Abstract:
Provided is an optical modulator for modulating light comprising: a superlattice structure having a plurality of interleaved narrow and wide bandgap semiconductor layers, wherein wave functions of energy states of electrons and holes in different narrow bandgap layers are coupled; and a power supply that applies voltage to the superlattice structure between a first non-zero voltage and a second non-zero voltage to modulate the light.

Description:
RELATED APPLICATIONS 
     The present application is a US National Application of PCT/IL01/00143, filed on Feb. 14, 2001. 
     FIELD OF THE INVENTION 
     The invention relates to optical shutters and in particular to optical shutters based on semiconductor superlattice or multiple quantum well structures. 
     BACKGROUND OF THE INVENTION 
     Optical modulators based on semiconductor multiple quantum well (MQW) or superlattice (SL) structures are known. 
     An MQW structure comprises a stack of thin layers of narrow bandgap semiconductor material alternating with layers of wide bandgap semiconductor material so that each layer of narrow bandgap material is sandwiched between two layers of wide bandgap material. The alternating structure forms a series of quantum wells located in the narrow bandgap layers that are capable of confining conduction band electrons and valence band holes. Each narrow bandgap layer has a quantum well that confines conduction band electrons and a quantum well that confines holes in the valence band. 
     Width of the quantum wells in a narrow bandgap layer is substantially equal to the thickness of the narrow band layer. Generally, thickness of the narrow bandgap layers, and as a result, width of the quantum wells is substantially less than a diameter of an exciton that may be generated as an intermediate state when a photon excites an electron from the valence band to the conduction band of the narrow bandgap material. Depth of the electron quantum wells is substantially equal to a difference between bottoms of the conduction bands of the wide bandgap material and the narrow bandgap material. Depth of the hole quantum wells is substantially equal to a difference between tops of the valence bands of the wide bandgap and narrow bandgap materials. 
     In an MQW structure, depths of the quantum wells and thickness of the wide bandgap layers are such that a wave function of an electron or hole trapped in a quantum well of a narrow bandgap layer generally extinguishes rapidly in the wide bandgap layers on either side of the narrow bandgap layer. As a result, in an MQW structure, electrons and holes confined in quantum wells of a narrow bandgap layer are substantially isolated from electrons and holes confined in other narrow bandgap layers. Hereinafter, the wide bandgap layers are referred to as “barrier layers” and the narrow bandgap layers are referred to as “quantum well layers”. 
     When an electric field is applied perpendicular to the planes of the layers in an MQW structure, energy levels of allowed wave functions for trapped electrons and trapped holes in the quantum wells of a same quantum well layer are displaced towards each other. As a result, a minimum amount of energy required to transfer an electron from the valence band to the conduction band and produce thereby an electron-hole pair is reduced and the absorption spectrum of the MQW is red shifted. The red shift is a result of a quantum confined Stark effect that reduces a minimum amount of energy required to excite an electron-hole exciton as an intermediate state in raising an electron from the valence band to the conducting band. Changes, on the order of 10,000 cm −1  in an absorption coefficient for light having a wavelength, hereinafter an “operating wavelength”, near an absorption edge of an absorption spectrum of an MQW structure can be realized by red shifting the absorption spectrum. 
     U.S. Pat. No. 4,525,687, the disclosure of which is incorporated herein by reference, describes a small aperture MQW optical shutter comprising 50 GaAs narrow band quantum well layers sandwiched between wide bandgap barrier layers formed from Ga (1-x) Al x As with x˜0.36. The layers are formed in the intrinsic part of a pin diode. PCT Publication WO 99/40478, the disclosure of which is incorporated herein by reference, describes a wide aperture MQW high frequency optical modulator. 
     Performance of an MQW modulator is limited, inter alia, by an escape time for electrons and holes trapped in quantum wells of the MQW. Once holes and electrons are trapped in the MQW quantum wells of an MQW modulator after the modulator interacts with a beam of light, the electrons and holes require a finite escape time before they leave the quantum well region of the modulator. The same quantum wells that provide the modulating effects of an MQW modulator generally retard removal of the electrons and holes from the quantum wells. Buildup of photo-induced electrons and holes in quantum wells tends to shield and reduce effectiveness of electric fields applied to the MQW that are used to red shift the absorption spectrum of the modulator. In addition, current in the MQW layers generated by motion of the photo-induced electrons and holes in response to electric fields applied to the modulator can cause ohmic heating of the layers. The heating can result in an unwanted shift in the absorption edge of the modulator&#39;s absorption spectrum. 
