Terahertz generation and detection using aperture antenna
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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.
Terahertz generation and detection using aperture antenna
- 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
- 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