Abstract:
An amorphous film of As 2  S 5  on a substrate is disclosed for use in various optical applications. The film can be shaped as a lens to focus light. By mounting an acoustic transducer on the film to produce surface or bulk acoustic waves light moving in the film can be deflected or modulated respectively. The film can also be used in anti-reflective, high reflectivity, and photoconductive coatings.

Description:
BACKGROUND OF THE INVENTION 
     The new technology of integrated optics is expected in coming years to provide new devices for communications and signal processing, especially in conjunction with fiber optic transmission systems. Integrated optics is an attractive approach to these devices because there is the potential for very high performance operation using optical processing principles, in structures that can be very small and rigid, and manufactured by batch fabrication techniques. 
     One of the components required for integrated optic signal processing devices is an efficient modulator, capable of manipulating the intensity or position of the light beam at very high frequencies. There are many methods of modulating light in optical waveguides, including electro-optic, magneto-optic, and acousto-optic with acousto-optic surface waves. An advantage of a waveguide modulator based upon acousto-optic interactions is that there is a greater number of available film materials that can be used, while electro-optic or magneto-optic modulators have relatively few. Furthermore, many good acousto-optical films can be deposited upon a choice of substrates, in amorphous form. This makes possible the fabrication of hybrid integrated optic circuits, in which each component may be chosen to be made of a material which will optimize its function. 
     One known thin film wave guide material is As 2  S 3 . This material has a high acousto-optical figure of merit (i.e., it is very efficient in modulating or deflecting light with sound), but it does not transmit light in the very useful wavelength of 0.6328 microns, produced by the helium-neon laser. 
     SUMMARY OF THE INVENTION 
     We have discovered that As 2  S 5 , a known chemical compound, can be deposited as an amorphous thin film having very excellent optical properties. This is surprising since As 2  S 5  is known to sublime and to decompose at 500° C. The scattering and absorption losses of As 2  S 5  are lower than those of As 2  S 3  films. The film is photoconductive and has a high acousto-optical figure of merit compared to many other thin film optical waveguide materials. The conditions required for the deposition of As 2  S 5  films are less stringent than those required for the deposition of As 2  S 3  films, which means the cost of preparation will be lower. It is also expected on theoretical grounds that As 2  S 5  will be less lossy acoustically than As 2  S 3 , though this has not yet been experimentally established. 
     For optical applications the most important advantage of As 2  S 5  films is that they are transmissive much farther into the visible range than are As 2  S 3  films. Films of As 2  S 5  transmit at from about 0.55 μm to beyond 5 μm, while films of As 2  S 3  transmit only from 0.9 μm to 11 μm. In particular, As 2  S 5  films transmit at 0.6328 microns, the wavelength of the helium-neon laser. The helium-neon laser is a very important commercial laser in optical transmission work and the discovery that As 2  S 5  thin films can be prepared and are transmissive at that wavelength is significant to the advancement of applications involving the helium-neon laser. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is a side view in section of an anti-reflective coating; 
     FIG. 2 is a side view in section of a high-reflectivity coating (i.e., a mirror); 
     FIG. 3 is an isometric view of a waveguide lens; 
     FIG. 4 is an isometric view of an integrated optic spectrum analyzer or other signal processor or a surface wave acousto-optical waveguide modulator; 
     FIG. 5 is an isometric view of a bulk acoustic wave optical phase modulator; and 
     FIG. 6 is a diagrammatic view of the operational components of a vidicon tube. 
    
    
     In FIG. 1 a film 1 of As 2  S 5  covers a second film 2 of a material having a lower refractive index than As 2  S 5 . The second film 2 is on a substrate 3 which may be glass (e.g., a lens) or other material. On top of film 1 is a third film 4 having a refractive index lower than films 1 and 2. To make the coating of the three films anti-reflective, films 2 and 4 are made one-fourth of a wavelength thick, and film 1 is made one-half of a wavelength thick. Light entering the coating is thereby prevented from being reflected back. The high index of refraction of As 2  S 5  makes it very useful in this application. Materials suitable for films 2 and 4 are well known in the art. For example, CaF 2 , MgF 2 , or PbF 2  could be used. 
     In FIG. 2 layers 5 of As 2  S 5  alternate with layers 6 of a material having a lower index of refraction such as CaF 2  to form a high reflectivity coating on substrate 7. At least one layer of each film is required for an anti-reflective coating and the greater the number of pairs of layers the greater will be the reflectivity. Each layer is one-half wavelength in thickness. Of course, all films must be transparent to the light; the substrate may also be transparent, depending on the application. Both the anti-reflective coating and the high reflectivity coating will be more effective the higher the index of refraction of the high index layer. The choice of such high index film materials transparent in the visible is extremely limited. 
     In FIG. 3 a lens 8 of As 2  S 5 , typically deposited through a mask, focuses light 9 moving through thin film optical waveguide 10 on substrate 11. The thin film optical waveguide has an index of refraction less than As 2  S 5 . Corning 7059 glass about 1 micron thick (depending on the design) may be used for waveguide 10. Substrates are typically of SiO 2 . Very high index materials are particularly desirable for such lenses, and there are very few suitable materials which are transparent in the visible. 
     In FIG. 4 an interdigital surface wave acoustic transducer 12 generates a surface acoustic wave in thin film optical waveguide 13 of As 2  S 5  on substrate 14. Light 15 moving in waveguide 13 at the appropriate Bragg angle, θ B , to the sound waves is deflected to various angles according to the frequency of the sound waves. The Bragg angle, θ B , is given by 
     