     U.S. Pat. No. 5,210,428 to K. Goossen, the disclosure of which is incorporated herein by reference, notes that an article published in Applied Physics Letters, Vol. 57, No 22, pp suggests that escape times in an MQW modulator may be reduced by decreasing quantum well depth and barrier layer thickness. The patent describes an MQW modulator having a particular configuration of shallow quantum wells that results in reduced escape times. To provide the shallow quantum wells the effective bandgap energy of barrier layers in the modulator is chosen to be less than the sum of a longitudinal optical phonon energy and an exciton absorption energy in the modulator. U.S. Pat. No. 5,436,756 to W. H. Knox et. al. describes reducing current from photo-induced electrons in an MQW modulator by seeding the quantum well region of the modulator with non-radiative recombination centers such as protons. 
     A superlattice (SL) structure also comprises a series of quantum wells that are formed by a stack of quantum well layers sandwiched between barrier layers. However, in an SL, as distinguished from an MQW structure, widths of the barrier layers and heights of the quantum wells are such that wave functions of electrons and holes in quantum wells are not confined to individual quantum wells. There is substantial tunneling of electrons and holes between quantum wells. The wave functions in the quantum wells are relatively strongly coupled and in effect form extended wave functions that span substantially the full height of the stack of quantum well layers and have energies that form bands of allowed energies. When an electric field is applied to the SL perpendicular to the layers in the SL, coupling of wave functions between quantum wells is reduced and energies of allowed wave functions of electrons and holes in a same quantum well layer are displaced away from each other. The displacement is a result of narrowing of the widths of the energy bands defined by the allowed wave functions. As a result, a minimum amount of energy required to transfer an electron from the valence band of the quantum well layers to the conduction band in the quantum well layers is increased and the absorption spectrum of the MQW is blue shifted. 
     Optical modulators comprising SL structures are described in U.S. Pat. No. 5,194,983 to P. Voisin, the disclosure of which is incorporated herein be reference. Absorption spectra showing blue shifts for an SL structure having 4 nanometers thick layers are shown in the patent for different electric fields applied to the SL structure. 
     SLs in which absorption spectra of the SLs are red shifted are also possible. In “red shift” MQW modulators, red shifts that are used to modulate light are provided by changing energy differences of transitions, “direct transitions”, that occur between allowed electron and hole energy states in a same quantum well layer. “Oblique” transitions between allowed states of electrons and holes in quantum wells in adjacent quantum well layers of an SL provide an absorption spectrum that is red shifted by application of an electric field. However, red shifts provided by oblique transitions generally result in changes in absorption coefficients for light that are substantially smaller than changes in absorption coefficients provided by “direct” red shifts. In order to provide desired On/Off ratios, SL modulators that use oblique transitions to modulate light must generally provide relatively long path lengths for the light through quantum well layers of the SL. U.S. Pat. No. 5,073,809 to E. Bigan et. al., the disclosure of which is incorporated herein by reference, describes a “red shift SL” modulator in which a quantum well layer functions as a core of a waveguide having sufficient length to provide a suitable On/Off ratio. 
     Because barrier layers are relatively thin in SLs, escape times for photo-induced electrons and holes in SLs are relatively short and SLs are not as sensitive to escape times as are MQW modulators. However, changes in absorption coefficients for light at an operating wavelength of an SL are substantially smaller than changes in absorption coefficients achievable for light at an operating wavelength of an MQW modulator. On/Off transmission ratios for SLs are therefore generally substantially less than On/Off transmission ratios achievable with MQW modulators. 
     SUMMARY OF THE INVENTION 
     Aspects of some embodiments of the present invention relate to providing an optical multilayer semiconductor modulator having a relatively reduced escape time for photo-induced electrons and holes and an On/Off transmission ratio comparable to that achievable with prior art MQW modulators. 
     An aspect of some embodiments of the present invention relates to providing an optical multilayer semiconductor modulator having relatively reduced current generated by motion of photo-induced electrons and holes in electric fields applied to the modulator. 
     An aspect of some embodiments of the present invention relate to providing an optical multilayer semiconductor modulator having a relatively large On/Off ratio per unit length of the modulator along the modulator&#39;s optical axis, compared to prior art MQW modulators. 
     An optical modulator, in accordance with an embodiment of the present invention, comprises an SL structure having quantum well layers sandwiched between relatively thin barrier layers and a power supply that biases the SL structure at a bias voltage at which the modulator functions similarly to an MQW modulator. 
