NL2006937C2

NL2006937C2 - A pulsed terahertz emitter.

Info

Publication number: NL2006937C2
Application number: NL2006937A
Authority: NL
Inventors: Gopakumar Ramakrishnan; Paulus Clemens Maria Planken
Original assignee: Univ Delft Tech
Priority date: 2011-06-15
Filing date: 2011-06-15
Publication date: 2012-12-18

Description

P94077NL00

Title: A pulsed terahertz emitter

FIELD OF THE INVENTION

The invention relates to a pulsed terahertz emitter.

The invention further relates to a method of manufacturing a pulsed terahertz emitter.

5 The invention still further refers to a spectroscopy instrument.

BACKGROUND OF THE INVENTION

Currently interest regarding suitable sources of terahertz (THz) radiation is increasing. Terahertz radiation may be used for different purposes 10 including safety surveillance and spectroscopy applications. A very common method of generating THz radiation is by optically exciting nonlinear optical materials or semiconductors by femto-second laser pulses. Pulses of broadband THz radiation are generated through various ultrafast processes happening on the excited materials. A good choice of the source material is essential for a 15 specific application. An exemplary embodiment of a terahertz emitter is known from US2011024650. This known terahertz emitting device comprises a wafer and a current source. The wafer includes silicon carbide and a dopant. In particular, the wafer may consist of 6H silicon carbide; a nitrogen dopant having a concentration of approximately 1018 cm 3; a boron dopant having a 20 concentration of approximately 1016 cm 3; and an aluminum dopant having a concentration of approximately 1015 cm 3. The current source is electrically coupled to the wafer. The wafer emits radiation having a frequency between approximately 1 THz and 20 THz when driven by the current source.

It is found to be disadvantageous that currently there in lack of 25 flexibility in physically shaping few-cycle THz emitter materials. The need for extended THz emitters in the form of shaped surfaces is not met. Although poled polymers may be used as nonlinear optical materials for the generation 2 of a few-cycle THz radiation with femto-second laser pulses, these polymers have a low damage threshold, which is disadvantageous.

Another disadvantage of the materials which are currently used for generation of THz radiation is their tendency to overheating and thermal 5 damage. A further disadvantage of state of the art THz emitters is that they often employ high cost materials, such as crystalline indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), low temperature grown GaAs, zinc telluride (ZnTe), gallium phosphide (GaP) and gallium selinide (GaSe).

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SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative source of THz radiation, notably of pulsed THz radiation.

To this end the pulsed terahertz emitter according to the invention 15 comprises a substrate surface on which a Cu20/metal interface layer is provided.

The invention is based on the insight that Cu20/metal (copper (Cu) or platinum (Pt) or gold (Au) or silver (Ag) or any other metal) interface is particularly suitable for generation of few cycle THz radiation when irradiated 20 with a femto-second laser pulses having a wavelength in the near-infrared range. Thereby Cu20/metal interface layer is capable of increasing the flexibility in the design of a THz emitter.

It is observed that a bulk CU2O has a center of inversion symmetry which means that nonresonant optical rectification processes as sources of THz 25 radiation are impossible in the bulk of CU2O. At the Cu20/metal interface, however, the symmetry is broken. It has to be noted that when a Cu20/metal interface is formed, the resulting Schottky barrier is not very pronounced and the interface acts almost as an ohmic contact. Based on this and since CU2O show little to no absorption around a wavelength of 800 nm, it is found to be 3 surprising that the Cu20/metal interface manifests itself as a reliable source of THz radiation when excited with the laser pulses of 800 nm.

The Cu20/metal interface layer may be deposited on many suitable substrates, such as curved surfaces. In addition, it is found that the 5 Cu20/metal interface layer may be deposited, for example by chemical means, on complex surfaces, such as those of a THz waveguide or a mirror. More in particular, a Cu20/metal interface may be deposited on different classes of materials, such as metals, glass and semiconductors. It can therefore easily be used for the generation of few-cycle THz pulses, with femto-second laser 10 source. When deposited on curved surfaces, such as mirrors or other optical elements, and when illuminated with a femto-second laser pulses, the curved surface combines the ability to emit THz radiation with an additional functionality, such as the ability to automatically focus THz pulses onto a small spot.

