Terahertz generation and detection using aperture antenna


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Terahertz generation and detection using aperture antenna

  1. 1. Overview onTerahertz Generation andDetection using PhotoconductiveAperture antennabyTanumoy Saha
  2. 2. What is Terahertz Radiation?Terahertz radiation, also called sub millimeterradiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, or THz, consists of electromagnetic wavesat frequencies from 40GHz to 4THz
  3. 3. Generation of Terahertz RadiationPhotoconductive techniqueNon-linear optical technique
  4. 4. Photoconductive technique•High energy photons creates charge carriers•Biased electric field applied accelerates charge carriers•Accelerating charge carriers emit radiation
  5. 5. V+V-
  6. 6. IntensityofExcitationlaserpulseCurrentdensityETHzTimeCurrent density Change due toformation of Charge Carries for ashort instant of timeDue to change in current densityETHz α dJ/dt
  7. 7. Requirements for Generating BroadbandTerahertz radiation•A photoconductive Antenna•Ultrashort Laser pulse source•DC source for Bias Voltage
  8. 8. Setup for Generating Terahertz Radiation
  9. 9. Photoconductive Antenna•For our application we use Semiconductors (GaAs)• Impurities are doped epitaxy is done for decreasing the lifetime of the carriers•Structural design and material properties of the Antennadictates efficiency of the THz radiaton that we will discuss inthe subsequent slides.
  10. 10. Types of Photoconductive Antennas on the Basis oftheir Design•Aperture antennas(Small and large compared towavelength)•Spiral Antennas•Bowtie Antennas•Dipole Antennas
  11. 11. Photoconductive Aperture AntennaMetal ContactsEpitaxial layer(carriers in thislayer has low life time thensubstrate layer)substrate layerLT-GaAsSI-GaAsl
  12. 12. Photoconductive Aperture AntennaLT-GaAsSI-GaAslWhere τr,epi= trapping time of the carriersin the epitaxial layerR = intensity reflectivity of thesurfacex = distance from surface ofsemiconductor to theobservation pointn(x,t) = carrier densityV+ V-Small Aperture Antenna(A<<λTHz)
  13. 13. LT-GaAsSI-GaAslV+ V-Photoconductive Aperture AntennaWhere nepi = carrier conc in theepitaxial layerTherefore we have
  14. 14. LT-GaAsSI-GaAslV+ V-Photoconductive Aperture AntennaSimilarly for substrate layer we have
  15. 15. Photoconductive Aperture AntennaLT-GaAsSI-GaAslV+ V-In presence of biased field the timeevolution of the velocity of carriersis given byWhere τrel = momentum relaxation timeE = local electric field
  16. 16. LT-GaAslV+ V-Photoconductive Aperture AntennaTime evolution of Polarization isgiven byWhere τrec = recombination time ofthe carriersJ(t) = surface current densitySI-GaAs
  17. 17. Now by the use of Maxwells equationelectric far field(i.e r>>λTHz) is given byWhere A = area of illumination of theexcitation pulser = distance from the center of theantenna to observation pointJs(t) = surface induced currentdensity = σ(t)Eeff(t)Photoconductive Aperture AntennaV+LT-GaAslV-SI-GaAs
  18. 18. EDCPhotoconductive Aperture AntennaLarge Aperture antenna (A >> λTHz)Then using the above approximation we haveWhere σs(t) = surface conductivityσd = threshold conductivity(conductivity at whichsubstance transfers from dielectric to metallic)
  19. 19. EDCPhotoconductive Aperture AntennaSurface conductivity is given byWhere I(t) is the instantaneous amplitude ofthe excitation pulseAnd v is the frequency of the excitation pulseAnd τ is the carrier life time
  20. 20. Photoconductive Aperture AntennaWhere I(t) is given by
  21. 21. Factors Effecting the efficiency ofaperture THz-PCAs•Trapping time of Carriers: Trapping time governs theFWHM of the carrier density thereby that of currentdensity J(t). Trapping time of the order of ps generateTHz spectrum•Effect of Laser pulse and Duration: High frequency andlow duration pulse(order of femto-seconds) generatewideband terahertz radiation•Effect of Electric field and Dipole apperture antenna:smaller aperture perfect dipole
  22. 22. Detection of Terahertz RadiationPhotoconductive techniqueNon-linear optical technique
  23. 23. ExcitationlaserpulseCarrierDensityTerahertzpulseCurrentdetectedtime
  24. 24. Detection of Terahertz RadiationDynamics of the Carriers is same as discussed earlier, Theonly difference is that instead of bias field we have thetime varying ETHz and we measure the time varyingcurrent which gives information of the frequency andamplitude of the THz radiation
  25. 25. Detection of Terahertz RadiationFFTCurrentdetectedtimeAmplitudeFrequency(THz scale)Frequency(THz scale)
  26. 26. Factors Effecting the efficiency ofdetector•Trapping time of Carriers: Trapping time governs theFWHM of the carrier density i.e the effective region ofdetectionSo for better detection τtrap<1/wTHz•Effect of Laser pulse and Duration: Amplitude dictatesthe rate of formation of effective charge carriers and soits density thereby increasing the resolution of detection•Dipole apperture antenna: small aperture moreeffective detection as it acts like perfect dipole
  27. 27. ConclusionSo in making terahertz antennas wefocus on factors effecting1. Life time of the carriers2. Mobility of the Carriers3. Density of carriers
  28. 28. Reference1. Broadband THz Generation from Photoconductive Antenna by QingChang1, Dongxiao Yang1,2, and Liang Wang12. Terahertz Photoconductive Antennas: Principles and Applications byDaryoosh Saeedkia3. 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