4-1 Introduction
In order to identify characterizations of our THz devices, power and electrical field measurement system at this range is needed. There are lots of tools could be used for this frequency range recently and following are brief introduction of some instruments in our experiments. First, power of THz radiation is measured by liquid-helium cooled bolometer. As for some devices designed in the sub –THz, for example, W-band, a RF power meter is usually used. Besides, to further study the radiated electrical field, there are different ways which are developed in recent years, such as photoconductive antenna and electro-optic sampling (EO sampling).
Far-infrared (FIR), submillimeter wave and millimeter wave can be detected by Bolometer. A hot electron bolometer is a device, which absorbs the incident radiation to change the electron’s temperature. Its resistance will respond for it correspondingly.
A traditional bolometer consists of a heat-sensitive detection element mounted inside a heat sink and physically supported by a thermally conductive physical supporter.
The most common systems are helium-cooled Si, Ge and InSb bolometer. It can measure the radiation power lower in nanowatt region, but it losses the information of phase and frequency. Because of this, using bolometer to detect the power of THz radiation usually accompanies Martin-Puplett interferometer [1]. By using combined system, we can obtain the interferogram of THz radiation [2].
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Using PC antenna as the detector is one of detecting THz radiation ways [3].
When the incident THz radiation illuminates the PC antenna, it induces transient current and then accelerate electron by probe beam. There are two factors determine the spectral bandwidth in this detector. One of them is the photocurrent response (i.e.
carrier lifetime) and the other one is the frequency dependence of the antenna structure [4-5]. In general, the low frequency cutoff of the detectors results from the collection efficiency of the dipole, while the upper frequency limit is determined by the photocarrier response. The photocurrent response is the convolution of the optical pulse duration and the impulse current of the photoswitch across the photoconductive antenna on pulse mode.
There are two advantages of Electro-Optical (EO) sampling, one is broad bandwidth spectrum and the other is easy to implement. Recently, technique of EO sampling has become an alternative to photoconductive (PC) detection [a6]. The Zinc-Blende crystal was used to measure THz radiation based on the Pockels effect [6]. When we vary the temporal delay between pump and probe beam, the synchronous probe beam will probe the transient change of refractive index result from THz radiation changing the refractive index. There is a trade-off in this method, thick crystal will introduce longer interaction length but reduce the frequency response.
4-2 Power Measurement 421 Bolometer
Figure 4-1 shows the looks of our bolometer and its structure drawing including
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a helium vessel. Table 4-1 shows specifications in details of the bolometer, which has a spectrum response 100cm-1 to 3 cm-1.
Figure 4- 1 Schematic diagram of a bolometer and its structure drawing
Table 1 Specification of the bolometer used in our measurement
Before using bolometer to detect THz radiation, we need to calibrate our meter firstly. The output of the bolometer is a voltage signal; it is not directly the power of the THz radiation. We use a blackbody radiation as the thermal source to radiate electric wave for the bolometer which we need to calibrate. According to the literatures [33-34], we can get the following equation (4-1).
77 100μmto 3000μm, in the response spectrum of our Si bolometer limited by the window filter. Rpeak is the peak response in the response spectrum. R is the
distance between the bolometer and the blackbody radiation source. A and BB A d are the area of aperture in the blackbody radiation source and the detective window of bolometer, respectively. R( )λ is the response spectrum of the bolometer which is assumed to have a rectangle shape due to that the response of the Si bolometer is nearly independent of the frequencies from cut-on to cut-off wavelength, although the response of the Si bolometer actually decrease with the decrease of the frequency slightly. Voutput is the voltage value which we obtain in lock-in amplifier.
F is the modulation parameter of chopper, which is about 0.5. There is the F
experiment setup in Figure 3.2-2. Then, M( ,T)λ is the absolute power spectrum of the blackbody radiation source, which is the function of wavelengths and temperatures. The formula is given as following equation (3.2-2).
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where the unit isWcm m-1μ -1. There is the experiment setup show after calibrate Bolometer, we know that the obtained response is about 50 mV per μW with the preamplifier gain set to 180 chopping frequency. So we can use the information of the voltage value we measure in experiment to calculate the energy
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power of THz radiation. According to our calibration result, the approximate calibration factor is around 0.1 μW/mV, which means absolute THz power in proportion to measured value of locking amplifier.
Figure 4- 2 Schematic of experiment setup for measure the electric characteristics of PC antenna.
4-3 Waveform of Spectrum Measurement
431 MartinPuplett Polarization Interferometer
Method & Setup
The design of the Martin-Puplett polarization interferometer which is based on a concept originally produced by Martin and Puplett in 1969 resembles the
Bolometer
Lock in amplifier Lock in amplifier Lock in amplifier
Pump pulse
Pump pulse
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well-know Michelson interferometer, which could be used as THz FTIR. The Martin-Puplett polarization interferometer takes advantage of the polarization of electromagnetic radiation. In the field of THz frequency range, it is the most interferometer system is used. It has also been choice as the spectrometer in sub-millimeter wavelength range. Comparing the classical Michelson interferometer, it offers several advantages. Such as, the modulation efficiency of the polarizing beam splitter is higher and more uniform under wide spectral range.
