Feedback enhanced light emitting device

• US 20040069995A1
• Filed: 05/08/2003
• Published: 04/15/2004
• Est. Priority Date: 05/08/2002
• Status: Abandoned Application

• Current Assignee: Zeolux Corporation
• Original Assignee: Zeolux Corporation
• Inventors: Koch, Gene C., Magno, John N.
• Application Number:
  US10/434,941
• Publication Number:
  US 20040069995A1
• Time in Patent Office: Days
• US Class Current: 257/80
• CPC Class Codes: G09G 3/2803 Display of gradations G09G3/288 takes precedence
• CPC Class Codes: G09G 3/32 semiconductive, e.g. using light-emitting diodes [LED]
• CPC Class Codes: H01L 33/465 with a resonant cavity structure
• CPC Class Codes: H01S 3/08059 Constructional details of the reflector, e.g. shape mirrors in general G02B5/08; mountings for mirrors G02B7/18
• CPC Class Codes: H01S 3/1065 using liquid crystals
• CPC Class Codes: H01S 3/1686 Liquid crystal active layer
• CPC Class Codes: H01S 5/183 having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• CPC Class Codes: H01S 5/36 comprising organic materials
• CPC Class Codes: H10K 2102/3031 Two-side emission, e.g. transparent OLEDs [TOLED]
• CPC Class Codes: H10K 50/816 Multilayers, e.g. transparent multilayers
• CPC Class Codes: H10K 50/828 Transparent cathodes, e.g. comprising thin metal layers
• CPC Class Codes: H10K 50/8426 Peripheral sealing arrangements, e.g. adhesives, sealants
• CPC Class Codes: H10K 50/85 Arrangements for extracting light from the devices
• CPC Class Codes: H10K 50/852 comprising a resonant cavity structure, e.g. Bragg reflector pair
• CPC Class Codes: H10K 50/856 comprising reflective means
• CPC Class Codes: H10K 50/858 comprising refractive means, e.g. lenses
• CPC Class Codes: H10K 50/86 Arrangements for improving contrast, e.g. preventing reflection of ambient light
• CPC Class Codes: H10K 59/12 Active-matrix OLED [AMOLED] displays
• CPC Class Codes: H10K 59/1201 Manufacture or treatment
• CPC Class Codes: H10K 59/1213 the pixel elements being TFTs
• CPC Class Codes: H10K 59/17 : Passive-matrix OLED displays
• CPC Class Codes: H10K 59/35 : comprising red-green-blue [RGB] subpixels
• CPC Class Codes: H10K 59/65 : OLEDs integrated with inorganic image sensors
• CPC Class Codes: H10K 59/8722 : Peripheral sealing arrangements, e.g. adhesives, sealants
• CPC Class Codes: H10K 59/876 : comprising a resonant cavity structure, e.g. Bragg reflector pair
• CPC Class Codes: H10K 59/878 : comprising reflective means
• CPC Class Codes: H10K 59/879 : comprising refractive means, e.g. lenses
• CPC Class Codes: H10K 59/8791 : Arrangements for improving contrast, e.g. preventing reflection of ambient light
• CPC Class Codes: H10K 71/00 : Manufacture or treatment specially adapted for the organic devices covered by this subclass

Abstract

A feedback-enhanced light emitting device is disclosed. A feedback element coupled to an emissive element allows the emissive element to emit collimated light by stimulated emission. Feedback elements that provide this function may include but are not limited to holographic reflectors with refractive index that varies at least in part periodically and continuously.

Claims

1. A feedback enhanced light emitting device, comprising:

a first feedback layer adapted to receive and reflect light;

a second feedback layer adapted to receive and reflect light, at least one of the first feedback layer and the second feedback layer having a refractive index profile that at least in part varies periodically and continuously along an axis normal or substantially normal to a plane of a respective feedback layer; and

an emissive layer disposed between the first feedback layer and the second feedback layer.

3. The device of claim 1, wherein the first feedback layer or the second layer or both the first feedback layer and the second layer comprises a hologram.

