Patent Application
20120148252
The present invention relates to using a wireless radio frequency (RF) signal, such as a GHz or THz frequency signal, to provide an optical signal carrying substantially the same information as the wireless RF signal. Information carried by the RF signal can be imparted on the optical signal using a method and device in accordance with the present invention.
Current communication systems technology allows for transfer rates up to about 1 Terabit/second (Tbit/s). Such systems are based on optical signals. A multitude of Gigabit/s (Gbit/s) emitters are combined to produce this high a transfer rate, via optical time-division multiplexing (OTDM). An obstacle on the path to higher transfer rates using single emitter are fundamental: direct THz-rate modulation of currents in for instance semiconductor laser diodes, or control voltages in phase-shift-based switches does not produce satisfactory effects due to intrinsically “large” RC time constants (resistance times capacitance) in such devices—“large” in the sense that the associated charge/discharge rate limits the modulation speed to perhaps 10 GHz to 40 GHz, orders of magnitude lower than 1 THz. Ultrafast optically induced switching of laser diodes is possible; however, the
THz modulation rates is ultimately difficult to achieve due to a slow recovery of the cold electron population. This process is too slow to provide modulation rates higher than about 100 GHz.
Although optical time-division multiplexing can combine multiple emitters each having “low” repetition rates (such as 10 Gbit/s) to achieve high transfer rates at optical wavelengths, no digital encoding of a THz signal onto an optical signal is currently possible.
EP 1 416 316 and GB 2 386 965 disclose interferometers for modulating an optical signal, using electrically responsive optical phase shifters or modulators in the respective branches. The phase shifters involve a quantum dot containing layer. The modulation signals are RF hard wired voltage signals received over an electrical wire and applied over the phase shifter by electrodes or metal contact layers deposited thereon.
It is a disadvantage that the driving circuit for supplying the hard wire signal to the phase shifters leads to an RC-constant that limits the modulation speed of the phase shifter to appr. 100 GHz.
It is an object of the invention to provide a method and a device for coherent detection and/or instantaneous encoding of wireless high-frequency radio frequency (RF) signals with frequencies reaching hundreds of GHz to THz onto an optical carrier signal.
It is another object of the invention to provide a method and a device for modulating optical signals, where obtainable speeds are not limited by RC time constants in electrical circuitry.
It is yet another object of the invention to provide a method and a device for (direct) encoding of wireless high-frequency RF signals onto an optical signal. This enables coherent detection of the wireless RF signal via encoding of it onto an optical signal that can then be detected in known ways.
The invention thereby provides solutions to problems of the prior art corresponding to these objects.
In a first aspect, the present invention provides a method for modulating an optical input signal having a first frequency, comprising:
The change in absorption and possibly phase at the first frequency caused by the RF signal can be used to modulate data from the RF signal onto the optical input signal, or be used to detect characteristics of the RF signal by transfer of these onto the optical input signal followed by detection of the optical output signal.
In a second aspect of the invention, a signal modulator or RF-to-optical encoder device is provided. The signal modulator can provide an optical output signal based on a wireless radio frequency modulation signal and an optical input signal, the optical output signal having a first frequency and the radio frequency modulation signal having a second frequency. The signal modulator comprises a semiconductor structure comprising:
In the following, a number of preferred and/or optional features, elements, examples, implementations and advantages will be summarized. Features or elements described in relation to one embodiment or aspect may be combined with or applied to the other embodiments or aspects where applicable. For example, structural and functional features applied in relation to a method implementation may also be used as features in relation to the device and vice versa. Also, explanations of underlying mechanisms of the invention as realized by the inventors are presented for explanatory purposes, and should not be used in ex post facto analysis for deducing the invention.
Application of an electric field to a quantum-confined system, such as for example quantum well, quantum wire, quantum dot, etc, along the the direction of the quantum confinement will lead to an effect known as quantum-confined Stark effect (QCSE). As a result of application of an eledtric field, F, the confinement potential for charged carriers—the electrons and the holes, is modified in a way that the optical transition energy decreases as a result of the band structure tilt, and the probability of the optical transition decreases as a result of larger spatial separation between the electron and hole wavefunctons. The probability of the optical transition is roughly proportional to the optical absorption coefficient.
According to the working principle of the present invention, the application of electric field to the semiconductor nanostructure region along the direction of quantum confinement will result, in particular, in the reduction of optical absorption coefficient of the nanostructure at the wavelength of the optical input signal.
Now, if the electric field is applied not in a static, but rather in a time-varying manner, then the corresponding modification of the confinement potential of the semiconductor nanostructure will instantaneously follow the time-varying electric field. Thus, for example, an optical absorption coefficient will be time-varying, as it will follow the modification of the confinement potential, as influenced by a time varying electric field.
