Solid state illumination device based on non-radiative energy transfer

Abstract

There is provided an illumination device comprising: a wavelength converting layer comprising a photon emitting donor configured to absorb energy to reach an excited state, and a photon emitting acceptor; an energy source configured to provide energy to the donor such that the donor reach the excited state; wherein the donor and the acceptor are selected and arranged at a distance from each other such that non-radiative transfer of excitation energy from the donor to the acceptor occur, and wherein the acceptor is configured to emit a photon at a second wavelength after the transfer of energy; the illumination device further comprising a periodic plasmonic antenna array, arranged on the substrate and embedded within the wavelength converting layer, and comprising a plurality of individual antenna elements arranged in an antenna array plane, the plasmonic antenna array being configured to support a first lattice resonance at the second wavelength, arising from coupling of localized surface plasmon resonances in the individual antenna elements to photonic modes supported by the system comprising the plasmonic antenna array and the wavelength converting layer, wherein the plasmonic antenna array is configured to comprise plasmon resonance modes such that light emitted from the plasmonic antenna array has an anisotropic angle distribution.

Claims

1. An illumination device comprising: a wavelength converting layer comprising: a donor configured to absorb energy to reach an excited state, and to emit light of a first wavelength, and an acceptor configured to emit light of a second wavelength, longer than the first wavelength, wherein the donor and the acceptor are selected and arranged at a distance from each other such that non-radiative transfer of excitation energy from the donor to the acceptor occurs such that the acceptor emits a photon at the second wavelength after the transfer of excitation energy, and; a periodic plasmonic antenna array embedded within the wavelength converting layer, wherein: the plasmonic antenna array comprises a plurality of individual antenna elements arranged in an antenna array plane, the plasmonic antenna array is configured to support a first lattice resonance at the second wavelength, arising from coupling of localized surface plasmon resonances in the individual antenna elements to at least one photonic resonance mode by combining the plasmonic antenna array and the wavelength converting layer, and light emitted from the plasmonic antenna array has an anisotropic angle distribution.

2. The illumination device of claim 1, wherein the acceptor has a first energy level (E.sub.A1) corresponding to the second wavelength and a second energy level (E.sub.A2) higher than the first energy level, and the donor has an energy level (E.sub.D) matching the second energy level (E.sub.A2) of the acceptor.

3. The illumination device of claim 1, wherein a donor concentration and an acceptor concentration are selected such that the non-radiative transfer of excitation energy from the donor to the acceptor has an efficiency higher than 0.9.

4. The illumination device of claim 1, wherein a ratio between a donor concentration and an acceptor concentration is at least 1-to-1.

5. The illumination device of claim 1, wherein a ratio between a donor concentration and an acceptor concentration is in a range of about 1-to-1 to about 5-to-1.

6. The illumination device of claim 1, wherein the donor and the acceptor are point emitters selected from at least one of a set of rare earth ions, a set of dye molecules, and a set of quantum dots.

7. The illumination device of claim 6, wherein the donor is a dye molecule or quantum dot emitting green/yellow light having wavelengths from about 500 nm to about 580 nm.

8. The illumination device of claim 6, wherein the acceptor is a dye molecule or quantum dot emitting red light having wavelengths from about 580 nm to about 630 nm.

9. The illumination device of claim 6, wherein at least one of the donor and the acceptor is a perylene dye molecule.

10. The illumination device of claim 1, wherein a donor concentration and an acceptor concentration are selected such that at least one of an extinction coefficient and a quantum efficiency of the wavelength converting layer is higher than for a wavelength converting layer comprising substantially only the acceptor.

11. The illumination device of claim 1, further comprising an energy source configured to provide energy to the donor such that the donor reaches the excited state.

12. The illumination device of claim 11, wherein the energy source is at least one of a photon emitter, an electron emitter, an x-ray emitter, a gamma-ray emitter, and an electron-hole pair.

13. The illumination device of claim 11, wherein the energy source is at least one of a light emitting diode and a solid state laser.

14. The illumination device of claim 1, wherein the plasmonic antenna array is configured to comprise at least one plasmon resonance mode being out-of plane asymmetric.

