EE243 Quantum Electronics SeminarTerahertzTHz ImagingGeneration of THzOptically Pumped THz LasersSlide 6Photoconductive EmittersTHz Quantum Cascade LaserSlide 9Free Space Electro-optic Crystal DetectorZnTe sensitivityBolometersTHz (Aperture-less) NSOMTHz NSOMWith a smaller NSOM tip…ReferencesSince we have time……the time derivativeAs Predicted from Diffraction TheorySpecial case of sub-wavelength gratingAccuracy of ApproximationEE243 Quantum EE243 Quantum Electronics SeminarElectronics SeminarRobb WaltersThomas J. Watson Laboratory of Applied PhysicsCalifornia Institute of TechnologyTopics in Terahertz OpticsQuickTime™ and aBMP decompressorare needed to see this picture.October 29, 2003 Quantum Electonics Seminar 2Terahertz•Frequencies from 0.3 to 30 THz (Semiconductor Max to CO2 Lasers)•Wavelengths: 30um to 1mm (bigger than many bio systems)•Quanta from 1 - 100 meV (lots of absorbers/emitters)•Why all the interest?1.TDS improvements2.Bio Imaging (CT)3.Homeland Defense4.LO for Astronomy (98% of photons)October 29, 2003 Quantum Electonics Seminar 3THz ImagingComparatively good penetration without damage to tissue as in xray tomography…Sensitivity to water content allows for discrimination of tissue types…October 29, 2003 Quantum Electonics Seminar 4Generation of THzQuantumCascadeLaser ~mW4.4 THzOne linePulsedNoPhoto-conductionHeterodyne Ti:Sapphire+NIRGHz homodyneRequires 10kG FieldOctober 29, 2003 Quantum Electonics Seminar 5Optically Pumped THz LasersManley-Rowe Efficiency Limit:half the ratio of frequencies (here, 4%)(Typically <0.1%; Record is 1%.)October 29, 2003 Quantum Electonics Seminar 6Optically Pumped THz Lasers•Currently deployed at the South Pole, airborne systems, included in a satellite scheduled for launch next year that will study the ozone layer (AURA)Spatial Mode Profile at 2.5THz.October 29, 2003 Quantum Electonics Seminar 7Photoconductive EmittersBandwidth is inversely proportional to pump pulse duration. (Laser pulse closes the circuit.)Dry N2October 29, 2003 Quantum Electonics Seminar 8THz Quantum Cascade LaserNature May 2002… 104 repetitions in active regionCurrentLaser is 180um by 3.1mmOctober 29, 2003 Quantum Electonics Seminar 9THz Quantum Cascade LaserProblems with heat.Duty cycle is 0.003% on solid lines; 0.5% on dashed.October 29, 2003 Quantum Electonics Seminar 10Free Space Electro-optic Crystal DetectorZnTe…comparable electro-optic figure of merit to LiTaO3 without the limitations associated with intrinsic birefringence and a reduced sensitivity to thermal fluctuations.October 29, 2003 Quantum Electonics Seminar 11ZnTe sensitivityTransmission of PTFE shows useful bandwidth of ZnTe.(blind spots)5.2 = ZnTe TO phonon8.0 = GaAs TO phonon (photoconduction source)Highest resolved absorption at 32.3 THz is a “symmetric stretching vibration of CF2”October 29, 2003 Quantum Electonics Seminar 12BolometersQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Fancy bolometers use superconducting elements at the edge of transistions and SQUIDs to measure current fluctuations…All require cryogenic cooling for acceptable performance.October 29, 2003 Quantum Electonics Seminar 13THz (Aperture-less) NSOMRecent work from RPI group. (APL 10/13/2003)October 29, 2003 Quantum Electonics Seminar 14THz NSOMScan of a 1um W tip held 200nm over 10um wide lines of Au.THz modulation is 0.2 to 0.6% of the total received power.Resolution is about 3m = /50After 10 minutes…October 29, 2003 Quantum Electonics Seminar 15With a smaller NSOM tip…100nm tip radius gives 150nm resolution (/1000)…butThey still see about 0.4% modulation, while they had predicted 1000 times less signal for a smaller spherical tip (0.1 R)They suggest local field enhancement due to “antenna” geometry (of 1000)October 29, 2003 Quantum Electonics Seminar 16References•OP THz Lasers:–A white paper by Eric R. Mueller of Coherent-DEOS–Industrial Physicist (8/9 2003 Edition) Article by the same guy•Photoconduction (Broadband) Emitters and TDS–(APL10/13, 2003) Shen et al: Ultrabroadband terahertz radiation from low-temperature-grown GaAs photoconductive emitters•THz Quantum Cascade Laser–Nature May 9, 2002 - Terahertz Semiconductor Heterostructure Laser•General discussion in Nature “News Feature” (August 14, 2003)•ZnTe ElectroOptic Detectos–APL paper from 1996: Wu et al: Broadband detection capability of ZnTe electro-optic field detectors•THz NSOM: (APL10/13) Chen et al: Terahertz imaging with nanometer resolution•Optics Letters 12/15/2001: Filin et al: Time Domain Differentiation of THz PulsesKersting and Zhang at RPIBeere at CambridgeThese papers come from:October 29, 2003 Quantum Electonics Seminar 17Since we have time…Zero Order Transmission=130umGratings:200nm thick gold lines, periods from 10-40um (subwavelength)Superluminal???October 29, 2003 Quantum Electonics Seminar 18…the time derivativeOctober 29, 2003 Quantum Electonics Seminar 19As Predicted from Diffraction TheoryFor m=zero 01October 29, 2003 Quantum Electonics Seminar 20Special case of sub-wavelength gratingExpand in (k*l) to 4th order…October 29, 2003 Quantum Electonics Seminar 21Accuracy of Approximation=130mAs the grating gets smaller, the approximation is more accurate (but the transmitted signal decreases