According to the invention, the time-varying electric field will be supplied to the structure comprising the semiconductor nanostructure by a wireless radio frequency signal, thus leading the modification of, in particular, the optical absorption coefficient of the semiconductor nanostructure region with the rate corresponding to the change of electric field in the incident RF signal. We will thus employ the instantaneous nature of the QCSE induced by the RF wireless signal to provide the modulation of the optical input signal that will be transmitted through, or reflected from, the semiconductor nanostructure region.
By this method, information carried by the RF signal is imparted on the optical input signal. The effects by which the modulation on the RF signal is imparted on the optical signal are almost instantaneous, and modulation changes on a subpicosecond timescale, as well as on longer timescales, can therefore be imparted on the optical input signal. In an alternative formulation, the effects of the QCSE can be formulated as when the modulation RF signal is provided to the radio frequency receiver element, it again provides a time-dependent electric field across the first semiconductor nanostructure element, which causes a change in a valence band state of the first semiconductor nanostructure element from a first valence band state (Eh,ψh), to a second valence band state (E′h,ψh′) and a change in a conduction band state of the first semiconductor nanostructure element from a first conduction band state (Ee,ψe) to a second conduction band state (E′e,ψe′), whereby a wavefunction overlap in the semiconductor nanostructure element changes from <ψh|ψe >to <ψh′|ψe′>, which results in a change in an absorption at the first frequency of the optical input signal in the semiconductor nanostructure region. Thus, it follows that for the absorption experienced by the optical input signal to change, the first frequency should correspond to a photon energy sufficient for exciting a charge carrier from the first valence band state to the first conduction band state or from the second valence band state to the second conduction band state.
The method and the modulator can therefore be used for providing an optical data signal having the same information as an incident RF signal, which is very advantageous in optical communications systems, where this provides a way for receiving a wireless radio frequency signal by converting it into a corresponding optical signal.
The invention provides the advantage of modulating the optical input signal directly by the incident, wireless RF signal without a recourse to an intermediate electrical circuit. Thereby, the modulation is instantaneous and can support much higher modulation rates.
The method and the modulator allows for encoding a wireless radio frequency signal onto an optical signal, independent of the polarization states of both wireless and optical signals, as well as independent of the mutual orientations of the polarizations of a wireless radio frequency signal, optical signal, and orientation of the semiconductor nanostructure element. Some, rather weak, polarization dependency in respect to the mutual orientation of polarization of the optical signal and the orientation of the semiconductor nanostructure element may be expected as a result of anisotropy of optical absorption of the semiconductor nanostructure, however this will not lead to complete absence of the effect of the instantaneous modulation of the optical signal. Also, some rather weak dependency of the effect of the instantaneous modulation may arise from spatial anisotropy of semiconductor nanostructure, which will again not lead to the complete absence of the modulation effect. We note here, that this polarization independence of the modulation method is advantageous compared to other methods, such as photoconductive detection and free-space electrooptic sampling. There, it is always possible to find a mutual orientation of the polarizations of RF and optical signals, and the orientation of the RF receiver element, where RF signal detection, or encoding of the RF signal on the optical signal will be impossible.
Apart from use in communication systems, the same principle of RF-to-optical coherent encoding can very advantageously be used in high-frequency RF spectroscopy, sensing and imaging systems, where it can form part of a sensor, with the RF signal from the spectroscopy/sensing/imaging system (or other type of RF signal source) and the optical input signal as inputs. The optical output signal obtained by using the invention will comprise features from the RF signal and can then be analyzed in place of analyzing the RF signal itself. Various imaging, sensing and spectroscopy at THz frequency are discussed for instance in B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology”, Nature Materials, vol. 1, p. 26-33 (2002).
The invention produces an optical output signal that carries spectral features corresponding to those of the RF spectroscopy output signal. Detection of the optical output signal using suitable equipment is then used in order to reveal the spectroscopic information contained in the RF signal.
The semiconductor nanostructure is a core element of the invention. A semi-conductor nanostructure as used in the present invention is a semiconductor element that has at least one dimension which is smaller than 1 micrometer. Electrons and holes experience restricted motion along this dimension; they are quantum-confined. The optical properties associated with quantum-confined electrons and holes, such as optical transition energies and optical transition probabilities, are affected by the quantum confinement potential and the masses of the electrons and holes (often approximated using “effective masses”). In such a semiconductor nanostructure, an external electric field applied along this dimension of quantum confinement will essentially instantaneously lead to the modification of the optical transition parameters for electrons and holes, for instance the optical transition energies and optical transition probabilities. Thus the optical properties of the semiconductor structure, associated with the optical transitions, will change essentially instantaneously as a result of applied external electric field. If the external electric field is supplied by the electric field component of an incident wireless electromagnetic RF signal, then the optical properties of the semiconductor element will be modulated as a function of the electric field strength of the incident RF signal. If the optical input signal is transmitted through the semiconductor structure simultaneously with an incident RF signal, then the transmission of the optical input signal is modulated by the incident RF signal, resulting in the encoding of the information carried by the incident RF signal onto the optical input signal. This provides means of encoding an information from an RF signal having very high frequencies, such as GHz and THz, onto an optical signal.
