1. Single Point THz Imagery Jaewook Ahn KAIST - physics Postech, March 2010 Thanks to collaborators Kanghee Lee Kyung Hwan Jin Prof. Jong Ye (Kaist – biosystem) Funding Image encryption and decryption through THz waveforms.

2. Source: Terahertz waves KAIST THz waves Pulsed THz Ultrafast laser based FEL accelerator THz RadioMicrowaveInfraredUVX-rays Visible Frequency (Hz) 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 10 17 = 1 THz = 300  m h = 33 cm -1 T = 48 K 4.1 meV THz ultrasonic BWO Photomixing CO2 pumped FIR QCL ~100 fs nW-  W 10-100 kV/cm

3. What’s special about THz Optics ?  Wavelength = ~100 m  MEMS fabrication  Laser micro-machining  Extremely broad bandwidth : df/f=1  Dispersion engineering  Sub-diffraction limit optics ~   Coherent Emission and Detection  Laser Induced Terahertz Emission  Amplitude and Phase measurement

4. The First Terahertz Imaging First THz imaging ???

5. Far-Infrared Imagery T. S. Hartwick, D. T. Hodges, D. H. Barker, and F. B. Foote, Applied Optics 15, 1919 (1976). Source: 0.3-1THz (Commercial HCN laser, etc) 1-10mW Detector: liquid helium –cooled GaAs. Future direction: coherent detection, “Rapidly advancing FIR technology indicates that an FIR imaging system can be developed for industrial, military, law enforcement, and medical applications in the next few years.”

6. The First Terahertz Imaging They predicted …

7. Hu and Nuss : Future Directions “Imaging with Terahertz Waves” Hu and Nuss, OL 20, 1716 (1995). 1.“In future implementations, the THz beam could be scanned acr oss the sample instead.” 2.“With current microelectronics fabrication technology, one should be able to fabricate a 100 x100 focal-plane array of photo-condu cting dipole antennas to replace the single dipole detector that w e used.” 3.“An obvious future improvement of the T-ray imaging technology will include the use of speech recognition algorithms for recognit ion of the THz waveforms in amplitude and phase.” THz beam over the sample …

8. Single-Pixel THz Camera 300 번 (30%) 600 번 (60%) “A single-pixel THz imaging system based on compresssed sensing” Chan, Charan, Takhar, Kelly, Baraniuk and Mittleman, APL 93, 121105 (2008) Array detector imaging…

9. Real-Time THz Imaging : QCL “Real-time terahertz imaging over a standoff distance” Lee, Qin, Kumar, Willams and Hu, APL 89, 11125 (2006).  Standoff operation (>25 m)  Real-time operation  QCL 50 mW power : bright source  uncooled microbolometer camera : low sensitivity  To use atmospheric windows at 4.9 THz, 1.5 THz, etc.  Images taken with 1 s (20 frames) : Res. <0.75 mm THz beam over the sample …

10. THz Reciprocal Imaging “Terahertz wave reciprocal imaging” Xu and Zhang, APL 88, 151107 (2006).  Single detector to read out 2D target.  2D signals are separated in timed sequence.  To avoid crosstalk : mod. freq. are prime numbers.  Need source array with each modulated at a different frequency. Still needs a lot of development …

11. Fresnel Lens THz tomography KAIST Targets are along the beam line. z= 3, 4, 7cm. Patterns are images at z’=6cm. The corresponding focal lengths are achieved at 0.75, 1.24, and 1.57 THz. Wang and Zhang (2002).

12. None of these have spectroscopic capability, and THz beams were used as a simple wave. Image encryption and decryption through THz waveforms ??? Here is how. Simple waveComplex wave KAIST - Physics THz CDMA imaging…

13. Image encryption and decryption through EM waveforms. KAIST += Analog Optical Computing Signal Processing Digital Image Recovery KAIST - Physics

14. THz Single point Imagery : First Look (a) Target (b) E(t,  ) (c) E(  ) (d) Sinogram (e) d  (f) d  (g) d  (h) Simulation (a)(b) (c)(d) (e)(f) (g)(h) KAIST - Physics (1) Fourier mask selects spatial frequencies of the object and maps into THz spectrum. E(k  )  E(  ) (2) Temporal waveforms deliver object function. E(x,y) -> E(t,  ) (3) Single waveforms for 2D imaging ? Anatomy of the procedure…

15. (a)(b) (c)(d) Images Encrypted in Waveforms (a) Target (c) E(  ) (b) E(t,  ) (d) Sinogram Sinogram : A visual representation of the raw data obtained in a computed axial tomography (CAT) scan. (wikipedia)computed axial tomography KAIST - Physics

16. (e) d  (f) d  (g) d  (h) Simulation (e)(f) (g) (h) KAIST - Physics (h) Images Decrypted from Waveforms

17. KAIST To understand how it works, we go back to the introductory optics textbook.

18. Abbe’s Theory of Image Formation  Double diffractions of the object at  o do form a spatial-frequency filtered image at  i.  Fraunhoffer Formula  Spatial frequencies : (k x,k y )=kf, /f). KAIST - Physics fD (  /f,  /f) tt ii oo U(x,y) U’(  ) V(x’,y’) S2S2 S1S1 S0S0 S- 1 S- 2 (x’/D, y’/D) Spatial filtering

19. THz Broadband ? E  E(x) KAIST - Physics (a) Conventional imaging (b) Broadband imaging Broadband nature may allow single point imagery. The question is how.

