OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 8 — Apr. 16, 2007
  • pp: 4427–4434
« Show journal navigation

Acoustic phase imaging with terahertz radiation

F. Buersgens, G. Acuna, and R. Kersting  »View Author Affiliations


Optics Express, Vol. 15, Issue 8, pp. 4427-4434 (2007)
http://dx.doi.org/10.1364/OE.15.004427


View Full Text Article

Acrobat PDF (483 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report on acoustic phase imaging of objects using terahertz radiation. The sensitivity of the technique is sufficient to detect objects at oscillation amplitudes down to about 300 nm. Such acoustic amplitudes are comparable to the human physiological perception level, which offers novel opportunities in security imaging.

© 2007 Optical Society of America

1. Introduction

The development of terahertz (THz) technology [1

1. T. Hartwick, T. Hodges, D. Barker, and F. Foote, “Far infrared imagery,” Appl. Opt. 15, 1919–1922 (1976). [CrossRef] [PubMed]

, 2

2. B. B. Hu and M. C. Nuss, “Imaging with THz waves,” Opt. Lett. 20, 1716–1718 (1995). [CrossRef] [PubMed]

, 3

3. D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, “T-ray tomography,” Opt. Lett. 22, 904–906 (1997). [CrossRef] [PubMed]

] has indicated many applications for sensing concealed objects. Recent works on THz imaging include continuous wave (CW) techniques [4

4. K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743–3748 (2002). [CrossRef] [PubMed]

, 5

5. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz imaging,” Electron. Lett. 37, 1461–1463 (2001). [CrossRef]

] or quantum cascade lasers [6

6. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, “Imaging with a terahertz quantum cascade laser,” Opt. Express 12, 1879–1884 (2004). [CrossRef] [PubMed]

]. The potential of passive imaging [7

7. R. Appleby, “Passive millimetre-wave imaging and how it differs from terahertz imaging,” Phil. Trans. R. Soc. Lond. A 362, 379–394 (2004). [CrossRef]

] and reflection imaging [8

8. D. Zimdars and J. S. White, “Terahertz reflection imaging for package and personnel inspection,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5411, 2004). [CrossRef]

] is investigated for security applications, and many works use THz techniques for sensing illicit substances [9

9. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003). [CrossRef] [PubMed]

, 10

10. L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, “Study of the THz spectrum of methamphetamine,” Opt. Express 5, 6750–6755 (2005). [CrossRef]

]. However, these approaches are only sensitive to specific materials such as metals or few organic substances.

This paper presents the fundamental physics of an alternative approach, which is promising for detecting concealed objects due to its sensitivity to virtually all objects. Items are detected by forcing them to perform a minute acoustic oscillation and by imaging their acoustic phase with THz radiation. Objects are not identified by their specific optical properties because the technique provides no spectral information.

It exclusively answers the question whether or not an object is present, which is the most relevant information in many areas of application. The advantage of our technique is its sensitivity to every object with mass.

In the following section we discuss the fundamental properties of our approach and present two experimental methods for imaging the acoustic phase of driven vibrations. The experimental data shown in section 3 demonstrate that THz imaging provides the acoustic phase of an object even at submicron oscillation amplitudes. In section 4, we discuss the technique with respect to possible applications for security imaging.

2. Method

Mechanically coupled objects respond to a driving acoustic vibration with a phase lag that is given by their masses, their individual coupling constants, and the damping of the system. The two objects illustrated in Fig. 1a) respond with oscillation amplitudes D 1,2 and acoustic phase ϕ 1,2. These quantities can be measured by detecting electromagnetic waves reflected from the surface of oscillating objects. It can be shown that the amplitudes are rather difficult to track in particular when the imaging radiation is subject to absorption in covering materials or when the optical signal is corrupted due to imaging aberrations in general. In contrast, the optical measurement of the acoustic phase is much more robust regarding optical absorption or imaging aberrations. Therefore, we exclusively focus on the spatial mapping of ϕ in order to sense the presence of objects by their phase lag.

