Abstract:
A high speed miniature tera- and gigahertz electromagnetic radiation on-chip spectrometer that comprises a tunable solid state 2D charge carrier layer or a quasi 2D charge carrier layer with incorporated single or multiple defects, at least first and second contacts to the charge carrier layer. Also the device includes an apparatus for measuring the device response between the first and second contacts, and an apparatus for a controllable tuning of at least one of the charge carrier layer parameters. The operation principle is based on the fact that radiation of different wavelengths excites distinct sets of plasma modes in the charge carrier layer.

Description:
BACKGROUND 
       [0001]    The region of electromagnetic spectrum lying in the giga (GHz)-terahertz (THz) frequency range has recently become of increasing interest in the various fields of science and technology. In part, such interest is caused by unique properties of GHz-THz radiation, which make such radiation appealing for a great number of useful applications. Terahertz radiation is non-ionizing, and thus, unlike X-rays, it is not harmful for biological tissues and DNA. In addition, most biological and chemical agents have resonant absorption lines in the THz region. Therefore, accurate and safe tomography of different human and other biological tissues may be enabled. Because terahertz radiation can penetrate fabrics and plastics, it can be used in security applications, e.g., to screen for concealed weapons. The high-frequency nature of the THz radiation makes it possible to utilize it in higher frequency computer and high-altitude telecommunication systems. Likewise, terahertz sensing and imaging may be used in many applications in the field of manufacturing, quality control and process monitoring. These applications generally take advantage of the properties of plastics, cardboard and other packaging materials being transparent to terahertz radiation, and thereby making it possible to inspect packaged products. Similarly, THz radiation affords additional tools of scientific research in a variety of fields, from submillimeter and millimeter astronomy investigations to solid state research. 
       SUMMARY 
       [0002]    The following is a summary description of illustrative embodiments of the invention. It is provided as a preface to assist those skilled in the art to more rapidly understand the detailed design discussion, but it is not intended in to limit any way the scope of the claims, which are appended hereto in order to particularly point out the instant invention. 
         [0003]    The present invention builds upon the technology and invention described and claimed in U.S. patent application Ser. No. 12/247,096, which is incorporated herein by reference. The embodiments disclosed hereafter utilize the present invention, which provides a new fast, miniature electromagnetic radiation spectrometer based on resonant excitation of plasmons in solid-state systems that contain tunable charge carrier layers with at least one incorporated defect. Depending on the size of the cavity, where the resonant plasmon excitation takes place, the operation frequency of the device may span the range from approximately 1 GHz to approximately 10 THz. The tunability of the charge carrier layer can be achieved by controllable sweeping of at least one of the following layer parameters: carrier density, the dielectric environment, the applied magnetic field, the effective carrier mass, and the size of the plasmonic cavity. The device can comprise one or more tunable solid state systems, each of which includes at least one charge carrier layer (electron or hole) with al least one intentionally incorporated defect and at least two potential contacts electrically connected to said layer or layers. 
         [0004]    To achieve said tunability, the apparatus has to also include a means for a controllable tuning of the one or more of the above listed charge carrier layer parameters. A “defect” may comprise any inhomogeneity introduced into the two-dimensional charge layer (which is described in detail in U.S. patent application Ser. No. 12/247,096). The spectrum of the incident radiation can be calculated via an analysis of the device response to an adjustment of a tunable parameter. Frequency sensitive real-time matrix cameras can be implemented on the basis of the disclosed basic spectrometer element because of its small size (normally on the order of a few micrometers) and a complete absence of any moving parts or components. 
         [0005]    Operation of the embodiments described herein may rely on the following principles:
       1. Incident radiation is coupled to potential probes and/or to a charge carrier layer and/or to the antenna structure, thereby inducing on them an alternating potential.   2. The alternating potential induces plasma waves, which propagate and resonate in the tunable plasmonic cavity. The cavity stands for the region on the crystal restricted by the boundaries, on which boundaries the plasmon dispersion undergoes a step. The cavity can be tuned by a continuous controllable adjustment (i.e. sweeping) of at least one of its parameters (e.g., the carrier density).   3. The oscillating plasmon electric field inside the device is rectified by the non-linear characteristic of the device, resulting in a DC response between different pairs of the potential probes. The non-linear device behavior is caused by the presence of at least one defect (as described in detail in U.S. patent application Ser. No. 12/247,096).   4. Detection of the device response/signal as a function of the tuned parameter(s) yields sufficient data to calculate the spectrum of the incident radiation.       
