WO2012080105A1

WO2012080105A1 - Antenna for transmitting and receiving ghz and/or thz radiation with an optimized frequency characteristic

Info

Publication number: WO2012080105A1
Application number: PCT/EP2011/072276
Authority: WO
Inventors: Christian Jansen; Maik Scheller; Martin Koch
Original assignee: Philipps Universität Marburg
Priority date: 2010-12-15
Filing date: 2011-12-09
Publication date: 2012-06-21
Also published as: EP2466686A1

Abstract

The invention relates to a photoconducting terahertz antenna consisting of a substrate material with a low-resistance applied, preferably concomitantly applied, metal structure for voltage supply or power supply and a photoconducting excitation locus, characterized in that, in addition to the low-resistance structure for voltage supply or power supply, a resonator or a plurality of resonators are also applied in the vicinity of the photoconducting excitation locus, which improves the frequency-selective antenna gain and/or the emission characteristic of the antenna. The properties brought about by the resonators can furthermore be configured such that they can be modulated electrically, optically or by using dielectrics. In addition, such structures can be used as sensor elements, since the resonance response of an antenna according to the invention is sensitive to applied probes.

Description

Patent application

TITLE

Antenna for transmitting and receiving GHz and / or THz radiation with optimized frequency characteristics

[Description and Introduction of the General Field of the Invention]

The present invention relates to a photoconductive antenna for emitting or receiving terahertz radiation which has an increased antenna gain for one or more frequency bands. This increased antenna gain according to the invention by in the vicinity of the photoconductive excitation

implemented resonators implemented. THz systems based on photoconductive

Antennas are used, for example, in non-destructive testing and safety technology. In comparison with the prior art, antennas designed according to the invention can decisively improve the performance of THz systems, inter alia with regard to the signal-to-noise ratio, and thus open up new fields of application for the THz technology in which the dynamic range of existing systems is insufficient.

[Prior Art] The invention relates to a photoconductive antenna for emitting or

Receiving terahertz radiation.

Terahertz radiation is electromagnetic radiation with a frequency of about 0.1 THz to about 100 THz. Applications for terahertz Systems can be found in the security sector (eg the detection of hidden dangerous goods or the identification of liquid explosives) or in the quality control of food and plastic products. Furthermore, molecular vibrations of some substances in the terahertz frequency range, so that in addition to the technical-commercial and a scientific interest in high-performance terahertz emitters and detectors for spectroscopic applications exists.

It is known in the art that photoconductive antennas can be used to emit and detect terahertz radiation. There are two different approaches:

On the one hand, the photoconductive antenna can be used as a photomixer for

Continuous wave emission and detection are used. In this case, the optical excitation takes place by the superimposition of at least two laser modes of different frequencies (see US Pat. No. 5,789,750 A). The

Differential frequency between the laser modes is emitted or detected by the antenna. On the other hand, the antenna can be used as a photoconductive switch for generation and detection of terahertz pulses. This switches a short laser pulse whose duration in the range of femtoseconds to

Picoseconds, the photoconductivity, so that terahertz radiation can be emitted or detected for a short time (see Patent WO 2008/054 846 A2). According to the state of the art, a photoconductive telescope antenna consists of a high-resistance, semiconductive layer, which has the shortest possible length

Charge carrier recombination time in the femto to picosecond range and the highest possible charge carrier mobility. On this layer, an antenna structure of an electrically conductive material is applied. Common technical embodiments of this antenna include, for example, spiral antennas (KR 10 2005 0015364 A, JP2001060821 A, JP2008028872 A, JP 58123203 A, CA 2 292 635, CA 2 575 130), bowtie antennas (US 2006/0152412 A1) and dipole antennas (US 5 729 017 A, WO 02/060017 A1). These structures have a photoconductive excitation site in the form of a gap in the

Metallization on. Its conductivity is determined by the incident laser radiation, since the photon energy of the laser is greater than the band gap of the semiconductive layer and thus free charge carriers are generated upon illumination (compare US Pat. No. 5,729,017 A, WO 03/047036 A1). In addition to the

Antenna structure are applied electrical contacts and leads on the semiconductive layer.

