WO2010052654A1

WO2010052654A1 - Lighting element comprising a light guiding structure with a surface guided mode and a phospor material, and a method of lighting

Info

Publication number: WO2010052654A1
Application number: PCT/IB2009/054900
Authority: WO
Inventors: Abraham R. Balkenende; Jaime Gomez Rivas; Vincenzo Giannini; Christophe R. C. Arnold
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-11-06
Filing date: 2009-11-04
Publication date: 2010-05-14

Abstract

The invention relates to a lighting element, a device using such a lighting element and a method of lighting. The lighting element comprises light guiding structure, having a thickness sufficiently thin to support a surface guided mode, in particular a Long Range Surface Mode (LRSM) mode, a TM0 mode or a TE₀ mode, and a phosphor material, arranged for absorbing and converting light from the light guiding structure. The light will remain confined in the guiding structure and may be efficiently absorbed by the phosphor material.

Description

LIGHTING ELEMENT COMPRISING A LIGHT GUIDING STRUCTURE WITH A SURFACE GUIDED MODE AND A PHOSPHOR MATERIAL, AND A METHOD OF LIGHTING

FIELD OF THE INVENTION

The invention relates to a lighting element, a device using such a lighting element and a method of lighting.

BACKGROUND OF THE INVENTION

High power (Watt-class) blue laser, typically irradiating in the 405-470 nm range, can be used to excite phosphors in order to obtain low etendue bright light sources. The color of these sources can be tuned by selecting the appropriate phosphors used. Contrary to directly projected laser light, the light emitted from such a laser- irradiated spot is not coherent nor monochromatic, eliminating the two main disadvantages of the use of laser sources, i.e. speckle and small band width. Also, the emitted light is diffuse rather than directional, thus lessening safety risks with respect to eye damage.

According to the current state of the art, for commonly used phosphor materials the thickness required for sufficient absorption of the excitation radiation from the laser is in the order of 100-1000 μm, as most suitable and commonly available phosphorous materials with an efficiency exceeding 90% exhibit a relatively low absorption coefficient in the order of 10-100 cm^"1 upon excitation in the 400-480 nm range. Laser based systems used for lighting operate at power levels typically in the order of 100 mW to 10 W. When this power is concentrated on a small volume or area of phosphor material, the Stokes losses inherent to the frequency conversion process results in a high local heat dissipation. For a typical irradiated volume of 20x20x100 μm , this occurs already at about 300 mW, whereas smaller excited volumes will further limit the usable power. As the thermal conductivity of most common phosphorous materials is low, in the order of 0.1-10 WK ¹In ¹, the dissipation of heat away from the irradiated spot is a limiting factor for the power that can be used and thus the maximum achievable amount of converted light. Upon overheating (the temperature of the phosphor may exceed 300 ⁰C), the efficiency of the phosphor drops significantly, resulting in additional power loss and uncontrolled further heating that may damage the device. Having phosphor materials with higher absorption coefficients would allow for the use of thinner phosphor layers and a lower amount of phosphors; unfortunately most materials that would be suitable, having an conversion efficiency of 80-95%, are relatively expensive; other, relatively cheap materials typically have a conversion efficiency of less than 80%.

It is an objective of the invention to improve laser-irradiated lighting. It is an additional objective of the invention to allow laser-irradiated lighting using commonly available organic and inorganic phosphors having a relatively low absorption coefficient.

SUMMARY OF THE INVENTION

The invention provides a lighting element, comprising a light guiding structure, having a thickness sufficiently thin to support a surface guided mode, in particular a Long Range Surface Mode (LRSM) mode, a Transverse Magnetic (TMo)mode or a Transverse Electric (TEo)mode, and a phosphor material, arranged for absorbing and converting light from the light guiding structure. As a guided surface wave mode will be induced upon coupling suitable light into the light guiding structure, the light will remain in the guiding structure and may be efficiently absorbed by the phosphor material, even if the absorption coefficient of the phosphor is relatively low at the used wavelength, e.g. in the order of 10-100 cm-1, and the phosphor is only used in small amounts/thin layers. Thin layers make it relatively easy to dissipate generated heat. The invention allows a spot size of the order of the size of the collimated laser beam, which may be in the 20-100 μm range at incident light with a wavelength of 440 nm, or alternatively, much smaller if the focal point is sufficiently extended in the propagation direction, e.g. by incoupling of a Bessel shaped beam profile.