     The bias voltage, in accordance with an embodiment of the present invention, generates an electric field in the SL structure that substantially decouples allowed wave functions of electrons and holes in quantum wells of different quantum well layers of the modulator. As a result, in the biased modulator, allowed electron and hole wave functions in quantum wells of the modulator are confined similarly to the way in which electron and hole wave functions are confined in a prior art MQW modulator. The biased SL structure is then operated, in accordance with an embodiment of the present invention, similarly to the manner in which a prior art MQW modulator is operated. Voltage applied by the power supply to the modulator is increased above the bias voltage to generate a red shift in an absorption spectrum of the quantum well layer material of the modulator. The red shift causes a substantial increase in an absorption coefficient for light at an operating wavelength of the modulator. Hereinafter, the bias voltage used to decouple wave functions in an SL structure, in accordance with an embodiment of the present invention, is referred to as a “decoupling voltage”. 
     The inventors have found that changes in an absorption coefficient for light at an operating wavelength in quantum well material of a “decoupled SL modulator” operated as an MQW like modulator, in accordance with an embodiment of the present invention, can be substantially equal to changes in absorption coefficients of quantum well material achieved in prior art MQW modulators. In addition, because barrier layers comprised in the modulator are thinner than barrier layers in prior art MQW modulators, there is also more quantum well layer material per unit length along the modulator&#39;s optic axis than there is in prior art MQW modulators. Response of the quantum well material to voltage applied to an SL or MQW modulator substantially determines changes in absorption coefficient of light at the operating wavelength of the modulator. To the extent that a modulator comprises more quantum well material, an On/Off transmission ratio of the modulator generally increases. As a result, for a given voltage, On/Off transmission ratios for a modulator in accordance with an embodiment of the present invention, can often be equal to or greater than On/Off transmission ratios obtained with some prior art MQW modulators. 
     Because barrier layers in the modulator are thinner than barrier layers in prior art MQW modulators electron and hole escape times in the modulator are generally shorter than electron and hole escape times in prior art MQW modulators. In addition, when voltage applied to the modulator by the power supply is reduced below the decoupling voltage to a voltage at which wave functions in different quantum wells are “re-coupled”, the modulator operates as an SL structure having even shorter electron and hole escape times. In the presence of a moderate electric field, which is generated by an appropriate voltage below the decoupling voltage used to morph the SL structure to the MQW structure, photo-induced charges are rapidly swept out of the SL structure. 
     In some embodiments of the present invention, barrier layers in the modulator are seeded with non-radiative recombination traps, such as non-radiative recombination traps known in the art that are generated by growing the barrier layers at low temperature. Quantum well layers are not seeded with traps. At the trap sites photo-induced electrons recombine relatively rapidly with photo-induced holes in a non-radiative recombination process. As a result, the recombination traps function to substantially reduce lifetimes of photo-induced electrons and holes in the modulator. The traps thereby contribute to rapid removal of photo-induced electrons and holes and reduction of current resulting from motion of the electrons and holes in electric fields applied to the modulator. 
     In some embodiments of the present invention, all of the barrier layers are seeded with non-radiative recombination traps. However, seeding layers with traps can be an expensive process. The inventors have found that effective reductions in lifetimes of photo-induced electrons and holes can be achieved and costs of seeding a modulator reduced by seeding only some of the barrier layers, in accordance with an embodiment of the present invention. For example, the inventors have found that seeding only every fourth layer in a modulator in accordance with an embodiment of the present invention, can provide substantial savings in fabrication costs of the modulator while still providing effective reduction in lifetimes of photo-induced electrons and holes in the modulator. 
     It is noted that whereas methods of seeding layers, in accordance with an embodiment of the present invention, are described for modulators having an SL structure, the methods are applicable to heterojunction structures in general, whether they are SL structures or MQW structures. 
     There is therefore provided in accordance with an embodiment of the present invention, an optical modulator for modulating light comprising: a superlattice structure having a plurality of interleaved narrow and wide bandgap semiconductor layers, wherein wave functions of energy states of electrons and holes in different narrow bandgap layers are coupled; and a power supply that applies voltage to the superlattice structure between a first non-zero voltage and a second non-zero voltage to modulate the light. 
     Optionally, at the first voltage the wave functions are decoupled and the modulator has an absorption spectrum having an absorption edge determined by transitions between energy states in a same narrow bandgap layer. Optionally, at the second voltage the absorption edge is red shifted. 
