A Study of the Lossy Mode Resonances during the Synthesis Process of Zinc Telluride Films

Abstract

Films of zinc telluride (ZnTe) were deposited on the surface of a chemically thinned section of an optical fiber by metalorganic chemical vapor deposition. The boundary values of temperatures and the concentration ratios of the initial tellurium and zinc precursors at which the synthesis of ZnTe coatings is possible are determined. The influence of the position of the thinned part of the optical fiber in the reactor on the growth rate of films on the side surface of the fiber was studied, on the basis of which, the parameters of the deposition zone were determined. By placing a section of an optical fiber with an etched cladding in the center of this zone, sensitive elements for refractometers were created. The principle of their operation is based on the dependence of the spectral position of the lossy mode resonance (LMR) maximum on the refractive index (RI) of the external medium. It has been found that even thin films deposited on a light guide in a continuous process have cracks. It is shown that the interruption of the deposition process makes it possible to avoid the appearance of defects in the zinc telluride layers even with the repeated deposition of the sensor. The sensitivity of the spectral position of the LMR to changes in the RI of aqueous sodium chloride solutions in the range from 1.33 to 1.35 for the first transverse electric and transverse magnetic LMRs was 6656 and 6240 nm per refractive index unit, respectively.

Keywords: ZnTe thin film; lossy mode resonance; metalorganic chemical vapor deposition; optical fiber sensor; refractometer.

Figure 1

The scheme of deposition of ZnTe films on the lateral surface of an optical fiber by the MOCVD method with parallel control of the transmission spectra of a dielectric light guide.

Figure 2

Scheme of a thinned section of a fiber light guide.

Figure 3

Spectral response in the wavelength range of 1250–1650 nm depending on the duration of the synthesis process of ZnTe film on the surface of a thinned optical fiber at the C_Te/C_Zn ratio equal to: (a) 1.13, (b) 1.38, (c) 1.49, (d) 1.80.

Figure 4

Transmission spectra of thinned optical fibers coated with thin ZnTe films for (a) TE and (b) TM LMRs at different C_Te/C_Zn ratios.

Figure 5

Dependence of (a) FWHM and (b) peak transmission on the C_Te/C_Zn ratio for the first TE and TM resonances with a maximum at a wavelength of 1450 nm.

Figure 6

Dependence of the spectral response on the deposition process duration in the wavelength range from 1250 to 1650 nm. The etched region of the optical fiber was located at a distance of (a) 0 mm, (b) 6 mm, (c) 14 mm, and (d) 20 mm from the furnace center.

Figure 7

(a) Dependence of optical transmission at a wavelength of 1450 nm on the duration of the deposition process, the spectral response of which is shown in Figure 6c. (b) Dependence of the coating RI at a wavelength of 1450 nm on the coefficient K_NLM.

Figure 8

The temperatures inside the reactor (red squares) and the growth rate of the ZnTe film on the surface of the optical fiber (black circles) depending on the position of the thinned area of the light guide relative to the furnace center.

Figure 9

(a) Spectral response as a function of process duration for a thinned optical fiber with a ZnTe coating in the wavelength range of 1250–1650 nm. (b) SEM image of the destroyed layer.

Figure 10

Spectral response in the wavelength range of 1250–1650 nm depending on the duration of the synthesis process of ZnTe film on the surface of a thinned optical fiber with an outer diameter of (a) 32.3 µm (b) 25.9 µm (c) 16.3 µm. (d) Dependence of the normalized rate of coating deposition on the outer diameter of the etched fiber region.

Figure 11

Photographs of sections of the thinned regions of optical fibers coated with ZnTe films of the thickness necessary to observe the first TM LMR in the near-infrared region. The layer is synthesized on the fiber (a) without and (b) with interruption of the process. The images are taken from different ends of the optical fiber etched section.

Figure 12

Transmission spectra of thinned optical fibers coated with zinc telluride when immersed in aqueous solutions of sodium chloride in the case of observation of the first (a) TE and (b) TM LMRs. (c) Dependences of the resonance maximum wavelength on the environment refractive index.