 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a side schematic view of an alternate embodiment where the plasmonic cavity is tuned by a back gate; 
           [0011]      FIG. 2  is a top schematic view of the device of  FIG. 1 ; 
           [0012]      FIG. 3  is a side schematic view of an alternate embodiment where the plasmonic cavity is tuned by a top gate placed on a dielectric layer; 
           [0013]      FIG. 4  is a top schematic view of the device of  FIG. 3 ; 
           [0014]      FIG. 5  is a side schematic view of an alternate embodiment where the plasmonic cavity is tuned by a gate placed directly on top of the structure; 
           [0015]      FIG. 6  is a top schematic view of the device of  FIG. 5 ; 
           [0016]      FIG. 7  is a side schematic view of an alternate embodiment where a series of plasmonic cavities of arbitrary shape is tuned; 
           [0017]      FIG. 8  is a top schematic view of the device of  FIG. 7 ; 
           [0018]      FIG. 9  is a side schematic view of an alternate embodiment where rectifying defects are placed away from the plasmonic cavity; 
           [0019]      FIG. 10  is a top schematic view of the device of  FIG. 9 ; 
           [0020]      FIG. 11  is a side schematic view of an alternate embodiment where tunability of the plasmonic cavity is implemented by sweeping its dielectric environment; 
           [0021]      FIG. 12  is a side schematic view of an alternate embodiment where tunability of the plasmonic cavity is implemented by sweeping magnetic field; 
           [0022]      FIG. 13  is a side schematic view of an alternate embodiment where tunability of the plasmonic cavity is accomplished by adjusting the size of the cavity; 
           [0023]      FIG. 14  is a side schematic view of an alternate embodiment with antennas to efficiently couple incident radiation to plasmons; 
           [0024]      FIG. 15  shows experimentally measured dependencies of the radiation induced photovoltage, as a function of electron density, and for different radiation frequencies. The size of the plasmonic cavity L=400 μm; 
           [0025]      FIG. 16  shows experimentally measured dependencies of the radiation induced photovoltage, as a function of electron density, and for different radiation frequencies. The size of the plasmonic cavity L=100 μm; 
           [0026]      FIG. 17  (bottom part) shows experimentally measured dependencies of the radiation induced photovoltage, as a function of electron density, and for different magnitudes of magnetic field.  FIG. 17  (top part) shows experimentally measured dependency of the radiation induced photovoltage, as a function of magnetic field magnitude; 
           [0027]      FIG. 18  demonstrates the radiation induced photovoltaic signal for an illustrative embodiments of the device of  FIGS. 3 and 4 ; 
           [0028]      FIG. 19  is a graph of the spectrometer response vs temperature in degrees Kelvin (K), and illustrates the feasibility of the proposed method at temperatures above liquid nitrogen point; 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Collective plasma excitations in low-dimensional charge carrier layers have attracted interest of the researchers and engineers for decades. On the one hand, the interest has been caused by a plethora of plasma-wave effects, which are interesting from the scientific point of view. It has been established that by properly designing the geometry and the parameters of a plasmonic cavity, it is possible to achieve the plasma frequency of the cavity in a specific teraherstz region. The basic characteristics of a plasmon are the frequency and the wave vector. They are related to each other by the dispersion relation. Plasma waves in the two-dimensional charge carrier layer possess a dispersion of the following form: 
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         [0000]    Here, n S  and m* are the density and the effective mass of the two-dimensional electrons/holes, respectively, while ∈ 0  and ∈(q) are the permittivity of vacuum and the effective permittivity of the surrounding medium, respectively, and ω p  denotes the frequency of the plasma wave. The dispersion is strongly influenced by the effective electric permittivity, which is a complex function of the plasmon wave vector q. In the device embodiments suggested herein, propagation of each plasmon is restricted to a certain part of the charge carrier layer—plasmonic cavity. This plasmonic cavity is defined by the geometric boundaries, within which the plasmon dispersion undergoes a step. If the geometric cavity size amounts to L, then, due to interference of plasma waves backscattered from the cavity borders, only plasmons with wave vectors q=nπ/L (n=1, 2, 3 . . . ) are excited. If the radiation frequency is fixed and one of the charge carrier layer parameters, e.g., electron/hole density n S , is swept, then the consecutive plasma resonances with wave numbers n=1, 2, 3 . . . result in the cavity. The resulting resonances can be used to generate a device response, e.g. photovoltage, photocurrent, photocapacitance, photoinductance or photoresistance signal, as described in U.S. patent application Ser. No. 12/247,096. The signal, as a function of the electron/hole densities, is characterized by a series of corresponding maximums. Such maximums are achieved when the parameters (e.g., electron or hole densities) are in specific ranges. The measured resonant response function is defined by the frequency of the incident radiation and can be determined based on the above equation (1). The shape of the resulting resonant function allows to calculate the frequency or the spectrum of the incident radiation. 