The semiconductor nanostructures can comprise a “quantum well” (quantum confinement in one dimension), and/or a “quantum wire” (quantum confinement in two dimensions), and/or a “quantum dot” (quantum confinement in three dimensions). However, the semiconductor nanostructures can have other, more complex shapes. The essential principles of the invention will nevertheless be the same for any kind of semiconductor nanostructure used.
Below, the principles of the invention are discussed based on quantum dots as an example of semiconductor nanostructures.
The semiconductor structure can advantageously comprise a waveguide structure adapted to guide the optical input signal from the first waveguide interface to the second waveguide interface of the semiconductor structure, the waveguide structure and the semiconductor nanostructure region being optically coupled so that at least a part of the optical input signal, when guided in the waveguide structure, has an overlap with the semiconductor nanostructure region.
Advantageously, the modulation radio frequency signal is a signal comprising information to be encoded onto the optical input signal, such as an output from a radio frequency telecommunication or data transmission process, radio frequency spectroscopy process, radio frequency sensing process, or radio frequency imaging process. Other applications can be envisioned. The data modulated onto the RF signal, and thereby transferred to the optical signal, may be both analogue and/or digital. The frequency of the wireless RF signal, i.e. the second frequency, is preferably in the range 5 GHz to 50 THz, such as in the range 5 GHz to 20 THz. Frequencies at the higher end of this interval are not possible in electrical hard wired signals, and it is therefore evident to the skilled person that the RF signal is an electromagnetic radiation signal.
A quantum dot is a 3-dimensional piece of a narrow-bandgap semiconductor, surrounded by a wide-bandgap semiconductor, dielectric, or vacuum. The dimensions of a quantum dot usually do not exceed 100 nm. The quantum dots can be of different shapes: spherical, disc-like, lens-like, pyramid-like, cubic, etc. The schematic of a disc-like quantum dot is represented in
We note here, that in some instances, a quantum well with a very short free-carrier lifetime and subject to the electric field of an incident RF signal, polarized parallel to the quantum well layers can be used instead of the quantum dot. The very short free carrier lifetime ca e.g. be provided by introducing of a number of trap states below the bangap. Such an arrangement can also result in the near-instantaneous modulation of the quantum well absorption by the electric field of an incident wireless RF signal by a Franz-Keldysh or bulk Stark effect. The absoprtion recovery speed will then depend on the efficient and fast enough capture of the electrons and/or holes photoexcited in the quantum well, onto the trap states.
By including a waveguiding structure (120) in the semiconductor structure, the modulation of the optical output signal can be increased because the overlap between the optical signal and the semiconductor nanostructure region is increased. The waveguide structure is adapted to at least partly guide the optical input signal from the first waveguide interface (121) to the second waveguide interface (122) of the semiconductor structure, and the waveguiding structure (120) and the semiconductor nanostructure region (112) are optically coupled so that at least a part of the optical input signal, when guided in the waveguiding structure (120), has an overlap with the semiconductor nanostructure region (112).
As described above, the invention has an advantage in that the modulation is not provided primarily through filling and emptying of quantum states, but through the change in wavefunction overlap that the RF signal causes.
The modulator according to the invention may be similar to a semiconductor optical amplifier or electro-absorption modulator, but adapted to provide the low-loss coupling of the RF modulation signal into the structure to have as much of the RF modulation signal enter the semiconductor nanostructure region. The reason is that in many applications and practical cases, the RF modulation signal is already quite weak. In some embodiments, the modulator in accordance with the second aspect of the invention can therefore be manufactured using the same equipment as that which is used for certain semiconductor optical amplifiers; the manufacturing just needs to be adapted so that a low-loss coupling of the radio frequency modulation signal into the semiconductor nanostructure region can be achieved.
The low-loss incoupling of an incident wireless modulating signal to the nanostructure region plays an important role in providing a high efficiency of inducing a quantum-confined Stark effect on the semiconductor nanostructures by an electric field of an incident wireless RF signal. The low-loss coupling is important to reduce or eliminate the reflection loss of the incident wireless RF signal at the surface of the RF-to-optical encoder device, preferably by providing an efficient impedance matching for the incident RF signal. The adaptation to low loss coupling of the RF modulation signal may be made in several different ways.
