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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 14 — Jul. 5, 2010
  • pp: 15236–15241
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Holographic grating based high sensitivity device for refractive index measurements

Domenico Donisi, Roberto Caputo, and Giovanni Cennini  »View Author Affiliations


Optics Express, Vol. 18, Issue 14, pp. 15236-15241 (2010)
http://dx.doi.org/10.1364/OE.18.015236


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Abstract

In this communication, we show how a short-pitch diffractive structure can be used as a low-cost high sensitivity device for refractive index measurements with sensitivity of 10−2. The device consists of a photo-resist diffraction grating put in optical contact with a hollow prism used as a container for a test material. Its main advantage is the possibility to monitor the composition of solids, fluids and gases in real time. Knowledge of optical parameters of a system with high accuracy can be vital when working in the biological/medical field.

© 2010 OSA

1. Introduction

Light based sensors are routinely used to analyze the composition of solids, fluids and gases. A common procedure is to use diffractometers or spectrophotometers in order to determine the chemical composition of a sample material. This kind of technology is reliable and routinely used but usually bulky and expensive. New possibilities for obtaining low cost, yet efficient, novel devices would be desirable. In the last 20 years, scientific research has shown a growing interest in realizing micro/nano structures in soft matter both for fundamental and industrial purposes [1

1. Y.J. Liu and X.W. Sun, “Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications,” Adv. OptoElectron. 2008, 684349 (2008) and references therein.

4

4. G. Strangi, V. Barna, R. Caputo, A. De Luca, C. Versace, N. Scaramuzza, C. Umeton, R. Bartolino, and G. N. Price, “Color-tunable organic microcavity laser array using distributed feedback,” Phys. Rev. Lett. 94(6), 063903 (2005). [CrossRef] [PubMed]

]. One of the main challenges is to involve these materials in the biological/medical field to obtain very sensitive devices. Scientific literature is plenty of examples in this direction. An interesting one is offered by Broer et al. who have successfully realized holographically sculptured structures working as highly selective membrane [5

5. A. M. Prenen, J. C. A. H. van der Werf, C. W. M. Bastiaansen, and D. J. Broer, “Monodisperse, Polymeric Nano- and Microsieves Produced with Interference Holography,” Adv. Mater. 21(17), 1751–1755 (2009). [CrossRef]

]. Crawford has shown that by combining soft matter and nanotechnologies it is possible to open new frontiers in optically probing biological systems [6

6. S. J. Woltman, G. D. Jay, and G. P. Crawford, “Liquid-crystal materials find a new order in biomedical applications,” Nat. Mater. 6(12), 929–938 (2007). [CrossRef] [PubMed]

]. Finally, Sutherland et al. suggest the use of diffraction gratings as a way for detecting hazardous agents in the environment [7

7. R. L. Sutherland, D. M. Brandelik, and C. K. Shepherd, “Device and Method for Detection and Identification of Biological Agents,” United States Patent No. 7,186,567 (2007).

]. In this paper, we show how a binary diffraction grating realized with photoresist materials can perform as an accurate and sensitive device for measuring the refractive index of a medium. An eventual implementation of such a device can find application in the biological/medical field as a low-cost solution for performing self-made analysis or in the food and agro-industrial sector to make on-site quality checks of natural products (Oil or wine, etc.).

The very first bench-top laboratory refractometer is due to Ernst Abbe [8

8. D. Malacara, Geometrical and Instrumental Optics, (Methods in Exp. Phys., Academic Press, 1988) Vol. 25.

]. This device is very accurate but, due to the use of a prism (or a hemisphere), the interval of measurable refractive indices is limited to the refractive index by which the prism is made. Another common way to measure the refractive index of a medium is to use ellipsometric techniques [9

9. R. M. A. Azzam, and N. M. Bashara, Ellipsometry and Polarized Light, (Elsevier Science Pub Co., 1987).

]. The optical setup involves a He-Ne laser, a polarizer and a quarter wave-plate. However, in order to perform a reliable measurement, the medium needs to be spread on a very thin layer. This situation becomes cumbersome when probing the temporal behavior of a system. Another possibility is offered by the intracavity laser refractometry in reflection (ILRR), recently developed by Gonchukov et al. [10

