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Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 17 — Aug. 22, 2005
  • pp: 6497–6503
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Imaging with THz quantum cascade lasers using a Schottky diode mixer

Stefano Barbieri, Jesse Alton, Colin Baker, Thomas Lo, Harvey E. Beere, and David Ritchie  »View Author Affiliations


Optics Express, Vol. 13, Issue 17, pp. 6497-6503 (2005)
http://dx.doi.org/10.1364/OPEX.13.006497


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Abstract

An imaging system in reflection geometry based on a multimode 2.9 THz quantum cascade laser as radiation source is reported. The beating between neighbouring longitudinal modes is detected using a room temperature point-contact Schottky diode as mixing element. We show that the technique can, in principle, give a dynamic range of 60 dB with a time constant of ~ 10 μs.

© 2005 Optical Society of America

1. Introduction

THz technology has attracted a great deal of interest in recent years in applications ranging from security screening to medical imaging [1–8

1. C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, “Developments in people screening using THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)

]. This is the consequence of the unique properties of THz waves, namely (i) their ability to penetrate different types of materials, from biological tissue, to plastic and cloth; (ii) their non-invasive nature; and (iii) their relatively short wavelength, allowing for a spatial resolution in the 100 μm range [3

3. S. Wang, B. Ferguson, D. Abbot, and X.-C Zhang, “T-ray imaging and tomography,” J. Biol. Phys. 29, 247 (2003) [CrossRef]

,4

4. See, for example, D. Mittleman, Sensing with THz radiation (Springer, Berlin, 2003)

].

Pulsed photoconductive THz generation/detection is a powerful technique which is now currently employed in commercial imaging systems used for research and to carry out online quality control [4–8

4. See, for example, D. Mittleman, Sensing with THz radiation (Springer, Berlin, 2003)

]. Applications such as security screening are instead more demanding in terms of system sensitivity [1

1. C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, “Developments in people screening using THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)

]. Indeed, compared to current values, the latter will need to be enhanced by several orders of magnitude to allow for the penetration of thick layers of material, an essential condition to successfully carry out passenger and luggage inspection. The exploitation of pulsed photoconductive generation will also prove challenging in applications requiring the ability to perform remote sensing on moving targets. In fact, in this case, the distance between the target and the detector must be tracked and measured with very high accuracy in real time, to allow for the time-resolved detection of the reflected THz pulses[1

1. C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, “Developments in people screening using THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)

]. In this context the use of sources emitting in continuous wave (CW) is certainly advantageous.

A compact CW imaging system for inspection applications has been recently demonstrated, using a 0.2 THz Gunn oscillator and a Schottky diode detector [9

9. N. Karpowicz, H. Zhong, C. Zhang, K.-I Lin, J-S. Hwang, J. Xu, and X.-C. Zhang, “Compact continuous-wave subterahertz system for inspection applications,” Appl. Phys. Lett. 86, 054105 (2005) [CrossRef]

]. At such low frequencies the penetration depth through most materials is large, for instance allowing to image objects inside a luggage [10

10. W. R Tribe, D. A. Newnham, P. F. Taday, and M. C. Kemp, “Hidden object detection: security applications of THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5434, 168 (2004)

]. In principle THz radiation can also be used to identify concealed substances that do not have a distinctive geometrical shape, by exploiting their spectroscopic properties For example many common plastic explosives have been shown to possess distinctive THz absorption features in the region between 0.5 and 3 THz [1

1. C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, “Developments in people screening using THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)

,2

2. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification of explosives using THz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241116 (2005) [CrossRef]

]. The availability of sources emitting in the lower end of this spectral window, i.e. in the range 0.5 – 2 THz, would be ideal for hidden explosives identification, since there are a number of spectral features, yet penetration through barriers is higher than at the upper end of the frequency range [10

10. W. R Tribe, D. A. Newnham, P. F. Taday, and M. C. Kemp, “Hidden object detection: security applications of THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5434, 168 (2004)

].

Millimeter-wave Gunn oscillators are commercially available with an output power of ~ 10 mW at ~ 100 GHz. To achieve operation at higher frequencies these devices are combined with multiplier chains, yielding power levels of a few hundreds of μW up to ~ 0.5 THz. As the emission frequency goes deeper into the submillimeter/THz range the power drops rapidly and prototype devices have been demonstrated emitting a few μW at ~ 2 THz [11

11. A. Maestrini, J. Ward, J. Gill, H. Javadi, E. Schlecht, G. Chattopadhyay, F. Maiwald, N. R. Erickson, and I. Mehdi, “A 1.7 – 1.9 local oscillator source,” IEEE Microwave and Wireless Comp. Lett 14, 253 (2004) [CrossRef]

]. This is in spite of the fact that power levels of the order of the mW are needed to pump room temperature heterodyne mixers in the 0.3 –2 THz band.

