Chapter 2 Basic Theory
2.2 The properties of the material
3 4
2.2 The properties of the material
2.2-1 Choosing and analysis the material
In order to generate high power THz radiation, two properties are necessary for the material. One of properties is long carrier lifetime, and the other is high carrier mobility. Because of them, PC antenna could be generated more current flow which will also increase THz radiation power. Therefore, we can give consideration to two conditions. When we create the more recombination centers in semiconductor, it can trap carriers by using recombination centers to decrease the carrier lifetime. To create the more recombination centers, we implant the more defect in substrate and from deep level recombination. However, implanting
Figure 2.1-4 The beating intensity with an angular frequency of Ω.
In the simulation, we set I1 equals I2
the more defect, the structure of the crystal will be destroyed and decrease the
ε : quasi-static dielectric constant 0
ε : relative dielectric constant r
= 0 r
According to the Equation (2.1-1) and (2.1-2), we can know that the longer carrier lifetime the more carrier mobility. It means that we decreasing the carrier lifetime by implanting defects and the carrier mobility also decrease at the same time. Because of it, we use the way which is thermal annealing process to improve it. The thermal annealing processes can rearrangement the crystal of material which implanted defects. It can increase some of the carrier mobility. However, the carrier lifetime also increase.
2.2-2 Dark current
In theory, there is no current generated from PC antenna when we applied voltage across antenna, when it does not illuminate laser beam yet. However, we can measure a few current in PC antenna, which we call dark current. In general, the theory of current transport of the metal-semiconductor junction potential barrier is mainly drift, diffusion, and tunneling effect. It’s contribution is in relation to intrinsic potential barrierqΦ , built-in electric potential B V and bi applied voltage. In order to avoid generating rectification effect and junction resistance on the metal-semiconductor junction, it is usually using the process of implant high concentration of defect in semiconductor to increase built-in electric potential and decrease the width of depletion region. So the carrier can easily through the junction potential as applying voltage. It is the process of Ohm contact.
2.2-3 Photocurrent
Comparing with dark current, photocurrent means that CW THz is generated from photoconductive antenna which applies voltage, when it illuminates laser beam. We use the two modes laser to incident into the gap on the photoconductor antenna, we define the optical generation rate g as the number of absorbed 0 incident photons per second, i.e photon flux (P0
hv) per unit volume, multiplied by the optical quantum efficiency ηopt , which accounts for reflections at the air-semiconductor interface, as well as for the finite thickness of the
photoconductive layer. Accordingly, we define the optical quantum efficiency as (1 tLT)
opt e α
η = Τ − − , where T is the transmission coefficient at the
air-semiconductor interface and tLT is the thickness of the sample layer. Hence, the generation rate becomes:
0 Where V is the elementary volume associated with the elementary photocurrent
I .If we consider n and p the densities of free photogenerated electrons and holes ph
(assumed spatially homogeneous), we can write the current continuity equation:
0 0
=τ is the electron recombination rate. In fact, in case of LT-GaAs and GaAs:O photoconductor one should replace the recombination rate with the trapping rate. The same equation holds for holes. For an uniformly illuminated photoconductor, the electron and hole current densities are given only by the drift components, since there is no diffusion due to the lack of spatial dependence (∂/
∂x ≣ 0): wherev,e,h is the field-dependent electron (hole) drift velocity. The total current density is J =Jn +Jp , where, at this point the holes contribution to the photocurrent is discarded due to their low mobility in material. Moreover, the low mobility of holes produces a small displacement of the hole population in the external applied field, whereas due to their much higher mobility a significant
number of electrons are displaced into the cathode (+); hence, for maintaining the space-charge neutrality in the illuminated gap and current continuity at the electrodes. A replenishment of electrons at the anode is necessary (i.e. electron injection). The DC photocurrent flowing into the external circuit is given by:
( )
DC
ph e
I = Aenv E (2.2-6) where A is the total contact area. With g independent of time, that is ∂/∂t ≣ 0 0 at steady state, from Eq. 2.2-4, the density of photogenerated electrons becomes:
( 0) and by substituting it into Eq. 2.2-6 we obtain:
( 0) ( )
We can regroup the terms in Equation 2.2-8, by denoting the primary photocurrent ( 0)
p
ph opt
I e P η hv
= and introducing the photoconductive gain
( )
=v E is the photocarrier transit time. In other words, the photoconductive gain is determined by how fast the electrons can transit across the electrodes and contribute to the photocurrent in the circuit before they recombine with the holes, or as in the case of material, they become trapped. In most of the existing designs of photomixers, the carrier transit time is neglected since the transittime for an electrode spacing of ∼1 µm is much larger than the carrier lifetime. Forexample, assuming a trapping time of ∼1 ps and a drift velocity of ∼4.4×106cm/s,this yields a transit time of ∼25 ps and a gain of about 0.04 (<<1), hence the low quantum efficiency of photoswitches. This large
difference between the carrier transit and lifetime means that the photomixer is a carrier lifetime dominated device.
