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Optics Express

Optics Express

  • Editor: C. Martijin de Sterke
  • Vol. 15, Iss. 9 — Apr. 30, 2007
  • pp: 5451–5459
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Optical functionalities of dielectric material deposits obtained from a Lambertian evaporation source

J. M. González-Leal  »View Author Affiliations


Optics Express, Vol. 15, Issue 9, pp. 5451-5459 (2007)
http://dx.doi.org/10.1364/OE.15.005451


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Abstract

The thickness profile of deposits obtained from Lambertian evaporation sources is highlighted concerning its transmission optical functionality, in the case of dielectric materials. Fresnel diffraction is used to characterize the lateral resolution and intensity on the optical axis of an input gaussian laser beam. Functionality similar to logarithmic axicons, with uniform lateral resolution and also uniform on-axis intensity, is theoretically derived. It is also shown for this particular optical structure that the intensity slope along the optical axis can be changed from positive to negative values by only changing the input beam width.

© 2007 Optical Society of America

1. Introduction

Effusion is a well-known process in physical vapour deposition [1

1. R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

], in which individual molecules at thermal equilibrium flow through a hole, without collisions. This phenomenon manifests in a matter irradiance (i.e., mass per unit of evaporating area and unit of time) being proportional to the cosine of the angle between the observer’s sight and the evaporation surface normal, in similar fashion to the optical phenomenon observed in Lambertian surfaces, whose light irradiance also follows the same angular dependence [2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

]. As a consequence of the cosine law, the radiance of matter or light, from a Lambertian evaporation source or a Lambertian reflectance surface, respectively, measured by the observer at any view angle around the source or surface, is constant, as the area saw by the observer also depends on the cosine of the view angle.

Differently to the Lagmuir isotropic free-evaporation sources, Lambertian sources provide a material distribution following the above-introduced Lambert cosine law [1

1. R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

,2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

]. This fact is widely used for the uniform deposition of films onto inner spherical surfaces. Evidently, the same fact makes the deposits non-uniform in thickness when collected onto flat substrates, and it is a well-known drawback for planar technologies. Conventional approaches to reduce thickness inhomogeneities in the thin-film deposits involve substrates rotating around an axis far from the evaporation axis, and large substrate-source distances [1

1. R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

,3

3. J. M. González-Leal, R. Prieto-Alcón, J. A. Ángel, D. A. Minkov, and E. Márquez, “Influence of the substrate absorption on the optical and geometrical characterization of thin dielectric films,” Appl. Opt. 41, 7300 (2002). [CrossRef] [PubMed]

]. Nonetheless, the thickness profile of deposits condensed onto transparent substrates placed close to the evaporation source, will be shown here to have technological interest.

2. Theory

2.1 Lambertian evaporation: cosine law

The effusion evaporation regime can be achieved using a Knudsen cell [1

1. R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

]. This device consists in a crucible jailed into a box having a small aperture on top. The device guarantees multiple reflections of the evaporated molecules at the inner-box sides, in such a fashion that thermalization, and in turn, maxwellian-boltzmann distribution for velocities is achieved inside the box. The small size of the cell aperture assures no molecules come back into the evaporation source, which makes the so-called sticking parameter σv = 1 in Hertz-Knudsen equation [1

1. R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

]. This results in the following mathematical relation for the evaporated mass about an exit angle of θ into the forward solid angle dω

dMe=Mecos(θ)π:
(1)

this cosine dependence for the mass emission being similar to that observed in the photon emission from Lambertian surfaces.

If eventually, a flat substrate is placed in the evaporation chamber as illustrated in Fig. 1, the mass of condensed material per unit area on the substrate surface can be derived from Eq. (1)

dMcdAc=Meπρ2cos(θ)cos(ψ),
(2)

on the basis of both mass conservation law, dMc = dMe, and the projection of the elemental area ρ2 onto the substrate at an angle of Ψ (see Fig. 1).

Fig. 1. Illustration of the cosine law for the evaporated-mass distribution from Lambertian evaporation source. A flat substrate is also drawn and relevant deposition parameters are indicated: a being the distance from the centre of the source to the substrate, ρ the modulus of the vector from the centre of the source to any point on the substrate surface, θ is the angle between a and ρ, and Ψ is the angle between ρ and the normal vector to the surface substrate.