     In some embodiments of the present invention, the optical modulator has an absorption spectrum having an absorption edge when zero voltage is applied by the power supply to the modulator and the absorption edge at the first voltage is blue shifted with respect to the absorption edge at zero voltage. Optionally, the absorption edge at the second voltage is red shifted with respect with respect to position of the absorption edge at the first voltage. 
     In some embodiments of the present invention, the second voltage is larger than the first voltage. 
     Optionally, following application of the second voltage, the power supply applies a voltage less than the first voltage to the modulator to remove electrons and holes generated therein by passage of the light therethrough. 
     In some embodiments of the present invention, substantially none of the narrow bandgap layers and at least one of the wide bandgap layers is seeded with non-radiative electron traps. Optionally, the at least one wide bandgap layers comprises all the wide bandgap layers. Optionally, the at least one wide bandgap layer comprises some but not all the wide bandgap layers. Optionally, the at least one wide bandgap layer comprises every other wide bandgap layer. Optionally, the at least one wide bandgap layer comprises every fourth wide bandgap layers. In some embodiments of the present invention the traps in a wide bandgap layer are generated by growing the wide bandgap layer at low temperature. 
     In some embodiments of the present invention, the thickness of the narrow bandgap layers is less than or equal to 10 nanometers. Optionally, the thickness of the narrow bandgap layers is substantially equal to 3 nanometers. 
     In some embodiments of the present invention, the thickness of the wide bandgap layers is less than or equal to 6 nanometers. Optionally, the thickness of the wide bandgap layers is substantially equal to 3 nanometers. 
     In some embodiments of the present invention, a ratio of thickness of a wide bandgap layer to a narrow bandgap layer is greater than or equal to one. Optionally, the ratio is greater than or equal to two. Optionally, the ratio is greater than or equal to three. 
     In some embodiments of the present invention, the number of narrow bandgap layers comprised in the superlattice structure is greater than 50. Optionally, the number of narrow bandgap layers comprised in the superlattice structure is substantially equal to 200. Optionally the number of narrow bandgap layers comprised in the superlattice structure is substantially equal to 300. 
     In some embodiments of the present invention, the superlattice structure is formed in an intrinsic region of a pin diode. 
     In some embodiments of the present invention, the first voltage is less than 30 volts. Optionally, the first voltage is less than 15 volts. Optionally, the first voltage is between 5 and 10 volts. Optionally, the first voltage is substantially equal to 25 volts. In some embodiments of the present invention, the second voltage is a voltage between 25–55 volts. 
     In some embodiments of the present invention, the narrow bandgap layers are formed form GaAs. In some embodiments of the present invention, the wide bandgap layers are formed from Al x Ga (1-x) As. Optionally, x is less than or equal to 0.7. Optionally, x is substantially equal to 0.3. 
     There is further provided in accordance with an embodiment of the present invention, a method of modulating intensity of a beam of light comprising: applying a non-zero voltage to a superlattice structure having a plurality of interleaved narrow and wide bandgap semiconductor layers so as to determine a transmittance for the light in the structure, wherein in the absence of voltage applied to the superlattice structure, wave functions of energy states of electrons and holes in different narrow bandgap layers are coupled; directing the light so that it is incident on the structure; and applying a second non-zero voltage different from the first voltage to the structure to change the transmittance and modulate thereby the light. 
     Optionally, at the first voltage the wave functions are decoupled and the modulator has an absorption spectrum having an absorption edge determined by transitions between energy states in a same narrow bandgap layer. Optionally, at the second voltage the absorption edge is red shifted with respect to position of the absorption edge at the first voltage 
     Alternatively or additionally, the absorption edge at the first voltage is blue shifted with respect to an absorption edge of an absorption spectrum that characterizes the modulator in the absence of voltage applied to the superlattice structure. 
     In some embodiments of the present invention, the second voltage is larger than the first voltage. 
     In some embodiments of the present invention, following application of the second voltage, a voltage less than the first voltage is applied to the modulator to remove electrons and holes generated therein by the passage of the light beam therethrough. 
     There is further provided in accordance with an embodiment of the present invention, an optical modulator comprising: a multilayer heterojunction structure comprising a plurality of interleaved narrow and wide bandgap semiconductor layers; and non-radiative traps located in at least one of the wide bandgap layers and substantially none of the narrow bandgap layers. 