15. The illumination device of claim 1, wherein a plurality of antenna elements that form the plasmonic antenna array are arranged in a substantially square array having a lattice constant of about 400 nm, or a substantially hexagonal array with a lattice constant of 470 nm.

16. The illumination device of claim 1, wherein the plasmonic antenna array comprises: a plurality of truncated pyramidal antenna elements having a top side width in a first range of about 110 nm to about 130 nm, a bottom side width in a second range of about 135 nm to about 160 nm, and a height in a third range of about 100 nm to about 160 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention.

(2) FIG. 1 schematically illustrates an illumination device according to an embodiment of the invention;

(3) FIG. 2 schematically illustrates an illumination device according to an embodiment of the invention;

(4) FIG. 3 schematically illustrates an energy band diagram of photon emitters in an illumination device according to an embodiment of the invention; and

(5) FIG. 4 schematically illustrates an antenna element of an illumination device according to an embodiment of the invention.

DETAILED DESCRIPTION

(6) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Like reference characters refer to like elements throughout.

(7) FIG. 1 is a schematic illustration of an illumination device 100 comprising a wavelength converting layer 104 comprising a plurality of donor 114 and acceptor 116 particles arranged in close proximity of a periodic plasmonic antenna array comprising a plurality of individual antenna elements 108 arranged in an antenna array plane defined as the plane of the base of the antenna elements. The donor 114 and acceptor 116 may also be referred to as fluorescent materials, photon emitters, emitters, phosphors or dyes. The donor 114 and acceptor 116 may for example be rare earth ions, dye molecules, quantum dots, or a combination thereof.

(8) The external energy source 102 for exciting the donor may for example be a photon emitter such as a light emitting diode or a laser. Even though the main purpose of the energy source 102 is to excite the donor 114, it is in some cases unavoidable that also the acceptor 114 is excited. In principle any external energy source may be used to excite the donor, such as an electron having sufficiently high energy, x-ray or gamma radiation, heat, injection of electron-hole pairs etc. Electrons may for example be emitted by a cathode ray tube (CRT), x-rays/gamma-rays may for example be provided from a vacuum tube, gamma ray (CT). For simplicity, and to explain the general concept using photons to stimulate the donor, the energy source will henceforth be referred to as a light source 102. Herein, the light source 102 is illustrated as a separately arranged light emitting device, such as a light emitting diode or a laser, arranged at a distance from the wavelength converting layer and from the remainder of the illumination device. However, the light source 102 may equally well be integrated such as in a semiconductor light emitting diode formed in a semiconductor substrate, where the antenna array and the wavelength converting layer is formed on top of the substrate.

(9) FIG. 1 further illustrate that the donors 114 are arranged in the wavelength converting layer 104 adjacent to the light source 102 to receive energy from the energy source 102 such that the donors 114 may absorb energy and reach an excited state.

(10) The zoomed in portion of FIG. 1 illustrates a donor 114 separated from an acceptor 116 by a distance r. The efficiency of non-radiative energy transfer from the donor to the acceptor is inversely proportional to the distance r.

(11) FIG. 2 further illustrates a periodic plasmonic antenna array comprising a plurality of individual antenna elements 108 arranged in an antenna array plane. The antenna array is arranged within the wavelength converting layer 104. Furthermore, the antenna array is configured to support lattice resonances at the second wavelength, corresponding to the wavelength of light emitted by the acceptor 116, arising from diffractive coupling of localized surface plasmon resonances in the individual antenna elements. It is further illustrated in FIGS. 1 and 2 how light 110 is emitted from a light emitting surface of the illumination device 100 within a limited angular range 112, i.e. an anisotropic distribution of the emitted light.

(12) FIG. 3 is a schematic illustration of energy band diagrams of the donor 114 and the acceptor 116 used to explain the concept of non-radiative energy transfer, in particular Frster resonance energy transfer (FRET) and the respective energy levels of the donor 114 and the acceptor 116. The terminology for energy levels in a semiconductor is used herein, where the energy band gap E.sub.G is defined by the difference between the valence band and the conduction band. However, the reasoning may be applied analogously to organic semiconductors where the valence band corresponds to highest occupied molecular orbital (HOMO) and the conduction band corresponds to the lowest unoccupied molecular orbital (LUMO), and the band gap as the HOMO-LUMO gap.