10. S. A. Gonchukov, Y. B. Lazarev, and A. A. Podkolzin, “Laser Refractometry of Biological Fluids,” Instrum. Exp. Tech. 43(6), 826–828 (2000). [CrossRef]

]. It is known that a slant reflection from an interface between two media introduces a phase shift between waves with different polarization. By measuring and comparing this shift with the one predicted by Fresnel theory, it is possible to estimate the refractive index of a medium put on the interface. A limit of this technique, is represented by the mode stability of the used laser. Moreover, the test medium needs to be put in direct contact with the glass interface where the reflection takes place.

sinα=nmatnPMMAsinγ
(1)

If we indicate with m the considered diffracted order, β is given by:
sinβ=mλΛ+nPMMAsinα=mλΛ+nmatsinγ
(2)
where λ is the wavelength of the probe beam. In case we choose the minus first order of diffraction (m = −1), a typical probe wavelength λ = 632.8nm (He-Ne laser), an incidence angle γ = 45° and a periodicity Λ = 500nm, we can plot the behavior of the diffracted angle β as a function of the refractive index of the medium.

From Fig. 2
Fig. 2 behavior of the diffracted angle made by the minus first order respect the grating normal versus the refractive index of the test material in the interval 1.00< nmat<2.00.
, we can notice that the obtained behavior is, in first approximation, linear in the interval considered for nmat. We can also write the angular variation of β respect to nmat as the derivative:

dβdnmat=sinγ1(mλΛ+nmatsinγ)2
(3)

Δβ=dβdnmatΔnmat
(4)

The minimum detectable Δnmat represents the sensitivity of our device. By using a high-resolution CCD camera (or a linear array photodiode) it is possible to detect the angular shift Δβ of the diffracted beam through the corresponding spatial shift Δx=LΔβ where L is the distance between the grating and the CCD camera or the sensor. The typical pixel size of a high resolution CCD camera is as low as 10μm. By assuming this value as the minimum detectable spatial shift Δx and combining its expression with those in Eqs. (3) and (4) we estimate the sensitivity of the device as Δnmatmin104. In the previous, we considered L = 10cm and used the values reported above for the angle γ, the probe wavelength λ, the grating periodicity Λ and the refractive index nmat. If a higher device sensitivity is necessary, it is enough to increase the distance L. In order to keep compact the size of an eventual device, it is convenient to fold the longer light path from the grating to the observation plane by using mirrors.

2. System design

It is worth to remind that the angle γ of the light incoming at the interface medium/PMMA is common to all considered media while the incidence angle α to the grating is not. Indeed, depending on the test material, this angle can assume all values up to αmax = 90°. For what concerns the minimum value for α (in case the test material is air), it can be chosen depending on the range of refractive indices we intend to investigate. We chose αmin = 32° which corresponds, for a grating depth d = 700nm, to the maximum achievable value for the diffraction efficiency (η = 70%). To this angle corresponds a prism angle γ = 52.25°. By using the condition nmat/nPMMAsinγ1 with this value for γ and nPMMA = 1.491, the measurable range of refractive indices results 1.00≤nmat1.88. In order to fabricate the grating with parameters as designed above, the procedure reported in [13

13. R. Caputo, L. De Sio, M. J. J. Jak, E. J. Hornix, D. K. G. de Boer, and H. J. Cornelissen, “Short period holographic structures for backlight display applications,” Opt. Express 15(17), 10540–10552 (2007). [CrossRef] [PubMed]

] has been followed.

