WO2007124549A2

WO2007124549A2 - Spatially phase modulation of laser beams

Info

Publication number: WO2007124549A2
Application number: PCT/BE2007/000037
Authority: WO
Inventors: Johan Hofkens; Jun-Ichi Hotta; Hiroshi Uji-I
Original assignee: Katholieke Universiteit Leuven
Current assignee: Katholieke Universiteit Leuven
Priority date: 2006-05-03
Filing date: 2007-05-02
Publication date: 2007-11-08
Anticipated expiration: 2008-11-03
Also published as: WO2007124549A3

Abstract

By present invention the fabrication of a phase shaper for generating a 'doughnut mode' laser beam using a thin, circular polymer film on a substrate has been demonstrated. The fabrication method is based on a combination of spin-coating and drop-casting. The alignment procedure to get ideal 'doughnut modes' is described. The intensity distribution at the focus is analyzed with single molecule spectroscopy.

Description

SPATIALLY PHASE MODULATION OF LASER BEAMS

Background and Summary

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to a phase shaper for spatially phase modulation of a beam of electromagnetic radiation and more particularly to a phase shaper for generating of a doughnut mode electromagnetic radiation distribution. A particular embodiment is a system and method for the fabrication of a thin polymer film based phase shaper for generating doughnut mode laser beams.

Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer's specifications, instructions etc.) are herby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

B. Description of the Related Art

Recently, spatially phase modulated laser beams find applications in the various field, such as stimulated emission depletion (STED) fluorescence microscopy , laser manipulation [2, 3], quantum communication [4], and so on.

Especially, in the field of the microscopy, "doughnut modes" which contain a three dimensional, sharp zero-intensity hole attract attention since it is indispensable for STED microscopy using reversible saturable optical fluorophore transitions (RESOLFT) [5]. STED microscopy has the potential to overcome the diffraction limit of optical microscope [I]. In order to realize "doughnut modes", several methods have been applied, amongst them the use of a circular pattern of vapor deposited MgF2 [l],of liquid crystal thin films [6], of polymer films patterned with electron beam [7] and the use of liquid crystal phase modulators[8].

All the phase modulation devices of the prior art require the use of complex machinery or complex techniques for their fabrication. Thus, there is a need in the art for an improved system to realise the doughnut modes and cost effect method system therefore.

Present invention provides this. It concerns the fabrication of a phase shaper using a polymer film on a substrate, by using a combination of spin-coating and drop-casting. Spin-coating and drop-casting are very easy and low-cost techniques. The method phase shapers for a broad range of wavelengths proposed by present invention can be easily made, just by varying the thickness of the circular polymer thin film.

SUMMARY OF THE INVENTION

The present invention concerns to a system or a method of spatially phase modulation of beams of light or of streams of photons or a similar propagation of waves, preferably of a wavelength in the range from about 4,000 to about 10000 angstroms.

The phase shaper of present invention is suitable for spatially phase modulation of light beams or light rays of one or more discrete frequencies and these phase shapers are suitable most particular for laser beams.

A particular embodiment is a system to generate intensity distributions in the doughnut mode. The present invention solves the problems of the related art on cost effective spatially phase modulation of laser beams by the development of a novel phase shaper, more particularly by the development of a novel a circular polymeric π phase shaper. The π phase retarding properties were confirmed by interferometric measurements. Next, we demonstrate the application of this phase shaper for generating laser beams with a doughnut shape intensity distribution at the focus. The latter was unambiguously confirmed by imaging single molecules.

In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to methods that allow for easy fabrication of phase shapers that can be used over a wide range of wavelengths just by controlling the concentration of phase retarding polymer and/or spin coating speed.

In one aspect of the invention, the phase shapers of present invention and the doughnut shaped beams that can be generated can be used in industrial application such as for example STED microscopy.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. Detailed Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.

Referring now specifically to the drawings, a space shaper for beams of radiation, such as laser beams can be produced according to an embodiment of the present invention is illustrated in FIG. 3 and whereof the effect on laser beams has been shown generally in FIG 2, FIG. 4, FIG 5, FIG 6 and Fig 7. The system has particular application in stimulated emission depletion (STED), fluorescence microscopy, and quantum communication and laser manipulation.

It will be apparent to those skilled in the art that various modifications and variations can be made in materials used for producing the phase shaper and the application of the present invention and in construction of the system and method without departing from the scope or spirit of the invention. Examples of such modifications have been previously provided.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. Present invention concerns the fabrication of a phase shaper using a polymer film on a substrate, by using a combination of spin-coating and drop-casting.