         θ.sub.B =sin -1λ/Λ 
    
     where λ is the light wavelength and Λ is the sound wavelength. A detector array 16 consists of many detectors which detect light deflected at various angles. In this way a radio frequency signal driving transducer 12 can be analyzed. The As 2  S 5  waveguide is particularly useful in this application because of its high acoustic-optical efficiency. 
     In FIG. 5 a bulk acoustic wave transducer 17 generates bulk acoustic waves which pass downward through As 2  S 5  waveguide 18 and into substrate 19. Light 20 moving in waveguide 18 is modulated (i.e., undergoes a phase change) by interaction with the acoustic wave. The acoustic wave frequency in both the bulk and surface wave modulators (FIGS. 3 and 4) may be between 1 MHz and 10 GHz and the As 2  S 5  waveguide thickness is typically about a micron though it may range from about 3000 A to about 3 microns. The high acousto-optical efficiency of As 2  S 5  also makes it particularly useful in this application. 
     In FIG. 6 an electron gun 21 scans As 2  S 5  film 22 on electrically conducting glass 23. A light image 24 is projected on glass 23 which is transparent to the light. The As 2  S 5  film is photoconductive, so when light is transmitted to the As 2  S 5  film, it becomes conductive and permits electrons from the electron gun to pass to glass 23 thus generating a signal 25. In this way the light image is encoded on signal 25. NESA glass is commonly used as the conducting glass. The components shown in FIG. 6 are, of course, enclosed in an evacuated tube as is well known in the vidicon tube art. 
     The As 2  S 5  compound may be prepared by weighing out stoichiometric amounts of arsenic and sulphur, and sealing these elements in evacuated silica-glass ampoules. The ampoules are placed in a split furnace and are heated to between about 480° to 500° C. and held at this temperature for a number of hours. They are shaken frequently during this period to promote homogeneity within the melt. When the run is completed, the ampoules are removed from the furnace and air cooled. The material thus formed is of a glassy nature. 
     Thin films of the As 2  S 5  are prepared by thermal evaporation. A measured amount of the material is placed in a tantalum resistively heated boat, in a vacuum, and the As 2  S 5  is brought to the glass softening point (˜300° C.). The rate of evaporation for best quality films is less than 2500 A per hour, and slower rates may be desirable. The material is stable and non-hygroscopic, but relatively soft and easily scratched. It can easily be polished to a good optical finish. 
     The following examples further illustrate this invention. 
     EXAMPLE 
     5.0417 grams of arsenic and 5.3942 grams of sulphur were weighed out and sealed under vacuum in a silica-glass tube. The tube was heated to approximately 500° C. in a split furnace. The melt was held at this temperature for 4 to 5 hours and agitated frequently. The ampoule was removed from the furnace and cooled. 
     To prepare a thin film 2 to 3 grams of As 2  S 5  were placed in a tantalum resistively heated boat in a vacuum and were heated to about 300° C. After about 4 hours a film about 1 μm thick had formed. Repeating the procedure for different lengths of time produced films having thickness from about 0.5 to about 1.3 microns. 
     The acoustic and optical properties of waveguide films will generally depend strongly upon film thickness, so that the material properties are better specified by reference to bulk measurements. Bulk acoustic and optical measurements were made on a clear, uniform section of the melt, approximately 1 cm long. Refractive index measurements were made on a prism-shaped sample by the method of minimum deviation. The index between 0.577 μm and 5.3 μm is shown in the following table. 
     
         ______________________________________   λ (μm)                 n______________________________________   0.5660        2.224   0.5770        2.150   0.5990        2.204   0.6328        2.199   0.6350        2.194   0.6580        2.186   1.554         2.145   2.66          2.130   3.38          2.128   5.3           2.119______________________________________ 
    
     The material is transmitting at higher or lower wavelengths, but was not tested beyond 5.3 μm. 
     The acoustic velocity was measured by the conventional pulse-ehco technique on a rod-shaped sample. Transducers were bonded with salol onto the rod ends, which were polished flat and parallel. Shear and longitudinal wave quartz transducers cut for 30 MHz were used. The measured longitudinal wave velocity is 2.22×10 5  cm/sec, and the measured shear wave velocity is 1.09×10 5  cm/sec. 
     These measurements may be used to estimate the acousto-optic figure of merit, M 2 , of As 2  S 5 . The figure of merit is defined as 
     
         M.sub.2 =n.sup.6 p.sup.2 /ρv.sup.3, 
    
     in which n is the refractive index, p the photoelastic constant, ρ the density, and v the acoustic velocity. The calculated value of the figure of merit, M 2 , is 160 (relative to fused quartz. This value is larger than that of most good optical quality film materials. 
     Using similar procedures As 2  S 3  was prepared as a compound and as a film and tested in the same manner. Preparation was more difficult because its lower vapor pressure makes deposition of high quality films more critically dependent on conditions of evaporation. The transmission range of As 2  S 3  is given in the literature (Y. Ohmachi, Journal of Applied Physics 44, 3928 (1973)) as 0.9 to 11 microns and it was confirmed that As 2  S 3  would not transmit at 0.6328 microns. Crystals of As 2  S 3  are monoclinic and occur naturally as the mineral &#34;orpiment.&#34; There does not appear to be a naturally occurring form of As 2  S 5  and it apparently does not form a crystal.