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Abstract. In this paper we present experimental results on preparation and characterization of colloidal CdSe quantum dots in organic solvent. CdSe QDs were synthesized following a modifi ed literature method. CdSe QDs were isolated by adding acetone to the cooled solution followed by centrifugation. CdSe QDs have been characterized by UV-Vis absorption and photoluminescent (PL) spectroscopy. The average CdSe particles size estimated from the UV-Vis absorption spectra was found to be in the range 2.28-2.92 nm which is in good agreement with PL measurements.Keywords: colloidal quantum dots, semiconductor, cadmium selenide, nanocrystals, photoluminescence. IntroductionAmong various semiconductor quantum dots (QDs) extensive research efforts have been developed over the last decade in the case of CdSe QDs because of their attractivity for various application in optoelectronics, photonics, medicine, etc. [1][2][3]. In spite of the problems related to the toxicity of Cd compounds, the possibility of tuning of the optical parameters of CdSe QDs makes them very attractive for technological development as well as for optoelectronics and biomedical applications. The size of QDs can be easily controlled by different ways, for example by variation of the reaction time, or the temperature of the solvents, by variation of the concentration of the reaction solvents, etc. This means a relatively simply and convenient technology for controlling the optical parameters of the nanocompozite structures for practical exploitation. For example, PL emission from colloidal CdSe quantum dots can be adjusted in a relatively wide spectral range from 465 to 670 nm [2]. The technology makes it possible to prepare different sized nanocrystals with high PL effi ciency and narrow PL band, while PL emission may cover a narrow spectral band, tuned from blue to read and even up to near infrared. Control and improvement of optical properties of CdSe quantum dots remain on the agenda of research in the fi eld of nanotechnology.

Optical hysteresis is a fundamental phenomenon that can lead to optical bistability and highspeed signal processing. Here, we present a theoretical and experimental study of the optical hysteresis phenomenon in amorphous As 2 S 3 chalcogenide based waveguide structures under surface plasmon resonance (SPR) conditions. The SPR structure is irradiated with low power CW Ar laser radiation at 514 nm wavelength, with photon energy near the optical band-gap of As 2 S 3 , in a Kretschmann-Raether configuration. First, we determined the incidence angle on the SPR structure for resonant coupling of the laser radiation within the waveguide structure. Subsequently, by setting the near resonance incidence angle, we analyzed the variation of the laser power reflected on the SPR structure with incident power. We demonstrated that, by setting the incidence angle at a value slightly smaller than the resonance angle, the increase followed by the decrease of the incident power lead to a wide (up to 60%) hysteresis loop of the reflected power. This behavior is related to the slow and persistent photo-induced modification of the complex refractive index of As 2 S 3 under 514 nm laser irradiation. The experimental and theoretical results are in good agreement, demonstrating the validity of the theoretical model presented here.

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