Depending on the wavelength of light and the type of material used, the required thickness of the light guiding structure is smaller than 1 μm, preferably smaller than 100 nm, in order to induce an effective surface guided wave (SW). A surface guided wave is defined here as an electromagnetic surface charge oscillation mode guided by a thin layer. This mode has the majority of the electromagnetic energy located in the medium surrounding the thin layer. The phosphor is preferably applied in a region where the SW intensity is at a maximum, preferably near the interface of the light guiding structure with adjacent material, either in the light guide itself or in the surrounding material. The light guiding structure may have all kinds of shapes, but is preferably a flat layer or has a periodic corrugation in the form of a grating. A grating can be used to efficiently couple the incident light into the SW. All different types of SWs (long-range surface polaritons, long-range surface plasmon polaritons, TMo modes or TEo) have similar characteristics: long propagation lengths and with most of the electromagnetic energy confined in the medium surrounding the thin layer.

It is advantageous if the lighting element is provided with light incoupling means arranged for coupling light from an external light source into the light guiding structure. Thus, the incident light may be coupled effectively into the light guiding structure. Preferably, the incoupling means are optimized, for instance by optimizing the angle of the incident light with the surface of the light guiding structure. Suitable incoupling means include prisms, lenses, microprisms, microlenses and grating-like structure, optionally provided with collimating means. In a preferred embodiment, the thickness of the light guiding structure is less than 100 nm. At these relatively small thicknesses, the p-polarized long range surface guided wave modes (LRSM) are most efficiently achieved.

In a preferred embodiment, at least part of the phosphor material is incorporated in the light guiding structure. Having the phosphor material within the light guiding structure allows for an efficient absorption of the light coupled in the SW surface mode, and also allows for relatively simple design of the lighting element. An optimum conversion efficiency is obtained in this configuration when the only absorbing material in the thin guiding layer is the phosphor material.

In another preferred embodiment, at least part of the phosphor material is arranged in at least one phosphor layer adjacent to the light guiding structure. As the surface guided wave modes (SW) are mostly confined to the interface between the light guiding structure and the phosphor material, this will allow for an efficient conversion by the phosphor material of the light coupled into the light guiding system.

Preferably, the phosphor layer has a thickness smaller than 1 μm. At such a thickness, a good conversion efficiency (typically at least 90%) is already easily achieved with phosphors having a relatively low absorption coefficient, while the relatively thin layer allows for efficient heat dissipation if the lighting element is mounted on a heat spreader. Otherwise, thin layers having a thickness smaller than 1 μm are not essential for heat dissipation; direct convection of heat to ambient is possible in case of confinement of the absorption to a relatively thin top layer. For some types of materials, thin layers may be more difficult to produce.

In a preferred embodiment, the light guiding structure is substantially made of an absorbing dielectric material or an absorbing semiconductor material. Preferably, the light guiding structure is substantially made of a material selected from the group consisting of silicon, germanium, III -V semiconductors, chalcogenide glasses. Thin layers of absorbing media at the wavelength of the incident light, such as silicon, germanium, chalcogenide glasses, support surface waves called long range surface polaritons. These components have suitable optical properties as well as good availability. In another preferred embodiment, the light guiding structure is substantially made of a non- absorbing or weakly absorbing dielectric material or a semiconductor material, preferably supporting TMo and TEo modes. Only phosphor absorbs the light in this system. This configuration is well suited when phosphor has a low absorption efficiency. The most suitable materials are selected from the group consisting of YAG, AI2O3, (meso)porous ZrO₂, (meso)porous TiO₂ and SiO₂.