The simplified schematic is shown in Figure 4-3.
We use the parabolic mirror to collect THz radiation and become the parallel beam, then we incident the parallel beam into the Martin-Puplett polarization interferometer system. The spectrometer utilized two wire grid polarizes which will discuss clearly later, used also as the polarizing beam splitters. The reflected and transmitted waves in two arms of the spectrometer have equal intensities while their directions of polarization are orthogonal to each other. The retro-reflector in each arm rotates the polarization of the incident light 180°. The orthogonal transmitted and reflected waves are recombined at wire grid polarizer #2, then propagated through the wire-grid polarizer #1 and be collected by a parabolic mirror. By scanning the movable retro-reflector, the interferogram can be measured by the bolometer and stored by the computer.
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Figure 4- 3 Schematic of a Martin-Puplett-type Fourier Transform Infrared Spectrometer (FTIR) system
Wire grids
For far-infrared spectroscopy, an effective polarizer can be made from an array of closely spaced parallel metallic wires, and it is the so-called wire grid.
When an incident electric field passes through the wire grid, it can be divided into one component parallel and one perpendicular to the wire. The parallel component induces a counteracting current in the metal and is thus reflected. In another part, the normal component can pass through pass the wire grid with a little attenuation.
If the thickness d of the wires and the spacing s are small compared to the
Lock- Lock -in Amplifier in Amplifier
Bolometer
Emitter Lock- Lock -in Amplifier in Amplifier
Bolometer
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wavelength of the incident wave, the modulus of the reflection coefficients for the electric field components parallel and perpendicular to the wires can be calculated by equation (4-3) and (4-4) [7]. In our experiment setup, the wire grid we used made of 10μm thick tungsten wire wound on a circular frame placed at a distance of 45μm. The reflectivity of the electric field for these parameters is plotted in Figure 4-4.While the parallel component is reflected nearly perfectly over the whole spectral range, the transmission of the normal component decreases towards higher frequencies.
According the experiment result, the frequency of PC antenna we measure would not be higher than 1.5THz. So we can ignore the THz power decrease from the wire grid.
Figure 4- 4 The reflectivity of the electric field dependent with frequency under (a) parallel and (b) perpendicular component.
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432 THz Time Domain Spectrometer
The THz time-domain spectroscopy system is shown in Figure 4-5. The incident pump pulse was focused by an objective lens on the biased gap of the PC antenna to generate THz radiation. The THz radiation was collimated and focused by a pair of off-axis parabolic mirror on a PC sampling detector. Which was also a PC antenna mounted on the back of a Si hemispherical lens. The PC detector was gated by femto-second probe beam pulses that were separated from the pump beam pulses by a beam splitter, and the DC photocurrent was induced by the incident electric field of THz radiation on the PC detector.
Figure 4- 5 Terahertz Time-Domain Spectroscopy
Using delaying the time of the probe pulse to the pump pulse, the time-domain
Emitter
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waveform of the electromagnetic pulse was obtained. The time resolution was limited by the carrier lifetime of the LT-GaAs used for the PC detector. To increase the signal-to-noise ratio, the pump beam was modulated with a mechanical chopper at 1 KHz, and output signal from the PC detector was measured with a lock-in amplifier and stored by the computer.
Figure 4- 6 A picture of measured photonic transmitter in a TDS system
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Reference
[1] D. H. Martin, and E. Puplett, “polarized interferometric spectrometry for the millimeter and submillimeter spectrum” Infrared Physics, Vol.10, pp. 105-109, 1969.
[2] O. Morikawa, M. Tonouchi, and M. Hangyo, “sub-THz spectroscopic system using a multimode laser diode and photoconductive antenna” Appl. Phys. Lett., Vol.75, No. 24, pp. 3772-3774, 1999.
[3] S. kono, M. Tani and K. Sakai, “Coherent Detection of mid-infrared radiation up to 60THz with an LT-GaAs photoconductive antenna” IEE, Proc-optoelectron, Vol.149, No. 3, pp. 105-109, 2002.
[4] Sang-Gyu Park, Michael R. Melloch, and Andrew M. Weiner, “Analysis of Terahertz Waveforms Measured by Photoconductive and electrooptic sampling”
IEEE J. Quantum Electronics. Vol.35, No. 5, pp. 810-819, 1999.
[5] S Kono, Masahiko Tani, and Kiyomi Sakai, “Ultrabroadband photoconductive detection: comparison with free space electro optic sampling” Appl. Phys. Lett., Vol.79, No. 7, pp. 898-900, 2001.
[6] Sang-Gyu Park, M. R. Melloch, and A. M. Weiner, “Comparison photoconductive sampling” Appl. Phys Lett., Vol.73, pp. 3184-3186, 1998.
[7] J. C. G. Lesurf. “Millimetre-wave optics, devices and systems.” Adam Hilger, January, 1990.
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