# Two-photon quantum well infrared photodetector

Band diagram of two-photon QWIP |

The two-photon quantum well infrared photodetector (QWIP) approach involves semiconductor quantum wells with three equidistant subbands, two of which are bound in the quantum well and the third state is located in the continuum (see the band diagram). The intermediate subband induces a resonantly enhanced optical nonlinearity, such that two-photon absorption becomes about six orders of magnitude stronger than in usual semiconductors. Applying an electric field across the quantum wells, this two-photon absorption leads to a photocurrent which depends quadratically on the incident power.

This quadratic detection can be exploited for autocorrelation measurements. Owing to the resonant enhancement of this optical nonlinearity, the time resolution of two-photon QWIPs is only limited by the sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of the intersubband polarization. These properties make two-photon QWIPs very promising for pulse diagnostics of pulsed mid-infrared lasers.

We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL) facility FELBE at the HZDR. Using a rapid-scan autocorrelation scheme at a scan frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity autocorrelation.

FEL autocorrelation at λ = 42 µm |

Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space charges generated in the detector.

We have demonstrated two-photon QWIP operation at various wavelengths from 5.5 to 42.0 µm. The concept has also been exploited to study modelocking in quantum cascade lasers.

## Publications

**C. Franke, M. Walther, M. Helm, H. Schneider**, *Two-photon quantum well infrared photodetectors below 6 THz*, Infrared Physics Technol. **70**, 30 (2015).

**C. Wang, L. Kuznetsova, V. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H. Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and F. Capasso**, *Mode-locked pulses from mid-infrared Quantum Cascade Lasers, *Opt. Express **17**, 12929-12943 (2009).

**H. Schneider, H.C. Liu, S. Winnerl, C.Y. Song, O. Drachenko, M. Walther, J. Faist, and M. Helm**, *Quadratic detection with two-photon quantum well infrared photodetectors, *Infrared Physics Technol. **52**, 419 (2009).

**H. Schneider, H. C. Liu, S. Winnerl, C. Y. Song, M. Walther, and M. Helm**, *Terahertz two-photon quantum well infrared photodetector, Opt. Express 17, 12279-12284 (2009)*

**A. Gordon, C. Y. Wang,, L. Diehl, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, H. C. Liu, H. Schneider, T. Maier, M. Troccoli, J. Faist, and F. Capasso, ***Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning, *Phys. Rev. A **77**, 053804 (2008).

**H. Schneider, H. C. Liu, S. Winnerl, O. Drachenko, M. Helm, and J. Faist**, *Room-temperature mid-infrared two-photon photodetector, *Appl. Phys. Lett. **93**, 101114 (2008).

**H. Schneider, O. Drachenko, S. Winnerl, M. Helm, T. Maier, and M. Walther**, *Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP, *Infrared Physics & Technology 50, 95 (2007).

**H. Schneider**, T. Maier, H. C. Liu, and M. Walther, *Two-photon photocurrent spectroscopy of electron intersubband relaxation and dephasing in quantum wells*, Appl. Phys. Lett. **91**, 191116 (2007).

**T. Maier, H. Schneider, H. C. Liu, M. Walther, and P. Koidl**, *Quantum well infrared photodetector with voltage-switchable quadratic and linear response, *Appl. Phys. Lett. **88**, 051117 (2006).

**H. Schneider, O. Drachenko, S. Winnerl, M. Helm, and M. Walther**, *Quadratic autocorrelation of free-electron laser radiation and photocurrent saturation in two-photon quantum-well infrared photodetectors, *Appl. Phys. Lett. **89**, 133508 (2006).