Coherent imaging techniques provide access to the acoustic phase, as for example in optical holographic interferometry [11

11. K. Stetson and R. Powell, “Hologram Interferometry,” J. Opt. Soc. Am. 56, 1161–1166 (1966). [CrossRef]

]. For our purposes radiation wavelengths are suitable that penetrate covering materials while providing sufficient spatial resolution. These requirements are fulfilled by few-cycle THz radiation or CW radiation in the upper GHz range. Shorter wavelengths may be absorbed or Mie scattered by covering materials, while longer wavelengths may not allow for sufficient spatial resolution. In particular, radiation of about 100 GHz is advantageous, because it can be generated using commercially available GHz devices such as Gunn oscillators. In order to highlight fundamental properties of acoustic phase imaging, we focus in the following on CW techniques. One straightforward approach for measuring the acoustic phase is homodyne mixing of the reflected radiation with a CW reference as it can be achieved for instance with a Michelson interferometer. As illustrated in Fig. 1b) the acoustic oscillation of the object has an amplitude of Dacoust, which changes the path length difference and leads to a modulation amplitude Amod. Integrating the mixing signal over time intervals larger than the optical oscillation period yields

Fig. 1. a) Schematic of two coupled objects, one of which is driven by an external force to perform a vibrational motion. The radiation reflected from the object’s surface contains information on amplitude D and phase ϕ of the individual oscillation. b) Interference signal depending on the path length difference Δ of an interferometer. Acoustic oscillation amplitude Dacoust and resulting modulation amplitude Amod are indicated. The amplitude of the interference signal is Aifg
Î(t)=Iobj+Iref2+IobjIref   ∙cos(2πλTHz[Δ+Dacoustsin(ωacoustt+ϕ)]),
(1)

where Δ is the difference in optical path length and Iobj and Iref are the intensities of the beam reflected from the object and of the reference beam, respectively. The oscillation of the object is described by amplitude Dacoust, angular frequency ωacoust, and phase ϕ. Since Dacoust/λ ≈ 0 the 1st order Taylor series expansion of the cosine at 2πΔλTHz leads to

Î(t)ωacoustIobjIref   ∙sin(2πΔλTHz)2πDacoustλTHzsin(ωacoustt+ϕ)
(2)
=Amodsin(ωacoustt+Φ),
(3)

when considering only the fundamental harmonic of ωacoust. This is equivalent to using a notch filter at the fundamental frequency of the acoustic drive. Most conveniently a lock-in amplifier can be used, which uses the driving frequency as reference. However, it has to be noted that in Eq. 3 the amplitude of the modulation signal Amod is a function of sin(…Δ). Taking into account that Amod by definition is greater than or equal to zero, yields the following relation between the measured phase Φ and the acoustic phase ϕ:

Φ={ϕifsin(…Δ)<0ϕπelse.
(4)

Most important of Eq. (4) is that the measured acoustic phase Φ does neither depend on spectral properties of the object, nor on optical properties of the environment. In particular, the phase is not affected by the reflectivity of the oscillating objects or by the absorption due to cover materials or air moisture.

The only requirement is that the amplitude of the sampling beam is sufficient for a phase measurement. The measured phase depends in a highly nonlinear manner on the difference in path length Δ yielding only two values. This is advantageous since many imaging aberrations are associated with a change in Δ. One example is a changing distance between object and imaging optics. The fact that only two phase values are possible makes the technique extremely robust. Both properties, the robustness of the technique and that the measured phase cannot be corrupted by absorption, facilitate the sensing of concealed objects.

Fig. 2. a) Schematic of the setup for homodyne interferometry. b) Schematic of the setup for time-resolved THz imaging.

Our experimental results were achieved using two techniques: i) a homodyne technique utilizing CW radiation (Figure 2a) and ii) a time-resolved THz technique (Figure 2b). For the CW technique we used a Gunn diode, which emits about 30 mW radiation at 87 GHz. Homodyne mixing is achieved using a Michelson interferometer with an arm length of about 0.5 m. In one arm of the interferometer the radiation is reflected by the object, which is acoustically driven by a piezo. The reflected beam and the reference beam are mixed in an amplified Schottky detector. The lock-in amplifier is referenced to the acoustic drive frequency of the piezo. Typical oscillation frequencies are of the order of 200 Hz. A maximum amplitude of the modulation signal Amod is obtained when the difference in path length is Δ = (2n - 1)λ/4. Additionally, the oscillation amplitude Dacoust of the object is estimated from the maximum modulation amplitude Amod and the amplitude of the steady state interferogram given by Aifg. Using the approximation DacoustAmodAifgλ2,, we deduced for the presented experiments oscillation amplitudes between 300 nm and 1.5 μm.

