WP5

Stimulated THz emission from Si:P and Si:Bi under resonant intracenter

optical pumping J.N. Hovenier, T.O. Klaassen, Delft University of Technology, The Netherlands; R.W. Zhukavin, D.M.

Gaponova, A.V. Muravjov, E.E. Orlova, V.N. Shastin, IPM, RAS, Nizhny Novgorod, Russia; S.G. Pavlov, H.-W. Huhers, Institute of Space Sensor Technology and Planetary Exploration, DLR, Berlin,

Germany; H. Riemann, Institute of Crystal Growth, Berlin, Germany, and A.F.G. van der Meer, FOM- Institute for Plasma Physics, Rijnhuizen, The Netherlands

Abstract - Frequency tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in hulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature under resonant pumping of odd parity impurity states

I. INTRODUCTION

There are two basic mechanisms that may cause population inversion of charge carriers between impurity states. The first one is based on the suppression of acoustical phonon assisted relaxation of the optically excited electrons over the localized states with the increase of the energy distance between the levels. Such a bottleneck effect occurs for the lower excited states of the impurity center. In Si:P it leads to the overpopulation of the 2p0 state (Fig.1,) A second mechanism for population inversion is predicted in Si:Bi due to the strong resonant coupling of both 2p0 and 2s excited states with the Is(A) ground state via inter-valley TO- and LO- optical

Si:P conduction

FELIX pumping

THz emission

Is(A) * -

Fig. 1. Energy level schemes and possible excitation and emission transitions for Si:P and Si:Bi.

phonons. The resulting spontaneous emission of optical phonons makes the lifetimes of the 2p0 and 2s states extremely short s), dumping carriers directly into the ground state. Such a scenario provides the depletion of the 2p0 and 2s states and leads to a negligible population of the ls(E,T) states. Hence, population inversion between the higher excited states (notably the 2p, state) and the 2s, 2p0, Is(E, T) states is expected. Recently THz lasing has been observed in Si:P and Si:Bi under CO2 laser pumping [1,2]. The (photo-ionization) COz laser pumping, has drawbacks. The absorption cross section is small (ca. 4 ~ 1 0 . ’ ~ cm2 for Si:P) compared to that of intracenter -i.e. between impurity states- optical transitions (up to 10.’’ cm2 ) and the photo- ionization leads to the creation of D- centers which are good absorbers of THz radiation. Both factors increase the pump threshold for stimulated emission. Using the frequency tunable Dutch Free Electron Laser FELIX enables the more efficient intracenter pumping and the investigation of the intracenter relaxation of photoexcited carriers. The FELIX radiation consisted of 6 - 8 ps long trains of (6 - 8 ps long) micro-pulses at a 1 ns time interval and maximum peak power of ahout 0.5 MW at a repetition rate of 5 Hz.

11. EXPERIMENTAL RESULTS

In Figs. 2 and 4 the stimulated emission of Si:P and Si:Bi as a function of the frequency of the FELIX excitation is shown for various pump power levels. Clear maximums are visible at frequencies where resonant pumping from the donor groundstate into an excited state occurs. In Si:P the lowest threshold pump power (-IkWicm’) is found for pumping the 2p0 state.

0-7803-7423-1/02/$17.00 02002 IEEE. -271 -

I 34 36 38 40 42 44 46 48 50

FEL photon energy (mev)

Fig.2. Si:P -- stimulated emission intensity as a function of FELIX excitation energy.

Emission in Si:P has been observed on the 2p0 - ls(T2) intra-center transition (1 8 1 cm-I), when pumping the 2p, state, the higher excited donor states or the continuum. Direct pumping of the 2p0 state leads to emission on the 2p0 -ls(E) transition (171 cm-') (Fig. 3) This observed difference of laser transition is not yet well understood, hut might be related to interactions between free carriers leading to a redistribution of population between the Is(E) and ls(T) states.

pumping in ? m the 2p. state ._ 2 10 m c al - .G n "

50 100 150 200 250 300 350 400

pumping in m the Zp, state ._ . m c (U

c - .- 0

50 100 150 200 250 300 350 400

wavenumber (mi')

Fig.3. Si:P : emission spectrum pumping different excitated states.

A similar behaviour is found in Si:Bi; now the lowest threshold is observed for pumping the 2p, state. Stimulated emission occurs here on the 2p, -ls(E) (191 cm-') and 2p, -+ls(Tz) (206 cm.') transitions, when pumping into the 2p, or higher excited donor states, or into the continuum (Fig.5). Surprisingly, under direct optical pumping of the short living 2p0 state, also

ZP, 3~,3~*,.4p,,.continuu

56 60 64 68 71 FEL photon energy (mev)

Fig. 4. Si:B --stimulated emission intensity in Si:B as a function of FELIX excitation energy.

stimulated emission is observed on the 2pO+ls(E) transition at 155 cm-I.

I pumping in -. 30 3 = 20 x .= m 5 10 .- c ._

0 50 100 150 200 250 300 350 40C

pumping in the 4p0 state

2 5 m c al - .- :lOL c 0 50 100 150 200 250 300 350 400

wavenumber (cm-')

Fig.5. Si:B : emission spectrum pumping different excitated states.

ACKNOWLEDGEMENTS This work was partly supported by the Deutsche Forschungsgemeinschaft and the Russian Foundation for Basic Research (RFBR) (joint grant 436 RUS 113120610 (R) and 00-02-04010), RFBR grants 02-02-16790, INTAS grant YSF 00-239, as well as the European Union TMR program "INTERACT" S.G. Pavlov gratefully acknowledges support

through an Alexander von Humboldt Stiftung.

REFERENCES

[I ] S.G. Pavlov et al. Phys.Rev.Lett. 84(2000)5220 [2] S.G. Pavlov et.al. submitted for be publication [ 3 ] V. N. Shastin, et.al. Appl. Phys. Lett.

80(2002)3512

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