1122 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 40, NO. 8, AUGUST 2004 Periodically Switched Nonlinear Structures for Frequency Conversion: Theory and Experimental Demonstration David Artigas, Edik U. Rafailov, Member, IEEE, Pablo Loza-Alvarez, and Wilson Sibbett Abstract—In this paper, we report on the analytical study of a first-order quasi-phase-matched structure based on a peri- odically switched nonlinearity. The general average equations describing parametric wave interaction for this structure are obtained. The theoretical results are then used to analyze the special case of a device based on a semiconductor Al Ga As waveguide for efficient frequency doubling in the mid-infrared to the far-infrared range. The necessary conditions to obtain an optimal configuration are discussed in both continuous wave and femtosecond-pulse regimes. Our analysis indicates that the conversion efficiency obtained should significantly exceed the efficiency obtained with a periodically poled lithium niobate crystal at long wavelengths. Finally, we include the preliminary results of the experimental demonstration of a waveguide device based on alternating domains of GaAs and Al Ga As. Index Terms—Nonlinear optics, parametric devices, quasi-phase matching, second-harmonic generation, ultrafast optics. I. INTRODUCTION T HERE is an increasing interest for obtaining compact tun- able optical sources for the near-infrared and, more par- ticularly, for mid-infrared wavelength ranges. Successful im- plementation of compact sources in these spectral regions can be expected to make a positive impact on a variety of appli- cations, such as remote sensing, atmospheric transmission, and photomedicine. The devices that are used at present are typically based on narrow-bandgap semiconductor lasers and quantum cascade lasers. However, these are restricted to low CW or av- erage pulsed output power levels at room temperature or may re- quire cooling to liquid nitrogen temperature. Additionally, they also have limited ( 140 nm) tunability. Frequency conversion must therefore regarded as a practical alternative. Progress in the development of electric-field poling techniques for the patterning of the domain structure of ferroelectric crystals has enabled the implementation of quasi-phase-matched (QPM) structures for efficient frequency Manuscript received October 20, 2003; revised April 6, 2004. The work of P. Loza-Alvarez was supported by the European Regional Development Fund and the Ministerio de Ciencia y Tecnología through the Ramón y Cajal program. This work was supported in part by the Generalitat de Catalunya, the Spanish Government under Grant TIC2000-1010, and the Engineering and Physical Sci- ences Research Council (EPSRC). D. Artigas and P. Loza-Alvarez are with the ICFO-Institut de Ciències Fotòniques and the Department of Signal Theory and Communications, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain (e-mail: david@tsc.upc.es). E. U. Rafailov and W. Sibbett are with the School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, U.K. Digital Object Identifier 10.1109/JQE.2004.831644 conversion interactions [1]. By employing this technique, it is possible to exploit, within the transparency range of the material, the largest available nonlinear coefficient. Such QPM devices have been fabricated using ferroelectric materials [2]–[4] and involving a wide variety of device structures [5]. There is also a major interest in fabricating semicon- ductor-based QPM structures because they offer several advantages over the conventional ferroelectric-based QPM counterparts. Notable advantages include larger nonlinear co- efficients ( 170 pm/V) [6], higher optical damage thresholds, and a larger transparency range with low absorption. Several attempts to fabricate semiconductor QPM devices in bulk [7]–[10] or waveguide [11]–[20] configurations have been explored already. These include the stacking of thin plates for first-order QPM [7], [8], epitaxial growth for orienta- tion-patterned GaAs films [9], [10], [21], [22], quantum-well intermixing induced by ion implantation [13]–[16], and peri- odic domain inversion by wafer bonding [17] and by regrowth techniques [19]. Analogously, several studies for optimizing the conversion efficiency in semiconductor waveguides have been reported [23], [24] and a number of candidates, such as AlGaAs [18], InSb [25], and GaN [26], have been identified for frequency up/down conversion, such that the operation range can be extended from the far-infrared to the visible. In this paper, we describe a technique that involves a pe- riodical switching of the nonlinear coefficient [periodically switched nonlinearity (PSN)] to produce a first-order QPM for frequency mixing applications. This technique applies to any material within its transparency range and can be used to exploit the largest available nonlinear coefficient in those cases where periodically poled QPM is not possible. We have undertaken a theoretical analysis of this structure, including the deduction of the governing averaged equations of the device and the required phase-matching conditions for efficient frequency conversion. A theoretical comparison of the special case of a semiconductor Al Ga As waveguide PSN with bulk periodically poled lithium niobate (PPLN) has been performed and this has shown a higher efficiency for the PSN structure in the mid-to-far-infrared range. The preliminary demonstration of the technique in a GaAs–AlGaAs waveguide structure is also reported. II. PERIODICALLY SWITCHED NONLINEARITY A simplified scheme of a PSN device is illustrated in Fig. 1. For the PSN effect to occur, the nonlinear optical coefficient in 0018-9197/04$20.00 © 2004 IEEE