Far infrared (Terahertz) Technology

Introduction to TerahertzTechnology and Applications

Terahertz Pulsed Imaging forNondestructive Inspection ofPharmaceutical Tablet

The polarization induced in the material

THz imaging

14th CLRC 2007

M. Jalal Khan, Jerry C. Chen, Sumanth KaushikMIT Lincoln Laboratory

Optical Detection of Terahertz using Nonlinear Parametric Upconversion

This work is sponsored by the United States Air Force under AF Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.

Second-order - (2)- Nonlinear Optical Interactions

• Frequency mixing via non-linear polarization current density

• Very fast

• Pump photons are down-converted to signal and idler

• Efficiency is highly sensitive to phase-matching

• Related work: Y.J. Ding, Solid St Electron, 2006, p. 1128

Detection of optical sideband photon implies presence of THz photon

Polarization = (1) E + (2) E2

Optical Detection of THz: Laboratory Implementation•THz source: Backward Wave Oscillator

(2.5 mW at 700 GHz)

•Optical Source: 1550 nm, 1 W avg, 10 ns pulses, 200 kHz rep rate

•Bulk GaAs, 4 mm thick

•Commercial telecomm thin film band-edge and band-pass filters

•New Focus PIN, NEP ~ 23 fW/Hz1/2

Optical Detection of THz: Experimental Results

•THz source is chopped; optical pump

is un-chopped

•Optical idler from mixing of Pump

and THz expected to have tones at

chopper frequency

•Chopper tones disappear when THz

is off.

•1.7 mW of THz power inside GaAs

•100 mW pump: SNR ~ 80 dB; 220

pW average idler power; intrinsic

photon conversion efficiency is 10-4 %

•1 W pump: 2.1 nW average idler

power; intrinsic photon conversion

efficiency is 10-3 %

Optical Detection of THz: Experimental Results

THz detection using bulk GaAs

• Successfully demonstrated THz detection in bulk GaAs by up-converting to optical frequencies

• 1550 nm pump was mixed with 700 GHz BWO source

• Expect and observe Idler generated at difference frequency, 1555.6 nm (1550 nm – 700 GHz)

• Terahertz-to-optical photon conversion efficiency of 0.001 %

Optical Detection of THz: Comparison with Theory

Experiment Theory

• 5 dB discrepancy between theory and experiment• Mode mismatch between optical pump and

multimode THz beam• Phase mismatch Dk uncertainty• Plane wave estimate used instead of Gaussian beams

approximation

690 pW

Improving Conversion Efficiency using Quasi Phase Matching

• Efficient nonlinear conversion requires phase-matched process

• Phase-match by reversing polarity of nonlinearity periodically

• Can be achieved by alternating orthogonal orientations

• Fabricated QPM crystal by diffusion-bonding GaAs pieces of orthogonal orientation.

• 20 pieces, 500 mm diffusion-bonded GaAs (DB-GaAs) crystal shown above

• Additional crystals of varying periods and aggregate lengths have been fabricated