In the case where the photoconductive telether antenna is used as an emitter, a bias voltage is applied to the antenna. If charge carriers are now generated by the laser radiation incident in the photoconductive gap, they experience an acceleration in the externally impressed electric field, which causes the emission of the terahertz radiation.

If the photoconductive telescope antenna is used as a detector, instead of a voltage source, a highly sensitive current amplifier is connected to the antenna. The charge carriers generated by the laser beam are accelerated by the incident terahertz field. The measurable photocurrent is thus a measure of the incident terahertz field strength. A disadvantage of terahertz antennas according to the prior art is in particular the inefficient conversion of laser to terahertz power. This results from the spectrally relatively flat but very broadband antenna gain. Depending on the laser source, the antennas may emit THz waves in a frequency range from 0.1 THz up to a few 10 THz, without the antenna gain in individual frequency ranges experiencing a noticeable increase caused by the metallization structure.

Another disadvantage of terahertz antennas according to the prior art are the static, non-modulatable nature of the antenna gain and the static, non-modulatable spatial radiation profile.

[Task]

The object of the present invention is to overcome the disadvantages of the prior art.

[Solution of the task]

This object is achieved by an antenna according to claim 1. Surprisingly, it has been found that such an antenna, which in addition to the features of a photoconductive terahertz antenna according to the prior art additionally has at least one arranged resonator in the vicinity of the photoconductive excitation locus, is suitable for improving the frequency-selective antenna gain and / or the emission characteristic. For this purpose, at least one of the resonators, measured from the excitation location, in a radius which corresponds at most twice the resonance wavelength, to be arranged away.

The resonators according to the invention consist of conductive areas

(Resistance below 1 k £ 2 / cm), which can be excited to an electrical oscillation at one or more resonant frequencies. The lateral dimension of the essentially planar resonators is at least 1/50, but not more than one, preferably 1/2 and more preferably one third to one fourth of the resonance wavelength.

In the case of an antenna for 300 GHz, a resonance wavelength of 1 mm results and with an antenna for 1 THz, a resonance wavelength of 300 μιτΊ results. Accordingly, according to the invention, the dimensions of the at least one resonator and the spatial proximity to the radiation location result. There are several resonator types, also in combination, can be used.

In particular, periodic arrangements of resonators (also known as frequency-selective surfaces or electronic-bandgap structures) can be used according to the invention in order to further increase the conversion efficiency from laser to terahertz power at the resonant frequency. Embodiments of the resonator elements include, among others symmetrical and asymmetrical single and double gap ring, as well

Rectangular resonators.

The conductivity of the regions forming the resonator structures is preferably achieved by metallization (for example with gold, titanium, copper, platinum, silver, aluminum, nickel, iron or a combination of these elements).

These can be applied wet, electrochemically, by evaporation (CVD, PVD, MOCVD), by doping or by printing processes (screen printing, gravure printing, inkjet printing, laser printing).

Particularly preferably, the resonators are in the same spatial level l o as the leads, so that no additional effort in the structuring of the antenna is formed.

By means of doping, switchable, electrically conductive resonators are directly in the

Semiconductor feasible. The circuit is performed electrically and or photoelectrically.

As the semiconductive material, gallium arsenide, indium gallium arsenide, indium aluminum arsenide, indium antimonide 20, or sapphire grown silicon films are preferably used singly or in combination. Other INA or ΙΙΛ / Ι

Workers may also be employed insofar as they are short

Have carrier lifetimes and a bandgap energy that is lower than the photon energy of the laser used, have. The semiconducting materials are preferably grown at low temperature (narrow. low temperature) or ion implanted to a short

To achieve charge carrier lifetime.

The antenna according to the invention is suitable for transmitting and receiving terahertz radiation. An advantage of emitting terahertz radiation is that the transmission power of the antenna in one or more narrow frequency band / bands (bandwidth of 1 to 100 GHz) is significantly enhanced by the use of the resonators. As a result, in these spectral regions the

Signal / noise ratio can be increased. If an antenna according to the invention is used to receive terahertz radiation, then it has an increased reception sensitivity in one or more narrow frequency bands (bandwidth from 1 to 100 GHz), which likewise achieves an improvement in the signal-to-noise ratio. If the antenna is excited by continuous wave laser radiation, it is particularly advantageous if the resonant frequency of the resonators is close to the resonator

Difference frequency of the exciting laser modes is because then a particularly efficient conversion of laser to terahertz performance is possible. It is favorable if the resonant structure increases the antenna gain in those

Attenuates areas in which unwanted frequency components would arise, which complicate the signal detection and / or evaluation.

