WO2012160418A1

WO2012160418A1 - Tunable plasmonic lens

Info

Publication number: WO2012160418A1
Application number: PCT/IB2011/052286
Authority: WO
Inventors: Yiting Yu; Hans Zappe
Original assignee: Albert-Ludwigs-Universität Freiburg
Priority date: 2011-05-25
Filing date: 2011-05-25
Publication date: 2012-11-29

Abstract

The present invention relates to the field of active plasmonics, and particularly, to a plasmonic lens with tunable focusing performance, such as the focal length, depth of focus (DOF), and full-width half- maximum (FWHM). The tunable plasmonic lens 100 according to the present invention comprises a plasmonic lens 101 and a tunable pinhole 102, wherein the plasmonic lens 101 focuses the incident electromagnetic (EM) radiation, and the tunable pinhole 102 controls the total number of functioning sub-apertures 103 of the plasmonic lens 101.

Description

Tunable plasmonic lens

Technical field

This invention relates to the field of active plasmonics, and particularly, to a plasmonic lens with tunable focusing performance, such as the focal length, depth of focus (DOF), and full-width half- maximum (FWHM).

^■ Background

Lenses are the basic optical elements, and are widely used in a variety of optical instruments and systems. In general, when a lens is fabricated, the focusing performance, including the focal length, depth of focus (DOF) and full-width half-maximum (FWHM), cannot be changed any more. By utilizing some special materials, such as liquid crystal (LC), a tunable lens can be realized by changing the refractive index of the material dynamically, mostly by applying an external voltage. In the last decade, a large number of micro tunable lenses have also been developed based on microelectromechanical systems (MEMS) technology. Besides the capability in three-dimensional (3D) control of the foci possessed by these tunable lenses, they are usually miniaturized, integrated and intelligent to some extent. As a result, they gain huge applications ranging from the high throughput scanning and confocal imaging systems, smart sensing, to lab-on-a-chip. However, no matter the conventional lenses or the LC-/MEMS-based tunable lenses, dielectric materials are adopted, and the resolution is mainly restricted by the diffraction limit.

On the other hand, surface plasmons (SPs) or plasmonics, which exploits the unique optical properties of metallic nanostructures to route and manipulate light at nanometer length scales, has become a research hotspot since the discovery of extraordinary optical transmission (EOT) through metallic nanohole arrays [T.W. Ebbesen et al., Nature 1998, 391 : 667] . SPs is a surface electromagnetic (EM) wave propagating along the interface of metal/dielectric, and is excited by the collective electronic oscillations, or the charge density waves. As the near-field evanescent wave can now be utilized for practical optical functionalities, the so-developed plasmonic lenses can realize the superfocusing beyond the diffraction limit. As an alternative to the conventional dielectric-based refractive lenses, plasmonic lenses are aimed for such applications as all-optical telecommunication, ultrahigh-resolution imaging, single-molecular biosensing, optical data storage, and nanolithography.

At the same time, efforts to develop a plasmonic device with tunable capability are in progress. So far, two different strategies exist. The first one is to modulate the dielectric property of environment surrounding the metallic material by electrical, optical, or thermal excitation [see for example: W. Dickson et al, Nano Lett. 2008, 8: 281 ; K. Meehan and R.E. Dessy, US Patent, Patent No: US 7193719 B2, Mar. 2007; G.A. Gibson, US Patent, Patent No: US 2009/0323171 Al , Dec. 2009; R.G. Beausoleil et al, US Patent, Patent No: US 2010/0278474 Al , Nov. 2010; R.A. Pala et al., Nano Lett. 2008, 8: 1506; G. Gagnon et al., J. Lightwave TechnoL, 2006, 24: 4391 ], as the behavior of SPs is highly sensitive to the dielectric constant at the metal/dielectric interface. The second one is to adjust the electronic property of metallic material itself by electrical, magnetic, or thermal excitation [see for example: N.I. Zheludev et al, US Patent, Patent No: US 6304362 Bl , Oct. 2001; H.A. Atwater et al, Int 'l Patent, Patent No: WO 2007/016675 Al, Feb. 2007; B. Lee and B. I. Choi, US Patent, Patent No: US 2010/0046060 Al , Feb. 2010; B.S. Passmore et al., Opt. Express, 2009, 17: 10224; V.V. Temnov et al., Nat. Photonics, 2010, 4: 107], as SPs is a surface EM wave that interacts with the oscillating electrons near the surface of metal. Obviously, both strategies are focused on the modulation of material properties. Therefore, for a practical application, a big problem may occur. That is, an appropriate material should be chosen, and its dielectric property (for the first strategy) or the electronic property (for the second strategy) can be adjusted by an external excitation. However, for a desired operating wavelength, such kind of materials may be not so easy to be found. Moreover, the tunable capabilities of so-developed plasmonic devices are usually limited because of the small adjustable range of material property.

