Quantum optoelectronics

Chip-scale frequency combs

A frequency comb is a light source whose lines are evenly spaced. Their regularity and ability to make precise measurements have revolutionized many fields, and for that reason the Nobel Prize in Physics was awarded for their development. Until recently, frequency combs were primarily based on mode-locked lasers, and could not be used in applications requiring compactness. However, this changed with the introduction of semiconductor frequency combs, which use a technology similar to what you might find in a laser pointer.

Although it is possible to naturally form combs in semiconductor lasers, this mode of operation is difficult to produce reliably or predictably over a wide dynamic range, since the dispersion could not be controlled by material growth to the necessary precision. We demonstrated that the concept of dispersion engineering, which had previously been well-established in ultrafast optics, was also valuable for semiconductor quantum cascade lasers. In doing this, we were able to demonstrate the first laser-based terahertz quantum cascade laser combs. We also showed how the temporal profile of these combs could be directly measured, solving a problem that had been standing in the field for decades. This led to the discovery that many different combs can produce frequency-modulated (FM) states, which we showed was an emergent property of many lasers.

Schematic of a frequency comb in a quantum cascade laser. A multimode laser is synchronized into a comb by an optical nonlinearity.

Double-chirped mirrors that compensate dispersion. Long wavelengths penetrate further into the cavity than short wavelengths. [4]

Picture of a MEMS-actuated mid-infrared QCL comb, which allows for chip-scale tuning of the comb. [2]


  1. Burghoff, D., “Unraveling the origin of frequency modulated combs using active cavity mean-field theory,” Optica 7, 1781–1787 (2020). (link)
  2. D. Burghoff, N. Han, F. Kapsalidis, N. Henry, M. Beck, J. Khurgin, J. Faist, and Q. Hu, “Microelectromechanical control of the state of quantum cascade laser frequency combs,” Appl. Phys. Lett., vol. 115, no. 2, p. 021105, Jul. 2019. (pdf)
  3. D. Burghoff, Y. Yang, D. J. Hayton, J.-R. Gao, J. L. Reno, and Q. Hu, “Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs,” Opt. Express, vol. 23, no. 2, p. 1190, Jan. 2015. (pdf, notes on SWIFTS)
  4. D. Burghoff et al., “Terahertz laser frequency combs,” Nature Photon, vol. 8, no. 6, pp. 462–467, Jun. 2014. (pdf, supplementarycover image)
  5. N.C. Henry, D. Burghoff, and J. B. Khurgin, “Mitigating offset frequency drift in frequency combs using a customized power law dispersion,” Opt. Lett., vol. 45, no. 13, pp. 3525–3528, Jul. 2020. (link)
  6. N. Henry, D. Burghoff, Q. Hu, and J. Khurgin, “Study of Spatio-temporal Character of Frequency Combs Generated by Quantum Cascade Lasers,” IEEE Journal of Selected Topics in Quantum Electronics, pp. 1–1, 2019.
  7. J. B. Khurgin, N. Henry, D. Burghoff, and Q. Hu, “Linewidth of the laser optical frequency comb with arbitrary temporal profile,” Appl. Phys. Lett., vol. 113, no. 13, p. 131104, Sep. 2018.
  8. N. Henry, D. Burghoff, Q. Hu, and J. B. Khurgin, “Temporal characteristics of quantum cascade laser frequency modulated combs in long wave infrared and THz regions,” Opt. Express, vol. 26, no. 11, p. 14201, May 2018.
  9. Y. Yang, D. Burghoff, J. Reno, and Q. Hu, “Achieving comb formation over the entire lasing range of quantum cascade lasers,” Opt. Lett., vol. 42, no. 19, pp. 3888–3891, Oct. 2017.
  10. P. Tzenov, D. Burghoff, Q. Hu, and C. Jirauschek, “Analysis of Operating Regimes of Terahertz Quantum Cascade Laser Frequency Combs,” IEEE Trans. THz Sci. Technol., vol. 7, no. 4, pp. 351–359, Jul. 2017.
  11. N. Henry, D. Burghoff, Y. Yang, Q. Hu, and J. B. Khurgin, “Pseudorandom dynamics of frequency combs in free-running quantum cascade lasers,” Opt. Eng, vol. 57, no. 01, p. 1, Sep. 2017.
  12. Y. Yang, D. Burghoff, D. J. Hayton, J.-R. Gao, J. L. Reno, and Q. Hu, “Terahertz multiheterodyne spectroscopy using laser frequency combs,” Optica, vol. 3, no. 5, p. 499, May 2016.
  13. P. Tzenov, D. Burghoff, Q. Hu, and C. Jirauschek, “Time domain modeling of terahertz quantum cascade lasers for frequency comb generation,” Opt. Express, vol. 24, no. 20, p. 23232, Oct. 2016.

Quantum engineering

Intersubband nanostructures are the key enabling technology for long-wavelength optoelectronics. Because mid-infrared and terahertz wavelengths are below the bandgap of most semiconductor materials, one cannot rely on interband transitions when manipulating them. Instead, one must rely on intersubband transitions, devices that exploit the quantum nature of electrons to make artificial atoms. Perhaps the most famous intersubband device is the quantum cascade laser (QCL), a semiconductor source of light at these wavelengths that is chip-scale. A key theme of our work is the design and characterization of new intersubband systems—particularly terahertz QCLs—as well as projects in other areas, such as non-von Neumann computing.

A basic quantum cascade laser design. The layers are nanometers thick, and as a result the quantum properties of electrons manifest in these structures. This enables interaction with long-wavelength light.

Gain measurements of a terahertz quantum laser as a function of device bias. Quantum effects (such as anticrossings) are evident in these spectra, and one can even find many of the relevant optical transitions.

Intersubband devices implementing optoelectronic nonlinearities for artificial neural networks (result to appear in Physical Review Applied)


  1. Y. Yang, A. Paulsen, D. Burghoff, J. L. Reno, and Q. Hu, “Lateral Heterogeneous Integration of Quantum Cascade Lasers,” ACS Photonics, vol. 5, no. 7, pp. 2742–2747, Jul. 2018. (link)
  2. D. Burghoff, Y. Yang, J. L. Reno, and Q. Hu, “Dispersion dynamics of quantum cascade lasers,” Optica, vol. 3, no. 12, p. 1362, Dec. 2016. (pdf, supplementary)
  3. N. Han, A. de Geofroy, D. P. Burghoff, C. W. I. Chan, A. W. M. Lee, J. L. Reno, and Q. Hu, “Broadband all-electronically tunable MEMS terahertz quantum cascade lasers,” Opt. Lett., vol. 39, no. 12, p. 3480, Jun. 2014. (link)
  4. D. Burghoff, C. Wang Ivan Chan, Q. Hu, and J. L. Reno, “Gain measurements of scattering-assisted terahertz quantum cascade lasers,” Appl. Phys. Lett., vol. 100, no. 26, p. 261111, Jun. 2012. (pdf)
  5. D. Burghoff, T.-Y. Kao, D. Ban, A. W. M. Lee, Q. Hu, and J. Reno, “A terahertz pulse emitter monolithically integrated with a quantum cascade laser,” Appl. Phys. Lett., vol. 98, no. 6, p. 061112, Feb. 2011. (pdf)