Spin-coating and drop-casting are very easy and low-cost techniques. The method phase shapers for a broad range of wavelengths proposed by present invention can be easily made, just by varying the thickness of the circular polymer thin film.

Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of the solvent is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates.

In order to make doughnut modes with a sharp three-dimensional intensity hole, a circular film of retardation material comprising a polymer film was coated on a substrate. The phase retardation is determined by the refractive index of the polymer and its thickness. The retardation generated by the polymer film is as follows,

Δ<* = — (n_polymer - n_aιr) (1)

/I₀

Here, Δφ, λa d, n_poi_ymer, n_aιr, are the phase retardation, the wavelength in the vacuum, the thickness of the polymer, the refractive index of polymer and the refractive index of air, respectively. The thickness of the polymer film d should be adjusted so that Δφ becomes π.

The diameter of the circular phase retardation material is also an important parameter, since perfect destructive interference at the focal point is indispensable to get the "zero" intensity hole of the 'doughnut'. If we assume the laser beam to be a plane wave, in order to make the zero-intensity hole at the centre of the beam, the diameter of the circle should be 1/V2 of the diameter of the laser beam. The amplitude distribution of a focused beam can be represented by the following expression [9],

Here, a, A/f, p are the radius of aperture, the amplitude at the aperture and the normalized distance from the origin in the aperture plane, respectively. And u, v are expressed as follows,

As we can see from eq. (2), the electric field at the focal point is proportional to the area of the aperture. Therefore, the electric field of the laser beam passing through the phase shaper E can be represented as superposition of two components (Fig. 1.).

The first component E₀ is the electric field of the incident laser beam, and the other component Ei is the π phase shifted electric field which has two times the amplitude of the non-shifted field. The intensity distribution |E|² of the phase shaped laser beam at the focus is shown in Fig. 2.

Examples

The strategy to make a circular thin polymer film pattern is as follows. First, we spin coat one polymer, and this layer will be used to generate the phase retardation. Then, we place a droplet of another polymer on top, creating a circular protection layer. After that, we do wet-etching with a solvent, removing the non protected hydrophobic polymer film.

Glass cover slips are used as a substrate They are carefully cleaned by consecutively applying a suitable solvent such as acetone, a sodium hydro oxide solution and milli-Q water.

After cleaning, the surface of the glass cover slip is modified to be hydrophobic by applying a silane coupling agent ((3- Aminopropyl)trimethoxysilane) solution in ethanol. Other suitable silan coupling agents are for instance all silan compounds that make the surface hydrophobic. This step is done to avoid detaching of the polymer film from the substrate during wet etching.

Actually any hydrophobic polymer can be used in order to generate the required phase shift.

We used two different hydrophobic polymers to demonstrate the general character of the method. Polymethylmethacrylate (PMMA) and polystyrene (PS) were selected since they are know to have excellent film forming properties. 1-20 wt% solutions of the selected polymer in toluene or chloroform are spin coated on the substrate. The thickness of the polymer film was controlled by changing the concentration of polymer solution and the rotation speed of spin coating.

As hydrophilic polymer, polyvinyl alcohol dissolved in water was used. The

PVA film forms the protection layer or mask for the spatial pattern of hydrophobic polymer film that one wants to generate. The size and the circular shape of the mask critically depend on the size of the droplet of the PVA solution placed on the hydrophobic polymer. Note that, due to dewetting, the droplet of aqueous polymer solution will keep its initial shape when placed on the hydrophobic retardation layer [10]. After drying overnight, the drop forms became a circle protection pattern. Then, the hydrophobic polymer in unprotected areas is removed using chloroform. The cover slip is dried with Ar gas. Next, the polyvinyl alcohol protection layer was removed by milli-Q water, and dried with Ar gas. A schematic representation of fabrication procedure of polymer film based phase shapers is shown in Fig. 3.

The spatial phase modulator was analyzed using an interferometer. A Mach-Zehnder type interferometer was used since it allows [9] direct observation of the phase shift of the wave front. The fabricated phase shaper was placed on an optical mount, and placed at the center of the laser beam in one arm of the interferometer. The phase shift introduced by different phase shapers made with different concentrations of PMMA solution is shown in Fig. 4. The phase shift by the phase shaper clearly depends on the concentration of the PMMA solutions, therefore, a phase shaper for any wavelength can be fabricated just by controlling the PMMA concentration in solution. A phase shaper for 633 nm was prepared and the interferogram of the polymer film phase shaper taken by a CCD camera is shown in Fig. 5.

For this particular phase modulator, a 7wt% PMMA toluene solution and 10 μl of a 10wt% PVA aqueous solution were used as polymer phase retardation material and protection layer, respectively. The diameter of the polymer film is 2.7 mm. The phase shift at the center the circular polymer structure is analyzed from the shift of the fringes, and it is determined to be π. The thickness of the polymer film is calculated to be 646 nm from eq. (1), taking into account a refractive index of 1.49 for PMMA.

