Dual-beam THz spectrometer

Abstract

A dual-beam terahertz, THz, spectrometer 100 receives an initial beam (pump beam) 122 from laser 102. A first optical element 104 splits the initial beam into first and second beams 124,126; a first T

Description

Field of invention The present application relates to a dual-beam THz spectrometer and a method of performing THz spectroscopy. In particular, the application relates to a dual-beam THz spectrometer with low aberration optics and off-axis multi-pixel photoconductive emitters for reduced systematic errors. Background Terahertz (THz) light has numerous applications in semiconductor chip inspection, industrial process monitoring, and biomedical imaging as it is non-ionising (unlike X-rays) and penetrates many common materials. However, existing technologies are inefficient and can lead to errors, artefacts and poorer quality results. In some existing technologies for performing THz spectroscopy, a user has to compare the THz electric field transmitted through the "sample" to a "reference" that has transmitted through air (without the sample), in order to determine (for example) the absorption and / or refractive index of the sample. Some existing THz spectrometers produce and detect one THz beam. The operator can measure the "sample" and the "reference" THz beams, sequentially, and analyse the data under the assumption that the incident THz beam was exactly the same in both cases. This can lead to errors in the analysed data, as the incident THz beam may change in its amplitude, arrival time, or position as a result of optical components changing or a variation in the experimental conditions (e.g. variation in the humidity level or temperature). It can take up to 20 minutes to acquire a sample scan and the same duration to acquire, at a later time, a reference scan. Double-modulation THz spectroscopy rapidly translates the sample into and out of the THz beam, resulting in a differential signal sensitive to subtle variations in amplitude and phase. While the sample is inserted and removed relatively fast, overall data acquisition is relatively slow as the modulation signal has to be acquired by averaging over a few repeats of inserting and removing the sample, for each time-delay in the THz waveform. This technique is thus not compatible with fast-scan THz data acquisition methods that acquire THz waveforms at rates of faster than a few Hz, such as rapid-scanning delay lines or asynchronous optical sampling. One existing method to avoid drift-like systematic errors includes self-referencing. However, this introduces systematic errors during data post-processing due to assumptions that are made about the system. For example, assumptions are made about the sample geometry, such as the absence of airgaps between materials. Another method includes THz ellipsometry which reduces systematic errors but, due to requiring almost double the components, is more expensive and complex and is more susceptible to changes in water vapour absorption. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. Summary of invention In order to mitigate at least some of the issues above, there is provided a dual-beam terahertz, THz, spectrometer configured to receive an initial beam from a laser, the spectrometer comprising: a first optical element configured to split the initial beam into a first beam and a second beam; a first THz source located on a substrate and configured to produce a first THz pulse on receipt of the first beam such that the first THz pulse propagates to interact with a sample; a second THz source located on the substrate and configured to produce a second THz pulse on receipt of the second beam such that the second THz pulse propagates to interact with a reference, wherein the first THz pulse and the second THz pulse propagate down the same optical path; and a detector configured to simultaneously detect the first THz pulse after interacting with the sample and the second THz pulse after interacting with the reference. The first beam and the second beam are preferably generated simultaneously. Such a dual beam THz spectrometer enables THz spectroscopy to be performed in a faster and more accurate way than is possible with current technologies. As both the first beam and the second beam are detected and measured at the same time and because the beams are positioned close together, the chances of the beams being subject to different conditions to one another is minimised. This ensures that the amplitude, arrival time and direction of the beams are the same such that higher quality results are achieved. Thus, the spectrometer reduces the influence of systematic errors in THz time-domain spectroscopy by the simultaneous measurement of both beams under the same conditions. The provision of two THz sources allows two THz beams to be formed that propagate along the same optical system. By using two THz beams propagating along the same optical path, this approach mitigates against long-term laser drifts that cause systematic errors while reducing system complexity and cost. Furthermore, as the THz sources are located on the same substrate and there is a single detector device, the entire system is more compact, more straightforward to use, more rugged and more reliable. Moreover, as both beams are detected simultaneously, the duration of time to acquire a sample scan and a reference scan is reduced. Optionally, wherein the first and second THz sources are both photoconductive emitter pixels. Optionally, wherein the first THz pulse and the second THz pulse are spatially separated at the sample and the detector. This prevents cross talk between the beams - the beams can overlap away from the