IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER/OCTOBER 2013 3300106 Design and Analysis of High-Speed, High-Contrast All-Optical Modulator Based on CdSe Quantum Dot-Doped Glass Leila Balaghi, Hamed Baghban, Mahboubeh Dolatyari, and Ali Rostami Abstract—To elucidate the theoretical foundation of an all- optical modulator (AOM) based on semiconductor quantum dots (QDs), numerical analysis have been performed, which is sup- ported by the rate equations. Coupled rate and propagation equa- tions have been solved toward investigating the carrier dynamics and optical behavior of the introduced AOM. A modulation depth (MD) of ∼96% has been achieved in the output probe signal at the telecommunication wavelength of 1522 nm through an active pla- nar waveguide design on silicon platform with a length of 200 μm for a pump power density of 5.6 MW/m 2 at the visible wavelength of 460 nm. Results indicate that the MD remains constant, until the pump frequency exceeds 71 GHz; the higher the pump frequency, the lower the MD. The throughput extinction ratio of the AOM is ∼15 dB at the mentioned roll-off frequency. The MD decreases to ∼45% while the modulation frequency reach to 1 THz. Also, the designed AOM based on cadmium selenide (CdSe) QDs operates with the switching energy of ∼10 fJ. Index Terms—All-optical modulation (AOM), extinction coeffi- cient, interband and intersubband absorption, modulation depth (MD), quantum dot (QD). I. INTRODUCTION N OWADAYS, a range of newly developed fundamental communication technologies have been developed to take a substantial step toward the high-speed optical communica- tion and signal processing. The crucial role that optical mod- ulators play in high-speed communication era is undeniable. These devices transmit information to light signals as they zip via kilometers of optical fibers at the center of data-sharing net- works [1], [2]. A leading candidate to achieve high-bit rate mod- ulation, might be all-optical modulators (AOMs), in which light is modulated by light [3], [4]. Such devices could perform signal processing purely in optical domain with low latency, where the Manuscript received November 30, 2012; revised January 7, 2013; accepted January 7, 2013. L. Balaghi is with the Photonic and Nanocrystal Research Lab (PNRL), Department of Electrical and Computer Engineering, University of Tabriz, Tabriz 51666, Iran (e-mail: rostami@tabrizu.ac.ir). H. Baghban and M. Dolatyari are with the School of Engineering- Emerging Technologies, University of Tabriz, Tabriz 51666, Iran (e-mail: h-baghban@tabrizu.ac.ir; m.dolatyari@tabrizu.ac.ir). A. Rostami is with the Photonic and Nanocrystal Research Lab (PNRL), Department of Electrical and Computer Engineering and the School of Engineering-Emerging Technologies, University of Tabriz, Tabriz 51666, Iran (e-mail: rostami@tabrizu.ac.ir). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2013.2240377 delay associated with optical–electrical and electrical–optical conversions is undesirable [4]. Ordinarily, amplitude modula- tion is adopted that each bit is specified by the presence or absence of a light pulse known as ON/OFF keying (OOK) [5]. Semiconductor quantum dots (QDs), offer many interesting applications for active controlling of light [3], [6]. There are certain criteria on which a modulator could be eval- uated including modulation depth (MD), modulation frequency, device size, manufacturing difficulty, fabrication cost, switching energy, device compatibility with electronic/photonic technol- ogy, etc. The vast studies on AOMs have been accelerated from 2004 and has reached to remarkable advances specially in 2010 and thereafter. Experimental demonstration of AOM based on ma- nipulated gold nanoparticles with MD > 74%, modulation fre- quency of ∼1 GHz, probe wavelength of 1550 nm, pump power density of 10 MW/m 2 , and length of 220 nm [7] can be implied as an instant. Potential of ultrafast all-optical modulation based on silicon nanoring with high MD, modulation frequency of ∼333 GHz, probe wavelength of 1515 nm, and pump energy of 0.88 pJ with 10 fs pulsewidth has also reported theoretically [8]. Also, an AOM being founded on plasmonic excitation of col- loidal cadmium selenide (CdSe) QDs synthesized chemically in solution with operating wavelengths of 514.5 (as pump signal) and 1426 nm (as probe signal) was reported with a low-power density (∼5 MW/m 2 ), which offers a MD of ∼12% and constant modulation frequencies up to 25 MHz [9], [10]. Since CdSe QDs can be tuned within the visible spectrum from wavelengths of 450–650 nm by controlling nanocrystal size, a variety of potential applications for CdSe QDs in photonic devices is opened up [6], [11]. Easy and low cost synthesis of CdSe QDs in different matrices on the other hand motivates research and device demonstration. These factors have led to considerable studies on ultrafast operation capability potential of popular QDs like CdE (E = S, Se, Te) doped in glass [12]. Beside the aforementioned specifications demanded from an AOM , there have been always a great worldwide interest in man- ufacturing low cost devices, building practical high-bandwidth AOMs and wavelength converters in a chip scale and also, highly integratable with silicon-based photonic devices. Thereupon, our intent is to introduce an active planar waveguide with sili- con platform doped with QDs as an AOM. Fig. 1 schematically illustrates the proposed CdSe QDs in SiO 2 matrix as the AOM. To analyze the performance specifications of the introduced AOM, different methods may be utilized. Although, the semi- conductor bloch equation is the most accurate approach for 1077-260X/$31.00 © 2013 IEEE