Seminar - Interband and quantum cascade laser frequency combs: From fundamentals towards monolithic spectrometers

May 28, 14:00, CEITEC C, room C2.11

Benedikt Schwarz  

Institute of Solid state electronics, Vienna University of Technology  

Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts. I will give an overview of our current work on interband and quantum cascade lasers (ICLs and QCLs) ranging from an introduction to fundamental laser physics to the realization of monolithic devices. I will highlight similarities and differences between these two types of lasers, show how both FM and AM type frequency combs can be realized and discuss why the ICL comb platform is perfect for the realization of miniaturized spectrometers.

Frequency combs consist of a pump laser and a non-linear mechanism that couples the modes [1]. In quantum cascade lasers, the non-linearity of the laser material can be used to generate frequency combs. Our results show that this concept can be also be applied to ICLs, despite the fact that their laser transition lifetime differs by more than two orders of magnitude[2]. This is possible because the effective upper state lifetime is sufficiently fast under operation. Different to traditional mode-locking, the comb state is dominantly frequency modulated (FM), which shows that phase-locking is governed by the same physical mechanism as in QCL combs.

Recent results revealed that RF injection allows coherent control of QCL and ICL frequency combs. This enables injection locking of the repetition frequency of the FM comb state, which provides an increased stability of the state and the missing knob for the mutual stabilization of two combs via control loops[3]. Even more exciting, the coherent control is not limited to locking a frequency modulated self-starting comb. Detuning the injection frequency far from the natural laser beatnote can completely change the overall behavior of the comb. 

Similarly to the occurrence of multiple normal modes in coupled oscillators, fundamentally different comb states can be externally enforced. Using this knowledge we are able switch ICLs to the amplitude modulated comb state, commonly referred to active mode-locking. There, the ICLs emit picosecond pulses. Even more exciting is the
fact that we can also apply this new insights into QCLs that were believed to be not capable of efficient pulsed emission via mode-locking. Our experimental results on QCLs clearly show that this previous picture needs to be reinterpreted. We were able to get pulse-width down to about 1ps and a peak-power enhancement of over 50 using a high-power QCL.

A key feature of both QCLs and ICLs is that the very same epilayer structure can also be used to integrate photodetectors on the same chip[4, 5]. The fact that QCLs and ICLs both rely on fast intersubband transport is a great advantage for high bandwidth multi-heterodyne detection. As an example, the on-chip detection capability of ICLs can exceed several GHz electrical bandwidth[2], while providing an excellent noise equivalent power of 2.5pW/Hz at room-temperature. Thus, the ICL technology provides all required properties for monolithic integration and can promote semiconductor based dual-comb spectroscopy from fundamental research to a broadly used sensing platform.

[1] A. Hugi, G. Villares, S. Blaser, H. C. Liu, and J. Faist, “Mid-infrared frequency comb based on a quantum cascade laser,” Nature, vol. 492, no. 7428, pp. 229–233, 2012.

[2] B. Schwarz, J. Hillbrand, M. Beiser, A. M. Andrews, G. Strasser, H. Detz, A. Schade, R.Weih, and S. Hofling, “A monolithic frequency comb platform based on interband cascade lasers and detectors,” arXiv preprint arXiv:1812.03879, 2018.

[3] J. Hillbrand, A. M. Andrews, H. Detz, G. Strasser, and B. Schwarz, “Coherent injection locking of quantum cascade laser frequency combs,” Nature Photonics, vol. 13, no. 2, pp. 101–104, 2018.

[4] B. Schwarz, P. Reininger, H. Detz, T. Zederbauer, A. M. Andrews, S. Kalchmair,W. Schrenk, O. Baumgartner, H. Kosina, and G. Strasser, “A bi-functional quantum cascade device for same-frequency lasing and detection,” Applied Physics Letters, vol. 101, no. 19, p. 191109, 2012.

[5] H. Lotfi, L. Li, S. M. S. Rassel, R. Q. Yang, C. J. Corr´ege, M. B. Johnson, P. R. Larson, and J. A. Gupta, “Monolithically integrated mid-IR interband cascade laser and photodetector operating at room temperature,” Applied Physics Letters, vol. 109, no. 15, p. 151111, 2016.