sample and detector but do not interact with one another. Optionally, further comprising a pre-sample imaging system for propagating THz pulses, wherein the pre-sample imaging system is positioned relative to the substrate and sample such that in use both the first THz pulse and the second THz pulse are both propagated through the pre-sample imaging system and have a diffraction-limited spot size at the sample and reference respectively. Optionally, further comprising a post-sample imaging system for propagating THz pulses, wherein the post-sample imaging system is positioned relative to the sample and detector such that in use both the first THz pulse and the second THz pulse are both propagated through the post-sample imaging system and have a diffraction-limited spot size at the detector. Optionally, wherein the pre-sample and / or post-sample imaging system comprises one or more mirrors or lenses configured to guide both the first and second THz pulses. Optionally, wherein the first optical element comprises diffractive optical element(s) positioned relative to the substrate such that in use the first beam causes excitation of the first THz source and the second beam causes excitation of the second THz source. Advantageously, this provides a small angle of separation configured to provide 3mm separation between the first beam and the second beam and splits the beam simultaneously with one optical element only. If a beam splitter is used, an additional mirror can be provided to guide the other beam and a glass window to compensate for the time delay. Optionally, wherein the first THz source and the second THz source are positioned proximal to but away from the optical axis. This allows the first THz pulse and the second THz pulse to form two independent focii at the sample plane and in the detector plane. Preferably the sources are positioned at distances between 1mm and 20mm from the optical axis. Optionally, wherein the detector comprises a first detector pixel on a second substrate and a second detector pixel on the second substrate. Advantageously, this provides a compact system comprising a single detector capable of detecting the first beam and the second beam at the same time. Optionally, wherein the detector is positioned proximal to but away from the optical axis. Optionally, further comprising a sampling plane configured to receive the sample and the reference. A single sampling plane capable of holding both a sample and a reference enables simultaneous measurement. Optionally, wherein the sampling plane is configured to hold the sample at a distance of 1mm to 20mm from the reference. Having the sample and reference positioned at this distance allows the first beam and the second beam to be closer together so as to better match the second beam conditions with the first beam conditions. For example, it minimises the chances of having differing path lengths from residue water vapour. Preferably, the distance is 3mm to 10mm. Optionally, wherein the laser is an infrared laser. Optionally, wherein the laser is a femtosecond laser. Optionally, wherein the spectrometer contains the laser. There is also provided a method of performing THz spectroscopy, the method comprising: providing, by a laser, an initial beam to a first optical element; splitting, by the first optical element, the initial beam into a first beam and a second beam; producing, using a first THz source located on a substrate, a first THz pulse from the first beam; producing, using a second THz source located on the substrate, a second THz pulse from the second beam; propagating the first THz pulse down an optical path to interact with a sample; propagating the second THz pulse down the optical path to interact with a reference; and detecting simultaneously, by a detector, the first THz pulse after interacting with the sample and the second THz pulse after interacting with the reference. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. Brief description of the drawings Embodiments of the present invention will now be described, by way of example only, with reference to the following figures in which: Figure 1 shows a schematic representation of a dual-beam THz spectrometer according to an aspect of the invention; Figure 2 shows an emitter according to an aspect of the invention; Figure 3 shows a process for performing THz spectrometry according to an aspect of the invention; Figure 4 shows a process for manufacturing an emitter according to an aspect of the invention; and Figures 5a-5f show a series of steps in manufacturing an emitter according to an aspect of the invention. Detailed description Figure 1 shows a schematic representation of a dual-beam terahertz (THz) spectrometer 100. The spectrometer 100 is a self-contained system and comprises a laser 102, an optical element 104, a mirror 106, a first THz mirror 108, an emitter 110, a first optical system 112, a sample plane 114, a second optical system 116, a second THz mirror 118, a second optical element 128, a second mirror 130 and a detector 120. The laser 102 can be supplied with the system or can be supplied separately. The laser 102 can be an infrared laser and / or a femtosecond laser. In one example, the laser can be a visible laser with a wavelength between 400nm to 710nm. In another example, the laser is a Yb laser with a wavelength of 1030nm. In another example, the laser is an Er: Fibre laser with a wavelength of 1550nm. In another example, the laser 102 can be a Ti: sapphire mode-locked laser oscillator with a lOOfs pulse duration and an 80MHz repetition rate, operating at 710nm to 910nm (with a central wavelength of 800nm). The laser is configured to produce an initial beam 122 (a pump