A relatively low-loss coupling can be obtained by providing only layers having a sufficiently low doping level (or doping levels) between the semiconductor nanostructure region and the radio frequency receiver element, that will reduce the absorption of the RF signal in the semiconductor structure by a free-carrier absorption effect, and by the increased interface reflectivity of the doped semiconductors, unless the doping level is chosen to provide the impedance matching for RF signal. Typically, the doping density should provide no more than about 1015 carriers per cm3. This number strongly depends on the types of materials used and the thickness of the layers. However, as will be described below, a certain, optimum doping level may lead to an enhancement of the RF signal incoupling at the surface of the structure. The person skilled in the art can design such layers appropriately given the teaching that the RF modulation signal should be coupled into the semiconductor nanostructure region with a low loss.
A low-loss coupling of the RF signal can be enhanced by providing an anti-reflection coating and/or impedance matching layer (or layers) at the radio frequency receiver element.
A generic configuration can be also used, where the angle of incidence of the wireless RF signal onto the device is the Brewster angle to the surface of the RF-to-optical encoder device, provided the polarization of the wireless RF signal is to a large degree linear.
As mentioned, the semiconductor structure can advantageously comprise a waveguide structure adapted to guide the optical input signal from the first waveguide interface to the second waveguide interface of the semiconductor structure, the waveguide structure and the semiconductor nanostructure region being optically coupled so that at least a part of the optical input signal, when guided in the waveguide structure, has an overlap with the semiconductor nanostructure region.
Advantageously, the optical input signal and the radio frequency modulation signal co-propagate. This increases the overlap and thus the modulation efficiency, i.e. the efficiency with which the information from the radio frequency modulation signal is imparted on the optical input signal.
Advantageously, when the optical input signal and the radio frequency modulation signal co-propagate in the semiconductor structure, their group velocities are identical or at least very similar in order to optimize the spatial and temporal overlap between them and thereby increase the modulation efficiency.
In a third aspect, a semiconductor modulator in accordance with the second aspect is used as part of an interferometer-based optical encoder for encoding an optical input signal with a radio frequency modulation signal. The encoder comprises:
Such an encoder enables a (near) background-free encoding of the optical input signal using the radio frequency modulation signal.
The same considerations that apply to the signal modulator also apply to the interferometer-based optical encoder. For instance, the coupling of the radio frequency modulation signal advantageously takes place with low loss. Similarly, co-propagation of the optical input signal and the radio frequency modulation signal can increase modulation efficiency, and so on.
Note that the invention does not depend on the absorption of the optical input signal being reduced or increased. The invention relies on the fact that there is a change at all (as opposed to no change). The discussion above applies to both ground and excited states in both the valence band(s) and conduction band(s), and the states involved in providing the change in absorption of the optical input signal need not be ground states, but may just as well be excited states.
a and 1b illustrate schematically a semiconductor structure for use in the present invention.
a-d schematically illustrate geometries for realizing a method in accordance with the present invention.
a and 3b schematically illustrate changes in wavefunctions in a quantum confinement region when an electric field is applied across the region.
a and 6b represent amplitudes of calculated electron and hole wavefunctions in the x-y plane for the quantum dot in
a and 7b represent amplitudes of calculated electron and hole wavefunctions in the x-y plane for the quantum dot in
a shows the dependency of the Stark shift on electric field applied along x axis for the quantum dot in
b shows the derivative of the Stark shift with respect to electric field applied along x axis for the quantum dot in
a-c illustrates various electrodes configurations for applying a pre-bias field to the semiconductor nanostructure region of an RF-to-optical encoder, for example such as that in
a and b show the electric field of an incident RF signal and the change in the reflectivity, AR, of the nanostructure region of the sample experienced by a laser pulse.
In the following, the invention is described through examples. The examples shall not be construed as imposing limits on the scope of the invention defined by the claims.
a schematically illustrates a semiconductor structure 110 for use with the present invention. It comprises material regions 131 and 132, with a semiconductor nanostructure region 112 of semiconductor nanostructure element(s) 112a, 112b. The semiconductor structure in fig. la also has a region 120 which can assist in increasing the interaction between an optical input signal propagating through the structure, and the nanostructure region that provides absorption. The structure 110 has a first optical, input interface 121 for coupling the optical input signal into the structure and a second optical, output interface 122 for coupling a non-absorbed part of the optical input signal out of the structure. The structure also has a receiver element 114, shown here as a flat element, for receiving a radio frequency signal and facilitate low loss coupling of the RF signal into the nanostructure region 112. The receiver 114 together with the rest of the semiconductor structure should be designed to efficiently receive radio frequency signals of at least one wavelength and couple the received signal into the nanostructure region to provide the electric field associated with the RF signal into the nanostructure region 112, or at least a part thereof. The first input optical interface 121 and the second output optical interface 122 can also be the same element, if the optical signal will enter the structure 110 through the interface, experience reflection within the structure 110, and leave through the same inteface, as is used in the proof-of-principle experiment reported later.