The time-resolved experiments were performed using a standard setup for electro-optic sampling [12

12. Q. Wu, M. Litz, and X.-C. Zhang, “Broadband detection capability of ZnTe electro-optic field sensors,” Appl. Phys. Lett. 68, 2924–2926 (1996). [CrossRef]

]. The setup has a bandwidth of about 3 THz and is described in detail in Ref. [13

13. A. Filin, M. Stowe, and R. Kersting, “Time-domain differentiation of terahertz pulses,” Opt. Lett. 26, 2008–2010 (2001). [CrossRef]

]. Imaging optics with a numerical aperture NA = 0.16 give a resolution of about 4λ, which corresponds to 600 μm, when considering a THz spectrum with a center frequency of 2 THz.

3. Results

Fig. 3. Acoustic phase sensed by homodyne mixing of radiation at 87 GHz in dependence on the optical path difference Δ and corresponding interferogram.

We investigated imaging properties such as spatial resolution or the required oscillation amplitude applying time resolved THz imaging. The temporal observation window of the electro-optic sampling technique was set to a delay where the THz signal exhibits a zero-crossing, maximizing the differential signal due to oscillations of the object in a similar manner as illustrated in Fig. 1b. Figure 4 shows two-dimensional THz data of an oscillating metallic membrane. The quadrants of the membrane are driven individually from the backside by four piezos. For both modes shown in Figure 4 alternating voltages with different phases are applied to the piezos, which translates to different acoustic phases of the quadrants. The spatial structure of the oscillation mode is reproduced by the THz data. The plot shows the oscillation amplitude instead of the acoustic phase, allowing for estimating the oscillation amplitude Dacoust from the modulation amplitude Amod and the THz field amplitude as discussed above. From the data and the noise floor, we conclude that oscillations of about 300 nm are well distinguishable at an integration time of 0.1 s per point.

In order to investigate the spatial resolution of acoustic phase imaging, we recorded images of a lamella structure as shown in Fig. 5 using time resolved THz measurements. The metallic structure has an overall size of 20 mm by 20 mm. The middle lamella is loaded with a weight on its back side and the lower lamella is stiffened at the end. The top finger of the lamella structure is unmodified for reference. Terahertz images were recorded from the front side. The finger structure is well reproduced in the image showing the field amplitude of the reflected THz radiation when the structure is at rest (Fig. 5 bottom left). All fingers appear similar and a spatial resolution of about 1 mm can be deduced from the image. As expected this image gives no clue about the items behind the fingers. In contrast (see Fig. 5 bottom right), the acoustic phase image unveils the mechanical differences between the fingers. The load of the middle lamella changes the acoustic response. The stiffening of the lower lamella leads to a completely different acoustic vibration. Along this finger the acoustic phase changes by nearly 180 degrees. Both examples show that different mechanical properties can be located by THz imaging of the acoustic phase. Although it may be impossible to identify the objects by their specific material properties, the phase data indicate the presence of objects behind two of the lamellas. Generally, the spatial phase pattern depends on the shape of the object, its orientation with respect to the incoming beam, and also on its acoustic mode. Therefore, the acoustic phase provides primarily information about the presence of an object and its coupling to the environment. The object’s shape becomes visible by phase jumps, which appear at the borderline to another object that oscillates with different phase. One example is the lamella’s finger tips oscillating at different phase. The phase image of the lamella structure shows a spatial resolution of about 2 mm, which is somewhat coarser as compared to the reflection image. The image indicates an acoustic phase even in the gaps between the fingers, which we attribute to the spatial wings of the signal reflected by the nearby fingers.

Fig. 4. Left: Two-dimensional THz mapping of the acoustic vibration of a metallic membrane driven at a frequency of 225 Hz. The amplitude of the oscillation is about 0.7 μm. Right: A different oscillation mode detected through a linen textile, which had no contact to the membrane.

