CN102279476B - High-speed electrically-modulating terahertz modulator - Google Patents

Abstract

The invention discloses a high-speed electrical control terahertz modulator, comprising a dielectric substrate (4), the dielectric substrate (4) is a material transparent to terahertz waves, and high electron mobility transistors ( 3) The formed array, the surface of the dielectric substrate and the surface of the high electron mobility transistor array are also attached with a frequency selective surface structure (2), the frequency selective surface structure is a patterned conductive film with a bandpass filter structure, and the conductive The thin film corresponds to each high electron mobility transistor and forms the source (b), drain (c) and gate (a) of the high electron mobility transistor respectively; the electron mobility of the high electron mobility transistor is in 1500cm ² /Vs or more. The invention realizes the high-speed modulation effect on the terahertz wave amplitude in the electric modulation mode, the modulation speed can be greater than 10MHz, and the relative modulation depth is greater than 50%.

Description

High-speed electronically regulated terahertz modulator

technical field

The invention particularly relates to a device capable of high-speed and high-efficiency modulation of the terahertz wave amplitude through an electrical control method, which can be applied to the fields of terahertz communication, terahertz imaging and the like.

Background technique

Terahertz (THz) waves are electromagnetic waves with a frequency in the range of 0.1 THz to 10 THz, and wavelengths in the range of 30 μm to 3 mm, between millimeters and infrared. Compared with electromagnetic waves in other bands, terahertz waves have low photon energy It has great scientific research value and broad market prospects in basic research fields such as physics, chemistry, and medical science, as well as applied research fields such as safety inspection, environmental monitoring, and communication. With the rapid development of THz technology, the gap in the THz gap is gradually being filled, and semiconductor materials combined with photonics and electronics applied to THz technology have achieved major breakthroughs in the development of this field, such as quantum cascade lasers, Schottky diodes and Bolometer. While developing corresponding THz light sources and high-sensitivity detectors, high-speed modulators are indispensable core devices. However, people are still relatively lagging behind in controlling and manipulating THz wave technology, so it is very urgent to realize high-speed THz modulators.

The traditional THz modulator is mainly a mechanical modulation chopper, and the modulation frequency is from several Hz to several kHz. In recent years, people have developed bandgap shifting THz modulators based on THz waves of artificial electromagnetic materials such as photonic crystals and metamaterials. The modulation depth can reach 30 dB and the modulation speed is about 10 kHz. However, The bandgap edges of these THz modulators are difficult to achieve steepness, and indicators such as modulation depth and insertion loss are not very ideal. The THz modulator based on the artificial structure of Metamaterials is also a research hotspot at present, but the modulation speed of such devices is still low, and the performance index is difficult to improve. In fact, at present, the modulation speed of THz modulators through electrical control is only a few hundred Hz ~ kHz, which is difficult to meet the requirements of THz applications in the field of communication and imaging. key.

Contents of the invention

The purpose of the present invention is to propose a high-speed electrically controlled terahertz modulator with a modulation depth of >50% and a modulation speed of >10 MHz, thereby overcoming the deficiencies in the prior art.

In order to realize the above-mentioned purpose of the invention, the present invention has adopted following technical scheme:

A high-speed electrically regulated terahertz modulator, comprising a dielectric substrate formed of a material transparent to terahertz waves, characterized in that: the surface of the dielectric substrate is distributed with an array formed by a plurality of high electron mobility transistors, A frequency selective surface structure is also attached to the surface of the dielectric substrate and the surface of the high electron mobility transistor array, and the frequency selective surface structure includes a patterned conductive film with a bandpass filter structure, and the conductive film corresponds to each high electron mobility Transistor parts respectively constitute the source, drain and gate of the high electron mobility transistor;

The electron mobility of the high electron mobility transistor is above 1500 cm ² /Vs;

The modulation speed of the high-speed electrically controlled terahertz modulator reaches above 10 MHz.

Specifically, the frequency selective surface structure is a grid-like grid structure formed by a metal film, and the sides of the grid-like grid structure correspond to each high electron mobility transistor and form the source of the high electron mobility transistor respectively. , a drain and a gate; the widths of the source and drain are 1 μm, respectively, and the width of the gate is 7 μm.