In the case of a substrate facing parallel to the cell aperture Ψ = 0, and the thickness profile of the deposit can be easily derived from the sketch illustrated in Fig. 1. In this case cos(Ψ) = cos(θ) = a/ρ = a/(a 2 + r 2)1/2, and Eq. (2) can be written as

dMcdAc=Meπa2(a2+r2)2.
(3)

As surface density scales thickness if volume density is constant, which is the case for deposits with homogeneous composition, the thickness profile of the deposited material will follow the same dependence on r as given in Eq. (3),

t(r)=A(1+(ra)2)2,
(4)

with A standing for the maximum thickness of deposited material at r = 0. For illustrative purposes a plot of Eq. (4) for A = 1 μm and a = 1 mm, is shown in Fig. 2.

2.2 Optical function analysis

If the deposit is made with a dielectric material with refractive index n, the amplitude transmission function of such a refractive optical structure would be, from Eq. (4) [2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

,4

4. A. T. Friberg, “Stationary-phase analysis of generalized axicons,” J. Opt. Soc. Am. A 13, 743 (1996). [CrossRef]

,12

12. J. W. Goodman, Introduction to Fourier optics (McGraw-Hill, Singapore, 1996).

]

φ(r)=A(n1)(1t(r))=A(n1)(11(1+(ra)2)2).
(5)

The light field at polar coordinates r’ and z on the image plane behind this refractive optical structure, can be analysed by solving Fresnel diffraction integral [2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

,4

4. A. T. Friberg, “Stationary-phase analysis of generalized axicons,” J. Opt. Soc. Am. A 13, 743 (1996). [CrossRef]

,12

12. J. W. Goodman, Introduction to Fourier optics (McGraw-Hill, Singapore, 1996).

]

U(ρ,z)=eikziλzeik2zρ2sU(r,z)eik2zr2eikzrρcos(ϕ)dϕdr.
(6)

Integration in Eq. (6) is performed over polar coordinates r and ϕ at the object plane, into the surface S. U(r, z) is the incident light field, λ the wavelength, and k = 2π/λ. U(r, z) can be determined from the amplitude transmission function φ(r) of Eq. (3). If gaussian intensity distribution with width σ at I 0/e2 is further considered for the input light, U(r, z) can be eventually written as

U(r,z)=U(r)=e(rσ)2eikφ(r).
(7)
Fig. 2. Thickness profile, t(r), corresponding to a deposit obtained from a Lambertian evaporation source. The analytical formula for t(r) and design parameters A and a are conveniently indicated.

On the other hand, as we are dealing with thickness profiles with circular symmetry, the integral over the angle ϕ can be shown to give the Bessel function of first kind and zero order [12

12. J. W. Goodman, Introduction to Fourier optics (McGraw-Hill, Singapore, 1996).

], and Eq. (7) can eventually be written as follows:

U(ρ,z)=eikzikzeik2zρ2se(rσ)2eik(r22zφ(r))J0(kρr2z)rdr=
=eikzikzeik2zρ2se(rσ)2eikf(r)J0(kρr2z)rdr,
(8)

where f(r) = r 2/(2z) - φ(r). In the case of the amplitude transmission function given by Eq. (3), the integral of Eq. (8) can be solved by the stationary-phase approximation [2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

, 4

4. A. T. Friberg, “Stationary-phase analysis of generalized axicons,” J. Opt. Soc. Am. A 13, 743 (1996). [CrossRef]

] with critical point

rc=a((4A(n1)za2)131)12
(9)

fulfilling the equation dfdr|rc=0. The solution for the intensity in the case of fully coherent illumination being, finally

I(r',z)e2(rcσ)2rc2z211z+φ''(rc)J02(krcr'2z).
(10)

2.3.1 Lateral resolution

The width of the light beam, w, behind the refractive optical structure under study, can be found solving equation

J02(krcw4z)=12,
(11)

where the full width at half maximum (FWHM) is assumed to characterize the lateral resolution. Plot of w(z) for a gaussian incident laser beam with σ = 2 mm, and values for the design parameters A = 5 μm, a = 1 mm and n = 2 , is shown in Fig. 3.

