IL126074A - Teraherz radiation detection method and apparatus - Google Patents

Abstract

An optically gated antenna detector comprising a metallic track (10A, 10B) and a first switch (14) providing a break in said track, which first switch is operable upon incidence of an energy pulse to provide a conducting link across said track, said detector further comprising a second switch (20) located behind the first switch such that the energy pulse incident on the first switch will later be incident on the second switch, and wherein said second switch is arranged to short said first switch upon incidence of the energy pulse. 3719 כ" ח בשבט התשס" ב - February 10, 2002

Description

Yimoi ronp i >fr pjim rrony TeraHerz Radiation Detection Method and Apparatus SELA SEMICONDUCTOR ENGINEERING LABORATORIES LTD. Q»yn n*noi> ο>3>ϊ α ntmn Ό Inventor: Dan Hashimshony yT : N3oan C.31109 TeraHerz Radiation Detection Method And Apparatus Field Of The Invention The present invention relates to a TeraHerz radiation detection method and apparatus and more particularly but not exclusively to such a method and apparatus for optically activated detection of radiation pulses in the femtosecond time scale.

Background Of The Invention The TeraHerz range in the electromagnetic spectrum has received considerable attention in recent years due to the potential of its use in a number of fields of growing interest. These include material processing and monitoring, impulse ranging and semiconductor characterization.

The field of interest includes the measurement of amplitude and phase of very short pulses in the TeraHerz range. "Very short" here means a small number of wavelengths. One of the known methods to achieve this is to use an optically gated dipole antenna detector. An application is in Time Domain Spectroscopy. In TeraHerz time domain spectroscopy, sub-picosecond pulses of THz radiation are measured after propagation through a sample and an identical length of free space. A comparison of the Fourier transforms of the pulse shapes gives the absorption and dispersion of the sample.

The detection of extremely short TeraHerz pulses, of the order of femtoseconds is achieved using semiconductor detectors. The conventional detector comprises a semiconductor detection region. The detection region is a gap between biased coplanar regions whose carriers are excited by an incident laser pulse. The TeraHerz beam is generated and collimated by a silicon lens and an off-axis paraboidal mirror. An identical set of optics at the receiver focuses the beam onto a micron scale dipole antenna fabricated on a radiation damaged silicon on sapphire chip. A transient bias is induced on the antenna by the incident beam. The previous photoexcitation of the gap ensures that carriers are available and thus the bias is able to induce a current. The incident beam may thus be sampled with sub-picosecond resolution.

The difficulty with this approach is that the lifetime of the carriers is considerably longer than the pulse time. A very important parameter for measuring incident pulses in the time domain is the measurement.

The main parameters that control the gate width are the laser pulse width and the intrinsic carrier lifetime. A state of the art laser pulse width can be as short as a few tens of femtoseconds. A typical carrier lifetime is controlled by the extent of radiation damage done to the semiconductor crystal of the detector, and the best that can currently be achieved is about half a picosecond. As the detector is not in a position to detect the next pulse until the majority of carriers excited by the previous pulse have reverted to their unexcited state, this tends to limit the gate width to about half a picosecond, very much longer than the limitation imposed by the laser pulse. This means that the time window or the time resolution that the device can provide is not limited by the laser pulse but by the electron-hole lifetime.

Summary of the Invention It is an object of the present invention to reduce the limitation on the temporal resolution of semiconductor detectors imposed by the carrier lifetime.

According to a first aspect of the present invention there is provided an optically gated antenna detector comprising a metallic track and a first switch providing a break in said track, which first switch is operable upon incidence of an energy pulse to provide a conducting link across said track, said detector further comprising a second switch located behind the first switch such that the energy pulse incident on the first switch will later be incident on the second switch, and wherein said second switch is arranged to short said first switch upon incidence of the energy pulse.

In an embodiment the second switch may link two metallic wings of the metallic track.

In a further embodiment the energy pulse is an optical pulse and the first and second switches are optically activated switches. Preferably a laser is used to provide the optical pulse.

The antenna detector in general works by setting up voltages within the antenna itself upon incidence of TeraHerz radiation. The voltages generate currents that circulate to attached measuring apparatus and qualities of the incident radiation may be detected. In embodiments of the invention detection occurs when the first switch is in an activated state and the second switch is not in an detection may occur.

According to a second aspect of the present invention there is provided an antenna detector controllable by two optically activated switches. The switches are arranged such that an optical pulse will be incident on a first of the switches to activate the antenna detector and then on the second of the switches to deactivate the antenna detector.

In an embodiment the first switch is operable to switch a primary electrical circuit in the antenna which includes measurement apparatus and the second switch is operable to short the primary electrical circuit.

