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Competing Inversion-Based Lasing and Raman Lasing in Doped Silicon

Date of news: 22 October 2018

Optically pumped lasers can create intense, coherent beams of light using two different mechanisms: inversion based and inversionless. The difference between the two lies in whether electrons decay from an overpopulated excited energy state to a lower one (inversion based) or whether a transition occurs from a low state to a higher one (inversionless). Typically, only one of the two mechanisms can work at a time. However, in the results presented here it is shown that both mechanisms can operate simultaneously and even at the same emission frequency.
The lasing medium is crystalline silicon doped with bismuth. Three of the available electronic levels in this system are utilized to create an energy cascade for laser pumping. Inversion-based photons originate from electrons dropping from the highest to the middle levels, while transitions from the lowest to the middle level mediate Raman scattering that, in turn, emits inversionless photons.
Time-resolved spectroscopy using the infrared ad THz free-electron laser FELIX at a frequency of 19 micrometer reveals that the system rapidly alternates between the two lasing mechanisms. Theoretical modelling shows that this behaviour arises from the ultrafast evolution of electron populations among all three energy levels. The change in the contribution to the total laser output from a particular mechanism occurs within just a few picoseconds.
This research might stimulate the search for similar simultaneous lasing mechanisms in other media.

1022 competing inversion

Temporal and spectral evolution of the Si∶ Bi laser emission when pumped at 19.02μm(65.2 meV) which is 0.6 meV above the long-lived 2p± state (color map with projections to time and photon energy axis). The emission lines at approximately23.8meV (5.78 THz) and at 24.5 meV (5.92 THz) correspond to the PIL 2p± 1s(E) transition and to the Raman emission from a virtual state into the 1s(E)state, respectively. TheFWHMFEL is about 0.25 meV and the spectral resolution of the Fourier-transform infrared spectrometer is 37μeV (0.3cm−1). The right upper graph shows the Si∶ Bi absorption spectrum compared with the FEL pump spectrum.

This work was partly supported by the German-Russian “Research on technological advances of radiation sources of photons and neutrons based on accelerators and neutron sources in cooperation with research organizations and universities of the Federal Republic of Germany” (InTerFEL project, No. BMBF05K2014 and No. RFMEFl61614X0008), The Netherlands Organisation for Scientific Research (NWO), as well as joint German-Russian project supported by the Deutsche Forschungsgemeinschaft (Project No. 389056032) and the Russian Foundation for Basic Research (RFBF Project No. 18-502-12077-DFG). Nils Deßmann acknowledges support by the Helmholtz Research School on Security Technologies.

S. G. Pavlov, N. Deßmann, B. Redlich, A. F. G. van der Meer, N. V. Abrosimov, H. Riemann, R. Kh. Zhukavin, V. N. Shastin, and H.-W. Hübers “Competing Inversion-Based Lasing and Raman Lasing in Doped Silicon” Phys. Rev. X 8 (2018) 041003

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Dr. Nils Deßmann