EE243 Quantum Electronics Seminar Robb Walters Thomas J. Watson Laboratory of Applied Physics California Institute of Technology Topics in Terahertz Optics

October 29, 2003Quantum Electonics Seminar1 Terahertz Frequencies from 0.3 to 30 THz (Semiconductor Max to CO 2 Lasers) Wavelengths: 30um to 1mm (bigger than many bio systems) Quanta from meV (lots of absorbers/emitters) Why all the interest? 1.TDS improvements 2.Bio Imaging (CT) 3.Homeland Defense 4.LO for Astronomy (98% of photons)

October 29, 2003Quantum Electonics Seminar2 THz Imaging Comparatively good penetration without damage to tissue as in xray tomography… Sensitivity to water content allows for discrimination of tissue types…

October 29, 2003Quantum Electonics Seminar3 Generation of THz Quantum Cascade Laser ~mW 4.4 THz One line Pulsed No Photo- conduction Heterodyne Ti:Sapphire +NIR GHz homodyne Requires 10kG Field

October 29, 2003Quantum Electonics Seminar4 Optically Pumped THz Lasers Manley-Rowe Efficiency Limit: half the ratio of frequencies (here, 4%) (Typically <0.1%; Record is 1%.)

October 29, 2003Quantum Electonics Seminar5 Optically 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, 2003Quantum Electonics Seminar6 Photoconductive Emitters Bandwidth is inversely proportional to pump pulse duration. (Laser pulse closes the circuit.) Dry N 2

October 29, 2003Quantum Electonics Seminar7 THz Quantum Cascade Laser Nature May 2002… 104 repetitions in active region Current Laser is 180um by 3.1mm

October 29, 2003Quantum Electonics Seminar8 THz Quantum Cascade Laser Problems with heat. Duty cycle is 0.003% on solid lines; 0.5% on dashed.

October 29, 2003Quantum Electonics Seminar9 Free Space Electro-optic Crystal Detector ZnTe …comparable electro-optic figure of merit to LiTaO3 without the limitations associated with intrinsic birefringence and a reduced sensitivity to thermal fluctuations.

October 29, 2003Quantum Electonics Seminar10 ZnTe sensitivity Transmission of PTFE shows useful bandwidth of ZnTe. (blind spots) 5.2 = ZnTe TO phonon 8.0 = GaAs TO phonon (photoconduction source) Highest resolved absorption at 32.3 THz is a “symmetric stretching vibration of CF 2 ”

October 29, 2003Quantum Electonics Seminar11 Bolometers 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, 2003Quantum Electonics Seminar12 THz (Aperture-less) NSOM Recent work from RPI group. (APL 10/13/2003)

October 29, 2003Quantum Electonics Seminar13 THz NSOM Scan 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 = /50 After 10 minutes…

October 29, 2003Quantum Electonics Seminar14 With a smaller NSOM tip… 100nm tip radius gives 150nm resolution ( /1000) …but They 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, 2003Quantum Electonics Seminar15 References OP THz Lasers: –A white paper by Eric R. Mueller of Coherent-DEOS –Industrial Physicist (8/ 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, 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 Pulses Kersting and Zhang at RPI Beere at Cambridge These papers come from:

October 29, 2003Quantum Electonics Seminar16 Since we have time… Zero Order Transmission =130um Gratings: 200nm thick gold lines, periods from 10-40um (subwavelength) Superluminal???

October 29, 2003Quantum Electonics Seminar17 …the time derivative

October 29, 2003Quantum Electonics Seminar18 As Predicted from Diffraction Theory For m=zero 0 1

October 29, 2003Quantum Electonics Seminar19 Special case of sub-wavelength grating Expand in (k*l) to 4th order…

October 29, 2003Quantum Electonics Seminar20 Accuracy of Approximation =130  m As the grating gets smaller, the approximation is more accurate (but the transmitted signal decreases too…)