(13) The donor 114 has a first energy level E.sub.D here corresponding to the donor band gap E.sub.GD. A photon emitted by the light source (or another energetic particle emitted by another energy source), having an energy E.sub.E higher than E.sub.D, is absorbed by the donor such that the donor reaches an excited state. Typically, when the photon absorbed by the donor has an energy higher than E.sub.D, the donor relaxes 302 to reach an excited state having the energy E.sub.D, from which a photon having a first wavelength .sub.1 can be emitted if the donor relaxes to the ground state by way of direct recombination. Once the donor is in an excited state having an energy E.sub.D, energy is transferred non-radiatively via FRET to the acceptor which has an energy level E.sub.A2 corresponding to E.sub.D. After the acceptor has been excited, it relaxes 304 down to the lower energy level E.sub.A1 after which a photon having the second wavelength .sub.2 is emitted via recombination when the acceptor relaxes to the ground state. As the band gap of the acceptor, E.sub.GA, is smaller than E.sub.GD, energy is lost in the wavelength conversion process such that the wavelength .sub.2 is longer than the wavelength .sub.1, and also longer than the wavelength of light absorbed by the donor. Thereby, for example blue light emitted by the light source 102 is shifted towards the red portion of the visible spectra. Any light emitted by the donor through direct recombination over the band gap E.sub.GD will be of the first wavelength .sub.1. As the donor for example may be a dye molecule emitting in the green/yellow wavelength range and the acceptor may be a dye molecule emitting in the red wavelength range, it is desirable to suppress emission of light directly from the donor, i.e. to have a FRET efficiency which is as high as possible. An increased FRET efficiency for a specific donor acceptor combination is achieved by tailoring the distance between donors and acceptors, by controlling the concentration of donor and acceptors in the wavelength converting layer 104, such that the average distance between a given type of donor and acceptor is suitable for FRET.

(14) For a single donor and a single acceptor, the FRET efficiency E.sub.FRET can be calculated by

(15) $E_{\mathrm{FRET}}=\frac{1}{1+{\left[{\left(\frac{R_0}{r}\right)}^6\right]}^{-1}}$
where it can be seen that in addition to the so called Frster radius R.sub.0, a number characterizing the probability for FRET between specific types of donor and acceptors, the efficiency of the transfer process depends strongly on the separation r between the donor and acceptor.

(16) In a layer as discussed here, many donors and acceptors are distributed randomly in a 3D space. Hence, to calculate the FRET efficiency for a specific donor, one has to take into account all the acceptors labeled i at a separation r.sub.i, with which the donor can interact, as shown by

(17) $E_{\mathrm{FRET}}=\frac{1}{1+{\left[{{}_i\left(\frac{R_0}{r_i}\right)}^6\right]}^{-1}}.$

(18) As the exact position of acceptors with respect to each donor is not known, the acceptor concentration c.sub.ACC can be used to describe the probability to find an acceptor at a certain distance to the donor rather than including the exact separation between donor and acceptor. In this way the FRET efficiency for a three-dimensional arrangement as a function of acceptor concentration c.sub.Acc can be calculated by comparing the intensity or number of photons emitted by the donor in the presence (I.sub.DA) and the absence (I.sub.D) of acceptors:

(19) $E_{\mathrm{FRET}}=1-\frac{I_{\mathrm{DA}}}{I_D}=1-\frac{I_D^0\exp \left[-t\text{/}{}_D-2{\left(t\text{/}{}_D\right)}^{\frac{1}{2}}\right]\mathrm{dt}}{I_D^0\exp \left[-t\text{/}{}_D\right]\mathrm{dt}}\mathrm{with}$$=\frac{\left(\frac{1}{2}\right)}{2}\frac{c_{\mathrm{Acc}}}{c_0}\mathrm{and}{c}_0={\left(\frac{4}{3}{R}_0^3\right)}^{-1}$