The transconductance of the high electron mobility transistor is above 300 mS/mm.

A high-speed electrical control terahertz modulation device is characterized in that it includes:

High-speed electrically regulated terahertz modulators as described above;

And, a modulation signal source for inputting a voltage signal for regulating channel conductance to the gate of the high electron mobility transistor in the high-speed electric regulation terahertz modulator.

The invention adopts the combination of the frequency selective surface structure and the high electron mobility transistor. The transistor is designed as a component of the frequency selective surface structure. By applying the pulse voltage signal to the gate of the transistor to control the high-speed change of the channel conductance on and off, In this way, the transformation of the frequency selective surface structure with filtering effect can be dynamically switched, and then the control of the reflection and transmission of electromagnetic waves can be realized.

Furthermore, the present invention uses an aperture-type frequency-selective surface structure to modulate THz waves. For the incident electromagnetic wave under resonance, the aperture-type FSS exhibits the characteristics of total transmission, forming a band-pass filter structure. The filtering mechanism can be described as follows: when low-frequency electromagnetic waves irradiate the aperture-type frequency-selective surface, a large range of electron movement will be excited, so that the electrons absorb most of the energy, and the induced current along the gap is small, resulting in a relatively small transmission coefficient. As the frequency of the incident wave continues to rise, the range of electron movement will gradually become smaller, and the current flowing along the gap will continue to increase, thereby improving the transmission coefficient. When the frequency of the incident electromagnetic wave reaches a certain value, the electrons on both sides of the slot just move back and forth driven by the electric field vector of the incident wave, forming a large induced current around the gap. Since electrons absorb a large amount of energy from the incident wave, they also radiate energy outward. The moving electrons radiate the electric field to the transmission direction through the gaps of the dipole slots. At this time, the reflection coefficient of the dipole slot array is the lowest, and the transmission coefficient is the highest. When the frequency of the incident wave continues to increase, the range of motion of the electrons will decrease, and the current around the gap will be divided into several segments. The electromagnetic waves radiated by the electrons through the slot gap will decrease, and the transmission coefficient will decrease. For the induced current generated on the metal plate far away from the gap, the electromagnetic field is radiated in the reflection direction, and because the electric field change cycle of the high-frequency electromagnetic wave restricts the movement of electrons, the radiation energy is limited.

Compared with the prior art, the present invention has at least the following advantages: it fully combines the frequency-selective surface structure’s effect on terahertz filtering and the high-speed gate electrical control characteristics of high-electron mobility transistors, and realizes the adjustment of terahertz wave amplitude by electrical modulation. Compared with the existing terahertz modulators such as choppers, photonic crystals and metamaterials with mechanical modulation methods, the modulation speed of the terahertz modulator of the present invention can be greater than 10 MHz, and the relative modulation depth is greater than 50%. That is, high-speed modulation of terahertz waves is realized.

Description of drawings

Fig. 1 is a schematic structural diagram of a high-speed terahertz modulator in a preferred embodiment of the present invention;

Fig. 2 is a schematic structural diagram of the high electron mobility transistor in Fig. 1;

Figure 3 is a schematic diagram of the principle of testing the high-speed terahertz modulator shown in Figure 1;

Fig. 4 is a graph showing the change in channel conductivity controlled by a pulse voltage signal applied to the gate of a transistor in a preferred embodiment of the present invention;

Fig. 5 is a graph of simulation results of a high-speed terahertz modulator in a preferred embodiment of the present invention;

The components shown in the above figures and their reference signs are: terahertz wave light source 1, frequency selective surface 2, high electron mobility transistor 3, dielectric substrate 4, modulation signal source 5, terahertz detector 6, grid Pole a, source b, drain c.

Detailed ways

The high-speed electrical modulation terahertz modulator of the present invention includes a dielectric substrate, a high electron mobility transistor array on the dielectric substrate, and a frequency-selective surface structure attached to the dielectric substrate and the surface of the transistor.

The foregoing dielectric substrate is formed of a dielectric material with relatively high transmittance to terahertz waves, and is easy to grow high electron mobility transistors and metal materials.