Among all the tunable plasmonic devices reported in publications and patents, lens application is rarely concerned, and up to now, to our knowledge, there are only two related publications [M.-K. Chen et al., Microwave Opt. Technol. Lett., 2010, 52: 979; A.E. Cetin et al, Opt. Lett., 2010, 35: 1980]. Chen et al. adopts the second strategy mentioned above, for THz wavelength based on the semiconductor material, InSb, which reveals metal properties (complex permittivity ε=ε' + is", with the real part ε'<0 and the imaginary part ε" of a relatively small value) at this wavelength. The method reported by Cetin et al. to control the focusing performance of the nanoslit-based plasmonic lens utilizes two extra light sources, and it provides an all-optical candidate.

■ Summary of the invention

A tunable plasmonic lens consists of two main components, a plasmonic lens to focus the incident EM radiation, and a tunable pinhole located in front of or behind the plasmonic lens to control the input or output area of the EM wave. The plasmonic lens is composed of an array of subwavelength sub- apertures, which can be holes or pillars in the shape of square, rectangular, circular, annular, elliptical, etc. The plasmonic lens is made of a negative dielectric constant material, such as a metal like gold, silver, chromium, aluminum, copper and nickel, or a semiconductor like indium antimonide (InSb), indium tin oxide (ITO) and aluminum zinc oxide (AZO). The tunable pinhole is made of a material that is opaque for the operating wavelength. There is a transparent window in the center of the tunable pinhole. The width or diameter of the transparent window can be adjusted by an external excitation, such as an electrical, magnetic, or thermal source. The operating wavelength can be ultraviolet (UV), visible, infrared, THz, etc., depending on the plasmonic material adopted. The plasmonic lens and the tunable pinhole can be integrated on a single chip by microelectromechanical systems (MEMS) or semiconductor mi cro-/nano fabrication technologies. The tunability of focal length of the tunable plasmonic lens can easily reach more than 100%.

■ Brief description of the drawings

FIG. 1 A is a schematic representation of the tunable plasmonic lens, with the tunable pinhole in front of the plasmonic lens.

FIG. IB is a schematic representation of the tunable plasmonic lens, with the tunable pinhole behind the plasmonic lens.

FIG. 2A is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength slits.

FIG. 2B is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength circular holes.

FIG. 2C is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength square holes.

FIG. 2D is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength annular holes. FIG. 2E is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength circular pillars.

FIG. 2F is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength square pillars.

FIG. 2G is a schematic representation of the plasmonic lens shown in FIG. 1 , composed of an array of subwavelength annular pillars.

FIG. 3A is a schematic representation of the tunable pinhole shown in FIG. 1, whose width can be adjusted.

FIG. 3B is a schematic representation of the tunable pinhole shown in FIG. 1 , whose diameter can be adjusted.

FIG. 4 is a schematic representation of one embodiment of the tunable plasmonic lens.

FIG. 5 is a discrete approximation of the required phase front to obtain a focal length of 5 μηι, for the plasmonic lens shown in FIG. 4.

FIG. 6A are the intensity patterns of electric field in the x-y plane for different cases, corresponding to different lens sizes of the plasmonic lens as shown in FIG. 5.

FIG. 6B are the intensity patterns of electric field in the x-y plane for different cases, corresponding to different widths of the tunable pinhole in front of the plasmonic lens of case d.

FIG. 6C are the intensity patterns of electric field in the x-y plane for different cases, corresponding to different widths of the tunable pinhole behind the plasmonic lens of case d.

FIG. 7 A compares the focal length for different cases according to FIG. 6.

FIG. 7B compares the maximum light intensity of focal point for different cases according to FIG. 6.

FIG. 7C compares the depth of focus (DOF) for different cases according to FIG. 6.

FIG. 7D compares the full-width half-maximum (FWHM) for different cases according to FIG. 6.

FIG. 8 is a schematic representation of the SOI-based process to fabricate the tunable plasmonic lens shown in FIG. 4, on a single chip.

FIG. 9 is a three-dimensional (3D) schematic representation of the tunable plasmonic lens shown in FIG. 4, by using the process described in FIG. 8.

