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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 1 — Jan. 14, 2013
  • pp: 968–973
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High performance terahertz quantum cascade laser sources based on intracavity difference frequency generation

Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi  »View Author Affiliations


Optics Express, Vol. 21, Issue 1, pp. 968-973 (2013)
http://dx.doi.org/10.1364/OE.21.000968


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Abstract

We demonstrate high power, room temperature, single-mode THz emissions based on intracavity difference frequency generation from mid-infrared quantum cascade lasers. Dual active regions both featuring giant nonlinear susceptibilities are used to enhance the THz power and conversion efficiency. The THz frequency is lithographically tuned by integrated dual-period distributed feedback gratings with different grating periods. Single mode emissions from 3.3 to 4.6 THz with side-mode suppression ratio and output power up to 40 dB and 65 µW are obtained, with a narrow linewidth of 5 GHz.

© 2013 OSA

1. Introduction

2. Design, growth and fabrication of THz QCL sources based on DFG

A row of ten devices with dual-period DFB gratings spanning a DFG frequency range from 3.3 to 4.6 THz are defined by electron beam lithography. The gratings were transferred into the cap layer with a grating depth of 200 nm, following the process in Ref [9

9. Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 98(18), 181106 (2011). [CrossRef]

]. Figure 1(b) is the scanning electron microscope (SEM) image of the grating shape after dry etching. The Fourier analysis of the extrapolated grating shape from the SEM picture shown in the inset of Fig. 1(b) gives two distinct peaks with a THz energy spacing around 4 THz. The other two satellite peaks with the same energy spacing away from the two main peaks correspond to the high-order Fourier series of the grating shape. The sample was processed into double-channel geometries with a 16-μm ridge width tapered into 60 μm with a taper angle of 1° toward one end. A narrow ridge width is crucial to secure operation in the fundamental transverse mode, which is then amplified in the tapered region. This is important to the THz generation based on DFG, because an optimal effective area of interaction is expected when both mid-IR wavelengths operate in the fundamental transverse modes. Any higher order mode will significantly reduce the THz signal intensity [10

10. M. Geiser, C. Pflügl, A. Belyanin, Q. J. Wang, N. Yu, T. Edamura, M. Yamanishi, H. Kan, M. Fischer, A. Wittmann, J. Faist, and F. Capasso, “Gain competition in dual wavelength quantum cascade lasers,” Opt. Express 18(10), 9900–9908 (2010). [CrossRef] [PubMed]

]. The surface grating is defined in the non-tapered region. The device with 3-mm cavity length is high-reflection (HR) and anti-reflection (AR) coated with Si3N4/Au (400/1000 nm) and Y2O3 (1000 nm), respectively.

All testing was performed at room temperature in pulsed mode operation, with a pulse width of 60 ns and a duty cycle of 1.5%. The THz power measurement system is similar to Ref [6

6. Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett. 99(13), 131106 (2011). [CrossRef]

]. Optical filters were used to differentiate the THz power from mid-IR powers. Spectral measurements were performed with a Bruker Fourier transform infrared (FTIR) spectrometer equipped with a mercury-cadmium-telluride (MCT) photodetector for mid-IR and a liquid helium-cooled silicon bolometer for THz signals. The far fields were obtained with a computer controlled rotational stage and a MCT detector.

3. Experimental results and discussions

The highest THz output power is obtained from a device operating at 4 THz. Figure 2
Fig. 2 Room temperature mid-IR and THz characteristics of a device operating at 4 THz. (a) Mid-IR spectra at different currents and EL spectrum from a reference mesa structure. (b) P-I-V characterization for the two wavelengths. Inset: far fields at different currents. (c) THz power as a function of current and mid-IR-power products (inset). (d) Normalized THz spectra at different currents.
shows the mid-IR and THz performances for this device. The mid-IR spectra and electroluminescence (EL) spectrum from a reference mesa structure are shown in Fig. 2(a). Considering that the free-carrier absorption in the waveguide has a λ2 dependence [11

