Fully-coherent terahertz detection method and system

Abstract

A method and a system for terahertz detection, using at least a first and a second electrodes separated by a centro-symmetric material. The system comprises at least a first and a second electrodes with conductive pads for connection to a voltage source, separated by a centro-symmetric material; the method comprising second harmonic generation in the centro-symmetric material by overlapping of a probe and a terahertz beams.

Claims

1. A terahertz detection system, comprising at least a first and a second electrodes separated by a centro-symmetric material, said electrodes having metallic contacts for connection to a voltage source, said voltage source being adapted to provide a bias field between said electrodes to said centro-symmetric material, said system inducing a second harmonic beam by propagation and overlapping of a probe and a terahertz beams in said centro-symmetric material.

2. The detection system of claim 1, wherein said centro-symmetric material has a nonlinear index comprised between about 10.sup.16 cm.sup.2/W and 10.sup.7 cm.sup.2/W and a breakdown voltage of at least 10.sup.9 V.Math.m.sup.1.

3. The detection system of claim 1, wherein said electrodes are separated by a distance in a range between 5 nm and 100 m.

4. The detection system of claim 1, wherein said material is fused silica.

5. The detection system of claim 1, wherein said material is diamond.

6. The detection system of claim 1, wherein said electrodes are made in one of a noble metal, a metal oxide, a metal hydroxide, a carbon material and a conductive polymer.

7. The detection system of claim 1, wherein said electrodes are made of one of graphite and graphene.

8. The detection system of claim 1, wherein said electrodes are coplanar gold layers deposited on a graded fused silica substrate, and said material is fused silica.

9. The detection system of claim 1, wherein said electrodes are coplanar gold layers deposited on a graded fused silica substrate via a chromium layer.

10. The detection system of claim 1, wherein said electrodes are coplanar gold layers of a thickness in a range comprised between about 40 and about 120 nm deposited on a graded fused silica substrate of a thickness in a range comprised between about 0.5 and about 10 mm via a chromium layer of a thickness selected in the range comprised between about 10 and 40 nm.

11. The detection system of claim 1, comprising at least a first and a second electrodes supported by a first contact pad interdigitated with a third and a fourth electrodes supported by a second contact pad facing said first contact pad, said first and said third electrodes being separated by a first distance, said third electrode and said second electrode being separated by a second distance, and said third electrode and said second electrode being separated by the first distance, the first distance being selected in a range between 1 to 5 micrometers, the second distance being selected in a range between 5 to 25 micrometers, and said second distance being of the order of 3 to 6 times said first distance.

12. The detection system of claim 1, wherein said voltage source is adapted to provide a bias field in a range between 10V and 300V.

13. A terahertz detection system, comprising: a terahertz beam source; a probe beam source; a detection unit comprising at least a first and a second electrodes separated by a centro-symmetric material; focusing optics adapted to overlap the terahertz beam and the probe beam and propagate them through the detection unit; and a voltage source adapted to provide a bias field between said electrodes to said centro-symmetric material, inducing a second harmonic beam by propagation and overlapping of the probe and the terahertz beams in said centro-symmetric material.

14. A method for terahertz detection, comprising generating a second harmonic beam by propagating overlapping probe and terahertz beams in a centro-symmetric material separating at least a first and a second electrodes connected to a voltage source adapted to provide a bias field between said electrodes to said centro-symmetric material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the appended drawings:

(2) FIG. 1 show steps of a method according to an embodiment of an aspect of the present invention;

(3) FIG. 2A is a schematic view of a system according to an embodiment of an aspect of the present invention;

(4) FIG. 2B shows a x-polarized optical probe pulse propagating through a metallic slit along the z-axis in the system of FIG. 2A;

(5) FIG. 3 is a schematic view of a system according to an embodiment of an aspect of the present invention; and

(6) FIG. 4 is a schematical view of a THz generation and detection system according to an embodiment of an aspect of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) The present invention is illustrated in further details by the following non-limiting examples.