^■ Detailed description of the invention

So far, some plasmonic lenses based on metallic subwavelength nanostructures have already been experimentally reported. However, all of them showed a large deviation of focal length by comparing the experimental results with the design. Now, it is well known that besides the SPs effect, diffraction also plays an important, sometimes a dominant role in determining the ultimate focal length. According to diffraction theory, when a plane wave radiates on a circular aperture with radius p, the output intensity / along the optical axis can be approximately calculated by using the Rayleigh-Sommerfeld integral, which gives

where Io is the initial

e wavelength. Accordingly, the maximum intensity can be derived at a position Z_m as

2

Z (2) From Eq. (2), we can see that the lens size has a large effect on the position of peak intensity, correspondingly the focal length of the lens. It is also suitable for a rectangular aperture, in which case the side length should be utilized. Therefore, if the lens size can be modulated dynamically, the resulting focal length for a specific wavelength can also be adjusted. This invention utilizes the size effect mentioned above, however in a totally different way.

FIG. 1 shows the schematic diagrams of the tunable plasmonic lens 100, which consists of two main components, a plasmonic lens 101 and a tunable pinhole 102. In FIG. 1A, the tunable pinhole 102 is located in front of the plasmonic lens 101. The tunable pinhole 102 contains a transparent window 104 in the center, whose width or diameter can be modulated by an external excitation. Thus, when an incident light 105 propagates to the tunable pinhole 102, the portion of 105A will be blocked, and only the portion of 105B can pass through and reach the plasmonic lens 101. The plasmonic lens 101 is composed of tens or hundreds of subwavelength sub-apertures 103, and has a different dimension from the transparent window 104. Correspondingly, only those sub-apertures within the portion of 103 A function to focus the incident light 105B to a focal point P. Therefore, by changing the dimension of the transparent window 104 dynamically, the area of the incident light 105B will be altered, and thus, the exposing size of 103 A or the total number of functioning sub-apertures 103 will also be changed, resulting in a position shift of the focal point P.

In FIG. IB, the tunable pinhole 102 is located behind the plasmonic lens 101. Therefore, the incident light 105 propagates to the plasmonic lens 101 first, and all the subwavelength sub-apertures 103 will be illuminated. However, because of the existence of the transparent window 104 in the tunable pinhole 102, only the portion except 103B can ultimately contribute to focusing. As in FIG. 1A, by changing the dimension of the transparent window 104 dynamically, the total number of functioning sub-apertures 103 can also be changed, and a position shift of the focal point P will thus be resulted.

FIG. 2 gives some schematic embodiments of the plasmonic lens 101. FIG. 2 A is a rectangular aperture and the subwavelength sub-apertures 103 are slits. FIG. 2B is a circular aperture and the subwavelength sub-apertures 103 are circular holes. FIG. 2C is a square aperture and the subwavelength sub-apertures 103 are square holes. FIG. 2D is a circular aperture and the subwavelength sub-apertures 103 are annular holes. FIG. 2E is a circular aperture and the subwavelength sub-apertures 103 are circular pillars. FIG. 2F is a square aperture and the subwavelength sub-apertures 103 are square pillars. FIG. 2G is a circular aperture and the subwavelength sub-apertures 103 are annular pillars. Besides, other shapes of the sub-apertures 103 can also be adopted, e.g. elliptical, cross-shaped, H-shaped, etc. All the sub-apertures 103 can be of the same or different dimensions. Various plasmonic materials can be utilized for the plasmonic lens 101 , depending on the operating wavelength concerned. For instance, gold and silver are the general materials for visible and near-infrared (NIR) wavelengths. Aluminum is a good material for blue and UV range. Indium tin oxide (ITO) and aluminum zinc oxide (AZO) are the materials for NIR, and indium antimonide (InSb) is a material for terahertz (THz).

FIG. 3 gives the schematic embodiments of the tunable pinhole 102. FIG. 3A contains a rectangular or square transparent window 104, whose width is w. By applying a transverse driving force 301 along the width direction at both sides, the width of the transparent window 104 can be modulated in the form of mechanical movement or thermal expansion of the material. The driving force 301 can be created by an external electrical, magnetic or thermal source. FIG. 3B contains a circular transparent window 104, whose diameter is d. By uniformly applying a radial driving force 302, the diameter of the transparent window 104 can be modulated in the form of mechanical movement or thermal expansion of the material. The driving force 302 can be created by an external electrical, magnetic or thermal source.