In Eq. 2.2-8, the electron drift velocity is a ected by the saturation effects and has the general form: fields one can assume a field-independent mobility, such that e( ) e eV
v E E
μ μ d
Replacing in Eq. 2.2-8, we obtain:
opt 0
If we now consider the photocurrent modulated at a THz frequency, we can write:
0
We can also use the ultra-pulse laser to incident into the gap on the photoconductor antenna, and generate the transient photocurrent to radiate THz electromagnetic pulse. The value of photocurrent is in relation to the bias value and the power of the laser beam. It can express as:
0 R is time-independence value of resistance. It can divide into two parts, one is 0
contact resistance and the other is load resistance. R(t) is time-dependence value of resistance which photoconductor illuminates laser beam. We set that there is an
incident beam on the photoconductor and using the photon energy generate the electron-hole pair. And then, the photoconductor parameter σ can be expressed as:
n p
=q( + ) n
σ μ μ Δ (2.2-13) where q is the charge of electron, μ and n μ are the mobility of electron and hole. p Δ is the density of carrier. If the average power of incident optical pulse is n P and photon energy is hν . The carrier generation rate G per unit volume can avg
express as: repetition rate of laser pulse and V is the total volume of activity region. 0
Under the steady state, the carrier generation rate G is:
pw From Equation (2.2-16), we can get the optical resistanceR . p
2 where A is the cross-section area of activity region and L is the length of activity region.
From Equation (2.2-12) and Equation (2.2-17), we can express the photocurrent as:
Chapter 3 Experiment Method and Setup
In this chapter, we introduce the samples which are fabricated for our PC antenna. And then, we introduce the experiment methods and setups to measure the emission properties of the THz radiation.
3.1 Sample preparation
We used the dipole antenna as the antenna structure in our experiment. Large signal and high bandwidth can be obtained from this kind of structure [38]. The photoconductive antenna structure is shown in Figure 3.1-1. Knon et al. proved that the spectral response of the PC antenna for broadband detection is mainly determined by the temporal behavior of the number of photo-excited carriers [22].
Antenna width Gap
Antenna length Transmission line width
Figure 3.1-1 Schematic diagram of the PC antenna
Recently, Salem et al. [32] improved the characteristics of GaAs substrate and generated THz waves by implanting oxygen ion. Normally, oxygen ion is considered as dust (a disadvantage) for electrical devices fabrication during the process, as its existence will make the contacts to be not Ohmic-contact. But, in the THz generation, it can become advantages. As the electrical level formed by oxygen ion in GaAs is close the Femi-level, it makes the oxygen ion implanted GaAs close to electrically neutral and has comparative high resistance. This kind of material also have high breakdown voltage; and short carrier lifetime by implanted under certain high dosage and annealed under suitable annealing temperature.
3.1-1 Preparation of the PC antenna
By using multi-implanted GaAs:O as the substrate and then fabricated dipole antenna on it. And our sample is fabricated by KeJian Chen who studies in CUHK.
From some groups [29-30, 32], the GaAs:O is further studied. They demonstrated that this kind of material really has some advantages for THz waves generation, and can generate higher power and higher frequency if we can optimize the implant dosage, annealing temperature and introduce some antenna design for high frequency purpose.
By optimize the dosage and the implanted energy to match the requirements.
Considering the cost of time for RBS facility, we prepared two kinds of materials which have different carrier life times and mobilities as the same dipole antenna
structure. In implanting process, multi-implants dosage concentrations is
13 2
2.5 10 ions/cm× (500Kev & 800Kev) and 4 10 ions/cm× 13 2(1200kev). After implantation process, we performed the thermal annealing by RTA (Rapid Thermal Annealing) under N2 gas environment and use a GaAs cap to prevent As ion absorption. In our experiment, we performed annealing at 500℃ for 60s. (For reference, under 500℃ for 60s annealing, the resistance of this GaAs:O substrate is close to 1.26 10 (× 7 Ω sq)) and we also measured the carrier life time of both materials by optical-pump probe measurement. The carrier life time of GaAs:O is 550fs and LT-GaAs is 700fs. The fitting curve are shown in figure 3.1-2
We also calculated the conductivity of GaAs:O and LT-GaAs. The conductivity of GaAs:O is 3.8x10 -2Ω-1cm-1 and LT-GaAs is 1.7x10-4Ω-1cm-1, the measurement are shown in figure 3.1-3.