4. The device of claim 1, wherein the first feedback layer or the second layer or both the first feedback layer and the second layer comprises a hologram of a plane wave light source.

5. The device of claim 1, wherein the emissive layer comprises an organic luminescent material.

6. The device of claim 1, wherein the emissive layer comprises at least one or more of, a cross-linked organic luminescent material, a cross-linked polymer luminescent material, a luminescent material comprising molecules having molecular weight range between that of a small molecule to a polymer, a small molecule luminescent material dissolved in a polymer host, a fluorescent material, a phosphorescent material, an organic and inorganic composite luminescent material, an inorganic luminescent material, and a liquid crystalline luminescent material.

7. The device of claim 1, further comprising:

a first electrode disposed between the first feedback layer and the emissive layer; and

a second electrode disposed between the second feedback layer and the emissive layer.

8. The device of claim 7, wherein the first electrode is an anode and the second electrode is a cathode.

9. The device of claim 7, wherein the first electrode is a cathode and the second electrode is an anode.

10. The device of claim 7, further comprising:

one or more buffer layers disposed between one or both of the first and the second feedback layers and one or both of the first electrode and the second electrode.

11. The device of claim 10, wherein the one or more buffer layers comprise at least transparent dielectric material.

12. The device of claim 10, wherein the one or more buffer layers are used to hermetically isolate the device from atmospheric contamination.

13. The device of claim 10, wherein the one or more buffer layers are disposed to provide spacing between the first feedback layer and the second feedback layer such that constructive interference and stimulated emission occur at one or more selected wavelengths.

14. The device of claim 1, further comprising a hole injection layer disposed between the first feedback layer and the emissive layer.

15. The device of claim 1, further comprising an electron injection layer disposed between the second feedback layer and the emissive layer.

16. The device of claim 1, further comprising a hole transport layer disposed between the hole injection layer and the emissive layer.

17. The device of claim 1, further comprising an electron transport layer disposed between the electron injection layer and the layer of light emissive organic material.

18. The device of claim 1, wherein at least one of first feedback layer and the second feedback layer comprises at least one or more of a plane mirror, a multilayer dielectric distributed Bragg reflector, a specular surface of an electrode, and a non-photonic crystal reflector.

19. The device of claim 1, wherein both the first feedback layer and the second feedback layer transmit no light at a peak wavelength of their spectral reflection bands and the light emissive material radiates light into band-edge laser modes.

20. The device of claim 1, wherein a level of light fed back from the first feedback layer and the second feedback layer is sufficient to initiate laser action.

21. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer comprises one or more of tuned thickness and tuned refractive index contrast to optimize an amount of light fed back into the emissive layer.

22. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer comprises at least one or more discontinuities in the continuously varying refractive index profile.

23. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer comprises a plurality of individual feedback layer refractive index profiles.

24. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer comprises refractive index profiles that have superimposed refractive index profiles with non-reflective functions.

25. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer comprises refractive index profiles having a dominant periodicity n/2 times a selected wavelength of the feedback light, where n is an integer.

26. The device of claim 1, wherein one or both of the first feedback layer and the second feedback layer is thinned in one or both of physical thickness and optical thickness to enable light to escape the device.

27. The device of claim 4, wherein optical thickness of one or both of the first feedback layer and the second feedback layer is thinned by varying a holographic exposure and a resulting refractive index contrast of the hologram.

28. The device of claim 1, further including one or more layers disposed between the first feedback layer and the second feedback layer, the one or more layers formed with a structure that is enabled to continue a refractive index alternation that comprises a photonic crystal structure in one or both of the first feedback layer and the second feedback layer.

29. The device of claim 28, wherein the one or more layers comprise at least one or more of electrode, charge carrier injection layer, charge carrier transport layer, carrier blocking layer, and the emissive layer.

30. The device of claim 28, wherein the refractive index alternation is produced holographically by fabricating the one or more layers using one or more of a photopolymer and photosensitive material and exposing the one or more of a photopolymer and photosensitive material to light.