The elements in
The receiver element 114 needs not necessarily be a separate element. The semiconductor structure itself can act as the receiving element. A prism or other suitable optical element can also be used as a receiver element. A coating on the semiconductor structure is also an option. This is a matter of design.
The receiver element 114, in all its embodiments, serves to reduce or eliminate the reflection loss of the incident wireless RF signal by providing an efficient impedance matching for the incident RF signal. Below, some preferred embodiments of receiver element 114 using special coating on the semiconductor structure 110 are described:
In some cases, it may be desirable to couple the radio frequency signal into the semiconductor structure codirectionally, or substantially so, with the optical input signal. In such embodiment, the first optical interface can be adapted to act as the receiver element 114; a suitable coating or structuring on the semiconductor structure, for instance formed on the first optical interface, can also be used as a receiver element.
The advantage of coupling the RF signal and the optical signal into the semiconductor structure codirectionally is in increase of the interaction length of the RF and optical signals while propagating through the part of the structure, comprising the semiconductor nanostructures, as shown in
a illustrates an optical input signal 201 to enter the structure 110, and an optical output signal (203) resulting after the optical input signal has propagated through the structure. A radio frequency signal such as 202a or 202b is received by the receiver element 114 and coupled into the semiconductor structure as illustrated by signals 210a and 210b, respectively. It reaches the nanostructure region where, as described previously, it provides changes in the optical absorption in least a part of the nanostructure region. The optical input signal 201 (also illustrated in this example as a guided signal 205) propagates from input interface 121 to output interface 122, and experiences the change in absorption in the structure under the influence of the electric field of the RF signal 210a, 210b.
b illustrates an RF signal 202c being coupled into the semiconductor structure codirectionally with the optical input signal 201 giving a signal 210c in the nanostructure region.
The electric field of the incident wireless RF signal applied to the semiconductor nanostructure elements can even be locally enhanced by using a receiver element 114 comprising at least one metallic nano slit and being positioned close enough to the semiconductor nanostructures so that it will lead to a local concentration of an electric field at the semiconductor nanostructures. This principle is demonstrated in the work Seo et al. “Terahertz field enhancement by a metallic nano slit operating beyond the skin-depth limit”, Nature Photonics, vol. 3, pp. 152-165 (2009). In this case, the efficiency of our method of modulation of optical properties of semiconductor nanostructures by the electric field of a wireless RF signal can be further increased, thus requiring weaker wireless RF signals to provide larger modulation of optical properties of semiconductor nanostructure. One possible embodiment using an incoupling element 114 comprising the metallic nano slits is shown in
a illustrates schematically the conduction band potential, Ec(x), of the semiconductor nanostructure, and the valence band potential, Ev(x), of the semiconductor nanostructure, along an axis, x, in the absence of an electric field, i.e. F=0. These potentials Ec(x) and Ev(x) provide the quantum confinement of electrons and holes, respectively, along the dimension x.
A photon having this energy can excite an electron from the quantum dot valence band state (Eh,ψh) to the quantum dot conduction band state (Ee, ψe), and will thus be absorbed with a probability that is proportional to the square of the overlap integral of the wavefunctions ψe and ψh.
b illustrates the situation in the presence of a non-zero electric field, F≠0. The conduction band energy along the x-axis is E′c(x), and the valence band energy along the x-axis is E′v(x). There quantum dot conduction band state has a wavefunction ψ′e and an eigenenergy E′e, and the quantum dot valence band state has a wavefunction ψ′h and an eigenenergy E′h. A corresponding transition energy is E′e−E′h. A photon having this energy can excite an electron from the quantum dot valence band state (E′h,ψ′h) to the quantum dot conduction band state (E′e,ψ′e), and will thus be absorbed with a probability that is proportional to the square of the overlap integral of the wavefunctions ψ′e and ψ′h.
a and 3b are schematic, and the quantum dot is illustrated as a one-dimensional potential well. As described previously, a quantum dot is characterized in that a portion of material is entirely surround by material(s) having higher bandgap energy than that of a quantum dot, that is, the potential well is three-dimensional. A necessary feature is the obtained shift of the electron-hole transition energy and/or the shift in optical transition probability (through a change in the overlap between electron and hole wavefunctions) with the change in applied electric field across at least one quantum confinement dimension of the nanostructure.