The advent of THz Quantum Cascade Lasers (QCLs) has opened new interesting perspectives for THz technology [12

12. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156 (2002) [CrossRef] [PubMed]

]. In these semiconductor laser sources the advantage of compactness is merged with the ability to generate CW powers of several tens of mW [13

13. S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, “2.9 THz quantum cascade laser operating up to 70K in continuous wave,” Appl. Phys. Lett. 85, 1674 (2004) [CrossRef]

,14

14. L. Ajili, G. Scalari, J. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, “High power quantum cascade lasers operating at λ = 87 and 130 μm,” Appl. Phys. Lett. 85, 3986 (2004) [CrossRef]

]. So far QCLs have been demonstrated with emission frequencies between 2.0 and 4.4 THz and maximum CW operating temperatures of 117K [15–16

15. B. S. Williams, S. Kumar, and Q. Hu, “Operation of THz Quantum cascade lasers at 164K in pulsed mode and at 117K in continuous-wave mode,” Opt. Express 13, 3331 (2005) [CrossRef] [PubMed]

]. Extending their operation at least down to 1.5 THz appears to be a realistic objective [17

17. Operation at 1.39THz has been achieved with a QCL subject to a magnetic field of 6T. Jerome Faist, private communication.

].

In this work we demonstrate raster scan imaging at 2.9 THz using a QCL and a point contact Schottky diode mixer (SDM) operating at room temperature. So far imaging with THz QCLs has been performed using liquid-helium cooled bolometers or pyroelectric detectors [18

18. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovannini, J. Faist, C. Kremser, and P. Debbage, “Imaging with a THz quantum cascade laser,” Opt. Expr. 12, 1879 (2004) [CrossRef]

,19

19. D. R. Chamberlin, P. R. Robrish, W. R. Trutna, G. Scalari, M. Giovannini, L. Ajili, and J. Faist, “Dual wavelength THz imaging with quantum cascade lasers,” Appl. Opt. 44, 121 (2005) [CrossRef] [PubMed]

]. Both detection techniques are inherently slow, with response times in the hundreds of ms range, rendering them impractical for many applications. Here we demonstrate a system with a 60 dB signal-to-noise ratio and a potential response time of 10 μs.

2. Experimental technique

Heterodyne mixing by pumping a GaAs membrane-diode with two far-infrared gas lasers and a THz QCL emitting at 4.7 THz was demonstrated by Barkan et al [20

20. A. Barkan, F. K. Tittel, D. M. Mittleman, R. Dengler, P. H. Siegel, G. Scalari, L. Ajili, F. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, “Linewidth and tuning characteristics of THz quantum cascade lasers,” Opt. Lett. 29, 575 (2004). [CrossRef] [PubMed]

]. Subsequently we have reported heterodyne mixing using two independent QCLs emitting at 3.3 THz and a GaAs-based point contact SDM [21

21. S. Barbieri, J. Alton, H. E. Beere, H. Linfield, D. A. Ritchie, S. Withington, G. Scalari, L. Ajili, and J. Faist, “Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode,” Opt. Lett. 29, 1633 (2004) [CrossRef]

]. In that case, owing to thermal instabilities in the devices, we found that the Intermediate Frequency (IF) signal experienced a fluctuation of the order of several tens of MHz. In addition, the IF amplitude and frequency stability suffered from unwanted optical feedback on the QCLs. These facts hinder the achievement of ultimate levels of sensitivity, by essentially limiting the possibility of IF filtering in a narrow band. Frequency stabilisation of THz QCLs by phase locking to a reference oscillator is certainly the most effective solution to this problem [22

22. A.L. Betz, R.T. Boreiko, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Frequency and phase-lock control of a 3 THz quantum cascade laser,” Opt. Lett. 30, 1837 (2005) [CrossRef] [PubMed]

]. In this work we adopted a simpler approach, based on the detection of the beating between adjacent Fabry-Perot modes from the same QCL, leading to an IF signal oscillating at the cavity round-trip frequency c/2NL, where c is the speed of light, N the group effective refractive index, and L is the cavity length. In comparison, the use of longitudinal modes from two independent QCLs, emitting at ν1 = nc/2N1L1 and ν2 = mc/2N2L2, considerably deteriorates the stability of the downconverted signal since (i) effective indices N1 and N2 are modified by independent thermal fluctuations, and (ii) integers n an m amplify the effect of changes of N1 and N2 on ν1, ν2 (n ~ 200, for a 3 mm long cavity at 2.9 THz).