Particularly preferably, the resonant structures include those which can be modulated in their quality (their spectral resonance width), for example via a Change in the impedance between two sub-segments of the resonator or a change in the impedance between adjacent resonator elements, and thus a modulation of the resulting antenna gain of

enable photoconductive antenna.

Further advantageous embodiments of the terahertz antenna according to the invention, which can tune the main emission and reception direction of the antenna by a spatial angle. This tunability can be, for example, by influencing the impedance between individual

Resonator segments or between a plurality of resonators can be achieved.

These changes in impedance can be effected by an optical excitation of a photoconductive excitation site within the resonator structure or by an electronic component which, in terms of its capacitance, its inductance or its ohmic resistance, is produced by applying a

Control signal can change.

Further advantageous embodiments of the terahertz antenna according to the invention with tunable resonant frequency. A tuning of the

Resonant frequency is among other things via electrical or optical

Switching ranges possible. For example, the resonators may consist of a plurality of ring segments, wherein two rings can be short-circuited to each other over a switching range, thus changing the To allow resonance condition. In this embodiment, the effective length of the resonator would be increased so that the resonant frequency would be shifted to lower frequencies. By several switchable

Resonator elements, it is thus possible to set different resonance frequencies.

It is also possible to achieve a tuning of the resonance frequency by means of dielectrics applied to the resonators, such as oils or polymers. The applied dielectrics have a different permittivity from air, so that the resonant condition of the resonators is shifted towards lower frequencies. When using an oil mixture of at least two portions with different Permittivitäten a continuous tuning of the resonant frequency is possible. Furthermore, liquid crystals (English Liquid Crystals) can be applied to the resonators. These change their dielectric properties when an electric field is applied. This will change the

Resonance condition of the resonators and thus a shift of

Resonance frequency causes.

Furthermore, structures according to the invention can be used as sensors which allow conclusions to be drawn about the sample material by changing the resonance properties when a sample is applied. In particular, the resonance frequency and the resonance width vary characteristically according to the material properties of the sample depending on the sample material. Particularly preferably, the leads of the antenna structure further

Matching elements, which minimize the parasitic resonances by back reflections from the electrical contacts. Furthermore, the matching elements can additionally be provided with loss elements, such as absorbing coatings and / or low-resistance metal structures, in order to further minimize the back reflections.

The embodiment according to the invention is explained below, wherein the invention comprises all the following preferred embodiments individually and in combination.

[Embodiments]

An arrangement of components (transmitter and receiver) is constructed which emits and detects THz radiation.

In Fig. 1, a photoconductive terahertz antenna is sketched. This consists of a semiconductive substrate (eg gallium arsenide, silicon on sapphire or indium gallium arsenide) 101, on which one or more electrical leads 102 are applied. To operate the antenna, laser radiation 104 is directed to a photoconductive region 106. The antenna structures according to the invention additionally include one or more conductive resonators 105 (see FIG. 2). The antenna can be switched as a receiver or as a transmitter. By applying a voltage 109 to the electrical leads 102, the antenna emits terahertz radiation and operates as a transmitter. Alternatively, a meter 103 may be connected to the leads 102 to detect terahertz waves.

The partial areas of the conductive resonators 105 can be connected via switching area 107. The conductivity of the switching ranges can be modulated by applying a voltage or a current to them. Further, the switching regions 107 may be realized in the form of photoconductive regions whose conductivity can be switched optically (e.g., by a laser). By modulating the switching ranges, the spectral characteristics of the resonators can be changed and thus the spectral antenna gain or the radiation characteristic of the antenna can be influenced. Particular preference is given to gap-ring resonators (FIGS. 2 to 5), a periodic arrangement of rectangular conductors (FIG. 6) or a periodic arrangement of gap-ring resonators (Frequency Selective Surface (FSS)) (US Pat. The resonators can have both angular and round shapes and can either be arranged laterally to the electrical supply lines (FIGS. 2 to 4) or can be embodied as part of these (FIG.