4. Discussion

Techniques for THz imaging of the acoustic phase promise applications in material characterization, but the main potential of the technique may be in security imaging. Many imaging technologies currently used for security applications are highly sensitive, such as metal detectors or X-ray scanners, but respond only to few materials or cannot be applied on humans because of health risks. The most important factors that make THz techniques attractive for contactless screening are [14

14. J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, “THz imaging and sensing for security applications-explosives, weapons and drugs,” Semicond. Sci. Technol. 20, S266–s280 (2005). [CrossRef]

]: i) Terahertz radiation is transmitted by many packaging materials and by common clothing [15

15. D. M. Mittleman, H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996). [CrossRef]

,16

16. J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, “Millimeter-wave, terahertz, and mid-infrared transmission through common clothing,” Appl. Phys. Lett. 85, 519–521 (2004). [CrossRef]

]. ii) Illumination by terahertz radiation causes no genotoxic effects because of the low photon energy [17

17. E. Berry, “Risk perception and safety issues,” J. Biol. Phys. 29, 263–267 (2003). [CrossRef]

, 18

18. M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M. D’Arienzo, “THz exposure of whole blood for the study of biological effects on human lymphocytes,” J. Biol. Phys. 29, 171–177 (2003). [CrossRef]

]. iii) According to Rayleigh’s criterion, the wavelength of THz radiation is small enough to image objects with sufficient spatial resolution.

Fig. 5. Top: Photograph of the front and backside of a metallic lamella structure. The middle stripe carries an additional load and the lower stripe is stiffened by a copper rod. Bottom left: Image of the THz field amplitude reflected from the lamella structure at rest. Bottom right: Image of the acoustic phase as sensed by using few-cycle THz pulses.

The major hurdle towards the development of future contactless scanners results from the vast number of potentially dangerous materials and items. The goal of many THz approaches in the field is to expand the number of detectable materials by sensing their spectral properties in the far infrared. However, sensing of specific material properties by their spectral response will cover only a fraction of all objects one could think of. Such technologies may not give sufficient confidence that any object with known or unknown properties is concealed. Another hurdle is that real life applications have to overcome corruptions of the signal, which may result from different cover materials that absorb or reflect part of the spectral information required for identifying a material. Optical aberrations of the imaging optics may also alter the spectral response of the system. Finally, it can be expected that the characteristic spectral signals of concealed objects may be minute compared to background and signal corruptions. Differential detection techniques would be helpful in order to enhance the signal quality, but require the modulation of the relevant signal. To the best of our knowledge, no applicable techniques exist for modulating the spectral properties in security applications.

According to the above considerations acoustic phase imaging differs from most THz techniques by the following properties:

  • 1. Completeness: The method is sensitive to all objects with mass subjected to a driving vibration. Acoustic phase imaging is not suited to identify an object by spectral information. It only provides insight into whether or not an object is concealed.
  • 2. Sensitivity: Acoustic phase imaging is a modulation technique. Differential detection methods allow for efficient suppression of noise and background signals, which may arise for instance due to covering materials.
  • 3. Robustness: The measured phase does not depend on the optical properties of cover materials, or water absorption in air. Additionally, the measured phase information depends on aberrations in a highly nonlinear manner and can take only two discrete values. Thus, different objects are easily distinguishable.

5. Conclusions

We have shown that THz imaging can be used to measure the acoustic phase of vibrating objects with resolutions comparable to those expected from the Abbe-Rayleigh diffraction criterion. An outstanding property of the developed technique is the capability of detecting virtually all objects. While the technique gives no insight into the object’s material properties it is capable of determining whether a concealed object is present. Differential detection schemes allow for a nearly background free signal and for efficient noise suppression. The measured acoustic phase depends only in a discrete manner on signal corruptions, which facilitates future applications.

Acknowledgments

The authors acknowledge technical help by C. Lang and thank Radiometer Physics GmbH for support. The work of G.A. is funded by the Deutsche Forschungsgemeinschaft (contract KE 516/1-1).

References and links

1.