Fig. 1. Layout of the experimental apparatus. Laser emission and IF signal spectra are also represented schematically. The separation of ~13 GHz between neighbouring longitudinal Fabry-Perot modes corresponds to a QCL cavity length of 3mm.

The experimental set up is shown in Fig. 1. The sample is mounted on motorised translation stages. Radiation from the QCL is collected, collimated, focused, and re-collected, after reflection on the sample surface, with the help of three 3inch-diameter, f/0.9 parabolic mirrors. The beam is then focused on a point contact GaAs-SDM (Farran CM(X)-5) biased with a regulated constant-voltage source. The device is mounted inside a 90° corner-cube reflector, with an extended “whisker” used to both point-contact the diode and act as a travelling-wave antenna. This leads to a main lobe of approximately 24° width (at the 10dB points), at an angle of 28° to the vertical. The IF signal is extracted with a bias-tee, fed into a two-stage broad-band preamplifier with a gain of 50 dB (MITEQ, JSD3 and JSD2 series), and recorded with a spectrum analyser (Agilent E4407B). As radiation source we used a back-facet gold-coated, 3mm-long THz QCL emitting at 2.9 THz (for details on laser performance see Ref. [13

13. S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, “2.9 THz quantum cascade laser operating up to 70K in continuous wave,” Appl. Phys. Lett. 85, 1674 (2004) [CrossRef]

]). This was mounted on the cold head of a continuous flow cryostat, and driven in CW at 10K with a dc power supply.

3. Results and discussion

In Fig. 2 we display the IF power spectrum obtained by placing a gold-coated mirror on the sample holder, and by driving the QCL at maximum power, i.e., approximately 20 mW. With resolution bandwidths of 1 kHz and 5 MHz, signal-to-noise ratios are of 80 and 50 dBm respectively. The line is centered at ~ 13 GHz, corresponding to the laser round-trip frequency. As expected this value is in agreement with the longitudinal mode separation obtained from the laser emission spectrum displayed in the inset (here the linewidth is limited by the maximum resolution, of 0.25 cm-1, i.e., 7.5 GHz, of the Fourier transform infrared spectrometer). Compared to the use of two independent QCLs we found a much improved line stability, with a maximum drift of the order of 1 MHz over a time scale of several hours. Here we note how this fact, together with the extremely narrow instantaneous linewidth of 10 kHz, obtained from the 1 kHz resolution bandwidth spectrum, are compatible with stable self-mode locked oscillations of the laser [23

23. A similar behaviour was found in mid-IR QCLs by measuring the beating between neighbouring Fabry-Perot modes with a fast quantum well photodetector. See A. Soibel, F. Capasso, C. Gmachl, M. L. Peabody, A. M. Serget, R. Paiella, D. L. Sivco, A. Y. Cho, and H. C. Liu, “Stability of pulsed emission and enhancement of intracavity second-harmonic generation in self mode-locked quantum cascade lasers,” IEEE J. Quantum. Electron 40, 197 (2004) [CrossRef]

]. Additional support to this hypothesis was found by analysing the voltage across the QCL with the help of a high frequency bias-tee. In this case (not shown) we measured an ac modulation superimposed to the constant dc bias, with a power spectrum essentially identical to the one reported in Fig. 2 (i.e., same center frequency and linewidth). Further investigations to clarify this issue are under way.

Fig 2. IF power spectrum detected at the output of the SDM with two different resolution bandwidths of the spectrum analyser. The QCL was driven in CW with a current of 1.4 A at T = 10K. The IF signal was amplified with a two-stage, 50 dB gain, 12–14 GHz low noise preamplifier (MITEQ, JSD3 and JSD2 series). Inset: QCL emission spectrum recorded with a Fourier transform infrared spectrometer (0.25 cm-1 resolution).

Fig 3. Examples of THz images. Left: Image of a gold pattern evaporated on top of a TPX window (100 μm2 pixels). Right: A razor blade imaged through a standard A4 paper sheet (200 μm2 pixels). Regions of different colours correspond to different intensities detected by the mixer as a consequence of the blade surface not being perfectly flat (see text). The light concentric rings visible on the image are produced by rings patterned on the surface of the circular 1mm thick polythene spacer placed between the blade and the paper. Additional non-uniformity of the horizontal edges of the blade (see for instance the bottom edge) is due to the translation stage not regaining exactly the same position in subsequent vertical scans.

