Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1997-03-31
2001-02-20
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S566000, C359S576000
Reexamination Certificate
active
06191890
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system for transferring a beam of electromagnetic radiation having a vacuum wavelength, said system comprising a first dielectric medium and a second dielectric medium with respective refractive indices.
BACKGROUND OF THE INVENTION
In Stephen Y. Chou and Wenyong Deng, “Subwavelength Amorphous silicon transmission gratings and applications in polarizers and waveplates”, Appl. Phys. Lett., 67 (6), pp.742-744, Aug. 7, 1995 there is described a fabrication and an experimental evaluation of subwavelength gratings consisting of a patterned layer amorphous silicon on a planar silica substrate. Measurements on this structure show (for normal incidence) a very different behaviour between TE and TM operation, and for one particular parameter set, transmission for the TM-mode is very low. Detailed calculations on this structure, according to the “Rigourous Coupled Wave Analysis” method show that for this particular configuration the specular TM reflection is as high as 91.6%.
It is to be noted that the period of the gratings is smaller than the wavelength in the substrate and superstrate and that this structure is to be illuminated from the low refractive index side.
In Henry L. Bertoni, Li-Hsiang S. Cheo, and Theodor Tamir, “Frequency-Selective Reflection and Transmission by a Periodic Dielectric Layer”, IEEE Transactions on Antennas and Propagation, Vol. 37, no.1, pp. 78-86, January 1989, there is described a dielectric structure exhibiting a complex behaviour in terms of reflection/transmission characteristics as a function of frequency, offering close to 100% reflection for properly chosen parameters.
The principle of operation relies on exciting a guided mode in a grating layer, propagating along the direction parallel to the layer direction. This waveguide mode is actually a leaky mode, due to the periodic nature of the waveguide. For properly chosen parameters, this excited mode couples for nearly 100% to zero order in reflection or transmission.
This implies that only structures with a high refractive index grating layer between a relatively low index substrate and superstrate are suitable. Furthermore this principle only works for oblique incidence.
U.S. Pat. No. 5,255,278 relates to semiconductor laser comprising a mirror having a dielectric layer with a grating structure etched in the dielectric layer whereon the metal layer is deposited. A high reflectivity is obtained by using the covering metal layer. This mirror is appropriate only for substrate-side emitting lasers.
SUMMARY OF THE INVENTION
The above cited systems are thus known. The present invention relates to a system for transferring a beam of electromagnetic radiation having a vacuum wavelength, said system comprising a first dielectric medium with a first refractive index wherein said beam can propagate, a second dielectric medium with a second refractive index and a dielectric layer in between said first medium and said second medium. The present invention aims at obtaining a very high reflectivity to the zero order reflective plane waves and a very low coupling to the zero order transmissive plane wave and also to the higher order reflective plane waves. To solve this problem, there is proposed according to the invention a system which is remarkable in that said dielectric layer has a periodicity of the dielectric properties parallel to said layer, said periodicity having a period that is smaller than the wavelength of the electromagnetic radiation in said second dielectric medium, said dielectric properties and said periodicity of said dielectric layer further being chosen such that their parameter cooperatively establish that said beam when incident on said dielectric layer is reflected for substantially 100% into the specular zero-order reflection, wherein the latter stands for more than 90%.
According to an embodiment of the invention, said dielectric layer has a sufficiently large variation of said dielectric properties within one repetition of said period such that said beam when incident on said dielectric layer excites at least two propagating modes within said layer, wherein further said beam preferably is substantially orthogonally incident on said dielectric layer.
According to a further embodiment of the invention, said dielectric layer has a periodicity of the dielectric properties in a plurality of directions, said periodicity having a period for each of said directions that is smaller than the wavelength of the electromagnetic radiation in said second dielectric medium.
According to a specific embodiment of the present invention said dielectric layer is a grating. Said grating can be a grated surface structure of the second dielectric medium, said dielectric layer being composed of the same material as said second medium.
According to a more particular embodiment, the present invention relates to an optical system comprising a highly reflective grating to order zero or a so-called Giant Reflectivity to order
0
(GIRO)-grating which is a periodically corrugated interface between a dielectric medium
1
with refractive index n
1
and another dielectric medium
2
with refractive index n
2
(<n
1
).
Upon incidence of a plane wave of a particular wavelength, polarization and angle of incidence from medium
1
exhibits a close to 100% reflectivity to the zero order reflective plane wave and close to 0% coupling to the zero order transmissive plane wave and to the higher order reflective plane waves, whereby the zero order reflectivity is much larger than the reflectivity of the equivalent plane interface between medium
1
and medium
2
. This will only happen for particular combinations of refractive indices n
1
and n
2
, angle of incidence, polarization, wavelength and grating parameters such as pitch or pitches, depth and shape. One necessary condition is that the pitch &Lgr; is at least smaller than the wavelength &lgr;
2
in medium
2
(with &lgr; being the vacuum wavelength) so that only zero order transmissive orders can exist. Another necessary condition is that the difference between n
1
and n
2
is sufficiently large such that within the grating region at least 2 propagative modes are excited by the incident plane wave.
Furthermore, in the case of a one-dimensionally periodic grating, the device has been found to work better for TM-polarization compared to TE-polarization. The grating parameters for optimal TM or TE-operation are different, thereby allowing said device to realize a large polarization selectivity for the zero order reflectivity, even for normal incidence.
An advantageous embodiment of the present invention thus comprises said grating having a one dimensional periodicity (A), wherein particularly a TM-polarization mode is set and/or wherein a TE-polarization mode is set, whereby the grating parameters are different for optimal TM- or TE-mode.
According to a specific embodiment of the invention, said grating has a substantially rectangular cross section. More particularly, said grating has a so-called filling factor which is comprised between 40 and 60%, preferably approximately 50%. More particularly still the second medium referred to the propagating direction of said beam, is formed by air or vacuum with a second refractive index n
2
being approximate to unity.
Herewith, according to a particularly advantageous embodiment of the invention, the grating parameters are chosen so as to be comprised within a minimum zero order transmission and maximum zero order reflection or a trade off between those two wherein said expressions are presented hereafter.
According to another particularly advantageous embodiment of the invention, the above mentioned giant reflective zero order grating is used as a mirror in a vertical cavity surface emitting laser (VCSEL), thereby replacing the more traditional Bragg quarter-wavelength stack, offering major advantages for the performance and fabrication simplicity of such devices.
Facing to the problem of dielectric mirrors with high reflectivity, there is provided a one dimensional
Baets Roel
Demeulenaere Bart
Dhoedt Bart
Goeman Stefan
Interuniversitair Micro-Elektronica Centrum vzw
Jr. John Juba
McDonnell Boehnen Hubert & Berghoff
Spyrou Cassandra
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