beam) and a probe beam 123. The initial beam 122 is split into a first beam 124 and a second beam 126 using the optical element 104. The optical elements 104 comprise one or more optical components that splits the pump beam in two, such as a beamsplitter, or a diffractive optical element. It can include a focussing element, such as a plano-convex lens, that creates smaller pump beams at the emitter substrate. In one example, the diffractive optical element has two primary diffraction orders with a full separation angle of 1.91° and the plano-convex lens as focal-length of 100mm. This enables the excitation of photoconductive emitter pixels 204, 206 (shown in Figure 2) using two beams spaced 3mm apart. The optical configuration pictured is an example where the THz radiation is collected from the same side as the incident laser beams. It is also possible to collect THz radiation transmitted through the emitter substrate. The first THz mirror 108 and the second THz mirror 118 can be fluorine-doped tin oxide THz mirrors. The first THz mirror 108 and the second THz mirror 118 are configured to reflect THz radiation while transmitting the pump laser beams. The first optical system 112 and the second optical system 116 each comprise a pair of off-axis parabolic mirrors (OAPMs) to guide the first beam 124 and the second beam 126 in free space whilst ensuring a diffraction-limited spot size at both the sample plane 114 and the detector 120 allowing independent first and second beams. The OAPMs have, for example, 76.2mm focal length and 50.8mm diameter. An alternative would be to use lenses, for example made from polytetrafluoroethylene or the polymer TPX. The emitter 110 is shown in Figure 2 and comprises two independent THz sources. For example, the THz sources may be photoconductive emitter pixels 204, 206 fabricated on the same substrate. Each pixel 204, 206 may be formed of interdigitated electrodes. The substrate is, for example, a semi-insulating Gallium Arsenide (SI-GaAs) substrate. The first pixel 204 is separated from the second pixel 206 by a distance 208 where the distance is calculated from the centre of the first pixel 204 to the centre of the second pixel 206. The distance 208 is chosen so as to spatially separate the beams produced from each pixel 204, 206 to prevent cross talk between the beams. In one example, the distance 208 from the centre of the first pixel 204 to the centre of the second pixel 206 is 3mm. Each pixel 204, 206 has an active area of 500pm x 500pm. The distance 208 can be adjusted by changing the photolithography process (described in more detail with respect to Figures 4 and 5a-5f). Alternative distances 208 can range from 0mm to 10mm and can be greater than 10mm for larger semiconductor wafers. The first pixel 204 and the second pixel 206 are positioned symmetrically about the optical axis and are offset from the optical axis to provide a convenient separation between a sample and a reference sampling area in the spectrometer's 100 sample plane 114. Where the distance 208 is 3mm, the pixels 204 and 206 are offset from the optical axis by ±1.5mm. Alternatively, the separation between the first pixel 204 and the second pixel 206 is in the range of 1mm to 20mm (or more). In one example, pixels 204, 206 can be mounted onto a copper-strip PCB board with bias and ground pins soldered individually for each pixel 204, 206. Returning to Figure 1, the sample plane 114 is configured to intersect the first beam 124 and the second beam 126. The sample plane 114 comprises a sample in the first beam 124 and a reference in the second beam 126. In some cases, the reference is the omission of the sample (in other words, there is nothing placed in the second beam 126). The closer together the first beam 124 and the second beam 126 are, and thus the closer together the sample and reference are, the better the second beam 126 matches the conditions of the first beam 124 and the more accurate the results are. For example, having the beams 124, 126 closer together minimises chances of differing path lengths from residue water vapour in the system. In one example, the sample and reference may be separated by 1mm to 20mm, preferably 3mm to 5mm. Similar to the emitter 110, the detector 120 is positioned away from the optical axis by a small amount. The detector 120 comprises a first detector pixel and a second detector pixel on a single substrate. The first detector pixel is configured to detect the first beam 124 and the second detector pixel is configured to detect the second beam 126. The second optical element 128 is configured to split the probe beam 123 into a first probe beam 123a and a second probe beam 123b. The second optical element 128 may comprise a diffractive optical element, or alternatively a beamsplitter such as a non-polarising beam cube (NP BC), and a mirror to split the probe beam 123 and guide the beams 123a, 123b to the second mirror 130. The probe beam 123 is preferably split 50:50. The first probe beam 123a and the second probe beam 123b are used to perform electro-optic sampling of the first beam 124 and the second beam 126. Figure 3 is a flowchart 300 of the steps performed in THz spectroscopy using the apparatus described with respect to Figures 1 and 2. At step 302, an initial beam 122 is provided by the laser 102. This initial beam 122 is then split at step 304 into a first beam 124 and a second beam 126 using optical element 104 which preferably comprises diffraction elements as discussed above with respect to Figure 1. Once the first beam 124 and the second beam 126 have been generated, each beam interacts with a THz source to produce a THz pulse at steps 306 and 308. For example, and looking