Abstract

We report the operation of a 2 THz quantum cascade laser based on a GaAs/Al0.1Ga0.9As heterostructure. The laser transition is between an isolated subband and the upper state of a 14 meV wide miniband. Lasing action takes place on a high order vertical mode of a 200 μm thick double-metallic waveguide. In pulsed mode operation, with a 3.16mm long device, we report a threshold current density of 115 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device shows lasing action in continuous wave up to 47K, with a maximum power in excess of 15 mW at T = 4K.

© 2006 Optical Society of America

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  1. 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]
  2. 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]
  3. B. S. Williams, S. Kumar, Q. Hu, and J.L. Reno "Resonant-phonon terahertz quantum cascade laser operating at 2.1 THz (λ= 141 µm)," Electron. Lett. 40, 431 (2004)
    [CrossRef]
  4. G. Scalari, S. Blaser, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, "Terahertz emission from quantum cascade lasers in the quantum Hall regime: evidence for many-body resonances and localization effects," Phys. Rev. Lett. 93, 237403-1 (2004)
    [CrossRef]
  5. G. Scalari, L. Sirigu, C. Walther, J. Faist, M. Sadowski, H. Beere, and D. Ritchie, "Lasing down to 1.45 THz in strong magnetic fields," 8th International Conference on Intersubband transitions in Quantum Wells, ITQW 2005, Cape Cod, MA, Abstract book (2005).
  6. H. Willenberg, G. H. Döhler, and J. Faist, "Intersubband gain in a Bloch oscillator and quantum cascade laser," Phys. Rev. B 67, 085315 (2003).
    [CrossRef]
  7. R. Ferreira, and G. Bastard, "Evaluation of some scattering times for electrons in unbiased and biased single- and multiple-quantum-well structures," Phys. Rev. B 40, 1076 (1989).
    [CrossRef]
  8. 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]
  9. 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)
  10. J. R. Gao, J. N. Hovenier, Z. Q. Yang, J. J. A. Baselmans, A. Baryshev, M. Hajenius, T. M. Klapwijk, A. J. L. Adam, T. O. Klaassen, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, "Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer," Appl. Phys. Lett. 86, 244104 (2005)
    [CrossRef]
  11. 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]
  12. T. Unuma, M. Yoshita, T. Noda, H. Sakaki, and H. Akiyama, "Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, ally disorder, and impurities," Appl. Phys. Lett. 93, 1586 (2003).
  13. . J. Alton, S. Barbieri, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, "Optimum resonant tunnelling injection and influence of doping density on the performance of bound-to-continuum THz quantum cascade lasers," Proc. SPIE Int. Soc. Opt. Eng. 5727, 65 (2005)
  14. The etch-stop layer beneath the active region is not relevant for this work, and was grown in order to allow the fabrication of a double metal waveguide. See for example: S. S. Dhillon, J. Alton, S. Barbieri, C. Sirtori, A. de Rossi, M. Calligaro, H. E. Beere, and D. A. Ritchie, "Ultra-low threshold current quantum cascade lasers based on double-metal buried strip waveguides," Appl. Phys. Lett. 87, 071107 (2005)
    [CrossRef]
  15. The dielectric constants of the doped GaAs layers were computed on the basis of the classical Drude model of the conductivity, with a scattering time of 1 ps in the AR, and of 0.1 ps elsewhere.
  16. C. Sirtori, F. Capasso, J. Faist, A. L. Hutchinson, D. L. Sivco, and A. Y. Cho, "Resonant tunnelling in quantum cascade lasers," IEEE J. Quantum Electron. 34, 1722 (1998)
    [CrossRef]
  17. More realistic calculations of the reflectivity are under way. See C. M. Herzinger, C. C. Lu, T. A. DeTemple, and W. C. Chew, IEEE J. Quantum Electron. 29, 2273 (1993).
    [CrossRef]
  18. Recently, we observed laser emission at 1.94 THz with devices processed from a nominally identical growth of the present QCL.
  19. These findings were confirmed by magneto transport measurements at low biases. At constant voltage we observed periodic oscillations of the current density as a function of 1/B, where B is the intensity of a magnetic field applied parallel to the growth axis. At any voltage, and down to 0.5V, we measured a constant periodicity, from which we derived a transition energy of 8.3 meV. C. Worral et al., unpublished data. For a description of the technique see: J. Alton, S. Barbieri, J. Fowler, J. Muscat, H. E. Beere, E. H. Linfield, A. G. Davies, D. A . Ritchie, R. Khöler, and A. Tredicucci, "Magnetic-field in-plane quantization and tuning of population inversion in a THz superlattice quantum cascade laser," Phys. Rev. B 68, 081303R (2003).
    [CrossRef]
  20. M. F. Pereira, Jr., S. -C. Lee, and A. Wacker, "Controlling many-body effects in the midinfrared gain and terahertz absorption of quantum cascade structures," Phys. Rev. B. 69, 205310 (2004).
    [CrossRef]
  21. M. S. Vitiello, G. Scamarcio, V. Spagnolo, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, "Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers," Appl. Phys. Lett. 86, 111115 (2005).
    [CrossRef]
  22. The pulsed mode Jth vs T curves of Fig. 5 are representative of several devices with different cavity lengths.