     Optionally, the at least one wide bandgap layers comprises all the wide bandgap layers. Optionally, the at least one wide bandgap layer comprises some but not all the wide bandgap layers. Optionally, the at least one wide bandgap layer comprises every other wide bandgap layer. Optionally, the at least one wide bandgap layer comprises every fourth wide bandgap layers. In some embodiments of the present invention, the traps in a wide bandgap layer are traps generated by growing the wide bandgap layer at low temperature. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with the same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
         FIG. 1A  schematically shows a cross sectional view of an optical modulator having an SL structure, in accordance with an embodiment of the present invention; 
         FIG. 1B  is a graph of experimentally determined absorption edges showing a red shift following a blue shift for an optical modulator similar to the optical modulator shown in  FIG. 1A , in accordance with an embodiment of the present invention; 
         FIG. 2A  schematically shows wave functions and energy levels of allowed energy states of electrons and holes in quantum wells of the SL structure of the optical modulator shown in  FIG. 1A  in the absence of an electric field applied to the modulator, in accordance with an embodiment of the present invention; 
         FIG. 2B  schematically shows allowed wave functions of electrons and holes in the SL structure of the modulator shown in  FIGS. 1A and 2A  and their energy levels, when a decoupling voltage is applied to the modulator; and 
         FIG. 2C  schematically shows effects on electron and hole wave functions in the modulator shown in  FIGS. 1A ,  2 A– 2 B when a voltage greater than the decoupling voltage is applied to the modulator. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1A  shows a schematic cross section view of an optical modulator  20  in accordance with an embodiment of the present invention. Optical modulator  20  comprises an epitaxial superlattice, i.e. SL, structure  22 , formed between heavily p doped layer  24  and heavily n doped layer  26 . SL structure  22  and heavily doped layers  24  and  26  form a pin diode  28 . SL structure  22  comprises a plurality of narrow bandgap “quantum well” layers  30  alternating with wide bandgap barrier layers  32 . Only a few of quantum well layers  30  and barrier layers  32  are shown in  FIG. 1A . It is noted that the words “narrow” and “wide” refer to bandgaps in layers  30  and  32  and not to thickness of the layers. 
     Narrow bandgap quantum well layers preferably have a thickness less than a diameter of an electron-hole exciton in the material of quantum well layers  30 . Typically, thickness of quantum well layers  30  is equal to or less than 10 nanometers. In order for electron and hole wave functions in different quantum well layers to be strongly coupled, preferably barrier layers  32  are substantially thinner than quantum well layers  30 . 
     Ohmic contact electrodes  34  and  36 , shown shaded, are provided on layers  24  and  26  respectively using methods known in the art. Electrodes  34  and  36 , as shown in  FIG. 1A , are optionally formed in a shape of a “picture” frame having an open central region  38 . Central regions  38  of electrodes  34  and  36  define apertures of the modulator through which light enters and/or leaves modulator  20 . Various other types of contact electrodes, such as those described in above referenced PCT Publication WO 99/40478 referenced above, may be used in the practice of the present invention. Light that is modulated by modulator  20  is schematically represented by wavy arrows  21  shown entering and leaving modulator  20 . 
     In some embodiments of the present invention, barrier layers  32  are seeded with non-radiative traps, such as traps generated by growing the layers at low temperature. In some embodiments of the present invention, all barrier layers  32  are seeded with traps. However, seeding can be an expensive process. Therefore, in some embodiments of the present invention only some of barrier layers  32  are seeded. By seeding only some barrier layers rather than all barrier layers, cost of seeding for modulator  20  can be reduced. In  FIG. 1A , only every fourth barrier layer  32  is seeded with traps, which are represented by circles  40 . Traps  40  serve as non-radiative recombination centers for photo-induced electrons and holes generated in modulator  20  and thus reduce their lifetime. 
     Modulator  20  comprises a power supply  50  that applies voltage to electrodes  34  and  36  to modify wave functions and energies of allowed electron and hole states in quantum wells of quantum well layers  30 . By modifying the energy levels of the allowed states, power supply  50  controls transmittance of modulator  20  to light at an operational wavelength of the modulator. 
     In accordance with an embodiment of the present invention, power supply  50  back biases pin diode  28  with a decoupling voltage, so as to decouple electron and hole wave functions in the quantum wells that are normally strongly coupled as a result of the SL structure of quantum well and barrier layers  30  and  32 . At the decoupling voltage, SL structure  22  has an absorption spectrum having an absorption edge that can be red shifted to modulate light at the operating wavelength of modulator  20 . In accordance with an embodiment of the present invention power supply  50  red shifts the absorption edge by increasing voltage applied to pin diode  28  above the decoupling voltage. As a result of the red shift, the absorption coefficient in material of quantum well layers  30  of modulator  20  increases and transmittance of the modulator decreases for light at the operational wavelength of the modulator. 