In the following, the quantum-confined Stark effect is illustrated for a disc-like quantum dot. The quantum dot material is In0.54Ga0.46As with a bandgap energy of 0.74 eV, and the barrier material is GaAs with a bandgap energy of 1.42 eV. The quantum dot is cylindrical and has a radius of 7.5 nm and a height of 5 nm. The effective masses for electrons and holes in the quantum dot material are assumed to be 0.045 m0 and 0.38 m0, respectively, where m0 is the rest mass of an electron. The effective masses in the barrier material are assumed to be 0.067 m0 for electrons and 0.51 m0 for holes. The ratio between the conduction band offset and the valence band offset between the quantum dot material and the barrier material is assumed to be 60/40.
Wavefunctions and eigenenergies for the ground state of electrons and holes in the quantum dot without and in the presence of the electric field are found by solving a Schrödinger equation. Methods for solving the Schrödinger equation can be found for example in Shun Lien Chuang, “Physics of Optoelectronic Devices”, John Wiley & Sons, Inc., pp. 94-95, (1995). The penetration of the wavefunctions for electrons and holes into the barriers is taken into account by the following approximation: introducing a larger effective radius of the quantum dot, which is different for electrons and holes, and then solving the Schrödinger equation for the quantum dot with infinite potential barriers, but having the larger effective dimensions. The particular choice of calculation approach is not essential to the scope to the invention. The calculations are merely included to illustrate the underlying principle.
a and 6b represent the amplitudes (in arbitrary units) of the calculated electron and hole wavefunctions in the x-y plane for the case where no electric field is applied along the x axis.
The overlap integral between the electron and hole wavefunctions is
This is reduced when the electric field is applied as a consequence of the displacements of the electron and hole wavefunctions. The wavefunctions at zero electric field and non-zero electric field, respectively, can be inserted into the equation above in order to calculate the overlap integrals in the two cases.
The optical transition strength, on which the absorption and recombination rates depend strongly, is proportional to the square, M2, of the overlap integral M.
of the square of the overlap integral M2 with respect to electric field. This derivative describes the change in the optical transition strength, which is proportional to M2, induced by a change in applied electric field, as a function of applied electric field. As can be seen from
has a minimum at the value of applied electric field of 24 kV/cm. If the quantum dot is biased at this level (“pre-biased”), then a small additional external RF signal having an electric field (co-polarized with the pre-bias electric field) will provide the strongest possible modulation of the optical transition strength. Use of such a pre-bias can thus assist in optimizing the modulation that the RF signal can incur on an optical signal having a frequency that is in the vicinity of the frequencies corresponding to the energies Ee−Eh and E′e−E′h. The pre-bias can be both static and time-varying.
Another manifestation of the QCSE is a change (typically a decrease) in the optical transition energy with applied electric field. This shift of the optical transition energy with applied electric field, relative to the optical transition energy without an electric field, is referred to as a Stark shift.
Generally, the RF signals 210a, 210b, 210c, which will modulate the optical transition strength in the quantum dot system, can be applied as an incident free-space propagating RF modulation signal as shown in
There are also fundamental differences in how to apply a wireless signal and a hard wired signals over a structure such as an optical semiconductor structure. As shown in e.g. EP 1 416 316 and GB 2 386 965, the application of hard wired signals requires two electrodes sandwiching the semiconductor structure, and imply limitations on the design and material selection of the semiconductor structure.
The application of wireless signals requires that the signal is coupled into the semiconductor structure 110 by the receiver element 114, preferably at low loss. Several different embodiments of receiver element 114 are described in the above, some of which involves a metallic antireflection coating on the side of the semiconductor structure 110 receiving the wireless RF.
In a further embodiment, it is preferred to include a reflector made from an electrically conducting material positioned on the opposite side of the semiconductor nanostructure region from the receiving element 114. Such reflector can reflect RF radiation that has already traversed the nanostraucture region back towards the nanostraucture region. A distance between the surface and the reflector can be made very small, so that the interaction between the RF signal and the QDs (placed between the surface and the reflector) will effectively be similar to a “point interaction”, not leading to a sufficient time delay between the interactions. Such reflector provides the advantage of increasing the in-coupling the RF signal to the nanostructure region.
Hence, it is preferred that any metallic layers comprised by the receiver element 114 only performs the function of receiving a wireless modulation RF signal and couples it into the semiconductor structure 110, preferably at low loss. Similar, it is preferred that a metal reflector opposite the receiver element 114 only performs the function of reflecting RF radiation back towards the nanostructure region. Hence, it is preferred that any such electrically conducting layers are not connected to electrical circuitry by hard wires, such as to electrical circuitry for providing hard wired modulation signals. As the person skilled in the art will know, it might be necessary to make a ground connection to such electrically conducting layers to avoid uncontrolled electrical bias, alternatively it may be used as one of two or more electrodes used to apply a DC bias field as described elsewhere.