In the experimental setup used in this work (see Fig. 1), both the local oscillator (LO) and signal “beams”, given by two neighbouring Fabry Perot lines, experience attenuation from the sample. This is unlike standard heterodyne detection, where radiation from the LO directly impinges on the mixer element. This way the mixer response scales with the amplitude and not with the power (as in the present work) of the detected signal, which is the reason for the extremely high levels of sensitivity achievable with this detection scheme. For instance, when pumped with an LO power in the 10 mW range, the present SDM has a nominal NEP of 10-19 W/Hz1/2 at room temperature, i.e., exceeding by several orders of magnitude the NEP of standard bolometers. In principle one could modify the present set up by generating two spatially separated beams from the QCL with the help of a beam splitter, and subsequently filter out one single FP line from each of them, however the required filtering selectivity goes beyond the capabilities of present THz technology. Another option is to recombine the two beams on the mixer and perform homodyne detection.

3. Conclusions

Acknowledgments

We thank Michael Kemp for careful reading of the manuscript. This work was partially supported by the European Community through the PASR 2004 Project TERASEC (Active Terahertz Imaging for Security). One of the authors (S.B.) acknowledges support from the Royal Society.

References and links

1.

C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, “Developments in people screening using THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)

2.

Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, “Detection and identification of explosives using THz pulsed spectroscopic imaging,” Appl. Phys. Lett. 86, 241116 (2005) [CrossRef]

3.

S. Wang, B. Ferguson, D. Abbot, and X.-C Zhang, “T-ray imaging and tomography,” J. Biol. Phys. 29, 247 (2003) [CrossRef]

4.

See, for example, D. Mittleman, Sensing with THz radiation (Springer, Berlin, 2003)

5.

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

6.

V. P. Wallace, R. M. Woodward, A. J. Fitzgerald, E. Pickwell, R. J. Pye, and D. D. Arnone, “Terahertz pulsed imaging of cancers,” Appl. Phys. Lett. 84, 2190 (2004)

7.

E. Pickwell, B. E. Cole, A. J. Fitzgerald, and V. P. Wallace, “Simulation of terahertz pulse propagation in biological systems,” Appl. Phys. Lett. 84, 2190 (2004) [CrossRef]

8.

A. J. Fitzgerald, B. E. Cole, and P. F. Taday, “Nondestructive analysis of tablet coating thicknesses using THz pulsed imaging,” J. Pharm. Sciences 94, 177 (2005) [CrossRef]

9.

N. Karpowicz, H. Zhong, C. Zhang, K.-I Lin, J-S. Hwang, J. Xu, and X.-C. Zhang, “Compact continuous-wave subterahertz system for inspection applications,” Appl. Phys. Lett. 86, 054105 (2005) [CrossRef]

10.

W. R Tribe, D. A. Newnham, P. F. Taday, and M. C. Kemp, “Hidden object detection: security applications of THz technology,” Proc. SPIE Int. Soc. Opt. Eng. 5434, 168 (2004)

11.

A. Maestrini, J. Ward, J. Gill, H. Javadi, E. Schlecht, G. Chattopadhyay, F. Maiwald, N. R. Erickson, and I. Mehdi, “A 1.7 – 1.9 local oscillator source,” IEEE Microwave and Wireless Comp. Lett 14, 253 (2004) [CrossRef]

12.

R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, “Terahertz semiconductor-heterostructure laser,” Nature 417, 156 (2002) [CrossRef] [PubMed]

13.

S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, “2.9 THz quantum cascade laser operating up to 70K in continuous wave,” Appl. Phys. Lett. 85, 1674 (2004) [CrossRef]

14.

L. Ajili, G. Scalari, J. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, “High power quantum cascade lasers operating at λ = 87 and 130 μm,” Appl. Phys. Lett. 85, 3986 (2004) [CrossRef]

15.

B. S. Williams, S. Kumar, and Q. Hu, “Operation of THz Quantum cascade lasers at 164K in pulsed mode and at 117K in continuous-wave mode,” Opt. Express 13, 3331 (2005) [CrossRef] [PubMed]

16.

C. Worral, J. Alton, M. Houghton, S. Barbieri, C. Sirtori, H. E. Beere, and D. A. Ritchie, “High power superlatticel quantum cascade laser emitting at 2 THz,” unpublished (June 2005).