In yet another preferred embodiment, the light guiding structure is substantially made of a metal or an alloy. Thin layers of metals support long range surface plasmon polaritons. Preferred metals are silver and aluminum.

In a preferred embodiment, at least part of the light guiding structure is confined between a first dielectric layer and a second dielectric layer having approximately equal refractive indices. The phosphor may be either incorporated in one of the dielectric layers, both of the dielectric layers and/or in the light guiding structure.

It is advantageous if the dielectric material has a refractive index lower than the refractive index of the incoupling medium, wherein the element is adapted to achieve incoupling to SWs by the evanescent transmitted field of totally internally reflected light. The incoupling medium, for instance a prism, is a relatively simple way to achieve effective incoupling.

It is preferred if the light guiding structure is a grating structure. Using a grating structure enables incoupling without a separate incoupling medium such as a prism. As less layers are needed, a grating structure improves the freedom of design for the light element.

In a preferred embodiment, the lighting structure is confined between an absorbing dielectric material, which absorbing dielectric material comprises at least part of the phosphor material, and a non-absorbing dielectric material. In such a configuration, the light is absorbed specifically in the layer comprising the phosphor. As only one of the layers surrounding the thin film is absorbing the light, this configuration implies a relatively low light conversion capacity, which is particularly advantageous if a low output power density is required. In another preferred embodiment, the lighting structure is confined between two absorbing dielectric material layers, which absorbing dielectric material comprises at least part of the phosphor material. Such a configuration produces a relatively large light conversion capacity, allowing for a relatively high converted light output capacity, and also allowing for emitting light from a relatively concentrated spot.

It is preferred if the absorption coefficient of the phosphor material is between 10-100 cm^"1 at a predetermined wavelength of light to be converted by the phosphor layer. In this range, the SW effect is optimally used to spread the conversion over a relatively large surface of the light element, very thin layers can be used that are easily cooled, and only a small amount of phosphor material is needed.

In another preferred embodiment, at least part of the phosphor material is incorporated in the light guiding structure, wherein the absorption coefficient of the phosphor material in the light guiding structure exceeds 100 cm^"1 at a predetermined wavelength of light to be converted by the phosphor material. In a preferred embodiment, the phosphor material is selected to convert light having a wavelength of 400-480 nm, preferably 440-450 nm. Such a system is particularly suitable for producing converted light in the visible spectrum.

It is advantageous, if the lighting element is provided with a heat spreader. A heat spreader can be used most advantageously, as the use of a relatively thin layer allows for a very efficient dissipation of heat towards the heat spreader. This allows the lighting element according to the invention to operate at relatively low temperatures at the location of light conversion, avoiding the diminished light conversion efficiency most phosphors experience at elevated temperatures.

The invention also provides a device comprising a lighting element according to the invention, wherein the light guiding structure of the lighting element is optically coupled to a light source, wherein the light source is selected to emit light of a wavelength convertible by the phosphor material.

Preferably, the light source is a laser. As a laser typically provides monochromatic polarized light at a small area, the use of the SW effect is easily optimized, and if the device comprises multiple lighting elements according to the invention, a laser allows for precise and controlled excitation of individual lighting elements. In a lighting system, multiple phosphors may be used, either mixed or to be addressed separately. In the latter case this lighting elements may be addressed separately either by using multiple lasers or by using a single laser in combination with laser beam control means. The invention also provides a method of lighting by the use of a device according to the invention, wherein the emitted spectrum is defined by selecting the wavelength of the light source and by selecting the composition of the phosphor material. Such a method may be employed for lighting purposes such as remote laser lighting, automotive lighting and laser based projection light sources, as well as sensing devices based on fluorescence or fluorescence microscopy.

The invention will now be further elucidated by the following non-limiting examples.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. Ia and Ib show a system according to the invention having an absorbing layer for guided surface wave in between phosphor materials.