     Following a period during which the absorption edge of modulator  20  is red shifted and photo-induced electrons and holes are generated by light at an operational wavelength incident on the modulator, the electrons and holes recombine and/or drift out of SL structure  22 . The escape time of the electrons and holes is shorter than in prior art MQW modulators because barrier layers  32  are relatively thinner than barrier layers in prior art MQW modulators. 
     In addition, in some embodiments of the present invention, following a period during which the absorption edge is red shifted and photo-induced electrons and holes are generated, power supply  50  back biases pin diode  28  with a moderate voltage that is less than the decoupling voltage. At the reduced bias voltage the electron and hole wave functions in the quantum wells of quantum well layers  30  are “recoupled”. As a result, photo-induced electrons and holes that may be trapped in quantum wells of SL structure  22  are more easily able to tunnel between the quantum wells and are relatively rapidly swept out of SL structure  22 . 
     Preferably, the density of traps  40  in barrier layers  32  is such that an average effective recombination path length resulting from the traps for an electron or hole being swept out at the reduced voltage is substantially shorter than the width of superlattice structure  22 . As a result, substantially all electrons and holes that are being swept out at the reduced bias voltage are trapped and recombine in barrier layers  32  that are seeded with traps and do not reach electrodes  34  and  36  respectively. 
     The inventors have experimentally verified a red shift in a modulator, in accordance with an embodiment of the present invention, similar to modulator  20 . The modulator had an SL structure  22  comprising 280 quantum well layers  30  and  281  barrier layers  32 . Narrow gap layers  30  were formed from GaAs and wide gap layers  32  formed from Al x Ga (1-x) As with x˜0.3. Both narrow and wide bandgap layers  30  and  32  were about 3 nanometers thick, resulting in a total thickness for SL structure  22  of about 1.68 microns. The modulator had an operating wavelength at about 787.5 nanometers. 
     Electron and hole wave functions in the modulator are decoupled when power supply  50  applied a voltage of about 25 volts to the modulator. The red shift was observed for voltages above the decoupling voltage.  FIG. 1B  is a graph of experimentally determined absorption edges  120 ,  121  and  122  for the modulator at 0 volts, 25 volts and 55 volts respectively. Absorption edge  121  obtained at 25 volts is blue shifted with respect to absorption edge  120  at 0 volts. Absorption edge  122  at 55 volts is red shifted with respect to both absorption edge  121  and absorption edge  120 . At voltages intermediate 25 and 55 volts, absorption edges are obtained that are red shifted with respect to absorption edge  121  less than the amount by which absorption edge  122  at 55 volts is red shifted. At an operating wavelength of 787.5 nanometers a difference between absorption edge  121  and absorption edge  122 , which is indicated by double arrowhead line  124  provides an On/Off transmission ration of about 4. 
     It is noted that modulators, in accordance with embodiments of the present invention, can have values for structural and operational parameters that are different from those given above. Decoupling voltages can be other than 25 volts, barrier layer thickness are not limited to thickness of about 3 nanometers and values of x other than 0.3 can be advantageous. Furthermore, modulators in accordance with embodiments of the present invention can have a number of quantum well layers that is more or less than 280 and provide On/Off ratios other than 4. Substantially any SL structure for which a decoupling voltage can be established and for which a voltage greater than the decoupling voltage red shifts an absorption edge of the SL structure can be used in the practice of the present invention. 
     Advantageous values for x are expected to be below 0.7 and advantageous thickness for barrier and quantum well layers are expected to be less than about 6 nanometers and 10 nanometers respectively. In addition, to increase On/Off transmission ratio per unit length along the axis of a modulator it is advantageous that quantum well layers have greater thickness than barrier layers. In some embodiments of the present invention a ratio of quantum well thickness to barrier layer thickness is greater than 2. In some embodiments of the present invention the ratio is greater than 3. Advantageous values for decoupling voltages are expected to be less than 30 volts. In some embodiments of the present invention a decoupling voltage is less than 15 volts. It is expected that an advantageous number of quantum well layers in a modulator in accordance with an embodiment of the present invention is equal to or greater than 50. In some embodiments of the present invention, the number of quantum well layers is greater than 200. In some embodiments of the present invention, the number of quantum, well layers is greater than 300. 