3. Device realization and characterization

For testing our idea, we prototyped the device by using the experimental set-up shown in Fig. 4(a)
Fig. 4 (a) Optical setup for device characterization; M, mirror; P1, P2 polarizers; S, sample; 0T, zero transmitted order; -1T, first transmitted order; α, incidence angle; β, diffracted angle. (b) Detail of the PMMA hollow prism used as a container for the test material. (c) SEM picture of the diffraction grating.
. A polarized He-Ne laser beam (λ = 633nm) is directed onto the hollow PMMA prism with an incidence angle γ respect the grating surface. Part of the beam is transmitted (0T) by the grating and part is diffracted (−1T) with an angle β. The grating is put in optical contact with the prism and both are mounted on a holder controlled by a translation and a rotation stage. By acting on the rotation stage it is possible to obtain the normal incidence of light on the entrance prism face. In Fig. 4(b), a photograph of the prism utilized for the experiment is reported, whereas a SEM image of the fabricated grating is shown in Fig. 4(c). The capacity of the device can be reduced or increased at will; in case a smaller volume is necessary, a smaller prism can be realized and, by conveniently reducing the spot size of the laser, the device functionalities remain the same. The fabricated grating has a periodicity Λ = 515nm which is compatible with results of simulations. In order to perform refractive index measurements, we estimated the angle by which light is diffracted by the grating by using the rotation stage where the sample is mounted. By substituting the measured value in Eq. (2), it is possible to calculate the refractive index value of the test material contained in the prism. Several media have been tested by using this method and obtained results, together with a comparison with corresponding refractive index values found in literature, are resumed in Table 1

Table 1. Measurements of the refractive index of several materials compared with corresponding values obtained with other techniques and found in literature

table-icon
View This Table
. Obtained values are compatible with literature ones within the experimental error that we estimated in 10−2. This sensitivity is way far from the one we predicted for our device (10−4) but, as anticipated above, in order to obtain such high sensitivity, it is necessary a more sophisticated implementation of the device that can be obtained by involving a high-resolution CCD camera for precisely measuring the position of the diffracted spot. Our aim, at the moment, was just to confirm the effectiveness of the technique and, in this view, obtained results are highly reliable.

4. Conclusions

In this paper, we have shown that by combining refraction and diffraction in an optical system it is possible to perform high sensitive analysis and refractive index measurements of a material in solid, liquid or gaseous phase. In more detail, the described system is represented by a diffraction grating put in optical contact with a hollow prism, used as a container of the test material. Probe light impinges on one of the prism faces and propagates throughout the test material. At the exit of the medium, the grating diffracts the light incident on it. The measurement of the diffracted angle can allow the calculation of the material refractive index with a sensitivity up to 10−4. An experimental implementation of this system has been set-up and the measurement of the refractive index of several materials has been taken. Even if this measurement has not been performed by using a high-resolution CCD camera, calculated values for the refractive indices are compatible, within the experimental error, with corresponding ones found in literature. An eventual device, based on this working principle can reveal itself as robust and low cost and find application in the healthcare or agroindustrial field. The main advantage of such a device is the absence of any contact between the grating surface and the test material. This preserves the grating functionalities ideally forever. Moreover, the simplicity and effectiveness of this sensor suggests a possible use as a research tool for measuring diffusion dynamics and local changes of the refractive index in the sample.

References and links

1.

Y.J. Liu and X.W. Sun, “Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications,” Adv. OptoElectron. 2008, 684349 (2008) and references therein.

2.

T. J. White, R. L. Bricker, L. V. Natarajan, V. P. Tondiglia, L. Green, Q. Li, and T. J. Bunning, “Electrically switchable, photoaddressable cholesteric liquid crystal reflectors,” Opt. Express 18(1), 173–178 (2010). [CrossRef] [PubMed]

3.

A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express 16(13), 9254–9260 (2008). [CrossRef] [PubMed]

4.

G. Strangi, V. Barna, R. Caputo, A. De Luca, C. Versace, N. Scaramuzza, C. Umeton, R. Bartolino, and G. N. Price, “Color-tunable organic microcavity laser array using distributed feedback,” Phys. Rev. Lett. 94(6), 063903 (2005). [CrossRef] [PubMed]

5.

A. M. Prenen, J. C. A. H. van der Werf, C. W. M. Bastiaansen, and D. J. Broer, “Monodisperse, Polymeric Nano- and Microsieves Produced with Interference Holography,” Adv. Mater. 21(17), 1751–1755 (2009). [CrossRef]

6.

S. J. Woltman, G. D. Jay, and G. P. Crawford, “Liquid-crystal materials find a new order in biomedical applications,” Nat. Mater. 6(12), 929–938 (2007). [CrossRef] [PubMed]

7.