8th Inter. Conf. on Intersub Trans 2005 (1)

G. Scalari, L. Sirigu, C. Walther, J. Faist, M. Sadowski, H. Beere, and D. Ritchie, "Lasing down to 1.45 THz in strong magnetic fields," 8th International Conference on Intersubband transitions in Quantum Wells, ITQW 2005, Cape Cod, MA, Abstract book (2005).

Appl. Phys. Lett. (6)

J. R. Gao, J. N. Hovenier, Z. Q. Yang, J. J. A. Baselmans, A. Baryshev, M. Hajenius, T. M. Klapwijk, A. J. L. Adam, T. O. Klaassen, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, "Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer," Appl. Phys. Lett. 86, 244104 (2005)
[CrossRef]

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]

T. Unuma, M. Yoshita, T. Noda, H. Sakaki, and H. Akiyama, "Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, ally disorder, and impurities," Appl. Phys. Lett. 93, 1586 (2003).

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 etch-stop layer beneath the active region is not relevant for this work, and was grown in order to allow the fabrication of a double metal waveguide. See for example: S. S. Dhillon, J. Alton, S. Barbieri, C. Sirtori, A. de Rossi, M. Calligaro, H. E. Beere, and D. A. Ritchie, "Ultra-low threshold current quantum cascade lasers based on double-metal buried strip waveguides," Appl. Phys. Lett. 87, 071107 (2005)
[CrossRef]

M. S. Vitiello, G. Scamarcio, V. Spagnolo, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, "Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers," Appl. Phys. Lett. 86, 111115 (2005).
[CrossRef]

Electron. Lett. (1)

B. S. Williams, S. Kumar, Q. Hu, and J.L. Reno "Resonant-phonon terahertz quantum cascade laser operating at 2.1 THz (λ= 141 µm)," Electron. Lett. 40, 431 (2004)
[CrossRef]

IEEE J. Quantum Electron (1)

More realistic calculations of the reflectivity are under way. See C. M. Herzinger, C. C. Lu, T. A. DeTemple, and W. C. Chew, IEEE J. Quantum Electron. 29, 2273 (1993).
[CrossRef]

IEEE J. Quantum Electron. (1)

C. Sirtori, F. Capasso, J. Faist, A. L. Hutchinson, D. L. Sivco, and A. Y. Cho, "Resonant tunnelling in quantum cascade lasers," IEEE J. Quantum Electron. 34, 1722 (1998)
[CrossRef]

Nature (1)

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]

Opt. Express (1)

Phys. Rev. B (4)

These findings were confirmed by magneto transport measurements at low biases. At constant voltage we observed periodic oscillations of the current density as a function of 1/B, where B is the intensity of a magnetic field applied parallel to the growth axis. At any voltage, and down to 0.5V, we measured a constant periodicity, from which we derived a transition energy of 8.3 meV. C. Worral et al., unpublished data. For a description of the technique see: J. Alton, S. Barbieri, J. Fowler, J. Muscat, H. E. Beere, E. H. Linfield, A. G. Davies, D. A . Ritchie, R. Khöler, and A. Tredicucci, "Magnetic-field in-plane quantization and tuning of population inversion in a THz superlattice quantum cascade laser," Phys. Rev. B 68, 081303R (2003).
[CrossRef]

M. F. Pereira, Jr., S. -C. Lee, and A. Wacker, "Controlling many-body effects in the midinfrared gain and terahertz absorption of quantum cascade structures," Phys. Rev. B. 69, 205310 (2004).
[CrossRef]

H. Willenberg, G. H. Döhler, and J. Faist, "Intersubband gain in a Bloch oscillator and quantum cascade laser," Phys. Rev. B 67, 085315 (2003).
[CrossRef]

R. Ferreira, and G. Bastard, "Evaluation of some scattering times for electrons in unbiased and biased single- and multiple-quantum-well structures," Phys. Rev. B 40, 1076 (1989).
[CrossRef]

Phys. Rev. Lett. (1)

G. Scalari, S. Blaser, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, "Terahertz emission from quantum cascade lasers in the quantum Hall regime: evidence for many-body resonances and localization effects," Phys. Rev. Lett. 93, 237403-1 (2004)
[CrossRef]

Proc. SPIE Int. Soc. Opt. Eng. (2)

. J. Alton, S. Barbieri, H. E. Beere, J. Fowler, E. H. Linfield, and D. A. Ritchie, "Optimum resonant tunnelling injection and influence of doping density on the performance of bound-to-continuum THz quantum cascade lasers," Proc. SPIE Int. Soc. Opt. Eng. 5727, 65 (2005)

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)

Other (3)

The dielectric constants of the doped GaAs layers were computed on the basis of the classical Drude model of the conductivity, with a scattering time of 1 ps in the AR, and of 0.1 ps elsewhere.

Recently, we observed laser emission at 1.94 THz with devices processed from a nominally identical growth of the present QCL.

The pulsed mode Jth vs T curves of Fig. 5 are representative of several devices with different cavity lengths.

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Figures (6)

Fig. 1.
Fig. 1.

Band diagram of 1 period of the active region under an electric field of 1.5 kV/cm. In red are the moduli squared wavefunctions of the upper (2) and lower (1) laser state. The green shaded areas represent the superlattice minibands. Highlighted are also the lower and upper miniband ground states g and 3 (thick green lines). Injection into state 2 occurs via resonant tunnelling from level g across a 5nm thick Al0.1Ga0.9As injection barrier. Starting from this barrier, from left to right, the layer sequence in nm is: 5/12.6/4.4/12.0/3.2/12.4/3.0/13.2/2.4/14.4/2.4/14.4/1.0/11.8/1.0/14.4, with the barriers in bold. The underlined layers are n-doped at levels of 1.3 × 1016 cm-3.

Fig. 2.
Fig. 2.

(a) 1-D mode intensity profile of the first, second and third order mode for the present 200μm-thick waveguide at f = 2THz (λ = 150 μm). The computed waveguide losses and overlap factor with the active region are respectively of 1.15 cm-1 , 0.045; 2.8 cm-1 , 0.145; 2.5 cm-1, 0.13. (b) Figure of merit χ = Γ/αw for the first three modes as a function of frequency. The waveguide thickness is 200 μm (c) Figure of merit χ = Γ/αw for the 2nd order mode at f = 2THz as a function of waveguide thickness.

Fig. 3.
Fig. 3.

V/J and P/J curves for a 3.16mm long device, with high reflectivity back-facet coating (SiO/Ti/Au). The V/J curve was collected in a three-terminal configuration. The laser was operated in pulsed mode with 250ns long pulses at a repetition rate of 80kHz. An additional 7Hz, 50% duty cycle slow modulation was superimposed to match the detector response time. Insets. Laser spectra recorded with a Fourier Transform Infrared Spectrometer with a resolution of 7.5 GHz (0.25 cm-1). The device was driven in continuous wave at T = 4K.

Fig. 4.
Fig. 4.

Low bias electroluminescence spectra collected with a Fourier Transform Infrared Spectrometer at T = 4K. The device was driven in pulsed mode with a 290 Hz repetition rate and 20% duty cycle. Inset. V/J curve. The coloured circles indicate the voltage and current density at which each spectrum was recorded.

Fig. 5.
Fig. 5.

CW output power vs current density characteristics as a function of the heat sink temperature. Inset. Threshold current density vs heat sink temperature in CW and pulsed mode for the same device of Fig. 3, and for the 2.9THz QCL of Ref. [11]. The solid lines are guides the eye.

Fig. 6.
Fig. 6.

Top. J/V curves at T = 4K of the present AR (red) and of the 2.9THz AR of Ref. [11] (blue). Devices were processed in rectangular 250×185 μm2 mesa structures. Bottom. Differential resistivity derived from the J/V curves. The drawings represent schematically two AR periods separated by the injection barrier, with the black arrow indicating the main current path. The shaded regions highlight the different transport regimes: (i) at low bias electrons transport between neighbouring minibands; (ii) at intermediate biases carriers are injected in the upper state via resonant tunnelling; (iii) at high voltages the structure breaks.

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