EE243 Quantum EE243 Quantum Electronics SeminarElectronics Seminar

Robb Walters

Thomas J. Watson Laboratory of Applied Physics

California Institute of Technology

Topics in Terahertz Optics

QuickTime™ and aBMP decompressor

are needed to see this picture.

October 29, 2003 Quantum Electonics Seminar 2

Terahertz• 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 improvements

2.Bio Imaging (CT)

3.Homeland Defense

4.LO for Astronomy (98% of photons)

October 29, 2003 Quantum Electonics Seminar 3

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, 2003 Quantum Electonics Seminar 4

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, 2003 Quantum Electonics Seminar 5

Optically Pumped THz Lasers

Manley-Rowe Efficiency Limit:

half the ratio of frequencies (here, 4%)

(Typically <0.1%; Record is 1%.)

October 29, 2003 Quantum Electonics Seminar 6

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, 2003 Quantum Electonics Seminar 7

Photoconductive EmittersBandwidth is inversely proportional to pump pulse duration. (Laser pulse closes the circuit.)

Dry N2

October 29, 2003 Quantum Electonics Seminar 8

THz Quantum Cascade Laser

Nature May 2002…

104 repetitions in active region

Current

Laser is 180um by 3.1mm

October 29, 2003 Quantum Electonics Seminar 9

THz Quantum Cascade Laser

Problems with heat.

Duty cycle is 0.003% on solid lines; 0.5% on dashed.

October 29, 2003 Quantum Electonics Seminar 10

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, 2003 Quantum Electonics Seminar 11

ZnTe sensitivity

Transmission 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 12

Bolometers

QuickTime™ and aTIFF (Uncompressed) decompressor

are 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 13

THz (Aperture-less) NSOMRecent work from RPI group. (APL 10/13/2003)

October 29, 2003 Quantum Electonics Seminar 14

THz 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 3m = /50

After 10 minutes…

October 29, 2003 Quantum Electonics Seminar 15

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, 2003 Quantum Electonics Seminar 16

References

• 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 Pulses

Kersting and Zhang at RPI

Beere at CambridgeThese papers come from:

October 29, 2003 Quantum Electonics Seminar 17

Since we have time…

Zero Order Transmission=130um

Gratings:200nm thick gold lines, periods from 10-40um (subwavelength)

Superluminal???

October 29, 2003 Quantum Electonics Seminar 18

…the time derivative

October 29, 2003 Quantum Electonics Seminar 19

As Predicted from Diffraction Theory

For m=zero

0 1

October 29, 2003 Quantum Electonics Seminar 20

Special case of sub-wavelength grating

Expand in (k*l) to 4th order…

October 29, 2003 Quantum Electonics Seminar 21

Accuracy of Approximation

=130m

As the grating gets smaller, the approximation is more accurate (but the transmitted signal decreases too…)