(20) To calculate the intensities I.sub.DA and I.sub.D, the donor intensity decays are integrated over time, indicated by t. I.sub.D.sup.0 denotes the donor intensity or photon count measured at time 0 and T.sub.D is the characteristic decay time or lifetime of the donor in absence of the acceptors. Furthermore,

(21) $\left(\frac{1}{2}\right)=\sqrt{}$
is the Gamma function and c.sub.0 describes a characteristic concentration for which a spherical volume with radius R.sub.0 contains on average 1 acceptor. At this concentration c.sub.0 a FRET efficiency of 72.38% is obtained.

(22) In order to use FRET as a mechanism to increase the absorption in a wavelength converting layer 104, e.g. for applications with plasmonics, but without altering the resulting emission spectrum, the FRET efficiency has to be high, such as at least 0.9, in embodiments higher than 0.95 or in other embodiments higher than 0.98. In this case very little or no donor emission will be detectable and the shape of the emission spectrum from the illumination device will completely resemble that of the acceptor. To achieve such high FRET efficiencies at reasonable acceptor concentrations (e.g. about 0.03 acceptors/nm.sup.3 corresponding to an average acceptor separation of 4.0 nm), donor-acceptor combinations with a relatively high Frster radius should be used, in an embodiment a Frster radius of 3.0 nm or higher. The acceptor concentration should not be too high as quenching of the acceptor quantum efficiency can reduce the overall system efficiency when the acceptors are too close to each other. In order to achieve relevant Frster efficiencies of about 0.95, a minimum acceptor concentration of 0.0295/nm.sup.3 for R.sub.0=3.0 nm, corresponding to an average acceptor separation of 4.0 nm, should be realized.

(23) Furthermore, in order to significantly enhance the absorption in the wavelength converting layer 104 the ratio of donors to acceptors should be at least 1:1, however higher ratios such as 4:1 or 5:1 would be even more beneficial. The maximum ratio can however be limited by the size of the donors and acceptors as it might be detrimental if donors start to be too close to each other and depending on the acceptor excited state lifetime and acceptor concentration, which determines the time that an acceptor is in a ground state and responsive to FRET.

(24) Moreover, a high concentration of donors would lead to a high absorbance around the pump wavelength, corresponding to the energy E.sub.E, which reduces the excitation of acceptors with high energy photons, which is favorable for the long term stability of the acceptor. However, a too high donor concentration may lead to saturation of the acceptors. Here, the coupling between the acceptor and plasmonic modes of the antenna array provides an additional advantage as the coupling reduces the excited state lifetime of the acceptor. Thus, it is desirable to have a donor concentration as high as possible without donor-donor interaction (e.g. concentration quenching).

(25) Additionally, the donor 114 is in embodiments configured such that an absorption band of the donor is capable of absorbing incident light within an energy range above the band gap, so that as much as possible of the incident light is absorbed. This is for example achieved through additional energy levels located above the conduction band energy level E.sub.D, or LUMO-level in the case of an organic molecule.

(26) In principle, the above described wavelength converting layer may be provided in combination with one or more additional wavelength converting layers, with or without plasmonic antenna arrays.

(27) In an example embodiment, a plasmonic antenna array configured to support plasmonic-photonic lattice resonances at a frequency range corresponding to red light may for example comprise antenna elements in the form of truncated pyramidal antenna elements 400 as illustrated in FIG. 4, having a top side 404 in the range of 110 to 130 nm, a bottom side 402 in the range of 135 to 155 nm, and a height 406 in the range of 100 to 160 nm. The sides are in this example the lengths of the sides of a square, but rectangles or triangles are also possible. Moreover, the antenna elements are arranged in a square array having a lattice constant of about 400 nm. Also a hexagonal array with a period of 470 nm will exhibit near normal enhanced emission.

(28) The antenna element may for example be made from aluminum fabricated on a fused silica substrate. The wavelength converting layer can for example be a polystyrene material comprising donor and acceptor particles spin-coated onto the substrate.

(29) Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

(30) In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.