2. What is Terahertz Radiation?
Terahertz radiation, also called sub millimeter
radiation, terahertz waves, terahertz light, T-rays, T-waves, T-
light, T-lux, or THz, consists of electromagnetic waves
at frequencies from 40GHz to 4THz
9. Photoconductive Antenna
•For our application we use Semiconductors (GaAs)
• Impurities are doped epitaxy is done for decreasing the life
time of the carriers
•Structural design and material properties of the Antenna
dictates efficiency of the THz radiaton that we will discuss in
the subsequent slides.
10. Types of Photoconductive Antennas on the Basis of
their Design
•Aperture antennas(Small and large compared to
wavelength)
•Spiral Antennas
•Bowtie Antennas
•Dipole Antennas
11. Photoconductive Aperture Antenna
Metal Contacts
Epitaxial layer(carriers in this
layer has low life time then
substrate layer)
substrate layer
LT-GaAs
SI-GaAs
l
12. Photoconductive Aperture Antenna
LT-GaAs
SI-GaAs
l
Where τr,epi= trapping time of the carriers
in the epitaxial layer
R = intensity reflectivity of the
surface
x = distance from surface of
semiconductor to the
observation point
n(x,t) = carrier density
V+ V-
Small Aperture Antenna(A<<λTHz)
16. LT-GaAsl
V+ V-
Photoconductive Aperture Antenna
Time evolution of Polarization is
given by
Where τrec = recombination time of
the carriers
J(t) = surface current density
SI-GaAs
17. Now by the use of Maxwells equation
electric far field(i.e r>>λTHz) is given by
Where A = area of illumination of the
excitation pulse
r = distance from the center of the
antenna to observation point
Js(t) = surface induced current
density = σ(t)Eeff(t)
Photoconductive Aperture Antenna
V+
LT-GaAsl
V-
SI-GaAs
18. EDC
Photoconductive Aperture Antenna
Large Aperture antenna (A >> λTHz)
Then using the above approximation we have
Where σs(t) = surface conductivity
σd = threshold conductivity(conductivity at which
substance transfers from dielectric to metallic)
19. EDC
Photoconductive Aperture Antenna
Surface conductivity is given by
Where I(t) is the instantaneous amplitude of
the excitation pulse
And v is the frequency of the excitation pulse
And τ is the carrier life time
21. Factors Effecting the efficiency of
aperture THz-PCAs
•Trapping time of Carriers: Trapping time governs the
FWHM of the carrier density thereby that of current
density J(t). Trapping time of the order of ps generate
THz spectrum
•Effect of Laser pulse and Duration: High frequency and
low duration pulse(order of femto-seconds) generate
wideband terahertz radiation
•Effect of Electric field and Dipole apperture antenna:
smaller aperture perfect dipole
22. Detection of Terahertz Radiation
Photoconductive technique
Non-linear optical technique
24. Detection of Terahertz Radiation
Dynamics of the Carriers is same as discussed earlier, The
only difference is that instead of bias field we have the
time varying ETHz and we measure the time varying
current which gives information of the frequency and
amplitude of the THz radiation
25. Detection of Terahertz Radiation
FFT
Current
detected
time
Amplitude
Frequency(THz scale)Frequency(THz scale)
26. Factors Effecting the efficiency of
detector
•Trapping time of Carriers: Trapping time governs the
FWHM of the carrier density i.e the effective region of
detection
So for better detection τtrap<1/wTHz
•Effect of Laser pulse and Duration: Amplitude dictates
the rate of formation of effective charge carriers and so
its density thereby increasing the resolution of detection
•Dipole apperture antenna: small aperture more
effective detection as it acts like perfect dipole
27. Conclusion
So in making terahertz antennas we
focus on factors effecting
1. Life time of the carriers
2. Mobility of the Carriers
3. Density of carriers
28. Reference
1. Broadband THz Generation from Photoconductive Antenna by Qing
Chang1, Dongxiao Yang1,2, and Liang Wang1
2. Terahertz Photoconductive Antennas: Principles and Applications by
Daryoosh Saeedkia
3. COMPARISON OF TERAHERTZ ANTENNAS by Di LI , and Yi HUNAG
4. Terahertz Spectroscopy Principles and Applications by Brian J. Thompson
5. Wikipedia
6. Electricity and Magnetism by DJ Griffiths
7. Solid State physics by Charles Kittel