Transparent metallo-dielectric photonic band gap structure

Details Transparent metallo-dielectric photonic band gap structure Transparent metallo-dielectric photonic band gap structure

1997-09-16

2001-07-17

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S344000, C359S580000, C359S585000, C359S586000, C359S587000, C359S588000, C359S589000, C257S451000, C372S045013

Reexamination Certificate

active

06262830

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to photonic signal devices. In particular, this invention relates to a transparent metal device that utilizes a photonic band gap structure to transmit a selected range of wavelengths of the electromagnetic spectrum, such as the visible range, and to reflect all longer wavelengths.
2. Related Art
Recent advances in photonic technology have generated a trend toward the integration of electronic and photonic devices. In particular, this advance is due to the increased desire to utilize and manipulate “photonic signals”, as opposed to electrical signals, to perform such functions as information transfer. A “photonic signal” is a generic characterization of light that includes the entire range of electromagnetic frequencies, from gamma to x-rays, from visible light to microwaves, down to radio frequencies and beyond. Photonic devices offer an array of advantages over conventional electronic devices. For example, they can provide enhanced speed of operation, reduced size, robustness to environmental changes, such as rapid temperature variations, increased lifetime, and the ability to handle high repetition rates. These structures can be made of semiconductor materials, ordinary dielectrics, or a combination of semiconductor and dielectric materials.
The intense theoretical and experimental investigations of these structures in recent years, photonic band gap (PBG) structures in particular, are evidence of the widely recognized potential that these new materials offer. These optical devices, whose operating principles are based on a combination of nonlinear medium response and the physics of the photonic band edge, are extremely compact in nature (only a few microns in length), and some have electronic counterparts. It is well understood that a medium becomes nonlinear when the index of refraction of the substance is no longer constant, and is function of the applied electromagnetic field.
For example, recent advancements in PBG structures have been made in the development of a photonic band edge nonlinear optical limiter and switch. See, “Optical Limiting and Switching of Ultrashort Pulses in Nonlinear Photonic Band-Gap Materials”, M. Scalora, et al.,
Physical Review Letters
73:1368 (1994) (incorporated by reference herein in its entirety). Also, advancements in photonic technology have been achieved with the development of the nonlinear optical diode. See, “The Photonic Band-Edge Optical Diode”, M. Scalora, et al.,
Journal of Applied Physics
76:2023 (1994) (incorporated by reference herein in its entirety). Additionally, a high-gain second harmonic generator based on these photonic principles has been achieved. See, “Pulsed second harmonic generation in photonic band gap structures”, M. Scalora, et al., to appear in
Physical Review A
, 1997 (incorporated by reference herein in its entirety).
Under ordinary circumstances, however, the medium response need not be nonlinear in order for the interaction of the electromagnetic waves with matter to be useful. For example, the photonic band edge delay line makes use of the linear properties of the structure to drastically reduce the speed of a light pulse propagating through the structure without causing distortion of the pulse or scattering losses. See, “Ultrashort pulse propagation at the photonic band edge: large tunable group delay and minimal distortion and loss”,
Physical Review E
54:1078R (1996) (incorporated by reference herein in its entirety).
To use a simple illustration, substances are usually characterized by the degree to which they conduct electricity. Thus, a distinction can be made between good conductors (such as metals), insulators (such as glasses), and semiconductors (such as gallium arsenide), which under the right conditions can display properties common to both metals and insulators. The propagation of light inside these substances strongly depends on their conductive properties: metals are highly reflective, as well as absorptive, at nearly all light frequencies of interest, from long radio waves to short-wavelength ultraviolet (UV) light. On the other hand, some dielectric materials may be transparent across the spectrum (a slab of window glass, for example).
For Ulis reason, metals are routinely used for radiation shielding purposes, as in the case of microwave oven cavities, or for their reflective properties, such as in conventional household mirrors. On the other hand, dielectric or semiconductor materials are used in integrated circuit environments, in waveguides and directional couplers, for example, because they allow the unimpeded propagation of light beams with minimal losses. Therefore, it would be highly desirable, under certain circumstances, to have access to a substance that can act as a shield (or filter) for a certain range of frequencies such as microwaves, and yet be transparent in the visible portion of the spectrum, i.e., a transparent metal structure.
SUMMARY OF THE INVENTION
The present invention generally relates to a device and method of creating an optical shield (or filter) based on a transparent metal photonic band gap (PBG) structure. In particular, the present invention provides an arrangement of alternating relatively thin or thick metal layers and refractive material layers deposited on an opaque or transparent substrate. This alternating metal/refractive material layer structure can provide a high degree of suppression of incident ultraviolet, infrared and microwave radiation, while still providing substantial transmission in the visible region of the electromagnetic spectrum. By utilizing a PBG structure, the shielding device can include thick metal layers to provide a greater degree of isolation of unwanted radiation than for similar metal layer based filters that are not designed to take advantage of photonic band gap effects.
According to one embodiment of the present invention, a transparent metal photonic band gap apparatus is provided to transmit a predetermined magnitude of visible radiation and to reflect a predetermined magnitude of ultraviolet, infrared, and microwave radiation. The apparatus includes a transparent substrate, a plurality of metal layers, and a plurality of interstitial layers.
A first metal layer is deposited on the transparent substrate. This metal layer can be any transition metal, preferably silver, aluminum, copper, or gold. Subsequent metal layers may be the same metal or different metals than the first metal layer. The thickness of the metal layer depends on the user application, and can range from approximately 2.5 nanometers (nm) up to 50 nm. Metal layers can be thicker than 50 nm, depending on the amount of visible transmission required.
The first interstitial layer is then deposited onto the first metal layer. Again, the thickness of the interstitial layer depends on the user application, and can range from approximately 2.5 nm up to several hundred or even thousands of nanometers. This interstitial layer can be selected from a group comprising semiconductor materials, ordinary dielectrics, and a combination of semiconductor and dielectric materials.
Subsequent metal and interstitial layers are arranged in a similar, alternating manner. The subsequent metal layers can be the same or different metals, depending on fabrication considerations and the desired transmission properties of the device. Similarly, the interstitial layers can be the same or different refractive materials, depending on fabrication considerations and the desired transmission properties of the device. This arrangement of metal/interstitial layers forms a device that exhibits a photonic band gap structure. By altering the thicknesses of the metal and interstitial layers, the device changes its transmission characteristics, such that different ranges and different magnitudes of transmission and reflection can be achieved.
According to a second embodiment of the present invention, the transmission range (in the visible region) of the transparent metal PBG device can be altered by app

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