However, such ground or bias connection does not represent a connection to electrical circuitry for providing modulation signals.
The quantum dot system can be pre-biased as discussed above. A pre-bias field can be supplied statically or time-varying, in just the same way as the RF modulation signal, such as by another incident RF signal or otherwise supplied electric field (for instance through electrodes, RF waveguide, and so on). A combination of these approaches can be used to provide non-linear mixing whereby more complicated pre-bias fields can be induced.
Since both the pre-bias field and the RF modulation signal provide the modification of absorption conditions for the input optical signal in the semiconductor nanostructure region, a non-linear mixing of the RF modulation signal and a pre-bias field (which can be both static and time-varying) can be achieved, which will provide the according modulation of the optical input signal.
Another effect of applying an electric field to a quantum-confined system, such as a quantum dot system, is the electrorefraction effect, which is a local modification of the index of refraction that occurs when an electric field is applied. This effect is, like the QCSE, also nearly instantaneous, and follows the applied electric field with a very short delay (perhaps a few femtoseconds). This can contribute a phase shift to an optical input signal propagating through the semiconductor structure 110. In this way, the transmitted optical signal is also phase-modulated by the externally applied electric field (RF modulation signal or pre-bias field), contributing further to the modulation of the optical input signal
The RF-to-optical encoder 1201 should initially be adjusted before the wireless RF modulation signal is applied to the semiconductor modulator 1202 in the first arm. The phase-shifter 1206 can contribute an adjustable phase change of the second part of the optical signal in order for the phase difference between the first part and the second part of the optical signal differ by 180 degrees (or as close as practically possible) at the coupling point 1214. The attenuator/amplifier is used to bring the intensities of the optical signals at the output of the two arms 1212, 1213 as close as possible in order to make it possible to obtain (a near perfect) signal extinction at the coupling point 1214 by (near perfect) destructive interference. In this case, before application of the wireless RF modulation signal to the semiconductor modulator 1202 in the first arm, the total transmission of the optical carrier through the encoder 1201 will be zero (or as close to zero as practically possible). An optical isolator (not shown) can prevent the reflected light 1233 from escaping through the input port in the opposite direction.
Now, when a RF modulation signal 1222 is applied to the semiconductor modulator of arm 1 in accordance with the description above, then the transmission through the arm 1 will change due to the modulation obtained as described previously. This will contribute to an intensity difference between the first and second optical signals (the signals going through arm 1 (1212) and arm 2 (1213)). The optical phase difference between the optical signals in arm 1 and arm 2 is still 180 degrees (or very close to), but due to the absorption by the semiconductor modulator, there will be an imperfect destructive interference between the signals at the output of the arm 1 and the arm 2. Thus, a part of the intensity of the input optical signal will be transmitted through the device even when the destructive interference is at its highest. The level of transmission is associated with the modulation strength of the optical signal in the semiconductor modulator resulting from the applied RF modulation signal. Thus, the wireless RF modulation signal is only applied to the semiconductor modulator 1202 in the first arm, no RF modulation signal is applied to the optical phase-shifter 1206 and the attenuator/amplifier 1204 in the second arm.
The invention can be modified because a change in absorption for the optical input signal is intrinsically related to a change in the index of refraction. In this case, the semiconductor nanostructure region changes a phase of the optical input signal and not so much an intensity of the optical input signal. The RF modulation signal causes a change in the index of refraction even below the band gap of the nanostructure region elements. Thus, in this case the first frequency can be below the energy difference between the first valence band state and the first conduction band state and also below the energy difference between the second valence band state and the second conduction band state. This means that the modulation of the optical input signal is mainly a phase modulation, and not predominantly an intensity modulation. Even if the electrorefractive effect is predominant, then a similar arrangement can be used. In this case, in the absence of the modulating RF signal, the transmission through the output port 1232 must be brought to zero or as close to zero as possible, by suitable equalization of the transmission intensities transmitted through 1212 and 1213 using an attenuator/amplifier 1204, and the phases of the signals of in 1212 and 1213 must be brought to the phase difference of 180 degrees (or as close to as possible), by suitable adjustment of the phase-shifter 1206. In this case, signals propagating through the arm 1 (1212) and the arm 2 (1213) will experience complete (or as close to as possible) destructive interference, thus the total transmission through the output port 1232 will be as close to zero as it is possible. Now, if a modulating RF signal is applied to the modulator 1212, due to electrorefractive effect a phase of the optical signal through the arm 1212 will be shifted, according to the electric field strength supplied by the RF modulating signal. In this case, the destructive interference of the optical signals through 1212 and 1213 will become incomplete, which will allow for the optical intensity to propagate through the output port 1232. This optical signal will be time-modulated by the RF signal, and again, will have zero- or near-zero background.
