Holographic recording in amorphous chalcogenide thin films

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Abstract

A review of the recent advances and developments in the practical application of chalcogenide materials is presented, focusing special attention on holography and lithography using amorphous chalcogenide thin films.

Introduction

Chalcogenide glasses are a group of inorganic glassy materials containing one or more of the chalcogen elements (S, Se or Te) in conjunction with more electropositive elements, most commonly As and Ge, but also P, Sb, Bi, Si, Sn and other elements. Chalcogenide glasses are more weakly bonded materials than oxide ones and both heteropolar (As–S) and homopolar (S–S and As–As) bonds, for example in the As–S system, can be formed. The chalcogenide glasses are bandgap semiconductors and are infrared transmitting. The maximum transmission wavelengths in the far-IR are close to: sulphides, 12 μm; selenides, 15 μm; tellurides, 20 μm [*1]. Chalcogenide glasses possess a high refractive index ranging from 2 to 4. Savage [2] has tabulated refractive indices for sulphide, selenide and telluride glasses containing As, Ge, Si and Sb measured at RT. The increase in refractive index with addition of Pb, Sn, and Bi has been reported.

During the past 10 years, research in the field of optical materials based on amorphous chalcogenide semiconductors has made significant advances. Much of this research is driven by applied interest and this field of research is extremely broad and active [3].

Among the glasses, the chalcogenide appears to be an interesting material for investigation of nanostructural properties because the chalcogenide glass possesses unique characteristics which are different from those in oxide and halide glasses, i.e. molecular (low-dimensional) structures and semiconductor properties [**4]. Nanometer-scale electrical recording has been demonstrated in amorphous GeSb2Te4 films using an atomic force microscope [*5]. The smallest recording mark obtained so far is ∼50 nm in diameter, which corresponds to a storage density of ∼50 Gbit/cm2.

Chalcogenide glass fibers based on sulfide, selenide, telluride and their rare earth doped compositions are being actively investigated worldwide [6], [*7]. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite these problems, current single mode and multimode chalcogenide glass fibers are enabling numerous practical applications. Some of these applications include laser power delivery, chemical sensing, imaging, scanning near field microscopy/spectroscopy, fiber IR sources/lasers, amplifiers and optical switches.

A new generation of optical fibers has been developed based on the large transparency domain of an original family of IR chalcogenide glasses transmitting from 2 to about 14 μm. This spectral range corresponds to the absorption bands of many organic compounds. So these fibers can be used as chemical sensors in many fields of applications such as biology, medicine, food, and the environment [8].

Photonic band gap (PBG) structures are essentially media with a periodic spatial modulation in refractive index and have interesting properties and applications [9]. Creating such structures in chalcogenide glasses would help realise the potential of PBG devices in the IR spectral region. An approach to fabricate a PBG in As–S glass features was studied using focused ion beam milling—a flexible technique that has found application in areas such as integrated circuit and mask/modification as well as micromachining.

The fabrication of As2S3 microlenses onto end surfaces of optical fibers was demonstrated, and their characteristics were evaluated [10]. These lenses were formed through photostructural transformations induced by He–Ne laser light, which was propagated in the fibers, so that the lenses were automatically positioned at the centre of the fiber cores. The lenses can be used at visible to infrared wavelengths, and the minimum focal length is about 10 μm. Bulk chalcogenide glasses formed using a moulding process to generate the spherical, aspherical and diffractive (binary) surface profiles of the optics for thermal imaging devices are one of the potential solutions for replacing the expensive single-crystalline germanium (Ge) lenses [11], [12].

Thin layers of chalcogenide semiconductors are used as a recording media for rewritable optical disks. The active layers of such disks are composed of a photosensitive phase-change material, for example, GeTeSb. A dual layer digital versatile disk-random access memory (DVD-RAM) with 8.5 GB capacity on one side of the optical disk is realized [*13].

Chalcogenide glasses doped with rare-earth elements are materials of large interest because of their potential applications in fiber amplifiers or near- and mid-infrared solid state lasers. Praseodymium-doped chalcogenide glasses possess many possible radiative electron transitions between discrete energy levels of the Pr3+ ions. These transitions are of interest for amplifiers in the near-infrared spectral region (the 1.3 or 1.6 μm band) and also for broad-band amplifiers in the 3–5 μm spectral region [14].

This paper describes some of the applications of amorphous chalcogenide semiconductor thin films in holography and lithography being developed in our laboratory as well as a review of the literature, which describes where chalcogenide glasses are being used and where they could potentially be used.

Section snippets

Holographic recording of transmission gratings

Illumination of amorphous chalcogenide thin films by bandgap light leads to appreciable changes of their optical properties. This phenomenon is well-studied and possesses both scientific and technological importance [**15]. The photo-induced atomic structure variations modify the electronic structure of the disordered system leading to changes in optical properties in the films—optical band gap (Eg), absorption coefficient (k) and refractive index (n). The photo-induced changes of refractive

Holographic recording of surface-relief gratings

The copying of surface-relief microstructures by replication techniques such as hot embossing, moulding or casting is a key technology for the low-cost mass-production of diffractive optical elements (DOEs) and other optical elements with micrometre- or nanometre-sized features. Replication technology is used for the commercial production of submicron grating structures (hot embossed diffractive foil and security holographic labels) and data storage microstructures (injection moulded compact

Fabrication of Bragg grating structures

Formation of Bragg grating structures in the waveguides is an important technology for the development of optical devices applicable to a wavelength division multiplexing (WDM) optical network. In particular, the access network requires a low cost and highly reliable waveguide device. Controlled recording of waveguides based on modification of the optical material properties by laser irradiation may have important implications, for example, in optical interconnects for high data-rate optical

Conclusions

The use of amorphous chalcogenide thin films in holography and lithography has probably only just begun, but already produced some promising results. Amorphous chalcogenide layers are thought to be one of the promising media for optical recording of information with high density due to the high resolution and refractive index values of these materials. The large values in photoinduced changes of the refractive index enable holographic recording in thin layers with high diffraction efficiency.

Acknowledgements

Financial support from the Latvian Science Council (grant 01.0816) is gratefully acknowledged.

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