Next Ray? T-Rays! Center for Terahertz Tera Hertz Wave

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1 Next Ray? T-Rays! X.-C. Zhang Center for Terahertz Tera Hertz Wave THz wave, also known as T-rays, electromagnetic radiation in a frequency interval from 0.1 to 10 THz, lies a frequency range with rich science but unexplored technology. Electronics Classical Waveguide transport industry microwaves THz Gap visible Photonics Quantum Lens and transition industry mirror x-ray γ -ray 10 0 dc Hz kilo mega giga tera peta exa zetta yotta Frequency (Hz) in different units: 1 THz ~ 1 ps ~ 300 μm ~ 4.1 mev ~ 47.6 o K in different units: 1 THz ~? ps ~? μm ~? mev ~? o K

2 Space Shuttle Columbia Tragedy, Feb. 1, 2003 bipod The Columbia tragedy was blamed on a chunk of foam that flew off the tank and struck a wing on the shuttle. Defects in the foam insulation likely caused the material to detach A Request from Lockheed Martin We manufacture the Space Shuttle External Fuel Tank and have been investigating different NDE methods for inspection of the foam insulation used on the tank since the Columbia accident several weeks ago. One of the technologies that we are investigating is THz imaging through one of our NASA partners at Langley Research Center. If I may, allow me to give a brief summary of the problem we are investigating and I'd like to see if you have any ideas how THz may be applied to this issue. As I'm sure that you are aware, it appears that one or more large sections of insulating foam broke away from the fuel tank and struck the left wing of the orbiter. This may have been the cause of the accident, so there is much interest in developing an NDE method to inspect this foam before we can return to flight. The types of things that we are interested in seeing are voids within the foam itself and delaminations between the foam and the aluminum substrate. In the particular region of interest the foam can get up to about 10 inches thick. I'd like your opinion on the applicability of THz to inspections for voids and delaminations through thick, closed cell foams

3 NASA Tested Four Imaging Modalities back-scattered x-ray imaging laser shearography imaging THz imaging microwave imaging Hz microwaves visible x-ray γ -ray Taking the Challenge Dr. Eric Madaras (NASA Langley Research Center) and RPI Researchers Defects were pre-built in the foam samples, 49 of 57 defects were detected in one of the panels

4 A Personal from a NASA Scientist Finding 49 out of 57 was impressive and I think that when I presented your results at the meeting at Marshall several weeks ago, it made a very solid impression on all present. As a matter of fact, I believe that terahertz imaging has moved up in the scheme of things at Michoud and Marshall. Three weeks ago, we participated in a blind study for finding flaws in foam in a sample with some complicating geometries. That sample was then dissected to document accuracy. The reported results that I received back yesterday were that we successfully identified 14 out of 19 flaws and that we had no false calls using terahertz. Although the seventy percent sensitivity may not sound great, it ranked with the best of the methods tested, and the no false call rate is very, very important to them Return to Flight and Beyond NASA s Implementation Plan for Space Shuttle To improve our ability to detect potential problems, NASA built backscatter x- ray and terahertz imaging prototypes, two alternative methods of advanced nondestructive inspection (NDI) of the ET foam. These two methods provide complementary data and may be used to screen for voids. Oct ,

5 THz Wave Imaging of Space Shuttle Foam 9-50 This photo was featured in Nature. 9 Why Should Anyone Care About THz Wave? Energy: objects at room temperature (300 K) emit thermal energy in this range (6 THz). Half the cosmic background radiation from the Big Bang is in the THz part of the spectrum. Signature: From GHz to THz frequencies, numerous organic molecules exhibit strong absorption and dispersion due to rotational and vibrational transitions. These transitions are specific to the targets and enable T-ray fingerprinting. Safer: T-rays have low photon energies (4 1 THz, one millions times weaker than a x-ray photon) and will not cause harmful photoionization in biological tissues