         [0030]    A spectroscopic analysis of incident giga-terahertz radiation has been conducted for a number of device embodiments. The spectrum of the incident radiation is obtained by tuning one of the parameters, which influence the plasmon dispersion or localization. For the first device family schematically depicted in the  FIGS. 1-10 , the parameter that is being tuned is the charge density in the plasmonic cavity. For other embodiments shown in the  FIGS. 11-13 , the parameter being tuned is either magnetic field, dielectric environment, size of the plasmonic cavity. For simplicity, in the following discussions one layer two-dimensional charge carrier system is used. However it is to be understood that the same principles and results also apply to other types of charge carrier layers specified in claims. 
         [0031]    Turning to the drawings,  FIGS. 1-2  show a solid state device  1  according to one embodiment of the instant invention. Device  1  includes a structure  2  with embedded back gate  3 , and a two-dimensional charge carrier layer  4 . The device charge carrier layer  4  includes a defect structure  5 . A defect and/or defects may be any inhomogeneity in a two-dimensional charge carrier layer and/or in its environment. For example, such a defect in the environment can be introduced by creating a step in the solid state crystal cap layer. In general, defects can be made or introduced using any approach now known or later developed. Another example of a defect can be realized in one or more of the following forms: an etched area, a charge carrier density inhomogeneity, a restriction or expansion, metallic layer (e.g., deposited on the structure), an impurity doping, a charge carrier mobility defect, a dielectric environment defect, a structural defect, etc. The device embodiment illustrated in  FIGS. 1-2  contains a defect structure  5  (charge carrier density inhomogeneity), which is created by gate  6  deposited on the top of the semiconductor crystal or any other suitable solid state device. By applying voltage to gate  6 , the region of the two-dimensional system under gate  6  can be depleted or enriched, which adds a density inhomogeneity to the two-dimensional charge carrier layer  4  and forms the defect. The device  1  terminates at each end with contacts  7  and  8 . Contact  9  is connected to back gate  3 . Back gate  3  is intended to sweep the carrier density in the charge carrier layer via field effect. 
         [0032]      FIG. 2  shows that defect  5  and contacts  7  and  8  confine the regions of the two-dimensional system of lengths L 1  and L 2 . Under incident radiation, these regions act as resonant plasmonic cavities tuned by the back gate potential. The known function of the device response to the tuned carrier density allows to calculate the radiation spectrum. 
         [0033]    Additionally, device  1  may include one or more charge carrier layers and/or contacts not shown in  FIGS. 1 and 2 . Further, the tunability of the plasmonic cavity by back gate  3  is only illustrative of various configurations for tuning the charge density in the cavity. To that extent,  FIGS. 3 and 4  show an alternative semiconductor device  10  according to another embodiment of the invention. In this embodiment, carrier density in the plasmonic cavity is tuned by top gate  11 . Top  11  gate is located on dielectric layer  12 , which isolates gate  11  from two low-lying gates  13  and  14 . These gates serve as boundaries for the plasmonic cavity of length L. By applying a voltage to gates  13  and  14 , the region of the two-dimensional system under the gates could be depleted or enriched, which adds inhomogeneity to the two-dimensional layer  4  and forms defects  15  and  16 . 
         [0034]    For certain defect types, dielectric layer  12  is not necessary. For example  FIGS. 5 and 6  illustrate such a case. In this embodiment  35  the gate  11 , which sweep carrier density in the plasmonic cavity is located directly on top of the structure  2 . The defects  36  and  37  can be implemented as two restrictions. They serve both as boundaries to the plasmonic cavity of length L and take part in the rectification process. 