Figs. 2a and 2b show reflection and absorbance at different layer thicknesses as a function of the incident laser angle for a system according to the invention having an absorbing layer for guided surface wave in between phosphor materials.

Fig. 3 shows the absorption as a function of the incident laser angle in a system using a phosphor material as the guiding layer.

Fig. 4a and 4b show the absorption as a function of the incident laser angle in a system according to the invention using a phosphor material as the guiding layer at different layer thickness.

Fig 5 shows the absorption in a phosphor layer for a system according to the invention having a dielectric layer for guided surface wave adjacent to the phosphor layer. DETAILED DESCRIPTION OF EMBODIMENTS

Example 1 : Absorbing layer for guided surface wave in between phosphor materials.

Figure Ia describes the x and z component of the Poynting vector amplitude of the electromagnetic field in a multilayer system (Rutile/Phosphor/Si/Phosphor, shown in Figure Ib) when using laser light at a wavelength of 440 nm, entering the system at an incident angle of 43.7 degrees with the plane of the layers. The system consists of a relatively thick first phosphor layer 2 of approximately 880 nm, on which a relatively thin silicon (Si) layer 3 is deposited. On the other side of the silicon layer, a second, relatively thin phosphor layer 4 of about 100 nm is deposited. For the phosphor layers, Cerium-doped yttrium aluminum garnet is selected. Finally, a rutile prism 5 is applied, through which the laser light 6 from a laser source 7 enters into the layer system 1 at a controlled angle θ. The size of the silicon layer 3 is exaggerated for clarity.

The silicon layer 3 preferably has a thickness under 100 nm for the best results, preferably under 20 nm. The first phosphor layer 2 can be extended to any desired thickness, even up to the mm range. The second phosphor layer is preferably kept under 1 μm, in which case about 100% of the incident light can be coupled into the guiding silicon layer 3 to be converted into a spectrum of the desired wave lengths by the phosphor material. The conversion efficiency depends on the type of phosphor material deposited on the thin layer, the thickness and type of material of the thin layer 3 and the thickness of the phosphor layer 4. This configuration is particularly useful when the absorption in the phosphor material is relatively low, i.e. when the absorption length in the phosphor is exceeding 10-100 μm or when the phosphor material cannot easily be applied as a thin film material. The phosphor layers 2, 4 convert the laser light 6 and the emitted light 9 comprises a spectrum according to the chosen phosphor composition. In lighting applications, the emitted spectrum 9 could for instance be white. For projection purposes, elements 1 having a specified color could be selected by aiming the laser 7 at the desired colors.

Figure Ia shows the typical behavior of a SW: a strong enhancement of the electromagnetic field at the two bounding interfaces in the phosphor material 2,4 adjacent to the silicon layer 3, and the electromagnetic field suppression inside the silicon thin layer 3. This implies that most of the energy of the SW is confined near the interface of the silicon thin film with the phosphor material.

The absorbing silicon layer 3 acts as the long range surface mode-supporting layer, and is in contact with a phosphor material in the adjacent layers 2, 4. Alternatively, phosphor material 2, 4 may also be available only at one side of the silicon layer 4; The relatively thick first layer 2 may be replaced with a non-absorbing dielectric having a refractive index comparable to the relatively thin phosphor layer 4 at the other side of the silicon layer 3. Another possibility is to replace thin phosphor layer 4 by a non-absorbing dielectric having a refractive index comparable to the refractive index of the thick phosphor layer 2. Suitable materials with a refractive index similar to that of Ce: YAG are for instance undoped YAG, A12O3, slightly (meso)porous ZrO2, or 40% (meso)porous TiO2. The incoupling medium 5 should have a higher refractive index than the phosphor layer 4 in order to enable effective incoupling of laser light 6 generated by an external laser 7. Coupling can be also achieved without an incoupling medium if light is scattered in the proximity of the thin layer. This scattering can be provided, for instance, by the thin film when it presents a corrugation. Very efficient scattering, and coupling efficiencies, can be obtained when this corrugation is a periodic structure or a grating.