Fig. 3. Lateral resolution (thick solid line) and light intensity (thick dashed line) along the axis for a representative refractive optical structure with design and input gaussian-beam parameters shown in the figure. Thin dashed line indicates the minimum lateral resolution, w min, reached at distance f 0, and thin solid line at a value of the resolution 5 % above w min, is also plotted to characterize the focal region Δf as described in the text (f 0 = 86 mm, w min = 74 μm, and Δf ≈ 43 mm).

A minimum lateral resolution, w min, is observed in Fig. 3, which appears at distance of f 0 from the refractive optical structure. This distance can be determined from the design parameters a, A and n, solving the equation

ddz(J02(krcw4z))|f0=ddz(rcz)|f0=0,
(12)

and it can be checked to be given by

f0=54125a2A(n1).
(13)

Finally, w min can be derived numerically by solving Eq. (12) at f 0, and it can be probed to give the following approximated relationship

wmin=w(f0)1.13216a255kA(n1).
(14)

2.3.2 Focal-region intensity

Under the assumptions of the stationary-phase approximation, the intensity will be modulated along the axis as

I(0,z)e2(rcσ)2rc2z211z+φ''(rc),
(15)

where φ''(rc)=d2φ(r)dr2|rc. The plot of the on-axis intensity normalized to the intensity value at distance f 0, I(0, z)/I(0,f 0), for a gaussian incident laser beam with σ = 2 mm, and values for the design parameters A = 5 μm, a = 1 mm and n = 2, is shown in Fig. 3

Finally, it is worth to calculate the first derivative of I(0, z) with respect z, at f 0, from Eq. (15), which is given by

dI(0,z)dz|f0A2(n1)2e25(aσ)2σ2(10σ23a2).
(16)

Equation (16) allows the minimum for w(z) and the minimum for I(0, z) can be made concurrent at f 0, in similar fashion as logarithmic axicons [10

10. J. Sochacki, A. Kolodziejczyk, Z. Jaroszewicz, and S. Bará, “Nonparaxial designing of generalized axicons,” Appl. Opt. 31, 5326 (1992). [CrossRef]

], through the following relationship:

σ0=(310)12a.
(17)

3. Discussion

3.1 Design parameters

Figure 3 illustrates the typical lateral resolution along the optical axis, of a dielectric deposit with thickness profile as given by Eq. (2). A significant depth of focus is observed in the plot, provided certain tolerance is allowed for this parameter around the minimum established by Eq. (14). If tolerance of 5 % is agreed, a depth of focus (or focal region), Δf, of about 40 mm would be attained for the refractive optical structure whose results are shown in Fig. 3.

In order to analyze the effects of the design parameters, plots of w(z) and I(0, z)/I(0, f 0) for two different values of the parameter A are shown in Figs. 4(a) and 4(b). As expected according to Eq. (14), the beam width decreases at the focal region with increasing the A value, and concurrently, the focal region makes shorter. This result is consistent with the typical behaviour observed in lenses when increasing the lens curvature, where the Airy spot makes smaller when increasing the numerical aperture, and depth of focus similarly makes shorter [2

2. M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

]. Also in consistence with the well-known behaviour occurring in spherical lenses, the increase of the refractive-index value leads to a higher lateral resolution, as observed in Figs. 4(c) and 4(d). In all cases illustrated in Figs. 3 and 4, a positive slope is observed in the intensity along z, within the focal region.

3.2 Intensity control

Equation (17) can be used as a useful way to control the optical function of a given refractive optical structure obtained from a Lambertian evaporation source. As pointed by Eq. (17), values of σ above and below σ0, change the slope of the intensity within the focal region from positive to negative, respectively, with no influence on the lateral resolution and focal region location. Such effect has also been reported for axicons and lens axicons fabricated by different means [8

8. B. P. S. Ahluwalia, W. C. Cheong, X.-C. Yuan, L.-S. Zhang, S.-H. Tao, J. Bu, and H. Wang, “Design and fabrication of a double-axicon for generation of tailored self-imaged three-dimensional intensity voids,” Opt. Lett. 31, 987 (2006). [CrossRef] [PubMed]

, 9

9. J. X. Pu, H. H. Zhang, and S. Nemoto, “Lens axicons illuminated by Gaussian beams for generation of uniform-axial intensity Bessel fields,” Opt. Eng. 39, 803 (2000). [CrossRef]

]. Plots of the w(z) and I(0, z)/I(0, f 0) for fixed values of the design parameters a, A and n, and values of σ at and around σ0 are shown in Fig. 6. From a technological point of view, such a kind of actuation on the input laser beam allows getting control on the intensity slope over the focal region. This result provides extra functionalities to the refractive optical structure.