15 When Cu20/metal interface layer is deposited on flat or curved surfaces and these surfaces are illuminated with femto-second laser pulses, they act as THz emitters. In conventional THz imaging or spectroscopy setups the generated THz light is collimated, steered and refocused using flat and curved metal mirrors. In cases when a focused THz beam is required, a 20 parabolic mirror with a Cu20/metal interface deposited on it can be used as a merged source and focusing element making supplementary focusing mirrors redundant. This is found to be an additional advantage of the THz emitter according to the invention. For the THz waveguides a Cu20/metal interface can be deposited on the inside of the waveguide. Rather than generating, 25 steering and focusing the THz pulses onto the waveguide (as is the case in the prior art), the invention enables generation of the THz radiation directly inside the waveguide. When a weakly focused THz pulse is used, the THz radiation may be generated inside the waveguide, simplifying the setup. Such a setup avoids power losses due to diffraction and absorption on the THz beam path 4 over which the THz pulses propagate to the waveguide and also avoids coupling losses when coupling the THz pulse into the waveguide.

Cu20/metal THz emitter may be easily formed by oxidizing copper at elevated temperatures, such as in the range up to 275 degrees Centigrade. If 5 the layer of copper to be oxidized is relatively thick, it automatically functions like a heat sink which more rapidly removes heat from the THz generation volume than conventional THz emitters based on absorption of femto-second laser pulses. Preferably, the laser is operated with a wavelength of about 800 nm corresponding to a near infrared wavelength range, because laser radiation 10 is not strongly absorbed by the CU2O and can reach the Cu20/metal interface.

A further advantage of using the Cu20/metal layer for manufacturing a THz emitter is that a layer of copper may be easily oxidized in air. It is found that a copper layer of about 50 - 300 nanometers is sufficient for manufacturing a reliable THz emitter. This is found advantageous as 15 material expenses are reduced.

In an embodiment of the pulsed terahertz emitter according to a further aspect of the invention the Cu20/metal interface layer is provided on a sub-layer of gold or silver.

It is found that gold or silver may be advantageously deposited on a 20 suitable substrate, such as a glass substrate for forming a suitable sub-layer to the semiconductor CU2O layer thereby forming a Schottky diode. It is found that a thickness of about 10 nanometer is sufficient for the gold sub-layer. Plasmonic effects at the laser wavelength may advantageously occur, by forming metal structures with dimensions of tens to hundreds nanometers, 25 which may further enhance the emission of THz light. This phenomenon is caused by increasing the interaction time between the laser pulses and the Cu20/metal interface and by the concentration of light at the wavelength of 800 nm.

5

It is further found to be particularly advantageous to electrically bias the resulting Cu20/metal. Those skilled in the art will readily appreciate which biasing voltage may be appropriate.

In a particular embodiment of the pulsed terahertz emitter 5 according to the invention, the sub-layer of gold or silver of a few nanometer thickness, comprises a percolated network.

It is found that a percolated network in the sub-layer metal may be advantageous for increasing intensity of the generated THz radiation. In an alternative embodiment the CU2O may be suitably doped to strengthen the 10 Schottky field.

The method of manufacturing a pulsed terahertz emitter according to the invention comprises the steps of oxidizing a layer of Cu at temperatures in the range of up to 275 degrees centigrade for obtaining a Cu20/metal interface.

15 Cu20/metal interfaces can be prepared on different substrates by depositing a copper layer on top of a suitable metal layer and subsequently by heating the resulting the structure at temperatures of about 275 degrees Centigrade in air. Copper layer gets fully oxidized to CU2O forming a semiconductor/metal interface. It is found that when such a structure is 20 illuminated with a femto-second near-infrared laser pulses, the Cu20/metal interface emits pulses of THz radiation. It is found that 280 nm of Cu completely oxidized on a 200nm gold surface can act as a very reliable source for the THz emission. However, even thinner layers emit substantially. It is further found that the emitted field amplitude may be about half of that 25 obtainable from a 300 micron thick GaP (110) crystal illuminated with femtosecond laser pulses.

It is further found that the thickness of the CU2O may be optimized for reducing the reflection of the femto-second laser pulse from the sample. Accordingly, the CU2O layer acts as an anti-reflection layer for the 800 nm 30 light, which is advantageous.

6

In an embodiment a block of copper may be selected for providing a CU2O top layer. For this purpose the block of copper may be heated in air at, for example 275 degrees centigrade to oxidize a top portion of the block. Accordingly a Cu20/metal interface is automatically formed. The thick copper 5 sheet under the oxidized top layer may act as a heat sink, which removes heat from the excited volume near the interface. It is found that such a system has a higher laser damage threshold then conventional THz emitters, such as GaAs, ZnTe and GaP.