Figure 2 shows calculated values for the reflection (R_p, fig 2a) and absorbance (Ap, Figure 2b) in the multiplayer system 1 in Figure 1 as a function of the thickness of the second YAG layer 4, at various incident angles θ. For these calculations, the lower Ce: YAG layer 2 was kept at a constant thickness of 1 micron, which for the purpose of the calculations is equivalent to any larger thickness. The results show that tuning of the thickness of the second YAG layer 4 enables total absorption of the incident light. In this specific system, the total absorption occurs at a thickness for the second layer 4 of approximately 100 nm. The maximum of the absorption (minimum of reflectance), is due to the excitation of a SW. This mode propagates parallel on the layers and it is confined in the YAG stratums. The optimum in Figure 2b illustrates that the invention enables a very good absorption of the light in thin layers of weakly absorbing phosphors when the incident light is coupled to a SW layer 3.

Example 2: System using a phosphor material as the guiding layer

In an alternative configuration, the phosphor material used may also be in the light guiding layer itself. Referring back to the system according to Figure 1 , this implies that both layers 2, 4 adjacent to the guiding layer 3 should be made out of non-absorbing dielectric materials, whereas instead of silicon another suitable material containing suitable light-conversion properties is to be used.

Figure 3 shows the calculated absorbance of a 20 nm thin film between two non-absorbing silicon dioxide (SiO₂) layers having rutile as an incoupling medium, varying the imaginary part of the thin film dielectric constant. The thickness of this layer is preferably less than 100 nm. Most if not all of the light in the system will eventually be absorbed and converted by the phosphor material. The calculated absorbance (absorbance A = I implies 100% absorption) of p-polarized light with a wavelength of 440 nm as a function of the incident angle θ on the surface of a Rutile/SiO2/phosphor/SiO2 system. Rutile is chosen in this example as material with a higher index of refraction than SiO2 to excite the SW via the evanescent field of the total internal reflection on the Rutile/SiO2 interface. The upper SiO2 layer (comparable to the second phosphor layer 4 in Figure. Ib) has a thickness of 180 nm in this example, the thickness of the phosphor layer is 20 nm. Absorption in the multiplayer takes only place in the phosphor material. The lower SiO2 layer in the calculation (which has a position comparable to the first phosphor layer 2 in Figure. Ib) is considered semi- infinite, but a typical value of 1 micron or larger would not give any different results. The absorbance is calculated as function of the angle of incidence θ and for different values of the imaginary part of the phosphor dielectric constant.

As shown in Figure 3, in this specific configuration an angle of incidence of approximately 32.7 degrees show a pronounced increase of the absorbance A. For an imaginary part of the dielectric constant (i) of 1, the absorbance is higher than 90%, even if the phosphor layer has only 20 nm in thickness. This extremely efficient absorption by such a thin layer is the result of the long optical path in the guiding layer enabled by the Long- Range Surface mode. The above material choices are again not critical, although the optimal incident angles and optimal layer thickness will vary dependent on the materials used.

Figure 4a shows the calculation of the phosphor layer absorption as a function of the incident laser angle (θ) for an identical alternative configuration as described above, for different combinations of a varying thickness of the upper SiO2 layer and a varying imaginary part of the dielectric constant of the phosphor. Nearly 100% absorption can be achieved in this configuration for a wide range of imaginary parts of the phosphor dielectric constant.

Figure 4b shows the absorption for a Rutile/SiO2/Phosphor/Air system. The SiO₂ layer has a thickness of 60 nm in this example, the thickness of the phosphor layer is 500 nm. The imaginary part of the dielectric constant of the phosphor is 0.05. In this configuration light can be absorbed very efficiently, even though the asymmetry in the dielectric constant of the two layers adjacent to the phosphor layer is significant.