31. The device of claim 28, wherein the refractive index alternation is produced by at least one or more of a cholesteric and chiral liquid crystal structure in a material used to fabricate the one or more layers.

32. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback layer and the second feedback layer comprises at least a material having at least a sinusoidally varying refractive index profile.

33. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback layer and the second feedback layer comprises a plurality of regions adapted to reflect a plurality of predetermined wavelength bands of light, with at least one of the plurality of regions adapted to reflect a predetermined spectral band of light centered on a wavelength different from a predetermined spectral band of light centered on a wavelength reflected by another one of the plurality of regions.

34. The device of claim 33, wherein one of the first feedback layer and the second feedback layer comprises at least a photonic crystal structure and a plurality of regions adapted to reflect a plurality of predetermined wavelength bands of light, with at least one of the plurality of regions adapted to reflect a predetermined spectral band of light centered on a wavelength different from a spectral band of light centered on a predetermined wavelength reflected by another one of the plurality regions, and with the plurality of regions in the first feedback layer registered to a corresponding plurality of regions in the second feedback layer reflecting spectral bands centered on the same wavelengths of light.

35. The device of claim 33, wherein the emissive layer comprises at least a plurality of regions adapted to emit a plurality of predetermined wavelength bands of light, with at least one of the plurality of regions adapted to emit a predetermined wavelength band of light different from a predetermined wavelength band of light reflected by another one of the plurality regions, and with each of the plurality of regions registered to corresponding regions in at least one of the first feedback layer and the second feedback layer with spectral reflection bands of light that at least in part overlap the corresponding emitter spectral emission bands.

36. The device of claim 35, wherein two or more of the plurality of regions in the emissive layer registered to corresponding regions in the feedback layers adapted to reflect a plurality of predetermined spectral bands of light centered on different wavelengths is adapted to emit predetermined different wavelength bands of light from the same broad spectral band emissive material that at least in part overlaps of all of the corresponding emitter spectral emission bands.

37. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises a hologram having a plurality of refractive index profiles superpositioned in the hologram.

38. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises a hologram having a plurality of refractive index profiles corresponding through Bragg'"'"'s law to a plurality of wavelengths of light superpositioned in the hologram.

39. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises one or more regions having constant refractive index.

40. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises material having a photonic crystal structure.

41. The device of claim 40 wherein both the first feedback layer and the second feedback layer combine to form at least in part a continuous photonic crystal structure.

42. The device of claim 41, wherein both the first feedback layer and the second feedback layer transmit substantially no light emitted by the emissive layer at the peak wavelengths of their spectral reflection bands and the emissive layer radiates light into band-edge light propagation modes of the photonic crystal.

43. The device of claim 42, wherein the emissive layer comprises an organic luminescent material.

44. The device of claim 42, wherein the emissive layer comprises at least one or more of, a cross-linked organic luminescent material, a cross-linked polymer luminescent material, a luminescent material comprising molecules having molecular weight range between that of a small molecule to a polymer, a small molecule luminescent material dissolved in a polymer host, a fluorescent ematerial, a phosphorescent material, an organic and inorganic composite luminescent material, an inorganic luminescent material and a liquid crystalline luminescent material.

45. The device of claim 42, further comprising:

a first electrode disposed between the first feedback layer and the emissive layer; and

a second electrode disposed between the second feedback layer and the emissive layer.

46. The device of claim 45, wherein the first electrode is an anode and the second electrode is a cathode.

47. The device of claim 45, wherein the first electrode is a cathode and the second electrode is an anode.

48. The device of claim 42, wherein a level of light fed back from the first feedback layer and the second feedback layer is sufficient to initiate laser action.

49. The device of claim 47, further comprising:

one or more buffer layers disposed between the a feedback layer and one or both of the first electrode and the second electrode.

50. The device of claim 49, wherein the one or more buffer layers comprises at least transparent dielectric material.

51. The device of claim 49, wherein the one or more buffer layers is used to hermetically isolate the device from atmospheric contamination.

52. The device of claim 49, wherein the one or more buffer layers is disposed to provide spacing between the first feedback layer and the second feedback layer such that constructive interference and stimulated emission occur at a selected wavelength or wavelengths.