T. Hartwick, T. Hodges, D. Barker, and F. Foote, “Far infrared imagery,” Appl. Opt. 15, 1919–1922 (1976). [CrossRef] [PubMed]

2.

B. B. Hu and M. C. Nuss, “Imaging with THz waves,” Opt. Lett. 20, 1716–1718 (1995). [CrossRef] [PubMed]

3.

D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, “T-ray tomography,” Opt. Lett. 22, 904–906 (1997). [CrossRef] [PubMed]

4.

K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743–3748 (2002). [CrossRef] [PubMed]

5.

T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, “Continuous-wave THz imaging,” Electron. Lett. 37, 1461–1463 (2001). [CrossRef]

6.

J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, “Imaging with a terahertz quantum cascade laser,” Opt. Express 12, 1879–1884 (2004). [CrossRef] [PubMed]

7.

R. Appleby, “Passive millimetre-wave imaging and how it differs from terahertz imaging,” Phil. Trans. R. Soc. Lond. A 362, 379–394 (2004). [CrossRef]

8.

D. Zimdars and J. S. White, “Terahertz reflection imaging for package and personnel inspection,” in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5411, 2004). [CrossRef]

9.

K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003). [CrossRef] [PubMed]

10.

L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, “Study of the THz spectrum of methamphetamine,” Opt. Express 5, 6750–6755 (2005). [CrossRef]

11.

K. Stetson and R. Powell, “Hologram Interferometry,” J. Opt. Soc. Am. 56, 1161–1166 (1966). [CrossRef]

12.

Q. Wu, M. Litz, and X.-C. Zhang, “Broadband detection capability of ZnTe electro-optic field sensors,” Appl. Phys. Lett. 68, 2924–2926 (1996). [CrossRef]

13.

A. Filin, M. Stowe, and R. Kersting, “Time-domain differentiation of terahertz pulses,” Opt. Lett. 26, 2008–2010 (2001). [CrossRef]

14.

J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, “THz imaging and sensing for security applications-explosives, weapons and drugs,” Semicond. Sci. Technol. 20, S266–s280 (2005). [CrossRef]

15.

D. M. Mittleman, H. Jacobsen, and M. C. Nuss, “T-ray imaging,” IEEE J. Sel. Top. Quantum Electron. 2, 679–692 (1996). [CrossRef]

16.

J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, “Millimeter-wave, terahertz, and mid-infrared transmission through common clothing,” Appl. Phys. Lett. 85, 519–521 (2004). [CrossRef]

17.

E. Berry, “Risk perception and safety issues,” J. Biol. Phys. 29, 263–267 (2003). [CrossRef]

18.

M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M. D’Arienzo, “THz exposure of whole blood for the study of biological effects on human lymphocytes,” J. Biol. Phys. 29, 171–177 (2003). [CrossRef]

19.

B. Harazin and J. Grzesik, “The transmission of vertical whole-body vibration to the body segments of standing subjects,” J. Sound and Vib. 215, 775–787 (1998). [CrossRef]

20.

International Standard, ISO 2631–1 (1997). Second edition.

21.

D. Dieckmann, “Einfluss vertikaler mechanischer Schwingungen auf den Menschen,” Internat. Z. angew. Physiol. einschl. Arbeitsphysiol. 16, 519–564 (1957). [CrossRef]

OCIS Codes
(110.3080) Imaging systems : Infrared imaging
(110.5120) Imaging systems : Photoacoustic imaging
(120.0280) Instrumentation, measurement, and metrology : Remote sensing and sensors
(330.4150) Vision, color, and visual optics : Motion detection

ToC Category:
Imaging Systems

History
Original Manuscript: January 26, 2007
Revised Manuscript: March 12, 2007
Manuscript Accepted: March 26, 2007
Published: April 3, 2007

Citation
Federico Buersgens, Guillermo Acuna, and Roland Kersting, "Acoustic phase imaging with terahertz radiation," Opt. Express 15, 4427-4434 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-8-4427