The aforementioned high electron mobility transistor array is a cell array composed of multiple transistors of the same shape and size, and the array period matches the frequency selective surface structure. These transistors can be prepared on a dielectric substrate by methods such as molecular beam epitaxy, and prepared into an array structure by inductively coupled plasma etching, photolithography, and the like.

The aforementioned frequency-selective surface structure is a network structure formed by a metal film of a certain size according to a special layout. For example, a gold film can be prepared by electron beam evaporation technology, and the gold film can be photoetched into a certain line width by micro-nano processing technology. A network structure formed by a specific layout, wherein the micro-nano processing technology includes cleaning dielectric substrates, splitting, thinning and polishing, electron beam evaporation and UV lithography, etc.

Specifically, let’s look at the structural features and functions of the high-speed terahertz modulator:

The function of the aforementioned frequency selective surface is to form a band-pass filter effect on the terahertz wave, and the band-pass frequency range of the frequency selective surface structure designed in the present invention is preferably 0.82-0.92 THz;

The role of the high electron mobility transistor is that the channel conductance presents two states of on and off under the regulation of the gate voltage, and realizes fast switching between two different frequency-selective surface structures;

The role of the dielectric substrate is to carry the core unit transistor of the modulator and the frequency selective surface structure, and has a high transmittance to the terahertz wave;

The role of the modulation signal source is to input a high-frequency voltage signal to the gate of the high electron mobility transistor to regulate the conductance of the channel.

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and a preferred embodiment.

Referring to Figures 1-2, the high-speed terahertz modulator of this embodiment includes a sapphire single crystal dielectric substrate 4 and an AlGaN/GaN high electron mobility transistor array epitaxially grown on the surface of the dielectric substrate, and the dielectric substrate 4 and the transistor array are attached There is a frequency selective surface structure 2, the frequency selective surface structure 2 is a patterned metal thin film, and the part corresponding to each transistor constitutes the source b, drain c and gate a of the crystal respectively.

The preparation, testing and analysis process of the high-speed electrical modulation terahertz modulator is as follows:

Step 1: Using the software FDTD Solution based on the finite difference in time domain, establish the model structure of the terahertz wave modulator, in which the metallic frequency selective surface structure 2 is set to 200 nanometers thick gold. By optimizing the gate G, source S, and drain D dimensions, the simulation obtains the maximum relative modulation depth when the channel conductance is turned on and off in a gated high electron mobility transistor. The best parameters obtained are: source width S = 1 micron, drain width D = 1 micron and gate width G = 7 microns, period cell spacing d = 50 microns, modulator relative modulation depth in the frequency range 0.82-0.92 THz 75%.

Step 2: Prepare a photolithography plate, use the inductively coupled plasma etching method, etch the transistor substrate mesa, use photoresist as a mask, etch out the transistor cell array, as shown in Figure 1, the area of each transistor 3 is 6 x 8 µm ² .

Step 3: Using ultraviolet exposure photolithography technology, prepare a photoresist inverse to the frequency selective surface structure on the etched transistor substrate mesa, with a thickness of about 1.5 microns.

Step 4: Deposit a gold film with a thickness of about 200 nm on the prepared photoresist substrate by using electron beam evaporation metal technology.

Step 5: The substrate prepared with the gold film is cleaned in acetone for about 30 minutes, and the photoresist and the gold film on it are washed away, leaving the frequency selective surface metal structure.

Step 6: The dielectric substrate is a sapphire single crystal substrate, the dielectric constant of this material is about 11.9, the area of the split is 10×10 mm ² , and the thickness is reduced to about 90 microns.

Step 7: Fix the modulator and place it between the terahertz light source 1 and the detector 6. The electrical control signal of the modulator is output from the signal source 5, and loaded to the gate of the transistor 3 through the coaxial line and metal leads to control high electron migration. The switching on and off of channel conductance in rate transistors. The terahertz detector sensitively detects the terahertz wave signal passing through the modulator.