53. The device of claim 42, further comprising a hole injection layer disposed between the first feedback layer and the emissive layer.

54. The device of claim 42, further comprising an electron injection layer disposed between the second feedback layer and the emissive layer.

55. The device of claim 42, further comprising a hole transport layer disposed between the hole injection layer and the emissive layer.

56. The device of claim 42, further comprising an electron transport layer disposed between the electron injection layer and the layer of light emissive organic material.

57. The device of claim 40, wherein the light emissive layer comprises a defect in a continuous photonic crystal formed by the first feedback layer and the second feedback layer.

58. The device of claim 57, wherein the defect comprises a phase-slip in spatial phase along one dimension of the photonic crystal of less than one wavelength.

59. The device of claim 57, wherein the light emitted from the light emissive layer emanates into a defect mode.

60. The device of claim 59, wherein the emissive layer comprises an organic luminescent material.

61. The device of claim 59, wherein the emissive layer comprises at least one or more of, a cross-linked organic luminescent material, a cross-linked polymer luminescent material, a luminescent material comprising molecules having molecular weight range between that of a small molecule to a polymer, a small molecule luminescent material dissolved in a polymer host, a fluorescent material, a phosphorescent material, an organic and inorganic composite luminescent material, an inorganic luminescent material and a liquid crystalline luminescent material.

62. The device of claim 59, further comprising:

a first electrode disposed between the first feedback layer and the emissive layer; and

a second electrode disposed between the second feedback layer and the emissive layer.

63. The device of claim 62, wherein the first electrode is an anode and the second electrode is a cathode.

64. The device of claim 62, wherein the first electrode is a cathode and the second electrode is an anode.

65. The device of claim 59, wherein a level of light fed back from the first feedback layer and the second feedback layer is sufficient to initiate laser action.

66. The device of claim 62, further comprising:

a buffer layer disposed between one or both of the first and the second feedback layers and one or both of the first electrode and the second electrode.

67. The device of claim 66, wherein the buffer layer comprises at least transparent dielectric material.

68. The device of claim 66, wherein the buffer layer is used to hermetically isolate the device from atmospheric contamination.

69. The device of claim 66, wherein the buffer layer is disposed to provide spacing between the first feedback layer and the second feedback layer such that constructive interference and stimulated emission occur at a selected wavelength or wavelengths.

70. The device of claim 59, further comprising a hole injection layer disposed between the first feedback layer and the emissive layer.

71. The device of claim 59, further comprising an electron injection layer disposed between the second feedback layer and the emissive layer.

72. The device of claim 59, further comprising a hole transport layer disposed between the hole injection layer and the emissive layer.

73. The device of claim 59, further comprising an electron transport layer disposed between the electron injection layer and the layer of light emissive organic material.

74. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer partially transmits light received from the emissive layer.

75. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises one or more of chiral and cholesteric liquid crystals.

76. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises opals, particulate agglomerates having structures akin to a crystalline lattice, middle phase lyotropic liquid crystals in a fluid or polymerized state, and self-assembled block copolymers.

77. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises a dielectric material of continuously varying composition.

78. The device of claim 1, wherein the first feedback layer and the second feedback layer are phase-locked or phase-registered.

79. The device of claim 78, wherein the phase-locking or phase-registration is performed interferometrically.

80. The device of claim 1, wherein the first feedback layer and the second feedback layer are formed holographically and simultaneously using one simultaneous exposure to an array of interference fringes formed by two beams of plane polarized light.

81. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises a hologram of plane waves written with linearly, circularly, or elliptically polarized light.

82. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprises a hologram fabricated by recording a refractive index profile in one or more of dichromated gelatin, dichromated emulsions, silver halide gelatine, silver halide emulsions, photopolymer material, positive photosensitive material, negative photosensitive material, and other photosensitive material.

83. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer is formed with a material with chiral centers.