to Figure 1, the first beam 124 and the second beam 126 propagate from the optical element 104 to the emitter 110 via the mirror 106 and through the THz mirror 108. The first beam 124 interacts with the first THz source (for example, provided by the first photoconductive emitter pixel 204) and the second beam 126 separately interacts with the second THz source (for example, provided by the second photoconductive emitter pixel 206) to provide THz beams 124, 126. Following the interaction with the photoconductive emitter pixels 204, 206 on emitter 110, the first beam 124 and the second beam 126 propagate to the sample plane 114 to interact with a sample and a reference, respectively. With reference to Figure 1, the first beam 124 and the second beam 126 propagate to the sample plane 114 via first optical system 112. Once the first beam 124 has reached the sample plane 114, it interacts with a sample at step 310. Once the second beam 126 has reached the sample plane 114, it interacts with a reference at step 312. Following the interaction at steps 310 and 312, the first beam 124 and the second beam 126 propagate to the detector 120 via THz mirror 118, for example. The process then continues at step 314 with detecting the first beam 124 and at step 316 with detecting the second beam 126. The first beam 124 may be detected by a first detector pixel on detector 120 and the second beam 126 is detected by a second detector pixel on detector 120. The detection at steps 314 and 316 occur simultaneously. Figure 4 is a flowchart 400 of the photolithography process for forming a multi-pixel emitter, such as emitter 110 shown in Figure 2. While we describe in detail one method to produce two THz sources that are on the same substrate, alternative variations using different photolithographic procedures may be employed. Figures 5a to 5f show schematic representations 500 of the process 400. An optional preparation step may be carried out before the steps shown in flowchart 400 where a bare substrate is cleaved into 1cm x 1cm squares and cleaned using IPA and acetone. The cleaning may be carried out for about 10 to 15 minutes. In a first step 402, a layer of photoresist 504 is spin-coated onto a substrate 502 as depicted in Figure 5a. The layer of photoresist 504 is a thin layer and may be roughly 2pm, for example. The photoresist 504 may be an image reversal photoresist (such as AZ®1524E). The photoresist 504 is then activated at step 404. This can be done by pre-baking the photoresist 504. For example, the photoresist 504 may be prebaked at 110°C to 115°C using a hotplate for about 60 seconds. This activates the photoresist and causes it to become insoluble in a developer. At step 408, the device pattern is imprinted on the photoresist as shown in Figure 5c with patterned sections 510. The device pattern is achieved using a mask 506 on the photoresist 504 as shown in Figure 5b and then exposing the device to UV light. The UV light preferably has a fluence of 90mJ / cm2. The development of the pattern is done using MF319 solution for a period of time, for example, 30-40 seconds. Optionally, the UV exposure is followed by post-baking at 120°C for a first period of time (e.g., 180 seconds) and flood exposure to UV light at a second fluence (e.g., 250mJ / cm2) for a second period of time (e.g., 40 seconds). This step improves the resolution and ensures that a pattern is imprinted. At step 410, a first layer 512 of metal is deposited on the substrate 502 and the photoresist 504 as shown in Figure 5d. This is achieved using e-beam evaporation. In one example, the first layer of metal is Titanium (Ti) and Gold (Au) of a certain thickness. For example, the Ti can have a thickness of 5nm and the Au can have a thickness of 300nm. The first layer 512 is a contact layer. Although Ti and Au have been specifically described, other materials can be used which achieve the same function. The first layer 512 is deposited to allow an electrical bias voltage to be applied to the semiconductor, which is needed to produce a THz pulse. This can be achieved by including bias contacts (for example, two contacts) on the first layer 512. At step 412, a second layer 514 is deposited over the first layer 512 as shown in Figure 5f. Again, this can be achieved using e-beam evaporation. In one example, the second layer is AI2O3 with a thickness of 200nm. The second layer 514 is an electrically-insulating layer. Although AI2O3 has been specifically described, other materials can be used which achieve the same function. The second layer 514 is an electrically insulating layer which prevents a third layer 516, 518, 520 from short-circuiting the bias contacts. At step 414, the third layer 516, 518, 520 is deposited over the second layer 514. The third layer 516, 518, 520 comprises an insulating portion and a shadow mask portion. The insulating portion can be AI2O3 with a thickness of 200nm and the shadow mask layer can be Ti with a thickness of 5nm and Au with a thickness of 300nm. Although AI2O3 and Ti have been specifically described, other materials can be used which achieve the same function. The third layer 516, 518, 520 prevents regions with opposing electric field being photoexcited. After each of steps 410, 412, and 414, any excess coating can be removed from the substrate 502 by rinsing the substrate with acetone and IPA for example. This is shown in Figure 5e with the excess metal removed from the first layer 512. 5 The above embodiments are provided as examples only. The scope of the invention is defined by the appended independent claims. The invention covers all modifications, variations and equivalents as fall within the scope of the appended independent claims.