Apart from semiconductor materials such as silicon used in the configuration according to Figure 1, also metal and dielectric layers can be used as guiding layer 3. Also the choice of the phosphor (host) material and the top and bottom layers can be varied over a large range of materials. The main implication is that the optimal thickness of the top layer and the optimal angle of incidence will be different.

Example 3: Non-absorbing layer for guiding a surface wave adjacent to a phosphor layer

Figure 5 shows the absorption in a 1 μm thick layer of phosphor in a multilayer system (Rutile/YAG/ SisNVPhosphor), for p- and s-polarizations, as a function of the incident angle θ of the laser light having a wavelength of 440 nm. The guiding thin film comprises silicon nitride. The YAG layer has a dielectric constant of 0.9. The imaginary component of the dielectric constant was only 0.005. It is shown that the two layers adjacent to the thin guiding film of silicon nitride in this configuration do not need to have the same dielectric constant in order to have an efficiently absorption of 100% of the incident light. The absorption of p-polarized light is due to the excitation of the TMo mode supported by the silicon nitride layer. Similarly, the absorption up to 90% of s-polarization arises from the excitation of the TEo mode. The absorption efficiency of the phosphor itself can be very low in this configuration and yet the system reaches an absorption of almost 100%.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Lighting element (1), comprising: a light guiding structure (3), having a thickness sufficiently thin to support surface guided mode (SW), in particular a Long Range Surface Mode (LRSM) mode, a Transverse Magnetic (TMo)mode or a Transverse Electric (TEo)mode , and - a phosphor material(2,4), arranged for absorbing and converting light from the light guiding structure.

2. Lighting element according to claim 1, characterized in that the lighting element is provided with light incoupling means (5) arranged for coupling light (6) from an external light source (7) into the light guiding structure (3).

3. Lighting element according to any of the preceding claims, characterized in that the thickness of the light guiding structure is less than 100 nm.

4. Lighting element according to any of the preceding claims, characterized in that at least part of the phosphor material is incorporated in the light guiding structure.

5. Lighting element according to any of the preceding claims, characterized in that at least part of the phosphor material is arranged in at least one phosphor layer adjacent to the light guiding structure.

6. Lighting element according to claim 5, characterized in that the phosphor layer has a thickness smaller than 1 μm.

7. Lighting element according to any of the preceding claims, characterized in that the light guiding structure is substantially made of an absorbing dielectric material or an absorbing semiconductor material.

8. Lighting element according to any of the preceding claims 1-6, characterized in that the light guiding structure is substantially made of a non- absorbing or weakly absorbing dielectric material or semiconductor material.

9. Lighting element according to any of the preceding claims 1-6, characterized in that the light guiding structure is substantially made of a metal or an alloy.

10. Lighting element according to any of the preceding claims, characterized in that at least part of the light guiding structure is confined between a first dielectric layer and a second dielectric layer having approximately equal refractive indices.

11. Lighting element according to claim 10, characterized in that the dielectric material has a refractive index lower than the refractive index of an incoupling medium, wherein the element is adapted to achieve incoupling to SWs by the evanescent transmitted field of totally internally reflected light.

12. Lighting element according to claim 10, 11 or 12, characterized in that the lighting structure is confined between an absorbing dielectric material layer, which absorbing dielectric material comprises at least part of the phosphor material, and a non- absorbing dielectric material layer.

13. Lighting element according to any of the preceding claims, characterized in that the lighting structure is confined between two absorbing dielectric material layers, which absorbing dielectric material comprises at least part of the phosphor material.

14. Device comprising a lighting element (1) according to any of the preceding claims, wherein the light guiding structure (3) of the lighting element is optically coupled to a light source (7), wherein the light source is selected to emit light of a wavelength convertible by the phosphor material (2, 4).

15. Method of lighting by the use of a device according to claim 14, wherein the emitted spectrum is defined by selecting the wavelength of the light source and by selecting the composition of the phosphor material (2, 4).