500 1000 1500 2000 2500 3000
-0.2
500 1000 1500 2000 2500 3000
-0.2
Figure 3.1-2 The measurement of carrier life time with both materials
3.1-2 The setup of PC antenna
After preparing the PC antenna, we put the PC antenna on the mount which we designed. We used objective lens to focus pump laser on the dipole antenna.
And then, we used the silicon lens to decrease the angle of the THz radiation outgoing the substrate. The type of our silicon lens is hyper-hemispherical lens, the radius is 6.75mm and the total thickness is 8.35mm. In order to make sure the right position of laser beam, we used two steps. For the first step, we used a thin glass and put it before the objective lens. Then, we can see the image on the screen reflected from the dipole antenna and check the location of focused laser beam on the sample. The detail experiment method is shown in the Figure 3.1-4. After checking the correct location the next step was to contact the multi-meter to the dipole antenna for measuring the resistance, and then optimized the objective lens Figure 3.1-3 The measurement of dark current and conductivity of GaAs:O and
LT-GaAs
and incident pump beam until search the minimum value of resistance was found.
3.2 Dual wavelength laser diode system
3.2-1 Laser diode characteristics
In order to measuring the better THz radiation power, we directly excited our PC antenna by using two frequency-independent circular laser diodes (SANYO DL-8032-001) with output power 57mW and wavelength around 830nm. The L-I curve of the laser diodes are showing in Fig. 3.2-1 and Fig 3.2-2
PC Antenna Afterimage
Thin glass
Figure 3.1-4 Schematic of focused the pump laser on the gap of dipole antennas.
0 50 100 150 200 0
10 20 30 40 50 60
Power (mW)
Current (mA) LD1
0 50 100 150 200
0 10 20 30 40 50 60
Power (mW)
Current (mA) LD2
The linewidth of LD1 and LD2 measured by Fabry-Perot interferometer are shown in Fig 3.2-3 and Fig 3.2-4, where their linewidth are 22MHz and 21MHz, respectively.
Figure 3.2-2 L-I curve of laser diode 2 (LD2), Ith=50mA Figure 3.2-1 L-I curve of laser diode 1 (LD1), Ith=50mA
0.02 0.04 0.06 0
2
Amplitude (a.u.)
scan time (s) FSR=2GHz
(0.006264s)
(0.00068s)
21.7MHz
0.04 0.06 0.08
0 1
Amplitude (a.u.)
scan time (s)
(0.00066s)
21.0MHz FSR:2GHz
(0.06284s)
In our experiment, we can get CW the THz radiation with different central frequency by tuning the wavelength of each LD. Therefore, by controlling the operation current and temperature, we can tune the wavelength difference between
Figure 3.2-3 Linewidth of LD1 measured by Fabry-Perot
Figure 3.2-4 Linewidth of LD2 measured by Fabry-Perot
the two laser diodes. The accuracy of the current driver (New port 505B) and the temperature controller (New port 325B) are ± 0.004mA and ± 0.1 ℃ respectively.
Figure 3.2-5 Temperature control at current 190mA for LD1.
The inset picture is Fabry-Perot interferogram of the laser diode 1.
Figure 3.2-6 Temperature control at current 190mA for LD2.
The inset picture is Fabry-Perot interferogram of the two laser diodes.
10 15 20 25 30
In figures 3.2-5 and fig 3.2-6, the wavelength shift caused by changing temperature are 11.6GHz/℃(LD1) and 13.2GHz/℃(LD2) ; the shift caused by shifting current are 2.3GHz/mA(LD1) and 2.1GHz/mA(LD2).
0.04 0.05 0.06
Figure 3.2-7 Current control at 23℃ for LD1.
The inset picture is Fabry-Perot interferogram of the laser1.
Figure 3.2-8 Current control at 23℃ for LD2.
The inset picture is Fabry-Perot interferogram of the laser2.
100 120 140 160 180 200 360.6
100 120 140 160 180 200 359.6
We used Fabry-Perot interferometer to measure the frequency stability of the two laser diodes. We recorded the fluctuation of frequency of two laser diodes on the oscilloscope around 20 minutes. The individual frequency shifts of LD1 and LD2 are 416 MHz and 554.MHz, respectively.
