Carriers density imaging by self-mixing interferometry in a THz quantum cascade laser L. L. Columbo, F. P. Mezzapesa, M. Dabbicco, M. Brambilla, and G. Scamarcio Dipartimento Interateneo di Fisica, Università degli Studi e Politecnico di Bari, via Amendola 173, I-70126 Bari, Italy lorenzo.columbo@uniba.it L. L. Columbo, F. P. Mezzapesa, M. Dabbicco, M.Brambilla, and G. Scamarcio CNR-IFN UOS Bari, via Amendola 173, I-70126 Bari, Italy M. S. Vitiello CNR-NEST - Istituto Nanoscienze and Scuola Normale Superiore, piazza San Silvestro 12, 56127 Pisa, Italy Abstract—We propose a THz imaging system based on self- mixing (SM) interferometry in a Quantum Cascade Laser (QCL) to map the distribution of free charges on a semiconductor surface. In our experiment a free electron plasma is photo- generated in a high resistivity n-type silicon wafer using a near infrared (NIR) continuous wave (CW) pump laser. A model based on Drude theory correctly reproduces the experimental results and in prospective promises a quantitative evaluation of free charges densities. Keywords— THz imaging; quantum cascade laser; self-mixing interferometry I. INTRODUCTION When a semiconductor laser is subject to optical reinjection from an external target (SM configuration) the intracavity laser field coherently interferes with the back reflected radiation carrying information about the target motion and/or its optical proprieties. This leads to a number of applications in metrology and sensing [1]. Coherent imaging that exploits the SM effect in THz QCLs is currently very promising in sensing and material processing applications mostly because of the THz QCLs high CW power, high sensitivity to optical reinjection and narrow linewidth [2, 3]. Moreover, because of the high value of the photon to carrier lifetime ratio (≈ 10 1 ps), and to the negligible linewidth enhancement factor (α ≈ 0.5 [4]), THz QCLs tolerate strong optical feedback levels without exhibiting dynamical instabilities such as mode-hopping, or coherence collapse [5]. Here we exploit these unique features, together with the free carriers dependence on the semiconductor dielectric response in the THz frequency range, to propose an innovative technique for imaging free carriers on a semiconductor surface via SM in THz QCLs. In the SM scheme the coherent superposition of the laser field with the radiation back reflected from the semiconductor target manifests itself as voltage variations across the THz QCL terminals. Hence, the THz QCL acts both as an emitter and a detector of changes in the semiconductor target reflectivity induced by a spatially modulated free carriers density. The Drude theory for free carriers allows to associate the measured SM signal with the corresponding carriers density variation. In our experiment, the latter is induced by photo-excitation with a reconfigurable pattern of a NIR pump. Compared to other optical techniques, such us plasma resonance and free carrier absorption, that shows better sensitivity and accuracy, or THz near-field nanoscopy and pump and probe microscopy that allows for higher spatial resolution [6-9], the advantage of the proposed THz imaging system consists in being more compact, detector-free and real- time. These features, together with the ultrafast response time of THz QCLs (few picoseconds) allows us to conceive future applications such us the direct investigation of the spatio- temporal free carriers distribution in active devices (organic transistors, photovoltaic cells, etc..). I. THE EXPERIMENT A. Free carriers imaging “pump-probe” set-up As described in Fig. 1, in the experimental set up [10] consists of a low power (≈ 40mW/cm 2 ) CW diode “pump” laser in the near infrared (λ IR = 832nm) whose emitted radiation passes trough spatial light modulator and illuminates a high resistivity n-type silicon wafer 1 mm thick. The modulated pattern of photo-excited charges induces a modulation of the silicon target reflectance  ௘௫௧ ൌ ቚ ா ೃ ா ಺ ቚ ଶ that is detected, in the SM configuration, using a “probe” beam at λ THz = 76.3 μm (3.93 THz) delivered by a as resonant-phonon single-mode 978-1-4799-0162-3/14/$31.00 ©2014 IEEE