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Advances in OptoElectronicsVolume 2008 2008, Article ID 467145, 2 pages

Editorial

Department of Physics, United States Air Force Academy, Colorado Springs, CO 80840, USA

Division of Advanced Electronics and Optical Science, Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

Department of Engineering Physics, McMaster University, Hamilton, ON, Canada L8S 4L7

Received 9 November 2008; Accepted 9 November 2008

Copyright © 2008 Yalin Lu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Artificially engineering ferroelectric andferromagnetic oxide materials’ domain structures provide many potentialopportunities to explore such materials’ extraordinary nonlinear optic, electroopticEO, and magnetooptic MO effects, to build unique photonic bandgapstructures, and to generate phonon-photon-coupled polaritons. Domainengineering can be applied onto such materials, either one-dimensinally or two-dimensionally, can be patterned on the above materials periodically, quasiperiodically, oraperiodically, or can be aligned along different crystalline orientations orby using complicated cascaded structures. Those commonly involved photonic materials can be versatile including ferroelectric-ferromagnetic oxide crystals, semicondcutors, electrooptic polymers, and so on, ineither bulk, thin film, or waveguide forms. Implementation of such domainengineering can be very versatile too, and it may follow direct crystalgrowth, superlattice growth, overgrowth on an already patterned structure,electrical field poling, E-beam writing, and so on. Potential applications ofsuch domain-engineeredmaterials for photonics are very widespread, including nonlinear frequencyconversion, EO modulation, optical bistability, acoustics, ultrasonic transducers,terahertz THz generation, fiber optics, left-hand materials, and so on.

A remarkable example of past researches in thisarea is the realization of the quasi-phase-matched QPM nonlinear frequency conversions usingperiodically poled crystals. Recently, development of new photonic andoptoelectronic components using suchadvanced domain-engineered materials, covering a broad range of operation frequenciesand having unique optical functions, has become very attractive. However, theeffort toward this direction has been crucially relying on the availability ofnew optical materials, new physical mechanisms, and new device designs. In thisspecial issue focusing on exploring such new aspects, we invited a few papersthat address the major issues in the area, summarized some of those recent progresses, and discussed those emergingopportunities of applications.

The first two papers ofthis special issue are related to the realization of such domain-engineeredstructures. The first article from M. Fujimura and T. Suhara is the speciallyinvited one, which reports a new formation method for making domain-invertedgratings in MgO:LiNb O 3 crystal at room temperature via applying anelectric field to the crystal under the irradiation of UV light. The results supportthe unique way to use a photoconductive cladding layer to suppress theexcessive lateral expansion of the domain-inverted regions. The formationprocess does not require the use of photolithography processing and allows afull room temperature operation. Therefore, it is simple and productive. Thesecond article describes the fabrication of proton-exchanged PE waveguides ondomain-inverted stoichiometric LiTa O 3 SLT crystals for guided-waveEO modulation applications. The extraordinary index change in SLT via PE with acoefficient of 0 . 2 5 × 1 0 − 1 2 c m 2 - s was found to be 0.017,which is comparable to that from the congruent LiTa O 3 crystal. Guided-waveEO modulation using such waveguides was also demonstrated.

The following four papersare discussing various issues in a few different nonlinear frequency conversionprocesses including second harmonic generation SHGand difference frequency generation DFG. Among them is the fifth article by Y. Wong et al. which is also a specially invited one. The third paper by S. Chu et al. reports an ~1W continueous-wave greenlight generation in a bulk periodically poled MgO:LiNb O 3 crystal,using the intracavity QPM design and excited by a diode-pumped Nd:Y O 4 laser. The paper that follows actually describes the SHG in an MgO-doped periodicallypoled congruent LiNb O 3 crystal and pumped by an efficient all-fiberQ-switched Yb-doped fiber laser which delivers high-output power and long-pulse width. The conversion efficiencyreaches 4.2% that agrees well with the theoretical simulation. The speciallyinvited article by Y. Wang et al. touches a veryunique side in the SHG process by studying the noise characteristics of bothharmonic and fundamental waves at relatively higher-power levels, through analyzing theirtime-domain and frequency-domain characteristics. Understanding the noisecharacteristics has strong impacts on many applications including coherentdetection, spectroscopy, free-space telecommunication, and so on. The fourth paper in thisgroup expands the scope of the interest to detect an optical signal bytransferring the signal from the optical frequencies to microwave frequenciesvia the second-order susceptibility-based DFG process. Thisfrequency-transition method not only offers the potential of a major detectionefficiency improvement, but also works well for both intensity-modulated andfrequency-phase-modulated optical signals. The study impact to improve themodulation bandwidth in optical fiber telecommunication will be deep.

The seventh and eightharticles in this special issue are directly focusing on EO modulators made fromdomain-engineered ferroelectric crystals. The seventh paper by H. V. Pham et al. discusses a new method to design traveling-wave EOmodulators with fully controlled frequency responses, using nonperiodicallydomain-reversed structures. Frequency responses of both magnitude and phase ofmodulation index will be artificially controllable using such new nonperiodicaldesigns. In this paper, several EO modulators for advanced modulation formatssuch as duobinary modulation and wideband single-sideband modulation areproposed. The eighth article actually discusses the estimation of the phasevelocity of a modulation microwave in a quasi-velocity-matched QVM EO phasemodulator, using the unique EO sampling method that should be very accurate andthe most reliable for measuring voltage waveforms on modulator electrodes.

Moving forward from the abovediscussions on frequency conversion and EO modulation, the last two papers ofthis special issue actually touch the terahertz THz wave generation in aspecially designed nonlinear optical fiber, and, even further, the realizationof negative optical refraction in a multilayered structure that modulated bothtunable dielectric and magnetic behaviors. For avoiding the common absorptionproblem in current nonlinear optical materials, a multicladding fiber designhaving a periodically poled LiNb O 3 fiber as the main core wasproposed. The generated THz waves via DFG will instantly be coupled to the outer cladding made from those polymeric materialshaving low absorption over a broad range of THz frequencies, and the opticalbeam will be maintained inside the main LiNb O 3 fiber core. The lastarticle proposes a new theory of realizing negative refraction by frequency-tuningto concurrence of both dielectric layer and the magnetic layer inside amultilayered structure. Negative refractive index will appear after theconcurrence frequency. This theory is significant, since the anticipatednegative index metamaterials will be flexible in fabrication, and have strongimpacts on emerging areas such as superlens, optical cloaking, and sensing.

Yalin LuHiroshi MurataChang-Qing Xu





Author: Yalin Lu, Hiroshi Murata, and Chang-Qing Xu

Source: https://www.hindawi.com/



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