         [0035]    Referring to  FIGS. 1 through 6 , it is understood that while device  1  is shown as having one defect  5  and devices  10  and  35  are shown as having two defects  15  and  16  ( 36  and  37 ), any number of defects and plasmonic cavities of different kinds may be used in any device embodiment. For example,  FIGS. 7 and 8  show device embodiment  17  having multiple gate defects  18  (formed by gates under voltage  19 ) placed at arbitrary positions along the two-dimensional charge carrier layer  4 . Gates  19  restrict a series of plasmonic cavities. It is understood that various defects  18  and gates  19  could be independently interconnected by metallization, by the two-dimensional charge wires or by any other method now known or later developed. These interconnections are not shown in the drawings to avoid confusion. In addition, although not shown, embodiments of the device in accordance with the instant invention may comprise arrays of elementary interconnected devices. 
         [0036]    In the device embodiments  1 ,  10 ,  35  and  17  the defects play a dual role. They form a plasmonic cavity, and they also rectify the oscillating plasmon electric field. At the same time, the rectifying defects may be placed away from the plasmonic cavity as it shown in  FIG. 9-10 . Device  20  ( FIG. 9-10 ) comprises a cavity, which is formed by tuning gate  11  and two density inhomogeneity defects  23  and  24 , which are separated from the cavity by small slits  38  and  39 . The density inhomogeneity defects  23  and  24  are produced by two gates  21  and  22 . This embodiment is particularly important for the device realization based on MOSFET structures. 
         [0037]    As discussed above, the tunability of a plasmonic cavity can be implemented in a number of ways. In device  25  ( FIG. 11 ) the dielectric environment of the plasmonic cavity is swept, for example, by means of a conducting microcantilever  26 . This cantilever serves as a floating gate with a controllable distance from the 2D charge layer. In arrangement  27  ( FIG. 12 ), the tunable parameter is the magnitude of the magnetic field. Also shown in  FIG. 12  is an independent source  28  for generating (by well-known methods) a magnetic field of a given magnitude, for example, in the direction of arrow  29 , or has at least a field component extending in the direction of arrow  29 . The tunability of the plasmonic cavity can also be accomplished by adjusting the size of the cavity. For example, device  30  ( FIG. 13 ) can have moving gates  31  and  32 , which change the size of the cavity. 
         [0038]    To couple the incident tera-gigahertz radiation to the plasma excitations, which propagate in the device, a system of antennas can be used. An example of such device with antennas  34  is shown in  FIG. 14   
         [0039]    Although all the embodiments described above comprise a two-dimensional charge carrier layer of a rectangular shape, many other shapes can be chosen and optimized. Also any number of contacts of arbitrary shape to charge carrier layer can be used. 
         [0040]    The device response (e.g. photovoltage, photocurrent, photocapacitance, photoinductance), induced by the incident giga-terahertz radiation has been experimentally observed in a number of device embodiments. Most experiments have been performed with the use of an 18-nanometer wide GaAs/AlGaAs quantum well, which was located 135 nm underneath the crystal surface. The first embodiment, which was experimentally studied by the inventors, is the same as device  1  and is further depicted in the inset of  FIG. 15 . The electron density in the device was tuned from 0.5×10 11  cm −2  to 4.5×10 11  cm −2  by applying voltage to the back gate. An n +  GaAs back gate was grown in-situ at a distance of d=765 nm below the quantum well. The two-dimensional electron layer, which is part of the device, has a shape of a stripe with the following geometrical dimensions: the stripe width W=50 μm and the length of the active plasmonic cavity L=400 μm. The sample was placed either in an oversized 16×8 mm waveguide or in an optical cryostat behind the window. In the case of the cryostat, terahertz radiation was focused at the sample by means of quasi-optical reflectors and lenses. A set of backward wave oscillators operating in the frequency range of 10 GHz to 1 THz and with a typical output power from 10 to 0.1 mW was used to illuminate the device with continuous wave radiation. 