17.

Operation at 1.39THz has been achieved with a QCL subject to a magnetic field of 6T. Jerome Faist, private communication.

18.

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

19.

D. R. Chamberlin, P. R. Robrish, W. R. Trutna, G. Scalari, M. Giovannini, L. Ajili, and J. Faist, “Dual wavelength THz imaging with quantum cascade lasers,” Appl. Opt. 44, 121 (2005) [CrossRef] [PubMed]

20.

A. Barkan, F. K. Tittel, D. M. Mittleman, R. Dengler, P. H. Siegel, G. Scalari, L. Ajili, F. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, “Linewidth and tuning characteristics of THz quantum cascade lasers,” Opt. Lett. 29, 575 (2004). [CrossRef] [PubMed]

21.

S. Barbieri, J. Alton, H. E. Beere, H. Linfield, D. A. Ritchie, S. Withington, G. Scalari, L. Ajili, and J. Faist, “Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode,” Opt. Lett. 29, 1633 (2004) [CrossRef]

22.

A.L. Betz, R.T. Boreiko, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Frequency and phase-lock control of a 3 THz quantum cascade laser,” Opt. Lett. 30, 1837 (2005) [CrossRef] [PubMed]

23.

A similar behaviour was found in mid-IR QCLs by measuring the beating between neighbouring Fabry-Perot modes with a fast quantum well photodetector. See A. Soibel, F. Capasso, C. Gmachl, M. L. Peabody, A. M. Serget, R. Paiella, D. L. Sivco, A. Y. Cho, and H. C. Liu, “Stability of pulsed emission and enhancement of intracavity second-harmonic generation in self mode-locked quantum cascade lasers,” IEEE J. Quantum. Electron 40, 197 (2004) [CrossRef]

24.

The 35 dBm residual dynamic range results from resolution bandwidth of 100 kHz used in the experiment, yielding a dynamic range of 70 dBm.

25.

G. Narayanan, N. R. Erickson, and A. W. Lichtenberger, “Development of multi-pixel heterodyne array instruments at submillimeter wavelengths,” Proc. Far-IR, Sub-MM, and MM Detector Technology Workshop, 203 (2002)

OCIS Codes
(110.3080) Imaging systems : Infrared imaging
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.5960) Lasers and laser optics : Semiconductor lasers

ToC Category:
Research Papers

History
Original Manuscript: June 16, 2005
Revised Manuscript: August 8, 2005
Published: August 22, 2005

Citation
Stefano Barbieri, Jesse Alton, Colin Baker, Thomas Lo, Harvey Beere, and David Ritchie, "Imaging with THz quantum cascade lasers using a Schottky diode mixer," Opt. Express 13, 6497-6503 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-17-6497