11. S. Slivken, A. Evans, W. Zhang, and M. Razeghi, “High-power, continuous-operation intersubband laser for wavelengths greater than 10 μm,” Appl. Phys. Lett. 90(15), 151115 (2007). [CrossRef]

], the mid-IR spectra are intentionally blue shifted by ~35 cm−1 with respect to the gain peak for a better power balance between the two wavelengths. Primary emissions at λ1 = 9.0 μm and λ2 = 10.22 μm are obtained at all working currents. The other two nontrivial emissions at λ1' = 8.06 μm and λ2' = 11.85 μm are attributed to the high-order Fourier series of the grating shape as shown in the inset of Fig. 1(b). The power-current-voltage (P-I-V) characterizations for the two wavelengths are shown in Fig. 2(b). Compared with results in Ref [6

6. Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett. 99(13), 131106 (2011). [CrossRef]

], the total power is enhanced by a factor of 2 and the input electrical power is reduced by about 40%. A proper optical filter is used to differentiate the two mid-IR wavelengths λ1 and λ2. The measured powers for λ1 and λ2 shown in Fig. 2(b) include the roughly 5% contributions from λ1' and λ2', which are estimated by comparing the peak intensities in the emitting spectra. The far fields (inset of Fig. 2(b)) of the total mid-IR output are single lobed and nearly diffraction-limited, which indicates that all the wavelengths are working in their fundamental transverse modes.

Figure 2(c) shows the THz spectra measurement results. The device exhibits stable single mode operation around 4 THz (λ ~75.4 μm). The spectrum linewidth is about 5 GHz, which is mainly limited by the resolution of the FTIR (0.125 cm−1). The SMSR is as high as 40 dB at high currents (>6.0 A), and the THz spectral position is very stable as the current changes. The THz current tuning rate is 0.60 GHz/A, which is about one order of magnitude smaller than the mid-IR tuning rates (5.4 and 6.0 GHz/A for λ1 and λ2). This indicates that the THz emission is relatively insensitive to current or temperature fluctuations, which is beneficial for many applications.

4. Conclusion

In conclusion, we demonstrate high power, room temperature, single-mode THz emissions based on intracavity DFG in mid-IR QCLs. The THz frequency is lithographically tuned by integrated dual-period DFB gratings with different grating periods. Narrow linewidth, single mode emissions from 3.3 to 4.6 THz with side-mode suppression ratio and output power up to 40 dB and 65 µW are obtained.

Acknowledgments

References and links

1.

B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef] [PubMed]

2.

M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]

3.

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(6885), 156–159 (2002). [CrossRef] [PubMed]

4.

S. Fathololoumi, E. Dupont, C. W. I. Chan, Z. R. Wasilewski, S. R. Laframboise, D. Ban, A. Mátyás, C. Jirauschek, Q. Hu, and H. C. Liu, “Terahertz quantum cascade lasers operating up to ∼ 200 K with optimized oscillator strength and improved injection tunneling,” Opt. Express 20(4), 3866–3876 (2012). [CrossRef] [PubMed]

5.

M. A. Belkin, F. Capasso, F. Xie, A. Belyanin, M. Fischer, A. Wittmann, and J. Faist, “Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation,” Appl. Phys. Lett. 92(20), 201101 (2008). [CrossRef]

6.

Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett. 99(13), 131106 (2011). [CrossRef]

7.

K. Vijayraghavan, R. W. Adams, A. Vizbaras, M. Jang, C. Grasse, G. Boehm, M. C. Amann, and M. A. Belkin, “Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett. 100(25), 251104 (2012). [CrossRef]

8.

M. A. Belkin, F. Capasso, A. Belyanin, D. L. Sivco, A. Y. Cho, D. C. Oakley, C. J. Vineis, and G. W. Turner, “Terahertz quantum-cascade-laser source based on intracavity difference-frequency generation,” Nat. Photonics 1(5), 288–292 (2007). [CrossRef]

9.

Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 98(18), 181106 (2011). [CrossRef]

10.