(8) FIG. 1 shows steps of a method 100 fabrication of a system according to an embodiment of an aspect of the present invention, in the case of two separate coplanar gold layers deposited on a 1 mm-thick UV graded fused silica substrate SiO.sub.2.

(9) A first layer of chromium is deposited on the UV graded fused silica substrate (step 120), then at least one gold layer is deposited on the layer of chromium (step 130), and a second layer chromium is deposited on top of the gold layer (step 140) in order to ensure a good contact of the gold electrodes with fused silica SiO.sub.2. Electrodes are then defined by UV lithography and etching (step 150) forming a gap therebetween. Then, fused silica SiO.sub.2 is deposited in the gap between the electrodes in order to fill it and to partially cover the electrodes (step 160) so that to avoid discharge in air. Finally pads intended for connection of the system to a high voltage source (HV) are opened by UV lithographic and etching (step 170).

(10) Any kind of quartz wafer available on the market can be used as a substrate. The substrate has a typical thickness in a range comprised between about 0.5 and about 10 mm. The first chromium layer has a thickness selected in the range comprised between about 10 and 40 nm, for example 30 nm, and the gold layer has a thickness selected in the range comprised between about 40 and about 120 nm, for example 100 nm.

(11) In step 150, by a UV photolithographic process, electrodes are defined by etching the whole layer of metal, i.e. chromium and gold, until the SiO.sub.2 substrate. The distance between the electrodes is in the range between 5 nm and 100 m, typically of a few tens of micrometers or less, for example 30 m.

(12) With a different process, such as e-beam lithography for example, the distance between the electrodes can be reduced below 1 m. In this case improvement in the signal is expected, due to field-enhancement effects of the THz field in the gap between the electrodes.

(13) As shown in FIG. 2, in a system 10, the gold electrodes are connected to a bipolar voltage modulator HV.

(14) Doubly resonant structures 20, i.e. enhancing both the THz and the optical probe, such as interdigitated electrodes 12, may also be fabricated. FIG. 3 illustrates electrodes 12a supported by contact pad 14a interdigitated with electrodes 12b supported by contact pad 14b. In this case, the width (w) of each electrode 12a, 12b ranges between about 5 and about 100 micrometers. The electrodes 12a and 12b are positioned asymmetrically in order to avoid the generation of second harmonic with opposite phase which would induce the cancellation of the effect: each electrode is separated from a first electrode by a distance (d.sub.1) selected in a range between 1 and 5 micrometers, and from a second electrode by a distance (d.sub.2) in a range between 5 and 25 micrometers, with d.sub.2 of the order of 3 to 6d.sub.1. The distance (D) between the end of each electrode 12a, 12b and the opposite contact pad 14b, 14a respectively is in the range between a few nanometers and about 10 micrometers. The small inter-electrode distance (d) allows increasing the applied bias and then enhances the electric field between the electrodes, which results in increasing the second harmonic SH signal and thus in improving the detection. Furthermore, the large area, related to width (w) and length between the contact pads, of the interdigitated electrodes 12a, 12b permits to work with much larger incident beams. The distance between the two opposite pads, i.e. the lengths of the electrode, can vary from hundreds of micrometers to few millimeters.

(15) Any kind of metal contact for the pads can be used. The most common ones can be an alloy of Aluminium/Silicium, Gold/Chromium or simply Platinum, Graphite, Rhodium, Copper, Lead, Silver. The deposited metal may be part of a CMOS compatible process.

(16) Diamond may be considered a centro-symmetric material of choice for the nonlinear medium due to its high nonlinear coefficient, good transparency properties at the wavelengths involved in the process and high breakdown voltage.

(17) Graphite or graphene may be used for the electrodes, or conducting materials or conductive materials used for conduction, i.e. noble metal, metal oxide/hydroxide, carbon materials, and conductive polymers.

(18) The substrate material use in the prototype was crystalline quartz. The substrate has to be transparent to THz frequencies and CMOS compatible such as for example high resistivity silicon, sapphire, crystalline quartz.