6 Where Would THz Wave Have The Most Impact? Defense: homeland security, chemical and biological agents detection, explosives detection, see-through-thewall, ambient breeze tunnel, imaging in space using satellites. Commercial: biomedical, such as skin imaging for cancer detection, forgery, mail inspection, luggage inspection, gas spectroscopy, THz tomography, non-contact & nondestructive method, such as structure damage in aircraft composites, NASA foam, etc Research: physics, plasma fusion diagnostics, electron bunch diagnostics, THz wave microscope, zero resistivity under THz radiation, left hand materials (LHM) at THz range, THz spintronics. 11 Far-Infrared Sources and Sensors Far-Infrared Sources: Free electron laser Gunn oscillators Photoconductive antenna (more power) Electro-optic crystal (optical rectification) Far-Infrared Detectors: Bolometer Pyroelectric detector Photoconductive dipole antenna (greater sensitivity) Electro-optic crystal (more bandwidth) This lecture highlights EO crystals as sources & sensors

7 THz Radiation by Carrier Transport (Real Current) reflected optical beam & THz pulse THz pulse E 1 E t (1) laser pulse + semiconductor hω E g (2) create transient current θ θ 1 0 optical beam E 2 θ 2 THz emitter J(t) = n(t) e μ E b (3) far field THz radiation (t) E THz J(t) t * Zhang and Auston, J. Appl. Phys. 71, 326 (1992). 13 THz Radiation by EO Crystal: Optical Rectification Input laser pulse I(t, ω, Δω) EO crystal THz pulse E THz (t, Ω) Δτ, Δω χ (2) P(t) 2 P(t) ETHz(t) 2 t Heisenberg s uncertainty relationship: ΔτΔω = 1 Δω Beating frequency Ω < Δω (laser bandwidth) Dielectric polarization: P(Ω) = χ (2) (Ω, ω+ω, ω) E(ω+Ω)E*(ω) E THz J(t) P(t) (t) = = χ t 2 t (2) 2 I(t) 2 t 14

8 Detection of a T-Ray (Electro-Optic Sampling) pellicle [1,1,0] [1,-1,0] Δi E THz detector Δi = i s s i p p polarizer λ/4 plate ZnTe THz beam E THz r E polarizer probe beam Q. Wu and X.-C. Zhang, Appl. Phys. Lett. 67, 3523 (1995) Sampling a THz Waveform Field (a.u) Time (ps) 16

9 Read a Book with a THz Beam Fundamentals of Photonics By Saleh and Teich about 500 pages EO Signal (μa) 1 0 Before the book Time Delay (ps) 40 The changes of amplitude, timing and shape of the waveform gave information of pages and water concentration in the book EO Signal (na) After the book Time Delay (ps) 17 Counting Pages with a THz Pulse THz Signal (a.u.) Time (ps) 360 pages 361 pages THz Signal (a.u.) pages 361 pages Time (ps) Typical time delay per printed page ranges from 130 to 200 fs. 14 AU$50 bill THz image 18

10 Counting with the Speed of Light Signal $8,000 to $9,000 one-bill increment Time Delay (ps) 5 Normalized Signal Time Delay (ps) 30 $9,000 $8,000 $7,000 $6,000 $5,000 $4,000 $3,000 $2,000 $1,000 $0 Amount of money versus THz signal time-delay. Refractive Index of a greenback ($100) is Time is Money About 4.5 bills / ps 12 random tests Dollars Experiment data Linear fitting 5 Slope: $450/ps Time Delay (ps) 20 Error (Number of Bill) Dollars 8000 Similar to the distance unit of "light year" which is a popular unit in astronomy, we could introduce a money unit: "T-Ray second". Since a THz pulse has 1 ps time delay for nearly every four and half bills, resulting 1 T-Ray second = $4.5 trillion on 20 $1 bill unit, a unit useful for the national debt ($5.7 trillion)

11 Signal and Noise Waveforms THz amplitude (a.u.) InAs Background (x10-4 ) Time Delay (ps) 21 FFT of Signal and Noise THz Amplitude (a.u.) InAs Background Frequency (THz) 22

