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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 17 — Aug. 22, 2005
  • pp: 6376–6380
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Recent progress in improving low-temperature stability of infrared thin-film interference filters

B. Li, S. Y. Zhang, J. C. Jiang, D. Q. Liu, and F. S. Zhang  »View Author Affiliations


Optics Express, Vol. 13, Issue 17, pp. 6376-6380 (2005)
http://dx.doi.org/10.1364/OPEX.13.006376


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Abstract

The degeneration of performance of an optical thin-film interference filter associated with the change of temperature is not acceptable. In this letter, we report a new progress in improving low-temperature performance of infrared narrow-band filters by using Pb1-xGexTe initial bulk alloy with appropriate Ge concentration x. It can be found that there exists a critical temperature for the investigated narrowband filter, at which the temperature coefficient of filter is exactly zero. Therefore, by means of controlling the composition in (Pb1-xGex)1-yTey layers, the temperature coefficient of filter can be tunable at the designated low-temperature. In our present investigation, when temperature varies from 300 to 85 K, a shift of peak wavelength of 0.05935 nm.K-1 has been achieved.

© 2005 Optical Society of America

The performance of an optical thin-film interference filter can be influenced by the change of ambient temperature. The peak or cut-off wavelength will shift and the pass-band shape and peak transmission will deteriorate due to the effects of varying temperatures. In particular, when the filter is used in light-wave communication systems or spaceborne remote sensing instruments, optical thin-film interference filter performance degeneration due to temperature changes is not acceptable. It becomes difficult to sustain the functional performance of non-dispersive narrow-band interference filters for precision spectroradiometric measurements from space [1

1. D. F. Heath, E. Hilsenrath, and S. Janz, “Characterization of a ‘hardened’ ultrastable UV linear variable filter and recent results on the radiometric stability of narrow band interference filters subjected to temperature/humidity, thermal/vacuum and ionizing radiation environments,” in Optical remote sensing of the atmosphere and clouds, J. Wang, B. Wu, T. Ogawa, and Z. Guan eds., Proc. SPIE 3501, 401–411 (1998). [CrossRef]

]. It is not practical to add an auxiliary temperature control to a narrow-band filter in order to maintain its stable optical performance in spaceborne remote sensing systems.

There are two factors that cause the instability of the performance of a thin-film interference filter accompanied by the change of temperature: one is the temperature-induced variations in the indices of refraction of layers; another is the variations in the physical thicknesses of layers. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of refractive index for substances employed for interference filters, it may be speculated that the shift of wavelength should be ascribed to the variations of temperature coefficients of indices of layers [2

2. H. Blifford, “Factors affecting the performance of commercial interference filters,” Appl. Opt. 5, 105–111 (1966). [CrossRef] [PubMed]

].

By classical design, Optical interference filters typically consist of multiple homogeneous layers of two materials with low and high refractive indices, nL and nH. As far as the infrared interference filters employed in the spaceborne remote sensing systems are concerned, as a rule, the materials are lead telluride (PbTe) for high-index layers, and either zinc sulphide (ZnS) or zinc selenide (ZnSe) for low-index layers [3

3. J. S. Seeley, R. Hunneman, and A. Whatley, “Temperature-invariant and other narrow-band IR filters containing PbTe, 4-20 μm,” in Contemporary infrared sensors and instruments, H. Kaplan and F. M. Zweibaum, eds., Proc. SPIE 246, 83–94 (1980).

,4

4. G. J. Hawkins, R. Hunneman, R. E. Sherwood, and B. M. Barrett, “Infrared filters and coatings for the high resolution dynamics limb sounder (6-18μm),” Appl. Opt. 39, 5221–5230 (2000). [CrossRef]

]. All materials are fully transparent over the infrared. The negative temperature coefficient is usual with filters having PbTe as one of the layer materials. This negative coefficient in PbTe is especially useful as it tends to compensate for the positive coefficient in ZnS or ZnSe.