20. Coherent Optical Computer TM f (  /f,  /f) ii tt U(x,y) M(  ) V(x’,y’) f ff  LtLt LiLi Tricks: KAIST - Physics

21. Coherent Optical Computer f (  /f,  /f) ii tt U(x,y) M(  ) V(x’,y’) f ff  LtLt LiLi M(  ) Spectrum at the image plane delivers the object shape. KAIST - Physics

22. Image encrypted in THz waveform (c) E(  )(b) E(t,  ) (b) (c) KAIST - Physics

23. Single-Point THz Imagery (a) Target (b) E(t,  ) (c) E(  ) (d) Sinogram (e) d  (f) d  (g) d  (h) Simulation (a)(b) (c)(d) (e)(f) (g)(h) KAIST - Physics (1) Fourier mask selects spatial frequencies of the object and maps into THz spectrum. E(k  )  E(  ) (2) Temporal waveforms deliver object function. E(x,y) -> E(t,  ) (3) Single waveforms for 2D imaging ? “Coherent Optical Computing for T-ray Imaging” K.Lee et al, submitted (2009).

24. KAIST

25. Decryption of Image from Waveform Angular resolution  KAIST - Physics Target Images are reconstructed by Inverse Radon transformation E’(x,y)Sinogram

26. Field of View KAIST (a) (b) (b’), where KAIST - Physics

27. THz Bandwidth & Image Resolution Inverse Radon Transformation is used to reconstruct the image. Terahertz Bandwidth:   max  THz KAIST - Physics Image Resolution

28. THz C(F?)DMA Imaging : proposal Simple maskCDMA mask N sets of spectral combs for diff. angular measurements. S 1 =   {1, N  +1, 2N  +1, 3N  +1,...} S 2 =   {2, N  +2, 2N  +2, 3N  +2,...} S 3 =   {3, N  +3, 2N  +3, 3N  +3,...} … S N  =   {N , 2N , 3N , 4N ,...} Total # of combs =  max    # of combs In each set = M T  frequency comb width N   comb width in each set N    max  KAIST - Physics

29. Using frequency: up to 1.8 THz Frequency resolution: 10GHz one set of combs With 45waveforms object 3 sets of combs With 15waveforms 5 sets of combs With 9waveforms 15 sets of combs With 3waveforms 45 sets of combs With 1 waveform ~4cm THz CDMA Imaging : Simulation KAIST - Physics

30. Using frequency: up to 1.8 THz Frequency resolution: 1GHz one set of combs With 45waveforms object 3 sets of combs With 15waveforms 5 sets of combs With 9waveforms 15 sets of combs With 3waveforms 45 sets of combs With 1 waveform ~4cm THz CDMA Imaging : Simulation KAIST - Physics

31. Using frequency: up to 1.8 THz Frequency resolution: 100MHz one set of combs With 45waveforms object 3 sets of combs With 15waveforms 5 sets of combs With 9waveforms 15 sets of combs With 3waveforms 45 sets of combs With 1 waveform ~4cm THz CDMA Imaging : Simulation KAIST - Physics

32. Simulation : Field of View    max  N    1GHz combs object 50 x 50 pixels ~4cm    max  N    100MHz combs Nyquist-Shannon sampling theorem limits the field of view. KAIST - Physics

33. Simpler variations (a)Waveforms could measured at once by (a-1) time separation with dense materials (a-2) frequency separation with multi-layers or modulations (b) Integrated array detector (a-1) (a-2) (b) THz CDMA Imaging KAIST - Physics

34. Summary  1.Single-pixel THz imagery has been demonstrated. 2.THz waves finds new applications in broadband coherent optical computing. 3.Code division multiple access protocol for “real” single-point THz imagery is under development. KAIST - Physics

35. Thanks to collaborators and students  THz System Development  Prof. Jong C. Ye (Kaist-biosystem)  Prof. Kihoon Jeong (Kaist-biosystem)  Dr. D.S. Yi (KRISS)  Laser Terahertz Emission Microscope  Prof. Y. D. Cho (Gist-IC)  Students  이강희, THz CDMA imaging  이민우, LTEM  한대훈, THz metamaterials KAIST - Physics