FIG. 4 shows a schematic embodiment of a tunable plasmonic lens 100, in which the tunable pinhole 102 is located in front of the plasmonic lens 101 , as described in FIG. 1A. The spacing between the tunable pinhole 102 and the plasmonic lens 101 is S (=200 nm). The focal length / is the distance from the focal point P to the output plane 401 of the plasmonic lens 101. For the case as described in FIG. IB, in which the tunable pinhole 102 is located behind the plasmonic lens 101 , we can just exchange the positions of the light source and the focal point P. In this case, the focal length / is the distance from the focal point P to the output plane 402 of the plasmonic lens 101. The plasmonic lens 101 is composed of an array of subwavelength nanoslits with different width. As the length-to-width ratio of the nanoslits is rather large, usually more than 30, a two-dimensional (2D) cross-section analysis is reasonable. The plasmonic lens 101 is made of gold with a thickness of 400 nm, and the operating wavelength is 650 nm. The permittivity of gold for this wavelength is z_m = -12.8915 + 1.2044Ϊ. The surrounding medium is air. In order to realize the focusing effect, the widths of all the nanoslits should be specially designed to construct a required phase front for a specific focal length f. The required phase front as a function of distanc as

where n is an integer. On the other hand, the phase retardation, induced when an incident light 105 passes through a metal-insulator-metal (MIM) nanoslit waveguide, can be derived by Real( fcf), where d is the thickness of metallic film and β is the propagation constant, which is determined by

where ko is the wave vector of light in free space, e_m is the permittivity of metal, and a is the slit width. As a result, the discrete approximation of the required phase front to obtain a focal length of 5 μιη can be achieved, as shown in FIG. 5. The designed widths for the nanoslits from the center are 40, 40, 40, 45, 45, 50, 55, 65, 80, 1 15, 15, 15, 15, 15, 15, 15, 15, 20, 20, 20, 25, 30, 35, 45, 65, 150, 15, 15, 15, 20, 20, 25, 35, 45 nm, respectively. The metallic walls between two adjacent nanoslits are all 80 nm.

FIG. 6A gives the finite-difference time-domain (FDTD) simulation results of the intensity patterns of electric field in the x-y plane for different cases a, b, c and d of the designed plasmonic lens in FIG. 5, corresponding to lens sizes of 2550, 4480, 6140, and 7800 nm, respectively. The total phase retardations are 0.45π, 1.42π, 2.51π, and 4.07π, respectively. Accordingly, the focusing performance, such as the focal length, maximum light intensity of focal point, depth-of-focus (DOF) and full-width half- maximum (FWHM), can be derived, as shown in FIG. 7. Obviously, as the lens size is increased, the focusing performance is also changing. For example, the focal length is increased from 2.42, 3.54, 4.765 to 4.955 μηι, for the cases a, b, c, and d, respectively. Though all the cases are from the same phase front required for the focal length of 5 μηι, the diffraction effect plays the dominant role at the beginning, for the small lens size, and then the SPs waveguide effect dominates.

FIG. 6B and FIG. 6C are the FDTD simulation results of the intensity patterns of electric field in the x-y plane for the tunable plasmonic lens 100, with the tunable pinhole 102 located in front of and behind the plasmonic lens 101 , respectively. The plasmonic lens 101 for all cases has the same lens size of 7800 nm, as case d in FIG. 5. The tunable pinhole 102 is made of gold (200 nm in thickness) and doped silicon (500 nm in thickness). Cases a b c and cf have different widths w of 2550, 4480, 6140, and 7800 nm for the transparent window 104, respectively. The derived focusing performance is also given in FIG. 7 for comparison, from which we can see that the focal lengths for cases a' to if in FIG. 6B are 2.345, 3.41 , 4.71 , and 4.92 μπι, respectively, and those in FIG. 6C are 2.765, 3.225, 4.88, and 4.965 μπι, respectively. As a result, for both FIGs. 6B and 6C, a dynamic modulation of the focal length is realized, with a tunability of 109.81 % and 79.57 %, respectively, calculated by (case if - case α') / (case α'). Other focusing performance is also modulated for different cases.

FIG. 8 gives a schematic representation of the SOI-based process to fabricate the tunable plasmonic lens shown in FIG. 4, and to demonstrate how the plasmonic lens 101 and the tunable pinhole 102 are integrated on a single chip. First, as shown in FIG. 8A, the substrate silicon is wet etched anisotropically. Then, in FIG. 8B, a gold film is deposited and patterned on the device silicon, and the pattern is further transferred to the bottom device silicon. After that, another layer of gold is deposited on the substrate side, and focused-ion-beam (FIB) milling is adopted to fabricate the nanoslits, FIG 8C. Finally, the sacrificial dioxide layer is etched, and all the structures are released, FIG. 8D. FIG. 9 shows a three- dimensional (3D) schematic representation of the tunable plasmonic lens 100, as shown in FIG. 4, by using the process described in FIG. 8. The driving force utilized to modulate the width of the transparent window 104 is provided by two electrostatic combs, which are widely used in the field of microelectromechanical systems (MEMS).

Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention.

List of References

100 tunable plasmonic lens

101 plasmonic lens

102 tunable pinhole

103 subwavelength sub-apertures

103A focusing portion of plasmonic lens

104 transparent window

105 incident light, incident electromagnetic radiation (EM) 105 A portion (blocked)

105B portion (pass-through)

301 transverse driving force

302 radial driving force

401 output plane

402 output plane

P focal point

w width

d diameter

S spacing

f focal length

Claims

1. A tunable plasmonic lens ( 100) comprising:

a lasmonic lens (101) and a tunable pinhole (102);

wherein the plasmonic lens (101) focuses incident electromagnetic (EM) radiation (105), and the tunable pinhole (102) controls the total number of functioning sub-apertures (103) of the plasmonic lens (101).

2. The plasmonic lens (101) as claimed in claim 1 , wherein the lens (101) is composed of an array of subwavelength sub-apertures (103).

3. The plasmonic lens (101) as claimed in claim 2, wherein the subwavelength sub-apertures (103) are holes.

4. The plasmonic lens (101) as claimed in claim 3, wherein the subwavelength sub-apertures (103) have a shape selected from the group consisting of square, rectangular, circular, annular, elliptical, slitlike, cross-like, and H-like shapes.

5. The plasmonic lens (101) as claimed in claim 2, wherein the subwavelength sub-apertures (103) are pillars.

6. The plasmonic lens (101) as claimed in claim 5, wherein the subwavelength sub-apertures (103) are selected from the group consisting of square, rectangular, circular, annular, elliptical, cross-like, and H-like pillars.

7. The plasmonic lens (101) as claimed in any of the preceding claims, wherein the lens (101) is made of a negative dielectric constant material.

8. The plasmonic lens (101) as claimed in claim 7, wherein the negative dielectric constant material is metal.

9. The plasmonic lens (101) as claimed in claim 8, wherein the negative dielectric constant material is selected from the group consisting of gold, silver, chromium, aluminum, copper, and nickel.

10. The plasmonic lens (101) as claimed in claim 7, wherein the negative dielectric constant material is a semiconductor.

11. The plasmonic lens (101) as claimed in claim 10, wherein the negative dielectric constant material is selected from the group consisting of indium antimonide (InSb), indium tin oxide (ITO), and aluminum zinc oxide (AZO).

12. The tunable pinhole (102) as claimed in claim 1 , wherein the pinhole (102) is located in front of the plasmonic lens (101) to control the input area of the EM wave.

13. The tunable pinhole (102) as claimed in claim 1 , wherein the pinhole (102) is located behind the plasmonic lens (101) to control the output area of the EM wave.

14. The tunable pinhole (102) as claimed in claim 12 or 13, wherein the pinhole (102) is made of a material that is opaque for the operating wavelength, and in the center, there is a transparent window

(104).

15. The tunable pinhole (102) as claimed in claim 14, wherein the transparent window (104) is square, rectangular, or circular.

16. The tunable pinhole (102) as claimed in claim 15, wherein the dimension of the transparent window (104) is adjustable by an external excitation.

17. The tunable pinhole (102) as claimed in claim 16, wherein the external excitation is selected from the group consisting of an electrical, magnetic, and thermal source.

18. The tunable plasmonic lens (100) as claimed in any of the preceding claims, wherein the EM wave has a wavelength in the range from ultraviolet (UV) to THz.

19. The tunable plasmonic lens (100) as claimed in any of the preceding claims, wherein the plasmonic lens (101) and the tunable pinhole (102) are integrated on a single chip by microelectromechanical systems (MEMS) or semiconductor micro-/nanofabrication technologies.

20. The tunable plasmonic lens (100) as claimed in claim 19, wherein the plasmonic lens (101) and the tunable pinhole (102) are integrated on a single chip based on SOI technology.

21. Method of focusing electromagnetic radiation (EM) using a tunable plasmonic lens (100), by carrying out the following steps:

providing the tunable plasmonic lens (100) comprising a plasmonic lens (101) with an array of subwave length sub-apertures (103), and a tunable pinhole (102) being opaque for the operating wavelength and having a transparent window ( 104) in the center;

providing incident light (105):

positioning the tunable pinhole (102) in front of or behind the plasmonic lens (101);

adjusting the dimension of the transparent window ( 104) by an external excitation:

focusing the portion of incident light passing the tunable plasmonic lens (100) to a focal point (P).