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References

  1. C. Baker, W. R. Tribe, B. E. Cole, and M. C. Kemp, �??Developments in people screening using THz technology,�?? Proc. SPIE Int. Soc. Opt. Eng. 5616, 61 (2004)
  2. Y. C. Shen, T. Lo, P. F. Taday, B. E. Cole, W. R. Tribe, and M. C. Kemp, �??Detection and identification of explosives using THz pulsed spectroscopic imaging,�?? Appl. Phys. Lett. 86, 241116 (2005) [CrossRef]
  3. S. Wang, B. Ferguson, D. Abbot, and X.-C Zhang, �??T-ray imaging and tomography,�?? J. Biol. Phys. 29, 247 (2003) [CrossRef]
  4. See, for example, D. Mittleman, Sensing with THz radiation (Springer, Berlin, 2003)
  5. B. B. Hu, and M. C. Nuss, �??Imaging with THz waves,�?? Opt. Lett. 20, 1716 (1995) [CrossRef] [PubMed]
  6. V. P. Wallace, R. M. Woodward, A. J. Fitzgerald, E. Pickwell, R. J. Pye, and D. D. Arnone, �??Terahertz pulsed imaging of cancers,�?? Appl. Phys. Lett. 84, 2190 (2004)
  7. E. Pickwell, B. E. Cole, A. J. Fitzgerald, and V. P. Wallace, �??Simulation of terahertz pulse propagation in biological systems,�?? Appl. Phys. Lett. 84, 2190 (2004) [CrossRef]
  8. A. J. Fitzgerald, B. E. Cole, and P. F. Taday, �??Nondestructive analysis of tablet coating thicknesses using THz pulsed imaging,�?? J. Pharm. Sciences 94, 177 (2005) [CrossRef]
  9. N. Karpowicz, H. Zhong, C. Zhang, K.-I Lin, J-S. Hwang, J. Xu, and X.-C. Zhang, �??Compact continuous-wave subterahertz system for inspection applications,�?? Appl. Phys. Lett. 86, 054105 (2005) [CrossRef]
  10. W. R Tribe, D. A. Newnham, P. F. Taday, and M. C. Kemp, �??Hidden object detection: security applications of THz technology,�?? Proc. SPIE Int. Soc. Opt. Eng. 5434, 168 (2004)
  11. A. Maestrini, J. Ward, J. Gill, H. Javadi, E. Schlecht, G. Chattopadhyay, F. Maiwald, N. R. Erickson, and I. Mehdi, �??A 1.7 �?? 1.9 local oscillator source,�?? IEEE Microwave and Wireless Comp. Lett 14, 253 (2004) [CrossRef]
  12. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and F. Rossi, �??Terahertz semiconductor-heterostructure laser,�?? Nature 417, 156 (2002) [CrossRef] [PubMed]
  13. S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, �??2.9 THz quantum cascade laser operating up to 70K in continuous wave,�?? Appl. Phys. Lett. 85, 1674 (2004) [CrossRef]
  14. L. Ajili, G. Scalari, J. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, �??High power quantum cascade lasers operating at λ= 87 and 130 μm,�?? Appl. Phys. Lett. 85, 3986 (2004) [CrossRef]
  15. B. S. Williams, S. Kumar, and Q. Hu, �??Operation of THz Quantum cascade lasers at 164K in pulsed mode and at 117K in continuous-wave mode,�?? Opt. Express 13, 3331 (2005) [CrossRef] [PubMed]
  16. C. Worral, J. Alton, M. Houghton, S. Barbieri, C. Sirtori, H. E. Beere, and D. A. Ritchie, �??High power superlatticel quantum cascade laser emitting at 2 THz,�?? unpublished (June 2005)
  17. Operation at 1.39THz has been achieved with a QCL subject to a magnetic field of 6T. Jerome Faist, private communication.
  18. J. Darmo, V. Tamosiunas, G. Fasching, J. Kröll, K. Unterrainer, M. Beck, M. Giovannini, J. Faist, C. Kremser, and P. Debbage, �??Imaging with a THz quantum cascade laser,�?? Opt. Expr. 12, 1879 (2004) [CrossRef]
  19. D. R. Chamberlin, P. R. Robrish, W. R. Trutna, G. Scalari, M. Giovannini, L. Ajili, and J. Faist, �??Dual wavelength THz imaging with quantum cascade lasers,�?? Appl. Opt. 44, 121 (2005) [CrossRef] [PubMed]
  20. A. Barkan, F. K. Tittel, D. M. Mittleman, R. Dengler, P. H. Siegel, G. Scalari, L. Ajili, F. Faist, H. E. Beere, E. H. Linfield, D. A. Ritchie, and A. G. Davies, �??Linewidth and tuning characteristics of THz quantum cascade lasers,�?? Opt. Lett. 29, 575 (2004). [CrossRef] [PubMed]
  21. S. Barbieri, J. Alton, H. E. Beere, H. Linfield, D. A. Ritchie, S. Withington, G. Scalari, L. Ajili, and J. Faist, �??Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode,�?? Opt. Lett. 29, 1633 (2004) [CrossRef]
  22. A.L. Betz, R.T. Boreiko, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, �??Frequency and phase-lock control of a 3 THz quantum cascade laser,�?? Opt. Lett. 30, 1837 (2005) [CrossRef] [PubMed]
  23. A similar behaviour was found in mid-IR QCLs by measuring the beating between neighbouring Fabry-Perot modes with a fast quantum well photodetector. See A. Soibel, F. Capasso, C. Gmachl, M. L. Peabody, A. M. Serget, R. Paiella, D. L. Sivco, A. Y. Cho, and H. C. Liu, �??Stability of pulsed emission and enhancement of intracavity second-harmonic generation in self mode-locked quantum cascade lasers,�?? IEEE J. Quantum. Electron 40, 197 (2004) [CrossRef]
  24. The 35 dBm residual dynamic range results from resolution bandwidth of 100 kHz used in the experiment, yielding a dynamic range of 70 dBm.
  25. G. Narayanan, N. R. Erickson, and A. W. Lichtenberger, �??Development of multi-pixel heterodyne array instruments at submillimeter wavelengths,�?? Proc. Far-IR, Sub-MM, and MM Detector Technology Workshop, 203 (2002)

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