(19) There is thus provided a system comprising conductive electrodes embedded in a centro-symmetric material, the distance separating the electrodes selected as short as 1 micrometer depending on the method used (see step 150 described hereinabove).

(20) In a set up for THz detection according to an embodiment of an aspect of the present invention illustrated in FIG. 4 for example, THz radiation is generated via optical rectification from a Zinc Telluride (ZnTe) crystal and focused on a detection system 10 described hereinabove for example. A probe beam (800 nm) is focused in order to make it overlap with the generated THz beam. The x-polarized optical probe beam and the THz beam then propagate through the metallic slits and into the SiO.sub.2 located between the gold electrodes along the z axis of the detection system 10. A bias field in a range between about 10V and about 300V, for example of 250V, is applied between the electrodes, allowing generating a second harmonic beam. The signal is collected by a photomultiplier (PM), thus permitting to retrieve the amplitude and phase of the THz wave. It is to be noted that, in absence of a low voltage source, a voltage up to 1 kV may be applied between the electrodes without damaging the unit.

(21) As people in the art may appreciate, detection systems of the present invention allow solving a number of problems identified in particular in the Air Biased Coherent Detection (ABCD) method at least for the following reasons.

(22) First, a solid state dielectric material with a breakdown voltage or dielectric strength higher than that of air, which is about 3.Math.0 10.sup.6 V.Math.m.sup.1, for example theoretically three orders of magnitude higher in the case of silica SiO.sub.2 with a value of about 10.sup.9 V.Math.m.sup.1 at room temperature, can be selected, thus allowing reaching an excellent signal to noise ratio (SNR), of about 100 under an applied bias field of 250V and of about 50 under an applied bias field of 50V for example.

(23) Secondly, since a small inter-electrode distance can be selected, in the range between 5 nm and 100 m, the applied voltage can be significantly lower, i.e. in a range between a few volts to hundreds of volts i.e. between about 10V and about 300V, compared to about 1000 V in the standard Air Biased Coherent Detection (ABCD) method, so that commercial low-voltage sources can be employed instead of more expensive high-voltage amplifiers.

(24) Thirdly, when selecting a centro-symmetric material, such as fused silica SiO.sub.2 in the above example, due to the high nonlinearity induced by the centro-symmetric material, commercially available fiber lasers may be used instead of expensive amplified laser systems delivering intense pulses.

(25) Moreover, the present detection system can be miniaturized and integrated due to its compact size.

(26) There is thus provided a detection system using metallic slits, for example one slit between two opposite electrodes as illustrated herein in FIGS. 1 and 2 for example, or multiple slits as illustrated herein in FIG. 3 for example, with a gap width in the range between about and 5 nm and about 100 m, filled with a material having a high nonlinear index, i.e. from n.sub.2=10.sup.16 cm.sup.2/W (as for SiO.sub.2 for example) to n.sub.2=10.sup.7 cm.sup.2/W (as for graphen for example), a high breakdown voltage i.e. of at least 10.sup.9 V.Math.m.sup.1, such as fused silica in the example illustrated herein. The slits are biased with an external voltage source in order to generate a bias field the layer of nonlinear material that is deposited in the slits between the electrodes and to induce a second harmonic pulse.

(27) The present detection system is thus a broadband terahertz detection system of reduced size compared to commercially available solutions. Furthermore, due the small inter-electrode distance, the required bias voltage can be significantly reduced with respect to commercially available solutions, and so the cost of the power supply as well. Since no high-voltage is required, the system is safer than similar commercially available solutions. The method of fabrication may be completely CMOS compatible, which is of interest in view of the production and commercialization of the system.

(28) Moreover, it can be contemplated using, instead of an intense probe generally delivered by a Ti:sapphire laser, a fiber laser, which again reduces the price of the necessary equipment thus increasing the accessibility of the method to a large number of scientists and companies.

(29) The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

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