     For example, the inventors expect that a modulator similar to that used to provide the experimental results shown in  FIG. 1B , but having narrow bandgap GaAs layers  30  about 8 nanometers thick instead of 3 nanometers thick, will have a decoupling voltage in a range from 5–10 volts. The decoupling voltage is reduced compared to the decoupling voltage in the “experimental modulator” because energy bands in the 8 nanometers narrow bandgap layers  30  (i.e. the quantum well layers) are narrower than those in the 3 nanometers quantum well layers of the experimental modulator. In addition, it is expected that as a result of the increased thickness of narrow bandgap GaAs layers  30 , that an On/Off transmission ratio for the modulator is expected to be about 20. 
     It is noted that whereas the above example of a modulator comprises a superlattice based on GaAs, superlattices comprised in modulators in accordance with embodiments of the present invention can be based on other III-V element combinations. 
       FIGS. 2A–2C  schematically illustrate effects of voltage applied to pin diode  28  by power supply  50  on energies and wave functions corresponding to lowest allowed energy states of electron and holes in quantum wells in SL structure  22 . 
     Each of  FIGS. 2A–2C  shows a graph of the energy of the top of the valence band and the bottom of the conduction band in SL structure  22  as a function of position along a direction perpendicular to the planes of quantum well layers  30  and barrier layers  32  in the SL structure. In the graphs, direction perpendicular to the layers is referred to as the z-direction and displacement along the z-direction is measured in arbitrary units along the abscissa. Boundaries between quantum well and barrier layers  30  and  32  are indicated with dashed lines  60 . Energy, in arbitrary units is measured along the ordinate. 
       FIG. 2A  schematically shows electron and hole quantum wells  62  and  64  respectively, which are formed in quantum well layers  30  of SL structure  22  as a result of differences in the bandgaps of quantum well layers  30  and barrier layers  32 . Energy of the top of the valence band in SL structure  22  as a function of z is indicated by a line  66 , which shows a difference in energy of the top of the valence band between quantum well layers  30  and barrier layers  32 . Energy of the bottom of the conduction band in SL structure  22  as a function of z is indicated by a line  68 , which shows a difference in energy of the bottom of conduction band between quantum well layers  30  and barrier layers  32 . A narrow bandgap of a quantum well layer  30  is indicated by arrowhead line  70  and a wide bandgap of an adjacent barrier layer  32  is indicated by an arrowhead line  72 . 
     In  FIG. 2A  power supply  50  does not apply a voltage difference between electrodes  34  and  36  of pin diode  28  and there is no electric field in SL structure  22 . Wave functions of allowed states of electrons in quantum wells  62  are strongly coupled and their energies form allowed energy bands for electron states in the SL structure. The wave functions at a given allowed energy in different quantum wells  62  are in resonance and “meld” together so that they appear as a single extended wave function that spans all the layers in SL structure  22 . Electrons tunnel relatively easily through barrier layers  32  and move relatively freely between quantum wells  62  in different quantum well layers  30 . As a result, the electrons are able to move relatively freely within substantially all the volume of SL structure  22 . 
     Amplitude of “melded” wave functions of a lowest allowed energy state of electrons in quantum wells  62  is schematically represented by width of a shaded band  74  shown in  FIG. 2A . An energy band of SL structure  22  determined by lowest energy states of the quantum wells is schematically represented in  FIG. 2A  by a shaded band  76 . Width of band  76  represents an energy spread of the states that define the energy band. Similarly, wave functions of holes in quantum wells  64  are strongly coupled and define energy bands of allowed states of holes in quantum wells  64 . Holes are also able to move relatively freely within substantially all the volume of SL structure  22 . (Though, because of their greater effective mass, generally, the holes are more sluggish and tend to be more confined by the quantum well structure than electrons.) Amplitude of coupled wave functions as a function of z of a lowest energy state for holes in quantum wells  64  is schematically represented by width of a shaded band  78 . A shaded band  80  represents an energy band of the lowest energy states of the holes in quantum wells  64 . 
     In the absence of an applied electric field, a minimum amount of energy “E o ” equal to an energy difference between the top of band  80  and the bottom of band  76  is required to raise an electron from the valence band to the conduction band of modulator  20 . A double arrowhead line labeled with the minimum energy E o  indicates graphically the minimum energy required to raise an electron to the conduction band. 