84. The device of claim 1, wherein the emissive layer comprises transparent electroluminescent material.

85. The device of claim 1, wherein the first feedback layer or the second feedback layer or both the first feedback and the second feedback layer comprise material patterned to reflect different wavelength bands of light.

86. The device of claim 1, wherein the emissive layer is patterned to emit different wavelength bands of light.

87. The device of claim 1, wherein the emissive layer comprises an electroluminescent material having spectral emission band that overlaps reflection bands of the first feedback layer and the second feedback layer.

88. The device of claim 1, wherein all light emitted by the device occupies a single light propagation mode.

89. The device of claim 88, wherein spacing between the first feedback layer and the second feedback layer is equivalent to λ

/2 excluding phase shifts due to reflection, λ

being a wavelength of the light in the single light propagation mode.

90. The device of claim 1, wherein light emitted by the device occupies two or more light propagation modes.

91. The device of claim 90, wherein one or both of the device substrate and cover are transparent and are used as spacers between the two feedback layers providing proper spacing between the layers to yield the desired laser mode spacing and spectral location.

92. The device of claim 1, further including:

a substrate on which one of the first feedback layer and the second feedback layer is disposed.

93. The device of claim 92, wherein the substrate comprises at least one or more of a flexible material, a rigid material, a glass, a metal, and a semiconductor material.

94. The device of claim 93, wherein the flexible material comprises one or more of a film of polyethylene terephthalate, (PET), polyethylene naphthalate (PEN), bisphenol A polycarbonate, and another engineering polymer.

95. The device of claim 1, further including:

a cover disposed on at least one of the first feedback layer and the second feedback layer.

96. The device of claim 1, wherein the refractive index profile comprises a profile intermediate between a square wave profile and a sinusoidal profile.

97. The device of claim 1, wherein a spectral reflection band of one or both the first feedback layer and the second feedback layer are chosen so as to generate stimulated emission that substantially overlaps neither a spectral excitation band nor a spectral absorption band of the emissive layer.

98. The device of claim 1, wherein a reflection band of one or both of the first feedback layer and the second feedback layer is spectrally narrower than an emission band of the emissive layer.

99. The device of claim 4, wherein the emissive layer comprises inorganic semiconductor material.

106. The device of claim 7, wherein the cathode comprises a transparent low work function material.

107. The device of claim 7, wherein the cathode comprises a first, very thin metal layer disposed towards the emissive layer and a second thicker layer comprising a transparent conductive material such as indium-tin oxide.

108. The device of claim 7, wherein the anode comprises a transparent high work function material.

109. The device of claim 79, wherein the second feedback layer is formed holographically using an aerial interference fringe pattern phase-locked or phase-registered to the first feedback layer interferometrically.

110. The device of claim 91, wherein a transparent spacer is fabricated between the first feedback layer and the second feedback layer to provide selected spacing between the first feedback layer and the second feedback layer to yield the selected laser mode spacing and spectral location.

111. The device of claim 42, further including one or more layers disposed between the first feedback layer and the second feedback layer, the one or more layers formed with a structure that is enabled to continue a refractive index alternation that comprises a photonic crystal structure in one or both of the first feedback layer and the second feedback layer.

112. The device of claim 111, wherein the one or more layers comprise at least one or more of electrode, charge carrier injection layer, charge carrier transport layer, carrier blocking layer, and the emissive layer.

113. The device of claim 57, further including one or more layers disposed between the first feedback layer and the second feedback layer, the one or more layers formed with a structure that is enabled to continue a refractive index alternation that comprises a photonic crystal structure in one or both of the first feedback layer and the second feedback layer.

114. The device of claim 113, wherein the one or more layers comprise at least one or more of electrode, charge carrier injection layer, charge carrier transport layer, carrier blocking layer, and the emissive layer.

115. The device of claim 42, wherein a level of light fed back from the first feedback layer and the second feedback layer is sufficient to initiate laser action.

116. The device of claim 57, wherein a level of light fed back from the first feedback layer and the second feedback layer is sufficient to initiate laser action.