         [0041]      FIG. 15  depicts typical functions of the photovoltage device response as a function of the electron density for three different frequencies. The photovoltage is measured between contacts  7  and  8 . The traces have been offset vertically for clarity and the arrows indicate the zero signal level when no radiation is incident on the sample. In each trace, a series of oscillations is observed. The maxima originate from the constructive interference of plasma waves with wave vectors q=nπ/L (n=1, 2, 3 . . . ) in the active plasmonic cavity formed between contact  7  and gated region  6  of the two-dimensional electron layer. The photovoltage oscillations in  FIG. 15  change with the frequency of the incident radiation. Hence, the device may serve as a “spectrometer-on-a-chip”. The frequency of radiation can be easily calculated from the density positions of the maxima, provided the plasmon spectrum is known. 
         [0042]      FIG. 16  demonstrates the same device  1  embodiment operation, but with another geometrical dimensions: the stripe width W=50 μm an and the length of the active plasmonic cavity L=100 μm. First, it is apparent from  FIG. 16  that plasmonic cavities with different sizes cover different frequency ranges. Second, the higher device operation frequency, the smaller the plasmonic cavity which is needed. This relationship between the operation frequency and the size of the cavity is mainly caused by the fact that the plasmon coherence length degrades strongly as the frequency increases. 
         [0043]    If an external magnetic field is introduced, equation (1) no longer describes the plasmon dispersion in the two-dimensional electron layer and certain changes in the photovoltage oscillations are bound to occur.  FIG. 17  illustrates the oscillations observed in the presence of a magnetic field. The bottom part of  FIG. 17  depicts the photovoltage device response is shown when the perpendicular component of the magnetic field is kept at a fixed level. Due to changes in the plasmon spectrum, the distance between oscillation maxima shrinks as the magnitude of the magnetic field increase. The top portion of  FIG. 17  shows the operation of the device embodiment  1 , in which the device tunability is implemented by means of sweeping the magnitude of the magnetic field. The photovoltage oscillations appear to be B—periodic, with a period inversely proportional to the radiation frequency. 
         [0044]      FIG. 18  illustrates a device, in which two gate defects  15  and  16  (e.g.  FIGS. 3 ,  4 ) are implemented on a single device  10 , and the density of the central plasmonic cavity is tuned by top gate  11 . The width of the plasmonic cavity equals to W=50 μm, and its length L=100 μm. The upper curve represents a device oscillatory response, when depleting voltage is applied to gate  13  and no voltage is applied to gate  14 . The bottom curve represents the opposite case when depleting voltage is applied to gate  14  and no voltage is applied to gate  13 . The oscillations reveal the spectrum of the monochromatic radiation of frequency 87.5 GHz incident on the device. The device embodiment discussed with respect to  FIG. 18  provides an opportunity to measure photo-response from a separate plasmonic cavity restricted by two easily-tuned defects  15  and  16 , as opposed to a defect and non-tunable contact boundary. Due to the ability to adjust two independent parameters, the latter device implementation provides a better way to control the plasma waves propagating in the plasmonic cavity. Better controllability of the device is illustrated by the fact that the oscillations change their polarity when the working defect is altered. 
         [0045]    The device temperature dependency of the spectrometer operation for a device embodiment  1  with plasmonic cavity size L=50 μm is presented in  FIG. 19 . The value plotted on the vertical axis represents an oscillation amplitude near n S =4×10 11  cm −2  at B=0.5 T for the radiation frequency 90 GHz. The oscillation period does not depend on the temperature, and the amplitude only slightly decreases when the temperature increases from 4.2 K to 70 K. For a successful operation an apparatus can comprises a device for cooling said solid state structure. That can be accomplished in a number of ways. For example, by using nitrogen cooling system or Peltier refrigerator. 
         [0046]    The experimental data above have been presented only illustrate the instant invention. It is not intended to be exhaustive or to restrict the scope of the invention to the disclosed examples and embodiments. While various aspects of the invention have been discussed in terms of an electromagnetic wave detection method, it should be understood that the disclosed findings, methods and discoveries may be utilized in other radiation-based technologies. These technologies may encompass generation, mixing, and/or frequency multiplication of radiation. 
         [0047]    Thus, the foregoing description of various embodiments of the invention has been presented only for the purposes of illustration and description and not to limit the scope of the claimed invention. Therefore, it must be understood that many modifications and variations may be possible within the scope of the instant invention. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the claims that follow.