12 1.6THz Quantum Transition: Caffeine (Gaussian 2003) 434 cm cm cm cm -1 Absorbance (a.u.) Wavenumber (cm -1 ) 23 Four Explosives Absorption Spectra α (cm -1 ) HMX 1.8THz α (cm -1 ) RDX 0.82THz 1.5THz 1.96THz α (cm -1 ) Frequency (THz) PETN 2.0THz α (cm -1 ) Frequency (THz) TNT 2.2THz Frequency (THz) Frequency (THz) 24

13 Four Non-Explosives Absorption Spectra 240 Sugar 240 Detergent α (cm -1 ) α (cm -1 ) α (cm -1 ) Frequency (THz) 240 Flour α (cm -1 ) Frequency (THz) Coffee Frequency (THz) Frequency (THz) 25 THz Spectroscopic Imaging Kawase s group in Japan demonstrated THz spectroscopic imaging introducing component spatial pattern analysis. The spatial distributions of the targets are obtained from THz multispectral image, using absorption spectra measured with a THz Parametric Oscillator Kodo Kawase, RIKEN

14 Biomedical Application: Cancerous Tissues Sample: (archived, skin tumor, dog): Connective tissueskin with Fat Tumor hairs 16 mm 34 mm Deflection coefficient at 2 THz: Masking / Tissue recognition at 2 THz: Courtesy by H. Roskos Photograph + Deflection loss < 0.05% and total loss > 95 % Movie showing THz images of surface features and skin cancer at depth overlayed on the clinical photograph. 27 Courtesy by Don Arnone NASA Panel #75 with Artificial Defects 0.2 THz image Map of 30 defects are identified 28

15 A Wilson Racquetball Racket in a Bag 0.6 THz image THz Image of a Shoe linear scale (side view) log scale (side view)

16 See-Through-The-Teapot (Empty and Half Filled) Leather Briefcase

17 DTRA and Navy s Visit THz Sensing and Imaging with a Fiber Laser IMRA Specifications Femtolite F-100 Center λ 810 nm Avg. Power 100 mw Pulse duration 150 fs Rep-rate 75 MHz Size 256 x 193 x 88 mm Absorbance (a.u.) THz THz Frequency (THz)

18 T-Rays Show Their See Through Ability New Focus in Science, 8/2/2002 STAR TIGER Passive imaging a 35 GHz could be in real-time. By QinetiQ Ugly: Privacy Issue? QinetiQ 36

19 94 GHz Images GHz Images

20 94 GHz Images GHz Images

21 94 GHz Images GHz Images

22 Defense & Security Sectors Need THz Technology Standoff distance detection of explosives Phase shift (a.u.) 2.5 m 10 m 20 m 30 m Frequency (THz) Measured phase shift φ of RDX(C-4) up to 30 meters Remote THz Sensing? Check the Weather Forecast Attenuation (db/km) versus Wavelength Attenuation db/km THz range α > 100dB/km 0.01dB/km Long distance THz sensing may be feasible by achieving low attenuation at the visible range (< 0.01 db/km) with an optical beam to generate and detect THz waves locally. 44

23 Generation & Detection of THz Wave by χ (3) Generation: E THz (t) χ (3) E 2ω (t) E* ω (t) E* ω (t) exchange Detection: E 2ω (t) χ (3) E THz (t) E* ω (t) E* ω (t) All-Air THz Photonics Filter λ/2 Waveplate Lens Si filter THz pulses Parabolic mirror Lens Filter Detector Delay Plasma BBO Lens Beamsplitter Laser pulse 120 fs, 800 μj 800 nm, 1 khz 46 In press, 2006.

24 BomDetec: Standoff Distance Detection Alert! > 10 meters THz radiation will be used for spectroscopic confirmation of a threat. Frequency resonances (signatures) of selected explosive and their related compounds will be tested versus the distance (up to 10 meters) and weather condition NATO and HSARPA Meetings

25 SPIE Professional, April, Conclusion Next a few years could be Golden Years for THz photonics Government & public have accepted the THz photonics Defense and security sectors need THz technology Industrial companies have started to invest in THz R&D Killer applications of THz technology have been identified The funding level and range are increased significantly. There are highly qualified research teams in THz field