Fig. 4. Lateral resolution (thick solid line) and light intensity (thick dashed line) along the axis for a refractive optical structure with design and input gaussian-beam parameters shown in the figure. Values for f 0 , wmin, and Δf are, respectively, 43 mm, 37 μm and 20 mm (a), 22 mm, 18.5 μm and 10 mm (b), 173 mm, 148 μm and 85 mm (c), and 58 mm, 49 μm and 28 mm (d).
Fig. 5. Lateral resolution (thick solid line) and light intensity (thick dashed line) along the axis for a refractive optical structure with design and input gaussian-beam parameters shown in the figure. Values for f 0 , w min, and Δf are, respectively, 777 mm, 222 μm and 400 mm (a), and 1382 mm, 296 μm and 680 mm (b).

Finally, as typically the gaussian laser beam width is fixed for a laser source, Eqs. (13), (14) and (17) can be used in similar way as design rules to match the specifications of this kind of refractive optical structure to a given working laser beam. For example, for a 532 nm wavelength gaussian laser beam with half-width at 1/e2 of σ = 2 mm (Coherent, Verdi V6), a refractive optical structure as those introduced here with focal region around 50 mm and uniform intensity, could be fabricated setting the design parameter with values a = 3.65 mm, A = 0.115 mm and n = 2. In this case, a minimum lateral resolution w min ≈ 12 μm, and a depth of focus Δf ≈ 37 mm, according to the 5 % of tolerance agreed above, can be achieved.

Fig. 6. Lateral resolution (thick solid line) and light intensity (thick dashed line) along the axis for a refractive optical structure with design and input gaussian-beam parameters shown in the figure. Values for f 0 , w min, and Δf are, respectively, 50 mm, 12 μm and 37 mm.

4. Concluding remarks

Acknowledgments

This work has been partly supported by the Spanish Ministry of Education and Science through the National Physics I+D+i Programme (Spain), under the FIS2005-01409 project, and by the Regional Ministry of Innovation, Science and Enterprise of Andalusia, through the Excellence Research Programme, under the FQM-0654 project. The author wants to thank the anonymous referee for her/his very valuable comments

References and links

1.

R. Glang, in: Handbook of Thin Film TechnologyL.I. Maissel and R. Glang eds., (McGraw-Hill, New York, 1983).

2.

M. Born and E. Wolf, Principles of optics: Electromagnetic theory of propagation, interference and diffraction of light 6th ed. (Pergamon Press, Oxford, 1989) p. 752.

3.

J. M. González-Leal, R. Prieto-Alcón, J. A. Ángel, D. A. Minkov, and E. Márquez, “Influence of the substrate absorption on the optical and geometrical characterization of thin dielectric films,” Appl. Opt. 41, 7300 (2002). [CrossRef] [PubMed]

4.

A. T. Friberg, “Stationary-phase analysis of generalized axicons,” J. Opt. Soc. Am. A 13, 743 (1996). [CrossRef]

5.

J. H. McLeod, “The axicon: A new type of optical element,” J. Opt. Soc. Am. 44, 592 (1954). [CrossRef]

6.

A. Burvall, K. Kolacz, Z. Jaroszewicz, and A. T. Friberg, “Simple lens axicon,” Appl. Opt. 43, 4838 (2004). [CrossRef]

7.

Z. Jaroszewicz, A. Burvall, and A. T. Friberg, “Axicon: the most important optical element,” Opt. Photon. News 16, 34 (2005). [CrossRef]

8.

B. P. S. Ahluwalia, W. C. Cheong, X.-C. Yuan, L.-S. Zhang, S.-H. Tao, J. Bu, and H. Wang, “Design and fabrication of a double-axicon for generation of tailored self-imaged three-dimensional intensity voids,” Opt. Lett. 31, 987 (2006). [CrossRef] [PubMed]

9.