The single crystal ingots of Pb1-xGexTe, which have a range of Ge concentration x from 0.08 to 0.75, were grown in our crystal-growth laboratory using a modified Bridgman method. The ingots were then crushed into small pieces and used as source bulk alloys for evaporation of thin films. The thin films were prepared by conventional thermal evaporation with a background vacuum 2.0 × 10-3 Pa. The silicon wafers polished on both sides, with a diameter of 10 mm and a thickness of 0.8 mm, were used as substrates. The substrate temperatures varying in the range from 100 to 300 °C were monitored. The thicknesses of the thin film were 2.0 μm and the errors are less than 2%. The optical transmission spectra of thin films were measured in the spectral range of 2.5-25 μm using a Perkin Elmer Spectrum GX Fourier-transform infrared spectrometer (FTIR) with a resolution of 8 cm-1 at normal incidence.

Since the component elements in an multi-component alloy system will evaporate at a different rate, which causes changes in compositions of thin films relative to the initial bulk alloys, it is in great necessity to measure directly the compositions in evaporated Pb1-xGexTe thin films. The compositions of thin films were analyzed using energy-dispersive X-ray analysis (EDAX) with a sensitivity limit for element detection of 0.1 wt.% in a Hitachi S-520 scanning electron microscope. The determination of element concentrations of lead, germanium and tellurium was performed by the analysis of the Pb Mα, Ge Kα, and Te Lα lines, respectively. Taking the uncertainties originating from inhomogeneity in thin films into account, for each sample measurements were repeated randomly at three different points on the surface of thin films, then the average of all counts for each element was taken as the standard intensity for that element. It is shown that Te concentration deviates from the stoichiometry in the form of either Te-rich character or Te-deficient one, together with a reduced Ge concentration x compared with initial bulk alloy. Therefore, the corresponding chemical compositions of evaporated Pb1-xGexTe thin films should be expressed by the formula (Pb1-xGex)1-yTey.

Subsequently, Pb1-xGexTe initial bulk alloy with appropriate Ge concentration x was selected to fabricate the thin-film narrow-band filters under the optimized processing condition. The design of filters is a simple 8-layer Fabry-Perot type, that is, one half-wave layer immersed in quarter wave layers is deposited on a germanium substrate of index 4.0 without rear surface antireflection, as follows:

substrate|LHLHLL¯HLH|air

where L and H represent, respectively, quarter-wave layers of ZnSe and (Pb1-xGex)1-yTey and the underlining signifies a spacer. The spectral characteristics of fabricated narrow-band filters were measured in the temperature range of 85-300 K using a bath cryostat (Oxford, DN1704). The measurements were reproducible, and no indication of hysteresis effect was observed.

The measured characteristics accompanied by the change of temperature for a filter fabricated with initial bulk alloy Pb0.79Ge0.21Te, from which the evaporated layer has the corresponding chemical composition (Pb0.8584Ge0.1416)0.4575Te0.5425, are shown in Fig. 1(a). The enlargement of the pass-band designed with peak wavelength of 11.30 μm is presented in Fig. 1(b). A comparison was made for the values of peak wavelength and peak transmittance versus temperature is given in the Fig. 2.

Fig. 1. (a) Low-temperature spectral characteristics of the filter with (Pb0.8584Ge0.1416)0.4575Te0.5425 layers; (b) Enlargement of low-temperature spectral characteristics of pass-band with peak wavelength of 11.30 μm
Fig. 2. Comparison of peak wavelength and peak transmittance versus temperature

Acknowledgments

The authors would like to thanks Mr. L. Zhang for his help with the measurements of transmittance spectra at low-temperature. This work was supported by the National Science Foundation of China (NSFC) under Grant No. 60378022.

References and links

1.

D. F. Heath, E. Hilsenrath, and S. Janz, “Characterization of a ‘hardened’ ultrastable UV linear variable filter and recent results on the radiometric stability of narrow band interference filters subjected to temperature/humidity, thermal/vacuum and ionizing radiation environments,” in Optical remote sensing of the atmosphere and clouds, J. Wang, B. Wu, T. Ogawa, and Z. Guan eds., Proc. SPIE 3501, 401–411 (1998). [CrossRef]

2.

H. Blifford, “Factors affecting the performance of commercial interference filters,” Appl. Opt. 5, 105–111 (1966). [CrossRef] [PubMed]

3.

J. S. Seeley, R. Hunneman, and A. Whatley, “Temperature-invariant and other narrow-band IR filters containing PbTe, 4-20 μm,” in Contemporary infrared sensors and instruments, H. Kaplan and F. M. Zweibaum, eds., Proc. SPIE 246, 83–94 (1980).

4.

