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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28465–28478

Theory of intrinsic linewidth based on fluctuation-dissipation balance for thermal photons in THz quantum-cascade lasers

Masamichi Yamanishi  »View Author Affiliations

Optics Express, Vol. 20, Issue 27, pp. 28465-28478 (2012)

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Intrinsic linewidth formula modified by taking account of fluctuation-dissipation balance for thermal photons in a THz quantum-cascade laser (QCL) is exhibited. The linewidth formula based on the model that counts explicitly the influence of noisy stimulated emissions due to thermal photons existing inside the laser cavity interprets experimental results on intrinsic linewidth, ~91.1 Hz reported recently with a 2.5 THz bound-to-continuum QCL. The line-broadening induced by thermal photons is estimated to be ~22.4 Hz, i.e., 34% broadening. The modified linewidth formula is utilized as a bench mark in engineering of THz thermal photons inside laser cavities.

© 2012 OSA

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(270.2500) Quantum optics : Fluctuations, relaxations, and noise
(300.3700) Spectroscopy : Linewidth
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Lasers and Laser Optics

Original Manuscript: October 10, 2012
Revised Manuscript: November 28, 2012
Manuscript Accepted: November 28, 2012
Published: December 7, 2012

Masamichi Yamanishi, "Theory of intrinsic linewidth based on fluctuation-dissipation balance for thermal photons in THz quantum-cascade lasers," Opt. Express 20, 28465-28478 (2012)

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  11. Equation (3).14) in reference [7] is applicable to THz-QCLs even near above thresholds since amplitude fluctuations are stabilized by very fast nonradiative relaxation of upper laser-state electrons as pointed out in Appendix B in reference [4] and also in Appendix A of this article.
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  21. The substantial reduction of dark currents by suppression of incoming thermal photons in a cooled low loss fiber coupled to a near infrared photomultiplier tube was in fact confirmed experimentally; T. Hirohata, Y. Negi, and M. Niigaki, Japan patent application No. 2009–047909 (2009) (will be published elsewhere).
  22. The Si doping level in injectors of the THz BTC-QCL is designed to be Ninj/S~1010 1/cm2 [3]. However, in the analysis, the lower doping density 5 × 109 1/cm2 is used by taking account of compensation by residual defect states. In fact, the assumed (effective) doping density 5 × 109 1/cm2 leads to good agreements in threshold and maximum currents between theory and experiments.
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  25. The band-structure computations for the 2.5 THz and 3.5 THz BTC-QCLs were performed by solving Schrödinger/Poisson equations; K. Fujita, private communication.

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