It will be appreciated that CU2O is a known semiconductor. When, 10 for example, a thin copper layer coated on a gold surface is heated an interface of CU2O and gold is formed. This is a semiconductor/metal interface which is also referred to as a Schottky interface. A Schottky interface acts as a diode and has a built-in electrical field. When such a device is excited with femtosecond laser pulses, THz pulses can be generated from Cu20/metal interfaces 15 through a third-order nonlinear process called electric-field induced optical rectification. The 800 nm light can also be absorbed near the interface, by impurities in the semiconductor and/or by internal photoemission from the metal into the semiconductor. Thus photo-excited charge carriers are accelerated by the electric field, or depletion field, present in the Schottky 20 interface giving rise to the emission of pulses of THz radiation.

A spectroscopy instrument according to the invention comprises the pulsed terahertz emitter as is discussed with reference to the foregoing.

These and other aspects of the invention will be described in more detail with reference to drawings wherein like reference signs are used for like 25 elements. It will be appreciated that the drawings are provided for illustrative purposes only and may not be used for limiting the scope of the appended claims.

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BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 presents schematically an embodiment of the THz emitter according to the invention.

5 Figure 2 presents schematically an embodiment of an optical element comprising the THz emitter according to an aspect of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 presents schematically an embodiment of the THz emitter 10 10 according to the invention. In this particular embodiment a layer of a suitable metal, such as gold or silver 3 is provided on a substrate 2. The layer of metal 3 is covered with a layer of Cu 4, which is allowed to oxidize as is described in the foregoing leading to a formation of a Cu20/metal interface layer 4, 3. It will be appreciated that the present drawing is schematic and is 15 not representative of relative dimensions. When a femto-second pulsed laser light 8 having a wavelengths of about 800 nm impinges on the THz emitter 10 THz radiation is generated.

It will be appreciated that CU2O is a known semiconductor. The formed semiconductor/metal interface 4, 3 acts as a diode and has a built-in 20 electrical field. When such a device is excited with femto-second laser pulses, light is absorbed near the interface by impurities in the semiconductor and/or by internal photoemission from the metal into the semiconductor. These carriers are accelerated by the electric field, or depletion field, present at the Schottky interface giving rise to the emission of pulses of THz radiation. It is 25 also possible to have a non-resonant third-order nonlinear optical process called electric field-induced optical rectification to take place at the interface leading to the emission of THz pulses.

It is further found that the emitted THz intensity may be enhanced by a plasmonic field which may be generated by the excitation of localized 30 surface Plasmon hot-spots at the Schottky interface. The surface plasmon may 8 be excited using a structured metal surface on which CU2O is coated. This can lead to enhancement of the nonlinear optical processes leading to the THz emission. This effect may be more pronounced when the metal layer 3 comprises a nanoscale percolated network. However, any structure of the 5 metal layer may be allowed, including the continuous layer, provided that a respective dimension of the said structure is in the order of 10 - 100 nm. Structures which are capable of enabling the plasmonic mode are nanoantennas and nanoscale optical resonators, for example.

Figure 2 presents schematically an embodiment of an optical 10 element comprising the THz emitter according to an aspect of the invention. In this particular embodiment a Cu20/metal interface 22 is provided on a concave surface of a mirror 21 and is conceived to be irradiated with suitable light pulses, for example the 800 nm light pulses 26. In the area 24 both the THz pulse and the 800 nm light may occupy the same volume in space. In order to 15 filter the 800 nm light from the generated radiation a filter element 25 may be provided. As a result, the generated THz radiation 23 is focused to a predefined focal area F. It will be appreciated, however that any suitable optical element may be provided with the Cu20/metal interface as is described before. Accordingly, the functionality of a THz emitter and a suitable optical element 20 is merged in a single device which is advantageous for simplifying the device architecture.

While specific embodiments have been described above, it will be appreciated that the invention may be practiced otherwise than as described. Moreover, specific items discussed with reference to any of the isolated 25 drawings may freely be inter-changed supplementing each outer in any particular way. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described in the foregoing without departing from the scope of the claims set out below.

Claims

1. Gepulste terahertz-zender, omvattende een oppervlak waarop een Cu20/metaal-interface-laag is voorzien.A pulsed terahertz transmitter, comprising a surface on which a Cu20 / metal interface layer is provided.