J. X. Pu, H. H. Zhang, and S. Nemoto, “Lens axicons illuminated by Gaussian beams for generation of uniform-axial intensity Bessel fields,” Opt. Eng. 39, 803 (2000). [CrossRef]

10.

J. Sochacki, A. Kolodziejczyk, Z. Jaroszewicz, and S. Bará, “Nonparaxial designing of generalized axicons,” Appl. Opt. 31, 5326 (1992). [CrossRef]

11.

R. Grunwald, U. Neumann, A. Rosenfeld, J. Li, and P. R. Herman, “Scalable multichannel micromachining with pseudo-nondiffracting vacuum ultraviolet beam arrays generated by thin-film axicons,” Opt. Lett. 31, 1666 (2006). [CrossRef] [PubMed]

12.

J. W. Goodman, Introduction to Fourier optics (McGraw-Hill, Singapore, 1996).

13.

H. Kogelnik,“Coupled-wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909 (1969).

OCIS Codes
(220.1250) Optical design and fabrication : Aspherics
(310.1860) Thin films : Deposition and fabrication

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: February 6, 2007
Revised Manuscript: February 27, 2007
Manuscript Accepted: March 24, 2007
Published: April 20, 2007

Citation
J. M. González-Leal, "Optical functionalities of dielectric material deposits obtained from a Lambertian evaporation source," Opt. Express 15, 5451-5459 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-9-5451


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References

  1. R. Glang, in Handbook of Thin Film Technology, L. I. Maissel and R. Glang, eds., (McGraw-Hill, New York, 1983).
  2. M. Born, E. Wolf, Principles of Optics: Electromagnetic theory of propagation, interference and diffraction of light, 6th ed. (Pergamon Press, Oxford, 1989) p. 752.
  3. J. M. González-Leal, R. Prieto-Alcón, J. A. Ángel, D. A. Minkov, E. Márquez, "Influence of the substrate absorption on the optical and geometrical characterization of thin dielectric films," Appl. Opt. 41, 7300 (2002). [CrossRef] [PubMed]
  4. A. T. Friberg, "Stationary-phase analysis of generalized axicons," J. Opt. Soc. Am. A 13, 743 (1996). [CrossRef]
  5. J. H. McLeod, "The axicon: A new type of optical element," J. Opt. Soc. Am. 44, 592 (1954). [CrossRef]
  6. A. Burvall, K. Kolacz, Z. Jaroszewicz, A. T. Friberg, "Simple lens axicon," Appl. Opt. 43, 4838 (2004). [CrossRef]
  7. Z. Jaroszewicz, A. Burvall, A. T. Friberg, "Axicon: the most important optical element," Opt. Photon. News 16, 34 (2005). [CrossRef]
  8. B. P. S. Ahluwalia, W. C. Cheong, X.-C. Yuan, L.-S. Zhang, S.-H. Tao, J. Bu, H. Wang, "Design and fabrication of a double-axicon for generation of tailored self-imaged three-dimensional intensity voids," Opt. Lett. 31, 987 (2006). [CrossRef] [PubMed]
  9. J. X. Pu, H. H. Zhang, and S. Nemoto, "Lens axicons illuminated by Gaussian beams for generation of uniform-axial intensity Bessel fields," Opt. Eng. 39, 803 (2000). [CrossRef]
  10. J. Sochacki, A. Kolodziejczyk, Z. Jaroszewicz, and S. Bará, "Nonparaxial designing of generalized axicons," Appl. Opt. 31, 5326 (1992). [CrossRef]
  11. R. Grunwald, U. Neumann, A. Rosenfeld, J. Li, P. R. Herman, "Scalable multichannel micromachining with pseudo-nondiffracting vacuum ultraviolet beam arrays generated by thin-film axicons," Opt. Lett. 31, 1666 (2006). [CrossRef] [PubMed]
  12. J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, Singapore, 1996).
  13. H. Kogelnik, "Coupled-wave theory for thick hologram gratings," Bell Syst. Tech. J. 48, 2909 (1969).

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