Claims

1. A dual-beam terahertz, THz, spectrometer configured to receive an initial beam from a laser, the spectrometer comprising:a first optical element configured to split the initial beam into a first beam and a second beam;a first THz source located on a substrate and configured to produce a first THz pulse on receipt of the first beam such that the first THz pulse propagates to interact with a sample;a second THz source located on the substrate and configured to produce a second THz pulse on receipt of the second beam such that the second THz pulse propagates to interact with a reference, wherein the first THz pulse and the second THz pulse propagate down a same optical path; anda detector configured to simultaneously detect the first THz pulse after interacting with the sample and the second THz pulse after interacting with the reference.

2. The dual-beam terahertz spectrometer of claim 1, wherein the first THz pulse and the second THz pulse are spatially separated at the sample and the detector.

3. The dual-beam terahertz spectrometer of any preceding claim, further comprising a pre-sample imaging system for propagating THz pulses, wherein the pre-sample imaging system is positioned relative to the substrate and sample such that in use both the first THz pulse and the second THz pulse are both propagated through the pre-sample imaging system and have a diffraction-limited spot size at the sample and reference respectively.

4. The dual-beam terahertz spectrometer of any preceding claim, further comprising a post-sample imaging system for propagating THz pulses, wherein the post-sample imaging system is positioned relative to the sample and detector such that in use both the first THz pulse and the second THz pulse are both propagated through the post-sample imaging system and have a diffraction-limited spot size at the detector.

5. The dual beam terahertz spectrometer of claim 3 or 4 wherein the pre-sample and / or post-sample imaging system comprises one or more mirrors or lenses configured to guide both the first and second THz pulses.

6. The dual beam terahertz spectrometer of any preceding claim wherein the first optical element comprises diffractive optical element(s) positioned relative to the substrate such that in use the first beam causes excitation of the first THz source and the second beam causes excitation of the second THz source.

7. The dual-beam terahertz spectrometer of any preceding claim, wherein the first THz source and the second THz source are positioned proximal to but away from the optical axis, preferably at distances between 1mm and 20mm from the optical axis.

8. The dual-beam terahertz spectrometer of any preceding claim, wherein the detector comprises a first detector pixel on a second substrate and a second detector pixel on the second substrate.

9. The dual-beam terahertz spectrometer of any preceding claim, wherein the detector is positioned proximal to but away from the optical axis.

10. The dual-beam terahertz spectrometer of any preceding claim, further comprising a sampling plane configured to receive the sample and the reference.

11. The dual-beam terahertz spectrometer of claim 10, wherein the sampling plane is configured to hold the sample at a distance of 1mm to 20mm from the reference.

12. The dual-beam terahertz spectrometer of any preceding claim, wherein the laser is an infrared laser.

13. The dual-beam terahertz spectrometer of any preceding claim, wherein the laser is a femtosecond laser.

14. The dual-beam terahertz spectrometer of any preceding claim wherein the spectrometer comprises the laser.

15. A method of performing THz spectroscopy, the method comprising:providing, by a laser, an initial beam to a first optical element;splitting, by the first optical element, the initial beam into a first beam and a second beam;producing, using a first THz source located on a substrate, a first THz pulse from the first beam;producing, using a second THz source located on the substrate, a second THz pulse from the second beam;propagating the first THz pulse down an optical path to interact with a sample;propagating the second THz pulse down the optical path to interact with a reference; and5 detecting simultaneously, by a detector, the first THz pulseafter interacting with the sample and the second THz pulse after interacting with the reference.s

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