Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2003-02-13
2004-11-02
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S321000, C359S246000, C359S248000, C359S276000, C359S563000, C359S299000, C257S017000, C257S021000, C385S003000, C385S014000, C250S351000
Reexamination Certificate
active
06813063
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical switch that utilizes a waveguide having a periodic structure, and narrow band absorption lines by exciton polariton.
BACKGROUND ART
In digital light communications, demands for the higher information transmission rate, i.e., the higher bit rate know no limits. In the conventional optical switch, the light emission source may be a laser diode whose current or voltage is controlled to turn its light emission on and off, thereby forming a “1” and a “0” digital light signal. Such a method of producing digital light signals by controlling the current passed through or the voltage applied to a laser diode, however, has its limit in decreasing the parasitic circuit capacitance and inductance and increasing the electron transit velocity, and hence has its limit in increasing the bit rate.
Attempts have also be made to rapidly produce digital light signals with the aid of a light modulator using an optical crystal such as LiNbO
3
(lithium niobate) with a laser light incident thereon to give rise to an electrooptic effect. This method has its limits, too, namely in matching the propagation speed of a light wave used and the propagation speed of a microwave applied for controlling the light wave, and the bit rate achievable there must be at most in the GHz (giga-hertz) order. Moreover, with the microwave propagation loss that cannot be ignored, the method has the problem that the 0 by 1 ratio in light intensity, namely the light intensity extinction ratio remains unsatisfactory.
Amid the state of the art, with the progress in crystal growth and semiconductor ultra-fine processing technologies made in recent years it has become possible to make a semiconductor quantum structure practically at will. This has also been followed by vigorous studies on the interaction between radiation field and elementary excitation of materials, using a photonic crystal or a semiconductor micro-resonator. Not only have the results of these studies contributed to the understanding of the fundamental physics, but also they are found applicable to the development of a high-performance optical device.
With the foregoing technological background, the present invention is aimed to provide an optical switch that has an excellent light intensity extinction ratio and is operable at a rate in the THz (tera-hertz) order, by applying to the switching of a light, a waveguide having a periodic structure and the excitation of exciton polariton as one form of the interaction between a radiation field and an elementary excitation of a material.
DISCLOSURE OF THE INVENTION
In order to achieve the object mentioned above, there is provided in accordance with the present invention an optical switch characterized in that it comprises: a polariton and photon interacting region made of a grating formed on a top face of a transparent substrate and a semiconductor layer with which the said grating is covered; a controllable light emitted from a free space and having a selected wavelength and with which the said polariton and photon interacting region is irradiated; and a control light for controlling transmissivity of the said controllable light through the said polariton and photon interacting region.
The said control light preferably has a wavelength that brings about an optical Stark effect of exciton without entailing an actual excitation of the said semiconductor layer.
The said grating is preferably formed so that its grating period corresponds to a length of m/2 of the wavelength of the said controllable light in the said semiconductor layer where m is a positive integer.
Further, the said semiconductor layer may be layered in a groove of the said grating to a predetermined depth.
The said semiconductor layer is preferably a semiconductor layer that is large in both exciton oscillator strength and exciton binding energy. The said semiconductor layer that is large in both exciton oscillator strength and exciton binding energy is preferably a semiconductor layer having a multiple quantum well structure made of units each of which comprises a pair of semiconductor quantum well and a barrier layer small in dielectric constant and separating the said semiconductor quantum wells from each other.
The said multiple quantum well structure that is large in both exciton oscillator strength and exciton binding energy is specifically of a laminar or layered inorganic-organic perovskite semiconductor that is expressed by chemical formula: (C
n
H
2n+1
NH
3
)
2
MX
4
where M=Pb or Sn and X=I, Br or Cl and n is a positive integer.
The said polariton and photon interacting region is preferably formed on its top face with a highly refractive transparent material for light confinement. The said highly refractive transparent material for light confinement is preferably a polymer.
In the optical switch of the present invention constructed as mentioned above, making the controllable light incident on the polariton and photon interacting region perpendicularly thereto causes a standing wave of the controllable light to be formed in a direction of the grating period of the grating. Photon of this standing wave is strongly coupled to exciton of the semiconductor to form exciton polariton and is thereby absorbed. Thus, the controllable light is prevented from passing through the polariton and photon interacting region.
On the other hand, making both the controllable light and the control light simultaneously incident on the polariton and photon interacting region allows the energy of exciton in the semiconductor layer to be rapidly changed by the control light causing the optical Stark effect to change the dispersion relation of polariton that is a state that exciton and photon are strongly coupled, thereby changing the photon energy of the standing wave strongly coupled with exciton of the semiconductor. Thus, the controllable light is prevented from coupling with exciton and is thereby allowed to pass through the polariton and photon interacting region. Selecting the grating period allows the wavelength of the light forming the standing wave to be selected.
Also, increasing the number of the grooves forming the grating allows setting the very narrow half-width of the wavelength of the light that forms the standing wave. Moreover, the control means for controlling the transmissivity by bringing about the third order optical nonlinear effect, preferably the optical Stark effect, permits controlling the controllable light so as to render it transmissible and nontransmissible very rapidly or at an ultra-high speed.
Further, the use of a semiconductor layer that is large in both exciton oscillator strength and exciton binding energy allows almost every photon of the standing wave to be converted to exciton polariton, thereby further raising the light intensity extinction ratio of the optical switch.
If the semiconductor layer that is large in both exciton oscillator strength and exciton binding energy is constituted by a semiconductor layer having a multiple quantum well structure made of units each of which comprises a pair of semiconductor quantum well and a barrier layer small in dielectric constant and separating the said semiconductor quantum wells from each other, then extremely high exciton oscillator strength and exciton binding energy are obtained.
Further, the use of a laminar or layered inorganic-organic perovskite semiconductor that is expressed by the chemical formula: (C
n
H
2n+1
NH
3
)
2
MX
4
where M=Pb or Sn and X=I, Br or Cl and n is a positive integer, provides for a multiple quantum well structure that is large in both exciton oscillator strength and exciton binding energy.
Further, if the polariton and photon interacting region is formed on its top face with a highly refractive transparent material for light confinement, then the improvement in the light confinement into the polariton and photon interacting region still further enhances the light intensity extinction ratio of the optical switch.
Constructed as mentioned above, the optic
Ben Loha
Japan Science and Technology Agency
Westerman Hattori Daniels & Adrian LLP
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