FIG. 4 shows the curve of channel conductivity change controlled by applying a pulse voltage signal to the gate of the transistor in this embodiment, wherein the change of channel conductivity is equivalent to the conversion of two frequency selective surface structures A and B. This is also the core idea of the present invention, that is, using the filtering effect of the frequency selective surface structure on the terahertz wave and the high-speed regulation performance of the gate voltage of the high electron mobility transistor on the channel conductance, the design of two The periodic structure A of the reflection effect and the periodic structure B of the transmission effect, the high electron mobility transistor is designed as a component of the periodic structure, and the channel conductance G is regulated by the gate modulation signal (V _g -V _b , V _g +V _a ) The rate change (G=0～4000 S/cm) realizes high-speed dynamic switching between structures A and B, and high-speed electrical modulation of the terahertz wave amplitude.

Figure 5 shows the simulation calculation results of this embodiment. In the designed frequency range of 0.82-0.92 THz, when the source and drain widths are 1 micron and the gate width is 7 microns, the modulation depth of the modulator reaches 75%. The grid modulation signal frequency of 10 MHz can be clearly reflected in the transmission signal, indicating that the grid modulation speed can reach at least 10 MHz.

Referring to Figure 3, when the terahertz wave emitted by the light source 1 is vertically incident on the modulator plane, only electromagnetic waves with a frequency range of 0.82-0.92 THz can pass through due to the band-pass filtering effect of the frequency selective surface. When the frequency is lower than 0.82 THz, The transmission rate is very low and close to zero (see Figure 5). When the frequency increases to the design frequency band, the electromagnetic wave can pass through the modulator, and the transmission rate increases rapidly. As the frequency of the electromagnetic wave continues to increase, the electromagnetic wave of a certain wavelength and the frequency selective surface structure occur. Resonance, the electromagnetic wave transmission rate reaches a maximum. From simulation and experiment result, adopt the frequency selective surface structure size that the present invention designs, modulator relative modulation depth reaches 75%, is applied to the 10 MHz modulation signal of gate and can clearly reflect on the radio frequency spectrum, illustrates that this modulator modulation The speed has reached 10 MHz, that is, "high speed" as described in this invention.

The high-speed electrical modulation terahertz modulator of the present invention fully combines the advantages of high-speed gate-controlled modulation characteristics of high-electron mobility transistors and frequency-selective surface structure filtering characteristics, has the characteristics of high modulation speed and large modulation depth, and can make up for the existing high-speed Shortcomings of terahertz modulators. The high-speed electrical modulation terahertz modulator device is simple to operate, low in cost, easy to integrate, and has great scientific research value and market prospects in terahertz communication and terahertz imaging.

Certainly, the present invention can also adopt other transistors with high electron mobility and high gate control capability, such as transistors composed of semiconductor materials such as AlGaAs/GaAs or Si, etc. any restrictions. All equivalent changes or modifications made according to the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (4)

1. A high-speed electrical control terahertz modulator, comprising a dielectric substrate (4), the dielectric substrate (4) is formed of a material transparent to terahertz waves, characterized in that: the surface of the dielectric substrate is distributed by a plurality of high An array of electron mobility transistors (3), the surface of the dielectric substrate and the surface of the high electron mobility transistor array is also attached with a frequency selective surface structure (2), and the frequency selective surface structure includes a patterned conductive a thin film, the conductive thin film corresponding to each high electron mobility transistor partially constitutes the source (b), drain (c) and gate (a) of the high electron mobility transistor; The electron mobility of the high electron mobility transistor is above 1500 cm ² /Vs; The modulation speed of the high-speed electrically controlled terahertz modulator reaches above 10 MHz. 2. The high-speed electrically controlled terahertz modulator according to claim 1, characterized in that: the frequency selective surface structure (2) is a mesh grid structure formed by a metal film, and the sides of the mesh grid structure Corresponding to each high electron mobility transistor, the source (b), drain (c) and gate (a) of the high electron mobility transistor are respectively formed; the width of the source and drain is 1 μm, and the gate The width of the poles is 7 μm. 3. The high-speed electrically controlled terahertz modulator according to claim 1, characterized in that: the transconductance of the high electron mobility transistor 3 is above 300 mS/mm. 4. A high-speed electrical control terahertz modulation device, characterized in that it comprises: The high-speed electrical control terahertz modulator according to claim 1; and a modulation signal source (5) for inputting control voltage signals to the gate and source of the high-speed electrical control terahertz modulator.

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