Terahertz radiation consists of electromagnetic waves between 40GHz and 4THz. It can be generated using photoconductive or non-linear optical techniques. Photoconductive generation works by using high energy photons to create charge carriers in a semiconductor antenna, which are accelerated by a bias electric field to emit terahertz radiation. Key components are a photoconductive antenna, ultrashort laser pulse source, and bias voltage source. Detection works similarly but measures the induced current from an incoming terahertz pulse. Important factors for efficient generation and detection include short carrier trapping/recombination times and high carrier mobility and density.

1. Overview on
Terahertz Generation and
Detection using Photoconductive
Aperture antenna
by
Tanumoy Saha

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

3. Generation of Terahertz Radiation
Photoconductive technique
Non-linear optical technique

4. Photoconductive technique
•High energy photons creates charge carriers
•Biased electric field applied accelerates charge carriers
•Accelerating charge carriers emit radiation

5. V+
V-

6. Intensityof
Excitation
laserpulse
Current
density
ETHz
Time
Current density Change due to
formation of Charge Carries for a
short instant of time
Due to change in current density
ETHz α dJ/dt

7. Requirements for Generating Broadband
Terahertz radiation
•A photoconductive Antenna
•Ultrashort Laser pulse source
•DC source for Bias Voltage

8. Setup for Generating Terahertz Radiation

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)

13. LT-GaAs
SI-GaAs
l
V+ V-
Photoconductive Aperture Antenna
Where nepi = carrier conc in the
epitaxial layer
Therefore we have

14. LT-GaAs
SI-GaAs
l
V+ V-
Photoconductive Aperture Antenna
Similarly for substrate layer we have

15. Photoconductive Aperture Antenna
LT-GaAs
SI-GaAs
l
V+ V-
In presence of biased field the time
evolution of the velocity of carriers
is given by
Where τrel = momentum relaxation time
E = local electric field

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

20. Photoconductive Aperture Antenna
Where I(t) is given by

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

23. Excitation
laserpulse
Carrier
Density
Terahertz
pulse
Current
detected
time

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