Next, we analyzed the intensity distribution of the phase modulated laser beam at the focus changing the ratio of the phase retarded over non retarded part of the laser beam. A plano-convex lens (f = 100 mm) was used to focus the phase shifted laser beam on a CCD camera. The alignment of the phase shaper is critical to the shape of the 'doughnut'. The alignment procedure is as follows. The diameter of the laser beam is adjusted to be -1.4 times the diameter of phase shifted area, and the position of the phase shaper is manipulated to make the intensity distribution at the focus to be symmetrical. Then, the diameter of the laser beam with respect to the diameter of the phase shifting area are precisely adjusted by an iris to get the maximum contrast ratio in the 'doughnut'. The effect of the beam size on 'doughnut shape' at the focus is shown in Fig. 6.

The intensity distribution of the phase modulated laser beam focused by a microscope objective (Olympus, numerical aperture 1.3, x 100) on a cover slip is evaluated by using single molecules as a probe for the electric field, since single molecules can be regard as ideal dipole emitter [H]. Scanning fluorescence images of single phenoxy substituted terylenediimide (TDI) molecules are shown in figure 7. The bin-time for each pixel is 5 ms, and the imaged areas are 10 μm x 10 μm or 2 μm x 2μm. Without phase shaper, the fluorescence originating from an individual molecule has a Gaussian intensity distribution, reflecting the intensity distribution of the applied excitation beam (Fig. 7(a)). When the polymer phase shaper is placed at the center of the laser beam, the images resulting from single molecules change drastically (Fig. 7(b)). Single molecules show up now as 'doughnuts', effectively reflecting the intensity distribution of the applied excitation beam. The intensity ratio of the center (of the hole) and the surrounding doughnut in Fig. 7 (c) is 1/20- 1/200.

Drawing Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

Fig. 1. is a schematic view showing an electric field of the laser beam passing through the phase shaper. (ao: radius of the laser beam, ai: radius of circular thin film)

Fig. 2. is a picture of a simulated intensity distribution at the focus (a) in x-y plane, (b) in x-z plane (λ = 633 nm, f = 2.3 mm, ao = 3 mm (incident laser beam radius), ai = 2.12 mm (phase shifted field)).

Fig. 3. provides a scheme of phase shaper fabrication.

Fig. 4. demonstrates the phase shift as a function of concentration of PMMA solution. The phase shift at the center the circular polymer structure is analyzed from the shift of the fringes. One fringe shift corresponds to 2π phase shift.

Fig. 5. is a picture of an interferogram of phase shaper recorded at 633nm. The diameter of the polymer film is 2.7mm.

Fig. 6. demonstrates the Intensity distribution of phase shaped laser beam, (a) Schematic of the diameter of the phase shaper (A) and that of the laser beam (B). The ratio of the diameter of the laser beam and that of the phase shaper (B/A) are (b) 1.30, (c) 1.39, (d) 1.41, (e) 1.48, and (f) 1.56, respectively.

Fig. 7. provides a fluorescence images of single TDI molecules, (a) without phase shaper, (b) with phase shaper, (c) zoom of one molecule in image b.

References to this application

1. Thomas A. Klar, Egbert Engel, and Stefan W. Hell, "Breaking Abbe's diffraction resolution limit in fluorescence microscopy with stimulated emission depletion beams of various shapes", Phys. Rev. E 64, 066613-1- 066613-9(2001).

2. D. W. Zhang and X.-C. Yuan, "Optical doughnut for optical tweezers", Opt. Lett. 28, 740-742 (2003).

3. Peter John Rodrigo, Vincent Ricardo Daria and Jesper Glϋckstad, "Realtime interactive optical micromanipulation of a mixture of high- and low- index particles", Opt. Express 12, 1417-1425 (2004).

4. Alois Mair, Alipasha Vaziri, Gregor Weihs and Anton Zeilinger, "Entanglement of the orbital angular momentum states of photons", Nature 412, 313-316(2001).

5. Michael Hofmann, Christian Eggeling, Stefan Jakobs, and Stefan W. Hell, "Breaking the diffraction barrier in fluorescence microscopy at low light intensities by using reversibly photoswitchable proteins", PNAS 102, 17565- 17569 (2005).

6. G Miyaji, K. Ohbayashi, K. Sueda, K. Tsubakimoto, and N. Miyanaga, "Generation of Vector Beams with Axially-Symmetric Polarization", The Review of Laser Engineering 32, 259-264 (2004).