Generally, a combination of QCSE and electrorefractive effects can be used to achieve the desirable RF-modulation strength of the transmitted optical signal at the output port 1232 using suitable optimization of the whole device and/or its elements.
The attenuator/amplifier 1204 can be made of the same material as the semiconductor modulator. The attenuation (or amplification) of the first optical signal in the arm 2 can be tuned by an applied electric field. This is well known, and an semiconductor electro-absorption modulator or semiconductor optical amplifier is well suited for this purpose. As a phase shifter 1206, a lithium niobate phase shifter or other suitable phase shifter can be used. The person skilled in the art will readily recognize that other attenuators (amplifiers) and phase shifters are suitable.
The waveguides, phase-shifter, and the attenuator can be realized on the same substrate (as schematically indicated in
a-c schematically show possibilities for pre-biasing the semiconductor nanostructure region of the signal modulator 1202 in
In
In
In
Numerical Simulation
The full line in
Proof-of-Principle Experiment
In the following, a description and results of a proof-of-principle experiment are presented.
In this experiment, a near single-cycle pulse of RF radiation (RF signal) was generated with the frequency spectrum covering the range 0.1-3 THz, by conversion of 80-fs long laser pulses with 800 nm central wavelength, provided by a pulsed laser, in an optical-to-RF conversion stage. The duration of the RF signal was not longer than 3 ps. The pulsed laser operated at the repetition rate of 1 kHz. Part of the laser output was frequency converted to the central wavelength of approximately 1040 nm (optical signal) using an optical parametric amplifier.
The duration of the optical signal was not longer than 100 fs. The RF signal was incident at normal incidence onto a sample representing the semiconductor structure 110. The optical signal was incident onto the sample at a small angle, and the power of the optical signal incident onto the sample, and reflected by the sample, was measured as a function of time delay between the RF and optical signal. The controlled time delay between the RF and optical signal was introduced using a variable optical delay line. The absolute value of the power of the optical signal incident and reflected onto the sample was measured using two optical detectors: reference and sample optical detectors, taking into account the optical signal transmission loss on the way to the sample. The schematic of the experimental setup is shown in
Also, no special receiver element for the incident RF signal was used, i.e. the receiver element was the sample surface itself.
The sample in this experiment was a QD structure, shown in
The RF signal, incident on the sample from a free space, experiences the reflection loss at the surface of the sample, propagates through the QD region, propagates through the dielectric mirror (which is not resonant with the RF frequencies), propagates through a thick GaAs wafer, and gets totally reflected off the metal reflector at the end of the sample, after which it propagates back, again propagating through the QD region, and then partially reflects back into the sample at the sample surface. Upon this reflection the elecrtric field of the RF signal re-entering the structure becomes smaller as the reflection coefficient of the sample surface is less than 1. The RF signal can also disperse, scatter, and attenuate as it propagates through the structure. Similarly, the optical pulse enters the device, passes through the QD region, reflects off the dielectric mirror (which does not affect the RF signal), passes through the QD region, and leaves the device, as the surface of the device is antireflection-coated for the wavelength of the optical pulse.
The distance between the QD region and the dielectric mirror is on the order of the optical signal wavelength, which leads to a negligibly small time delay for the interaction of optical pulse with QDs on the way in and out of structure (point interaction) The schematic showing the timing of interactions between the RF signal, and QDs, resulting in the corresponding change in the absorption in the QDs at the frequency of the optical signal is shown in
In
The first results of our proof-of-principle experiment are shown in
In
Finally, in
The modulator or RF-to-optical encoder device according to a preferred embodiment of the invention can be used as an ultrafast coherent detector for high-frequency radiation up to the THz range. The device operates at room temperature, and is capable of instantaneous encoding of incoming THz signal onto an optical signal transmitted through or reflected off the device (or probing the device). This optical signal is then analyzed/handled in a conventional manner, and it carries the information/features of the detected THz signal.
Compared to other coherent detection schemes, such as free-space electrooptic sampling (FEOS) and photoconductive sampling (PCS), the method and device according to the invention provides the advantage of being virtually independent of the mutual orientations of polarizartions of optical and THz signals, and of the the crystallographic axes of the semiconductor. Hence, the device according to the invcention is virtually polarization-insensitive.
Applications
The invention can advantageously be applied for many different applications, and the exact set-up applicable in each application will be within the capabilities of the person skilled in the art. Some of the envisioned applications are:
In preferred embodiments, the invention provides a signal modulator adapted for any one or more of these applications, as well as the use of a a signal modulator according to the invention for the purpose of any one or more of these applications.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09167373.1 | Aug 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK10/50200 | 7/29/2010 | WO | 00 | 2/28/2012 |