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. T. Hartwick, T. Hodges, D. Barker, and F. Foote, "Far infrared imagery," Appl. Opt. 15, 1919-1922 (1976). [CrossRef] [PubMed]
  2. B. B. Hu and M. C. Nuss, "Imaging with THz waves," Opt. Lett. 20, 1716-1718 (1995). [CrossRef] [PubMed]
  3. D. M. Mittleman, S. Hunsche, L. Boivin, and M. C. Nuss, "T-ray tomography," Opt. Lett. 22, 904-906 (1997). [CrossRef] [PubMed]
  4. K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, "Alloptoelectronic continuous wave THz imaging for biomedical applications," Phys. Med. Biol. 47, 3743-3748 (2002). [CrossRef] [PubMed]
  5. T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, and K. Donhuijsen, "Continuous-wave THz imaging," Electron. Lett. 37, 1461-1463 (2001). [CrossRef]
  6. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovanni, J. Faist, C. Kremser, and P. Debbage, "Imaging with a terahertz quantum cascade laser," Opt. Express 12, 1879-1884 (2004). [CrossRef] [PubMed]
  7. R. Appleby, "Passive millimetre-wave imaging and how it differs from terahertz imaging," Phil. Trans. R. Soc. Lond. A 362, 379-394 (2004). [CrossRef]
  8. D. Zimdars and J. S. White, "Terahertz reflection imaging for package and personnel inspection," in Terahertz for Military and Security Applications, R. Hwu and D. Woolard, eds. (SPIE, Vol. 5411, 2004). [CrossRef]
  9. K. Kawase, Y. Ogawa, Y. Watanabe, and H. Inoue, "Non-destructive terahertz imaging of illicit drugs using spectral fingerprints," Opt. Express 11, 2549-2554 (2003). [CrossRef] [PubMed]
  10. L. Ning, S. Jongling, S. Jinhai, L. Laishun, X. Xiaoyu, L. Meihong, and J. Yan, "Study of the THz spectrum of methamphetamine," Opt. Express 5, 6750-6755 (2005). [CrossRef]
  11. K. Stetson and R. Powell, "Hologram Interferometry," J. Opt. Soc. Am. 56, 1161-1166 (1966). [CrossRef]
  12. Q. Wu, M. Litz, and X.-C. Zhang, "Broadband detection capability of ZnTe electro-optic field sensors," Appl. Phys. Lett. 68, 2924-2926 (1996). [CrossRef]
  13. A. Filin, M. Stowe, and R. Kersting, "Time-domain differentiation of terahertz pulses," Opt. Lett. 26, 2008-2010 (2001). [CrossRef]
  14. J. F. Federici, B. Schulkin, F. Huang, D. Gray, R. Barat, F. Oliveira, and D. Zimar, "THz imaging and sensing for security applications-explosives, weapons and drugs," Semicond. Sci. Technol. 20, S266-s280 (2005). [CrossRef]
  15. D.M. Mittleman, H. Jacobsen, and M. C. Nuss, "T-ray imaging," IEEE J. Sel. Top. Quantum Electron. 2, 679-692 (1996). [CrossRef]
  16. J. Bjarnason, T. Chan, A. Lee, M. Celis, and E. Brown, "Millimeter-wave, terahertz, and mid-infrared transmission through common clothing," Appl. Phys. Lett. 85, 519-521 (2004). [CrossRef]
  17. E. Berry, "Risk perception and safety issues," J. Biol. Phys. 29, 263-267 (2003). [CrossRef]
  18. M. R. Scarfi, M. Romaneo, R. D. Pietro, O. Zeni, A. Doria, G. P. Gallerano, E. Giovenale, G. Messina, A. Lai, G. Campuria, D. Conglio, and M. D’Arienzo, "THz exposure of whole blood for the study of biological effects on human lymphocytes," J. Biol. Phys. 29, 171-177 (2003). [CrossRef]
  19. B. Harazin and J. Grzesik, "The transmission of vertical whole-body vibration to the body segments of standing subjects," J. Sound and Vib. 215, 775-787 (1998). [CrossRef]
  20. International Standard, ISO 2631-1 (1997). Second edition.
  21. D. Dieckmann, "Einfluss vertikaler mechanischer Schwingungen auf denMenschen," Internat. Z. angew. Physiol. einschl.Arbeitsphysiol. 16, 519-564 (1957). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited