terahertz generation and detection using aperture antenna
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Overview onTerahertz Generation and
Detection using Photoconductive Aperture antennaby
Tanumoy Saha
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
Generation of Terahertz Radiation
Photoconductive technique
Non-linear optical technique
Photoconductive technique•High energy photons creates charge carriers
•Biased electric field applied accelerates charge carriers
•Accelerating charge carriers emit radiation
V+
V-
Inte
nsity
of
Exci
tatio
n la
ser p
ulse
Curr
ent
dens
ityE TH
z
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
Requirements for Generating Broadband Terahertz radiation•A photoconductive Antenna
•Ultrashort Laser pulse source
•DC source for Bias Voltage
Setup for Generating Terahertz Radiation
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.
Types of Photoconductive Antennas on the Basis of
their Design
•Aperture antennas(Small and large compared to
wavelength)
•Spiral Antennas
•Bowtie Antennas
•Dipole Antennas
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
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)
LT-GaAs
SI-GaAs
l
V+ V-
Photoconductive Aperture Antenna
Where nepi = carrier conc in the epitaxial layer
Therefore we have
LT-GaAs
SI-GaAs
l
V+ V-
Photoconductive Aperture Antenna
Similarly for substrate layer we have
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
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
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 pulser = distance from the center of the antenna to observation pointJs(t) = surface induced current density = σ(t)Eeff(t)
Photoconductive Aperture Antenna
V+
LT-GaAsl
V-
SI-GaAs
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)
EDC
Photoconductive Aperture Antenna
Surface conductivity is given by
Where I(t) is the instantaneous amplitude of the excitation pulseAnd v is the frequency of the excitation pulseAnd τ is the carrier life time
Photoconductive Aperture Antenna
Where I(t) is given by
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
Detection of Terahertz Radiation
Photoconductive technique
Non-linear optical technique
Exci
tatio
n la
ser p
ulse
Carr
ier
Den
sity
Tera
hert
z pu
lse
Curr
ent
dete
cted
time
Detection of Terahertz RadiationDynamics 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
Detection of Terahertz Radiation
FFT
Curr
ent
dete
cted
time
Ampl
itude
Frequency(THz scale)Frequency(THz scale)
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
Conclusion
So in making terahertz antennas we focus on factors effecting1. Life time of the carriers2. Mobility of the Carriers3. Density of carriers
Reference
1. Broadband THz Generation from Photoconductive Antenna by Qing Chang1, Dongxiao Yang1,2, and Liang Wang12. Terahertz Photoconductive Antennas: Principles and Applications byDaryoosh Saeedkia 3. COMPARISON OF TERAHERTZ ANTENNAS by Di LI , and Yi HUNAG4. Terahertz Spectroscopy Principles and Applications by Brian J. Thompson5. Wikipedia6. Electricity and Magnetism by DJ Griffiths7. Solid State physics by Charles Kittel
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