7. Y. Miyamoto, M. Masuda, A. Wada, and M. Takeda, "Electron-beam lithography fabrication of phase holograms to generate Laguerre-Gaussian beams", in Optical Engineering for Sensing and Nanotechnology (ICOSN '99), I. Yamaguchi ed, Proc. SPIE 3740, 232-235 (1999).

8. Takeshi Watanabe, Yasunori Igasaki, Norihiro Fukuchi, Makoto Sakai, Shun-ichi Ishiuchi, Masaaki Fujii, Takashige Omatsu, Kimihisa Yamamoto, and Yoshinori Iketaki, "Formation of a doughnut laser beam for super- resolving microscopy using a phase spatial light modulator", Opt. Eng. 43, 1136-1143 (2004).

9. M. Born and E. Wolf, Principal of Optics, (Pergamon Press, Oxford, New York, Seoul, Tokyo, 1980) Chap. 7, Chap. 9.

10. Olaf Karthaus, Lars Grasjo, Norihiko Maruyama, and Masatsugu Shimomura, "Formation of ordered mesoscopic polymer arrays by dewetting", Chaos 9, 308-314 (1999).

11. J. Enderlein, "Theoretical study of detection of a dipole emitter through an objective with high numerical aperture," Opt. Lett. 25, 634-636 (2000).

Claims

SPATIALLY PHASE MODULATION OF LASER BEAMS

Claims

What is claimed is:

1) A method for creating a spectral phase shaper for spatially phase modulation of a beam of electromagnetic radiation characterized in that it comprises spin coating and edging of a circular hydrophobic polymer film on a substrate.

2) The method of claim 1, further comprising the steps of 1) spin-coating of a first phase retardation layer comprising an hydrophobic polymer on a substrate, 2) drop-casting of a second protection layer comprising an hydrophilic polymer on said first polymeric layer 3) wet-etching said film pattern with a suitable solvent to removing the non protected hydrophobic polymer film 4) drying the substrate comprising the layers 5) removing the second protection layer.

3) The method of claim 1 or 2, further comprising substrate a suitable solvent such as acetone, a sodium hydro oxide solution and milli-Q water before spin coating

4) The method of claim 1 or 2, further comprising rendering the surface of the substrate for spin coating of the polymeric layer to be hydrophobic by applying a silane coupling agent solution.

5) The method of claim 4, wherein the silane couping agent is ((3- Aminopropyl)trimethoxysilane) and the solvent is ethanol.

6) The method of any of the claims 1 or 5, wherein the first or the first layer comprises hydrophobic polymers selected from a Polymethylmethacrylate (PMMA) polymer or a polystyrene (PS) polymer.

7) The method of claim 6, wherein the hydrophobic polymers are spin coated on the substrate from a 1-10 wt% solutions.

8) The method of any of the claims 1 to 7, wherein the hydrophobic polymers are solved in toluene or chloroform.

9) The method of any of the claim 1 to 8, wherein the thickness of the polymer films is controlled by changing the concentration of polymer solution and the rotation speed of spin coating.

10) The method of any of the claim 1 to 9, wherein the hydrophilic polymer is polyvinyl alcohol

11) The method of claim 10, wherein the polyvinyl alcohol is drop-casted from a watery solution.

12) The method of claim 11, wherein the polyvinyl alcohol film forms the protection layer or mask for the spatial pattern of hydrophobic polymer film.

13) The method of any of the claim 1 to 12, wherein the hydrophobic polymer in unprotected areas is removed using chloroform.

14) The method of any of the claim 1 to 13, further comprising a step of drying the substrate comprising the polymers with a gas, preferably an inert gas such Ar gas or nitrogen gas

15) The method of any of the claim 1 to 14, further comprising removing the protection layer by a suitable fluid such as milli-Q water

16) The method of any of claim 15, further comprising drying the exposed hydrophobic polymer firm with an inert gas, preferably a water soluble gas such as Ar gas or nitrogen gas.

17) A spectral phase shaper polymeric film for spatially phase modulation of beam of electromagnetic radiation comprising a circular thin polymer film phase shaper with π phase retarding properties, obtainable by the method of any of the claims 1 to 16.

18) A spectral polymeric phase shaper polymeric film for spatially phase modulation of beam of electromagnetic radiation, obtainable by the method of any of the claims 1 to 16.

19) A spectral phase shaper polymeric film for generating of a doughnut modes electromagnetic radiation distribution, obtainable by the method of any of the claims 1 to 16.

20) An optoelectronic device for emitting of spatially phase modulated of light beams that comprise the spectral phase shaper polymeric firm of any of the claims 17 to 19.