The two switches are preferably located a predetermined distance apart and separated by a material of predetermined refractive index. This has the effect of defining a required switching response time. The antenna is thus designable with preset response times which can be manufactured to order for specific applications.

Brief Description Of The Drawings For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which, Fig. 1 is a longitudinal cross section through part of an antenna used for the detection of TeraHerz radiation, Fig. 2 is a graph of current against time for antennas used for the detection of TeraHerz radiation, and Fig. 3 is a graph of current against time for an antenna of the kind shown in Fig. 1.

Description Of The Preferred Embodiments Figure 1 is a longitudinal cross-section through an antenna used for the detection of TeraHerz radiation. A metal track 10 is laid on the surface of a substrate 12. The metal track is divided into upper 10a and lower 10b sections by a first silicon switch 14 of doped semiconductor. Behind the , whose function is to perform switching. Behind the layer of silicon 16 is a transparent layer 18. In practice a semi-transparent epitaxial layer of saphire may be used. Behind the transparent layer is a second silicon switch 20 which is preferably identical to the first silicon switch 14. A first metal shorting wing 22 extends from the upper metal track 10a and a second metal shorting wing 24 extends from the lower metal track 10b. The two metal shorting wings 22 and 24 face each other across the second silicon switch 20.

In operation, a laser pulse is incident on the first silicon switch 14 in the normal way and carriers in the semiconductor are raised into excited states and an open circuit is created along the metal track 10. The TeraHerz radiation is then incident on the antenna and is detected in the normal way. That is to say the voltages induced in the aerial by the radiation are able to set up currents which can be detected by measurement apparatus attached to the aerial.

The laser pulse then continues past the first semiconductor switch 14 and proceeds through transparent silicon layer 16, substrate 12 and transparent sapphire layer 18 until it reaches the second silicon switch 20. Here it excites carriers in the same manner creating a short circuit from the first silicon switch 14, through the first shorting wing 22, the second silicon switch 20 and the second shorting wing 24 and back to the first silicon switch 14. The short circuit thus formed means that the voltage induced on the antenna by the TeraHerz pulse drives current around the shorting wings and not to the measurement part of the circuit. This allows the carriers to dissipate and effectively resets the detector for the next pulse independently of the lifetime of the carriers. The delay involved in the reset operation is the time taken by the laser pulse to pass from the first 14 to the second semiconductor switch 20 plus the rise time of the laser pulse plus the response time of the shorting circuit.

Fig. 2 is a graph of current against time for the device shown in Fig. 1. The upper curve shows the normal fall in current without the use of the second silicon switch and the lower curve shows the more rapid fall in current associated with use of the invention. It will be appreciated that the invention does not lead to an instantaneous fall in current but the rate of fall is considerably higher than in the prior art.

Fig. 3 is another graph of current against time for the device shown in Fig. 1. The whole of the cycle is shown including the rise time and the steady state period, as well as the current fall altering the distance between the first 14 and second 20 silicon switches. It may also be altered by varying the transparent material used in the layer 18 for another material of different refractive index.

Claims (9)

Claims

1. An optically gated antenna detector comprising a metallic track and a first switch providing a break in said track, which first switch is operable upon incidence of an energy pulse to provide a conducting link across said track, said detector further comprising a second switch located behind the first switch such that the energy pulse incident on the first switch will later be incident on the second switch, and wherein said second switch is arranged to short said first switch upon incidence of the energy pulse.

2. An antenna detector according to claim 1 wherein the second switch links two metallic wings of the metallic track.

3. An antenna detector according to either of claim 1 and claim 2 wherein the energy pulse is an optical pulse and the first and second switches are optically activated switches.

4. An antenna detector according to claim 3 wherein the optical pulse is a laser pulse.

5. An antenna detector according to any preceding claim operable for the setting up of voltages by incident TeraHerz radiation and wherein the voltages generate currents that circulate to attached measuring apparatus when the first switch is in an activated state and the second switch is not in an activated state.

6. An antenna detector controllable by two optically activated switches, the switches arranged such that an optical pulse will be incident on a first of the switches to activate the antenna detector and then on the second of the switches to deactivate the antenna detector.

7. An antenna detector according to claim 6 wherein the first switch is operable to switch a primary electrical circuit in the antenna which includes measurement apparatus and wherein the second switch is operable to short the primary electrical circuit.

8. An antenna detector according to any preceding claim wherein the two switches are located a predetermined distance apart and separated by a material of predetermined refractive index so as to define a required switching response time.

9. An antenna detector substantially as hereinbefore described with reference to the accompanying drawings. For the Applicant, Sanford T. Colb & Co. Advocates & Patent Attorneys C: 31109