2. Gepulste terahertz-zender volgens conclusie 1, waarbij het 5 oppervlak een deel van een golfgeleider vormt.A pulsed terahertz transmitter according to claim 1, wherein the surface forms a part of a waveguide.

3. Gepulste terahertz-zender volgens conclusie 1, waarbij het oppervlak een deel van een optisch element vormt.The pulsed terahertz transmitter according to claim 1, wherein the surface forms part of an optical element.

4. Gepulste terahertz-zender volgens conclusie 3, waarbij het optisch element gebogen is.The pulsed terahertz transmitter of claim 3, wherein the optical element is curved.

5. Gepulste terahertz-zender volgens één van de voorgaande conclusies, waarbij een dikte van de Cu20/metaal-interface-laag 10-1000 15 nanometer is.The pulsed terahertz transmitter according to any of the preceding claims, wherein a thickness of the Cu 2 O / metal interface layer is 10-1000 nanometers.

6. Gepulste terahertz-zender volgens één van de voorgaande conclusies, waarbij de Cu20/metaal-interface-laag is voorzien op een sublaag van koper, goud, zilver, platina of ander geschikt metaal. 20A pulsed terahertz transmitter according to any one of the preceding claims, wherein the Cu 2 O / metal interface layer is provided on a sublayer of copper, gold, silver, platinum or other suitable metal. 20

7. Gepulste terahertz-zender volgens één van de voorgaande conclusies, voorts omvattende een femtoseconden-laser voor het exciteren van de Cu20/metaal-interface-laag.A pulsed terahertz transmitter as claimed in any one of the preceding claims, further comprising a femtosecond laser for exciting the Cu20 / metal interface layer.

8. Gepulste terahertz-zender volgens conclusie 7, waarbij de laser werkzaam is bij een golflengte van 800 nm.The pulsed terahertz transmitter according to claim 7, wherein the laser operates at a wavelength of 800 nm.

9. Gepulste terahertz-zender volgens één van de voorgaande 5 conclusies, waarbij de Cu20/metaal-interface-laag elektrisch voorgespannen is.The pulsed terahertz transmitter according to any of the preceding claims, wherein the Cu20 / metal interface layer is electrically biased.

10. Gepulste terahertz-zender volgens conclusie 6, waarbij de sublaag van goud of zilver een gepercoleerd netwerk omvat.The pulsed terahertz transmitter of claim 6, wherein the gold or silver sublayer comprises a percolated network.

11. Gepulste terahertz-zender volgens één van de voorgaande conclusies, waarbij het koperen oxide is gedoopt.The pulsed terahertz transmitter according to any of the preceding claims, wherein the copper oxide is doped.

12. Werkwijze voor het fabriceren van een gepulste terahertz-15 zender, omvattende de stap van het oxideren van een laag Cu bij temperaturen in de range van 100-275 graden Celsius voor het verkrijgen van een Cu20/metaal-interface.12. A method for manufacturing a pulsed terahertz transmitter comprising the step of oxidizing a layer of Cu at temperatures in the range of 100-275 degrees Celsius to obtain a Cu20 / metal interface.

13. Werkwijze volgens conclusie 12, waarbij de Cu20/metaal-20 interface wordt voorzien op een laag van een verder metaal.The method of claim 12, wherein the Cu20 / metal-20 interface is provided on a layer of a further metal.

14. Werkwijze volgens conclusie 13, waarbij voor het verdere metaal koper, goud, zilver of platina wordt geselecteerd.A method according to claim 13, wherein copper, gold, silver or platinum is selected for the further metal.

15. Werkwijze volgens één van de voorgaande conclusies 12-14, waarbij de Cu20/metaal-interface wordt voorzien op een werkoppervlak van een instrument.The method of any one of the preceding claims 12-14, wherein the Cu20 / metal interface is provided on a work surface of an instrument.

16. Werkwijze volgens conclusie 15, waarbij voor het 30 werkoppervlak een oppervlak van een golfgeleider of een optisch element wordt geselecteerd.16. A method according to claim 15, wherein a surface of a waveguide or an optical element is selected for the working surface.

17. Werkwijze volgens conclusie 16, waarbij het optisch element een spiegel is met een concaaf of convex werk oppervlak.The method of claim 16, wherein the optical element is a mirror with a concave or convex working surface.

18. Spectroscopisch instrument, omvattende de gepulste terahertz- zender volgens één van de voorgaande conclusies 1-11.A spectroscopic instrument comprising the pulsed terahertz transmitter according to any of the preceding claims 1-11.