FIG. 9 shows the spectral intensity of a signal measured in a THz time-domain spectrometer, wherein on the one hand a resonant-structure photoconductive antenna according to the invention and on the other hand a reference antenna according to the prior art was used as emitter. By doing Frequency range around 500 GHz, a significant increase in the emitted radiation, caused by the resonant structure can be seen.

Figure 10 shows schematically the spectral intensity of the signal of a THz time domain spectrometer in which a resonant structure photoconductive antenna according to the invention is used as a sensor. By applying a sample 108, the resonant frequency of the resonator structure can be changed. Based on the waveform can be deduced on the dielectric properties of the sample material.

[Illustration legends and reference list]

Fig. 1: side diagram of the detector (without 109) or the transmitter (without 103) without resonators

Fig. 2: front view of the antenna - symmetric resonators with switching areas: arranged laterally of the photoconductive gap, rectangular resonator design

Fig. 3: Front view of the antenna - split-ring resonators as a sensor configuration with applied sample, arranged laterally from the photoconductive gap, round design

Fig. 4: front view of the antenna - asymmetric resonators with switching areas: arranged laterally from the photoconductive gap, rectangular resonator design

Fig. 5: side view of the antenna -Sensor configuration with applied sample on the resonators Fig. 6: front view of the antenna - resonators integrated into the electrical leads, rectangular design

Fig. 7: front view of the antenna - resonators integrated into the electrical leads, strip shape

Fig. 8: front view of the antenna - resonators arranged laterally from the photoconductive gap. Several resonators in a periodic arrangement (frequency-selective surface) with switching areas.

9 shows a diagram with measured data of the emitted radiation as a function of the frequency of an antenna according to the invention in comparison with the emitted radiation of a structure according to the prior art.

10 shows a diagram with simulation data of the emitted radiation as a function of the frequency of an antenna according to the invention in sensor configuration. By applying the sample, the resonance properties change.

Claims

[Claims]

Photoconductive antenna for transmitting and receiving electromagnetic radiation in the terahertz frequency range, at least consisting of a substrate material, a low-resistance power supply structure or a photoconductive structure applied thereon

Stimulation site, which is characterized in that in addition to the

a low-resistance structure for power supply or power supply additionally a resonator or a plurality of resonators are applied in the vicinity of the photoconductive excitation locus on the substrate material, which the frequency-selective antenna gain and / or the

Improve the radiation characteristic of the antenna.

An antenna according to claim 1, characterized in that at least one of the resonators with respect to the quality and / or the resonant frequency can be switched.

An antenna according to claims 1 to 2, characterized in that at least one of the resonators is arranged in the same spatial plane as the supply lines.

An antenna according to claims 1 to 3, characterized in that the circuit of at least one resonator via switching areas, which consist of semiconducting materials, which by the application of a

Voltage or by irradiation by means of light in an electrically conductive state.

5. An antenna according to claims 1 to 4, characterized in that the circuit of at least one resonator by the application of a dielectric substance, for example an oil, a polymer or a liquid crystal takes place.

6. An antenna according to claims 1 to 5, characterized in that the circuit of at least one resonator is designed so that this allows a modulation of the frequency-selective antenna gain of the antenna. 7. An antenna according to claims 1 to 6, characterized in that the circuit of at least one resonator is designed so that this allows a modulation of the spatial radiation range of the antenna.

8. An antenna according to claims 1 to 7, characterized in that symmetric or asymmetrical single or multiple gap ring or rectangular resonators are used as resonators.

9. An antenna according to claims 1 to 8, characterized in that a periodic or aperiodic arrangement of a plurality of identical or different resonator elements is used.

10. An antenna according to one of claims 1 to 9, characterized in that the arrangement has at least one resonator with a lateral dimension which corresponds at most to the resonant wavelength, preferably at most half the resonant wavelength and more preferably at most one third to one quarter of the resonant wavelength.

11. Use of an antenna according to one of claims 1 to 10 as a sensor for a sample, wherein by applying the sample to the antenna the

Resonance properties and thus both the transmission and the reception characteristic of the antenna to be changed.