119. The device of claim 112, wherein the reflector layer comprises at least a second hologram layer.

124. The device of claim 1, wherein a distance between the first feedback layer and the second feedback layer is such that the space between the feedback layers comprise a cavity in which light of one or more desired wavelengths constructively interfere.

125. The device of claim 1, wherein light reflected by one or both of the first feedback layer and the second feedback layer stimulates emission of light from the emissive layer.

126. The device of claim 125, wherein the stimulated emission of light results in substantial collimation of light emitted by the device.

127. The device of claim 125, wherein the stimulated emission of light results in laser action.

2. A feedback enhanced light emitting device, comprising:

a first feedback layer adapted to receive and reflect light;

a second feedback layer adapted to receive and reflect light, at least one of the first feedback layer and the second feedback layer having a refractive index profile that at least in part varies periodically and continuously along an axis normal or substantially normal to a plane of the first feedback layer, at least one of the first feedback layer and the second feedback layer adapted to reflect one or more predetermined wavelength bands of light at least along an axis normal or substantially normal to the plane of a respective feedback layer; and

an emissive layer disposed between the first feedback layer and the second feedback layer, the emissive layer adapted to emit light, the emissive layer further adapted to provide stimulated emission caused by the one or more predetermined wavelength bands of light reflected from one or more of the first feedback layer and the second feedback layer.

100. A method of fabricating a feedback element coupled to an emissive element in a device for emitting light, comprising:

disposing a layer of polymer on a substrate; and

exposing the polymer to light to record one or more interference patterns in the polymer.

101. The method of claim 100, further comprising cross-linking the polymer.

102. A method of fabricating a feedback layer, comprising:

successively exposing holographic patterns on holographic material using photomasks in both image and reference beams of a holographic set up to form a plurality of regions reflecting a plurality of different wavelength bands of light.

103. A method of fabricating a feedback layer, comprising:

successively exposing holographic patterns on holographic material using a single photomask in one of an image beam and a reference beam to form a plurality of regions reflecting a plurality of different wavelength bands of light, the holographic material having a selected irradiance threshold of exposure such that energy of an unpatterned beam alone does not cause refractive index change in the material.

104. A method of fabricating a feedback enhanced light emitting device, comprising:

forming a substrate;

forming a first feedback layer;

forming a first electrode on the first feedback layer;

forming an emissive layer on the first electrode;

forming a second electrode on the emissive layer; and

forming a second feedback layer on the second electrode.

105. The method of claim 104, wherein by the emissive layer is formed by forming a layer of photo-cross-linkable material on the first electrode and then exposing it to light to cross-link it.

117. A feedback enhanced light emitting device, comprising:

a first means for reflecting light;

a second means for reflecting light, at least one of the first means and the second means having a refractive index profile that at least in part varies periodically and continuously along an axis normal or substantially to a plane of a respective first and/or second means; and

a third means for emitting light at least as a result of receiving the light reflected by at least one of the first means and the second means.

118. A feedback enhanced light emitting device, comprising:

a hologram layer;

a reflector layer; and

at least a luminescent material disposed between the hologram layer and the reflector layer.

120. A feedback enhanced light emitting device, comprising:

a photonic crystal structure layer;

a reflector layer; and

at least a luminescent material disposed between the photonic crystal structure layer and the reflector layer.

121. A feedback enhanced light emitting device, comprising:

a first reflector;

a second reflector;

and at least a luminescent material disposed between the first and the second reflector layers, wherein the first reflector and the second reflector combine to form at least in part a continuous photonic crystal structure.

122. A feedback enhanced light emitting device, comprising:

a first reflector;

a second reflector;

and at least a luminescent material disposed between the first and the second reflector layers, the at least a luminescent material comprising a defect in a continuous photonic crystal formed by the first reflector and the second reflector.

123. A feedback enhanced light emitting device, comprising:

a first feedback layer adapted to receive and reflect light;

a second feedback layer adapted to receive and reflect light, at least one of the first feedback layer and the second feedback layer having a refractive index profile that varies periodically and continuously along an axis normal or substantially normal to a plane of a respective feedback layer; and

an emissive layer disposed between the first feedback layer and the second feedback layer.