figure shows the generation of the Airy-Bessel wave packets. An Airy pulse with a Gaussian spatial profile is generated by impressing a large cubic spectral phase on a femtosecond pulse from a mode-locked laser. The spatial profile is then simply converted from a Gaussian to a Bessel beam by an axicon lens. The propagation of an Airy-Bessel light bullet in a dispersive glass is shown. Its nonspreading nature was verified for propagation through seven diffraction lengths and five dispersion lengths of the glass. The peak intensity of the corresponding Gaussian pulse and beam would fall by a factor of 250 during propagation through the same material. We expect that these versatile localized wave packets will find a wide range of scientific applications. t Andy Chong ([email protected]), William H. Renninger and Frank W. Wise are with the department of applied physics at Cornell University, Ithaca, N.Y., U.S.A. Demetrios N. Christodoulides is with the College of Optics/CREOL, University of Central Florida, Orlando, Fla., U.S.A. References 1. Y. Silberberg. Opt. Lett. 15, 1282-4 (1990). 2. Sonajalg et al. Opt. Lett. 22, 310-12 (1997). 3. G.A. Silviloglou et al. Opt. Lett. 32, 979-81 (2007). 4. A. Chong et al. Nat. Photonics 4, 103-6 (2010). 5. J. Durnin et al. Phys. Rev. Lett. 58, 1499-1501 (1987). www.osa-opn.org