R. L. Sutherland, D. M. Brandelik, and C. K. Shepherd, “Device and Method for Detection and Identification of Biological Agents,” United States Patent No. 7,186,567 (2007).

8.

D. Malacara, Geometrical and Instrumental Optics, (Methods in Exp. Phys., Academic Press, 1988) Vol. 25.

9.

R. M. A. Azzam, and N. M. Bashara, Ellipsometry and Polarized Light, (Elsevier Science Pub Co., 1987).

10.

S. A. Gonchukov, Y. B. Lazarev, and A. A. Podkolzin, “Laser Refractometry of Biological Fluids,” Instrum. Exp. Tech. 43(6), 826–828 (2000). [CrossRef]

11.

M. Born, and E. Wolf, Principles of Optics (Pergamon, New York, 1980).

12.

Grating solver development company. www.gsolver.com.

13.

R. Caputo, L. De Sio, M. J. J. Jak, E. J. Hornix, D. K. G. de Boer, and H. J. Cornelissen, “Short period holographic structures for backlight display applications,” Opt. Express 15(17), 10540–10552 (2007). [CrossRef] [PubMed]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(120.4290) Instrumentation, measurement, and metrology : Nondestructive testing
(120.5710) Instrumentation, measurement, and metrology : Refraction

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: June 9, 2010
Revised Manuscript: June 21, 2010
Manuscript Accepted: June 21, 2010
Published: July 1, 2010

Virtual Issues
Vol. 5, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Domenico Donisi, Roberto Caputo, and Giovanni Cennini, "Holographic grating based high sensitivity device for refractive index measurements," Opt. Express 18, 15236-15241 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-14-15236


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References

  1. Y. J. Liu and X. W. Sun, “Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications,” Adv. OptoElectron. 2008, 684349 (2008) and references therein.
  2. T. J. White, R. L. Bricker, L. V. Natarajan, V. P. Tondiglia, L. Green, Q. Li, and T. J. Bunning, “Electrically switchable, photoaddressable cholesteric liquid crystal reflectors,” Opt. Express 18(1), 173–178 (2010). [CrossRef] [PubMed]
  3. A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating,” Opt. Express 16(13), 9254–9260 (2008). [CrossRef] [PubMed]
  4. G. Strangi, V. Barna, R. Caputo, A. De Luca, C. Versace, N. Scaramuzza, C. Umeton, R. Bartolino, and G. N. Price, “Color-tunable organic microcavity laser array using distributed feedback,” Phys. Rev. Lett. 94(6), 063903 (2005). [CrossRef] [PubMed]
  5. A. M. Prenen, J. C. A. H. van der Werf, C. W. M. Bastiaansen, and D. J. Broer, “Monodisperse, Polymeric Nano- and Microsieves Produced with Interference Holography,” Adv. Mater. 21(17), 1751–1755 (2009). [CrossRef]
  6. S. J. Woltman, G. D. Jay, and G. P. Crawford, “Liquid-crystal materials find a new order in biomedical applications,” Nat. Mater. 6(12), 929–938 (2007). [CrossRef] [PubMed]
  7. R. L. Sutherland, D. M. Brandelik, and C. K. Shepherd, “Device and Method for Detection and Identification of Biological Agents,” United States Patent No. 7,186,567 (2007).
  8. D. Malacara, Geometrical and Instrumental Optics, (Methods in Exp. Phys., Academic Press, 1988) Vol. 25.
  9. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, (Elsevier Science Pub Co., 1987).
  10. S. A. Gonchukov, Y. B. Lazarev, and A. A. Podkolzin, “Laser Refractometry of Biological Fluids,” Instrum. Exp. Tech. 43(6), 826–828 (2000). [CrossRef]
  11. M. Born and E. Wolf, Principles of Optics (Pergamon, New York, 1980).
  12. Grating solver development company. www.gsolver.com .
  13. R. Caputo, L. De Sio, M. J. J. Jak, E. J. Hornix, D. K. G. de Boer, and H. J. Cornelissen, “Short period holographic structures for backlight display applications,” Opt. Express 15(17), 10540–10552 (2007). [CrossRef] [PubMed]

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