Figure 3.2-9 Frequency fluctuation of LD1 in 20 minutes
FSR
Table 3-1 is a brief summary of the characteristics of the two laser diodes
3.2-2 Dual-wavelength laser diodes system
LD1 LD2
wavelength ~830nm ~830nm
Power 57mW@190mA 57mW@190mA
linewidth 22 MHz 21 MHz
Current v.s.
frequency 2.3GHz/mA 2.1GHz/mA
Temperature v.s.
freqency 11.6GHz/℃ 13.2GHz/℃
Frequency
fluctuation 416MHz@20mins 554MHz@20mins
Figure 3.2-10 Frequency fluctuation of LD2 in 20 minutes
FSR
Table 3.1 A brief summary of the characteristics of the two laser diodes
Our dual-wavelength light source system is presented in figure 3.2-7. It is composed of two frequency-independent laser diodes and some proper optics. The polarization of the LD is S polarization. The 40X objective lens are used to focus the spot size of the two LD, when light passes through the isolator, the polarization of the incident light will rotated to45 to prevent the optical feedback 0 from other optics, when the light passes through the half-wave plate, the polarization will be changed from linear polarization to elliptical polarization.
We measure the output spectrum by optical spectrum analyzer (OSA, MS 9030, resolution 0.01nm) is shown in figure 3.2-8.
λ/2 plate
λ/2 plate λ/2 plate polarizer
isolator
isolator
Laser diode 1
Laser diode 2
Figure 3.2-9 schematic of dual-wavelength system
To optimize beating efficiency, we need RF spectrum to check it. Because the maximum frequency can be detected from our spectrum analyzer is 3GHz, we must adjust the frequency difference of two laser diodes lower than 3GHz.The low frequency beating signal is shown in figure 3-.2-8
.
0.5 1.0 1.5 2.0 2.5
-70 -60 -50 -40
Relative Power (dBm)
Frequency (GHz)
Beating at 2.3GHz T1=21.2℃
T2=12.3℃
832 833 834 835
-65 -60 -55 -50
power (dBm)
wavelength (nm) λ1=831.82nm λ2=834.42nm at 23℃ 190mA
Figure 3.2-11 The beating signal of the PC antenna Figure 3.2-10 The spectrum of 2 laser diodes system
3.3 Experimental setup of generation and detection of THz
3.3-1 Experimental setup
The emitted THz radiation was collimated and focused by a pair of off-axis parabolic mirrors onto the detector. We used a High-Voltage Source Meter (Model2410) to provide bias to the PC antenna and measure the current-voltage curve. To measure total THz power, we used a 4.2 K liquid-helium-cooled Si bolometer as our detector, which was carefully calibrated with a blackbody radiation source. The experiment setup for generation and detection of THz radiation is shown in Figure 3.3-1.
MOPA Pulse
Figure 3.3-1 Schematic of experimental setup for measure the electric characteristics of PC antenna
3.3-2 The calibration of the Si bolometer
We need to calibrate our meter first before using bolometer to detect THz radiation. The output of the bolometer is a voltage signal. We used a blackbody radiation as the standard thermal source to the bolometer for calibration. Due to the literatures [40-41], we can get the following equation (3.3-1).
2 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 are the area of aperture in the blackbody radiation source and the detective d
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. F The experiment setup is shown in figure 3.3-1. 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 (3.3-1).
4
where the unit isWcm m-1μ -1. There is the experiment setup show after calibrate bolometer, we know that the obtained response is about 5.26 mV per μW with the preamplifier gain set to 200 chopping frequency. So we can use the information of the voltage value we measure in experiment to calculate the energy power of THz radiation.
3.3-3 Semiconductor Laser Amplifier (MOPA)
The output power of single mode laser diodes is very limited and not sufficient for a variety of applications. In order to have higher output power, the output beam of the laser diodes is coupled into an amplifier with a tapered gain region. For this, the collimated beam of the laser diodes is focused on the entrance facet of the amplifier laser diode. The amplifier has an anti-reflection coating on both facets to prevent laser emission without seeding.
Because of the broad gain profile, the so-called” Tapered amplifiers” have a wide tuning range of some ±20 nm depending on the center wavelength.
We use TOPTICA Photonics AG BoosTA semiconductor Laser Amplifier and also test amplifier gain. Figure 3-2 shows that gain of amplifier with different wavelengths.
3.4 The time & frequency domain of THz pulse
In order to measure the time and frequency domain of THz properties generated from PC antenna, we used Time-Domain Spectroscopy system (THz-TDS) which can get time and frequency domain THz radiation information.
3.4-1 Terahertz Time-Domain Spectroscopy (THz-TDS)
Figure 3.4-1 shows that Terahertz Time-Domain Spectroscopy (THz-TDS) experimental setup. 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
359.5 360.0 360.5 361.0 361.5 362.0 13
14 15
relavtive intensity (dB)
Frequency (THz)
Gain
Figure 3.3-2 Frequency dependence of amplifier gain
PC detector. Using delaying the time of the probe pulse to the pump pulse, the time-domain 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.
PC detector. Using delaying the time of the probe pulse to the pump pulse, the time-domain 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.