G. J. Hawkins, R. Hunneman, R. E. Sherwood, and B. M. Barrett, “Infrared filters and coatings for the high resolution dynamics limb sounder (6-18μm),” Appl. Opt. 39, 5221–5230 (2000). [CrossRef]

5.

D. K. Hohnke, H. Holloway, and S. Kaiser, “Phase relations and transformations in the system PbTe-GeTe,” J. Phys. Chem. Solid 33, 2053–2062 (1972). [CrossRef]

6.

Q. T. Islam, “Ferroelectric transition in Pb1-xGexTe: extended X-ray-absorption fine-structure investigation of the Ge and Pb sites,” Phys. Rev. Lett. 59, 2701–2704 (1987). [CrossRef] [PubMed]

7.

B. Li, J. C. Jiang, S. Y. Zhang, and F. S. Zhang, “Low-temperature dependence of mid-infrared optical constants of lead germanium telluride thin film,” J. Appl. Phys. 91, 3556–3561 (2002). [CrossRef]

8.

B. Li, S. Y. Zhang, J. C. Jiang, B. Fan, and F. S. Zhang, “Improving low-temperature performance of infrared thin-film interference filters utilizing the intrinsic properties of IV-VI narrow-gap semiconductors,” Opt. Express , 12, 401–404 (2004), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-3-401. [CrossRef] [PubMed]

OCIS Codes
(120.2440) Instrumentation, measurement, and metrology : Filters
(160.2260) Materials : Ferroelectrics
(310.3840) Thin films : Materials and process characterization
(310.6860) Thin films : Thin films, optical properties

ToC Category:
Research Papers

History
Original Manuscript: July 15, 2005
Revised Manuscript: August 4, 2005
Published: August 22, 2005

Citation
B. Li, S. Zhang, J. Jiang, D. Q. Liu, and F. Zhang, "Recent progress in improving low-temperature stability of infrared thin-film interference filters," Opt. Express 13, 6376-6380 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6376


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References

  1. D. F. Heath, E. Hilsenrath, and S. Janz, �??Characterization of a �??hardened�?? ultrastable UV linear variable filter and recent results on the radiometric stability of narrow band interference filters subjected to temperature/humidity, thermal/vacuum and ionizing radiation environments,�?? in Optical remote sensing of the atmosphere and clouds, J. Wang, B. Wu, T. Ogawa and Z. Guan, eds., Proc. SPIE 3501, 401-411 (1998). [CrossRef]
  2. H. Blifford, �??Factors affecting the performance of commercial interference filters,�?? Appl. Opt. 5, 105-111 (1966). [CrossRef] [PubMed]
  3. J. S. Seeley, R. Hunneman, and A. Whatley, �??Temperature-invariant and other narrow-band IR filters containing PbTe, 4-20 µm,�?? in Contemporary infrared sensors and instruments, H. Kaplan and F. M. Zweibaum, eds., Proc. SPIE 246, 83-94 (1980).
  4. G. J. Hawkins, R. Hunneman, R. E. Sherwood, and B. M. Barrett, �??Infrared filters and coatings for the high resolution dynamics limb sounder (6-18µm),�?? Appl. Opt. 39, 5221-5230 (2000). [CrossRef]
  5. D. K. Hohnke, H. Holloway, and S. Kaiser, �??Phase relations and transformations in the system PbTe-GeTe,�?? J. Phys. Chem. Solid 33, 2053-2062 (1972). [CrossRef]
  6. Q. T. Islam, �??Ferroelectric transition in Pb1-xGexTe: extended X-ray-absorption fine-structure investigation of the Ge and Pb sites,�?? Phys. Rev. Lett. 59, 2701-2704 (1987). [CrossRef] [PubMed]
  7. B. Li, J. C. Jiang, S. Y. Zhang, and F. S. Zhang, �??Low-temperature dependence of mid-infrared optical constants of lead germanium telluride thin film,�?? J. Appl. Phys. 91, 3556-3561 (2002). [CrossRef]
  8. B. Li, S. Y. Zhang, J. C. Jiang, B. Fan, and F. S. Zhang, �??Improving low-temperature performance of infrared thin-film interference filters utilizing the intrinsic properties of IV-VI narrow-gap semiconductors,�?? Opt. Express, 12, 401-404 (2004), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-3-401">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-3-401</a> [CrossRef] [PubMed]

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