     In  FIG. 2B  power supply  50  applies a decoupling bias voltage between electrodes  34  and  36 , in accordance with an embodiment of the present invention. The decoupling voltage generates an electric field “ε o ” in SL structure  22  that modifies the shape of quantum wells  62  and  64  and decouples wave functions of electron and hole states in different quantum wells. Direction of electric field ε o  is indicated by the direction of a block arrow labeled by ε o  and potential energy increases in the positive z direction. As a result of the applied field ε o  electron states in adjacent quantum wells  62  are shifted out of resonance. The electron wave functions are decoupled and become relatively localized to regions of quantum wells  62 . In addition, widths of energy bands (e.g. widths of energy bands  76  and  80  shown in  FIG. 2A ) of wave functions in quantum wells  62  decrease and each energy band becomes an energy level with well defined energy. 
     Amplitude of “confined” electron wave functions for electrons in lowest energy states of quantum wells  62  are schematically represented by tapered bands  90  located in the quantum wells. Energy levels of of the lowest electron energy states in quantum wells  62  are schematically represented by lines  92 . 
     Hole wave functions are similarly decoupled and confined to regions of quantum wells  64  and hole energy bands converge to energy levels in the presence of ε o . Amplitude of confined hole wave functions for holes in lowest energy states of quantum wells  64  are schematically represented by tapered bands  94  located in the quantum wells. Energy levels of the lowest hole energy states in quantum wells  64  are schematically represented by lines  96 . 
     An energy difference between the centers of a hole energy band  76  and an electron energy band  80  in a same quantum well layer shown in  FIG. 2A  is substantially the same as an energy difference between corresponding energy levels  92  and  96  shown in  FIG. 2B . However, an energy difference between the top of energy band  76  and the bottom of energy band  80  in  FIG. 2A  is slightly less than an energy difference between energy levels  92  and  96 . As a result, slightly more energy is required to raise an electron to the conduction band when power supply  50  applies a decoupling voltage to pin diode  28  and as in prior art there is a blue shift in the absorption spectrum of modulator  20 . A minimum energy E 1 , which is greater than E o , is now required to raise an electron to the conduction band in modulator  20  in the presence of electric field ε o . A double arrowhead line labeled with E 1  indicates graphically the minimum energy E 1 . 
     In  FIG. 2C  power supply  50  applies a voltage to pin diode  28  that is greater than the decoupling voltage to generate and electric field ε 1 &gt;ε o  in SL structure  22 , in accordance with an embodiment of the present invention. Electric field ε 1  operates on wave functions  90  and  94  in modulator  20  that are already confined to their respective quantum wells  30  and have their energy bands reduced to energy levels by electric field ε o . The inventors have found that an effect of the increase of electric field from ε o  to ε 1  on the energy levels of the confined lowest energy states of quantum wells  62  and  64  is similar to the quantum confined Stark effect generated by an electric field in an MQW modulator. An electron energy level  92  in a quantum well  62  shown in  FIG. 2B  is shifted towards lower energies and moves closer to the bottom of the quantum well (i.e. the bottom of the conduction band in the quantum well layer in which the quantum well is located). Similarly, an energy level  96  for holes in a quantum well  64  shown in  FIG. 2B  is shifted towards lower energies and moves closer to the top of the quantum well. (It is to be noted that energy of hole states decreases towards the top of the valence band and when energy of a hole state approaches the top of the valence band the hole state approaches the bottom of its quantum well.) The shifted energy levels resulting from the increase in electric field from ε o  to ε 1  are schematically shown as energy levels  100  and  102  in  FIG. 2C . 
     The shifting of energy bands  92  and  96  that “transform” energy levels  92  and  96  into energy levels  100  and  102  respectively reduce a minimum energy required to raise an electron from the valence band to the conduction band of a quantum well layer  30 . The minimum energy is equal to an energy difference, E 2 &lt;E 1 , between energy level  102  and energy level  100  in the layer. Energy difference E 2  is shown graphically in  FIG. 2C  by a double arrowhead line labeled by E 2 . 
     As a result of the reduction in energy required to raise an electron to the conduction band of a quantum well layer  30  the absorption spectrum of modulator  20  is red shifted. The inventors have found that the red shift, in accordance with an embodiment of the present invention, can provide an On/Off transmission ratio for light at the operational wave length of modulator  20  that substantially approaches or exceeds On/Off ratios achieved by some prior art MQW modulators. 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. 
     The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.