M. Geiser, C. Pflügl, A. Belyanin, Q. J. Wang, N. Yu, T. Edamura, M. Yamanishi, H. Kan, M. Fischer, A. Wittmann, J. Faist, and F. Capasso, “Gain competition in dual wavelength quantum cascade lasers,” Opt. Express 18(10), 9900–9908 (2010). [CrossRef] [PubMed]

11.

S. Slivken, A. Evans, W. Zhang, and M. Razeghi, “High-power, continuous-operation intersubband laser for wavelengths greater than 10 μm,” Appl. Phys. Lett. 90(15), 151115 (2007). [CrossRef]

12.

P. K. Tien, R. Ulrich, and R. J. Martin, “Optical second harmonic generation in form of coherent Čerenkov radiation from a thin-film waveguide,” Appl. Phys. Lett. 17(10), 447–450 (1970). [CrossRef]

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3490) Lasers and laser optics : Lasers, distributed-feedback
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(190.4223) Nonlinear optics : Nonlinear wave mixing
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: November 7, 2012
Revised Manuscript: December 8, 2012
Manuscript Accepted: December 10, 2012
Published: January 9, 2013

Citation
Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, "High performance terahertz quantum cascade laser sources based on intracavity difference frequency generation," Opt. Express 21, 968-973 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-968


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References

  1. B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater.1(1), 26–33 (2002). [CrossRef] [PubMed]
  2. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics1(2), 97–105 (2007). [CrossRef]
  3. 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,” Nature417(6885), 156–159 (2002). [CrossRef] [PubMed]
  4. S. Fathololoumi, E. Dupont, C. W. I. Chan, Z. R. Wasilewski, S. R. Laframboise, D. Ban, A. Mátyás, C. Jirauschek, Q. Hu, and H. C. Liu, “Terahertz quantum cascade lasers operating up to ∼ 200 K with optimized oscillator strength and improved injection tunneling,” Opt. Express20(4), 3866–3876 (2012). [CrossRef] [PubMed]
  5. M. A. Belkin, F. Capasso, F. Xie, A. Belyanin, M. Fischer, A. Wittmann, and J. Faist, “Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation,” Appl. Phys. Lett.92(20), 201101 (2008). [CrossRef]
  6. Q. Y. Lu, N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett.99(13), 131106 (2011). [CrossRef]
  7. K. Vijayraghavan, R. W. Adams, A. Vizbaras, M. Jang, C. Grasse, G. Boehm, M. C. Amann, and M. A. Belkin, “Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers,” Appl. Phys. Lett.100(25), 251104 (2012). [CrossRef]
  8. M. A. Belkin, F. Capasso, A. Belyanin, D. L. Sivco, A. Y. Cho, D. C. Oakley, C. J. Vineis, and G. W. Turner, “Terahertz quantum-cascade-laser source based on intracavity difference-frequency generation,” Nat. Photonics1(5), 288–292 (2007). [CrossRef]
  9. Q. Y. Lu, Y. Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “2.4 W room temperature continuous wave operation of distributed feedback quantum cascade lasers,” Appl. Phys. Lett.98(18), 181106 (2011). [CrossRef]
  10. M. Geiser, C. Pflügl, A. Belyanin, Q. J. Wang, N. Yu, T. Edamura, M. Yamanishi, H. Kan, M. Fischer, A. Wittmann, J. Faist, and F. Capasso, “Gain competition in dual wavelength quantum cascade lasers,” Opt. Express18(10), 9900–9908 (2010). [CrossRef] [PubMed]
  11. S. Slivken, A. Evans, W. Zhang, and M. Razeghi, “High-power, continuous-operation intersubband laser for wavelengths greater than 10 μm,” Appl. Phys. Lett.90(15), 151115 (2007). [CrossRef]
  12. P. K. Tien, R. Ulrich, and R. J. Martin, “Optical second harmonic generation in form of coherent Čerenkov radiation from a thin-film waveguide,” Appl. Phys. Lett.17(10), 447–450 (1970). [CrossRef]

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