We investigate the recombination dynamics of photoexcited carriers in proton-bombarded InP crystals using near-infrared pump-THz-probe spectroscopy. The carrier lifetime is directly related to the proton dose and hence to the induced trap density: In weakly damaged samples we observe a saturation of the trap states for high-excitation densities. For highly damaged samples the time-dependent THz transmission can be explained by taking into account an Auger-assisted trapping process.

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<'ONllNLJlJM CONTKIULI'I'ION 'l'0 I3XCITONIC I~OUR-WAVE MIXING I). I3ikcdal. K.E. Siiyed. V.G. Lyssciiko. ;itid J.M. tlv;ttii Mikroclcklroiiik Cciitrcl. 'l'lic 'rcchaical Univcrsily 0 1 I)eniii;irk IIK-?X(N) Lyiigby. Dciiiiiark. Fax: +45 45 XX 77 62. Eiii:iil: D.tii(fi~iiiic.tltu.tlk . stiites. I ' Fig. 1. Tcinsicnt kwr-wiivc mixing spctruiii at zero time delay.I I 11. Rldiiiiiiin.'l'. Mcicr. F. von Plcsrca. r i d I'hys. Rev. h i t . 70, 3027 (IW3). 121 U. Sicgncr. M A . Mycck. S. Gluisch. iintl D.S. Chcnib. I'hys. Rev. Lctt. 74.470 (IWS). 1311'. kijiiiiai wid Y. Tiiira~ i. Pliyr SK. J;ip 47. 16?11 (IY7Y).Fig.2. lnicnsily d t h c transici lour wiive iiiiaiits r~p a l ai ihc heavy holc cxcilonic rcscmarcs. 16:30 bTu14 Generation of u l t r a s h o r t far-infrared pulses t u n a b l e around 5 THz M. Sailer. Ch. Mcssncr and K.A. 11opfcl ln.sfi/u/,fiir I~~i~e~inienrcrli~hysik Univer.si/ii/ Innshrnck. ?i.cIinikers/r. 25 A 4 0 2 0 Inwhr-rick. Aii.s/r-iti Phone: -C+43/5 I2/507-6332 PAX: ++43/5 I2/507-292 I Several opto-electronic techniques utilizing femtosecond laser pulscs h a w hccn recently developed to generate broadband 'fl-lz pulses in free space 11-31, One method IS the use of high speed photoconducting micro-strip antennae [2]. Our approach is the oplical rectification of fs-laser pulses in a nonlinear organic crystal. We use the organic salt dimethyl amino 4-Nmethylstilbazolium tosylate (DAST) because of its high nonlinear optical susceptibility x'*' The source of the ultrashort optical pulses was a cw-Ar+-laser pumped, mode-locked Tksapphire laser with a repetition rate of 76 MHz and a FWHM or 100 fs at a central wavelength of 850 nm. The duration of the pulses could be varied by means of an external group velocity dispersion control consisting of a prism pair The laser pulses were focussed on the DAST crystal The emitted Terahertz-radiation was collimated by an off-axis parabolic inlrror and coupled to a liquid He-cooled Ge:Ga photoconductive detector with a noise-equivalent power (NEP) of about l o i 4 WRlz'" Its specu;il response.covers the range of 2.5 to 8 THz. which reaches above the detection limit of about 3 THz for state of the art high speed photoconductive antennae. To absorb the residual laser light as well as the generated second harmonic, a cooled Si-filter was placed in front of the detector. Using a lock-in amplifier we measured a THz signal with an average poner of about 50 nW. The spectral characterization of the far infrared (FIR) pulses was accomplished by use of a Fourier transform Michelson interferometer in Martin-Puplett-configuration [4]. To our knowledge we have been the first to analyse this FIR radiation in the spectral range above 3 THz by means of a photoconductive detector.Femtosecond pulse-trains with a THz repetition rate were successfully synthesized and utilized to substantially narrow the spectrum of the FIR pulses. By varying the delay between the laser pulses the FIR spectrum could be tuned within the spectral range of the detector.Thus we present a coherent ultrashort ...

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190 / lQEC'98 / THURSDAY AFTERNOON I I 480 490 500 510 520 Wavelength (nm) QThG19 Fig. 3. Gain spectral band measured at zero probe delay (filled triangle) and PL spectrum (empty triangle) at high excitation fluence. The fitting of the gain curve is a Gaussian line. The fitting of the PL spectrum is obtained from Eq. (2). over the probe spatial intensity distribution. At high excitation fluence, the ASE term becomes the dominant mechanism of exciton population depletion. Figure 3 shows the gain band gprobe measured at zero probe delay and fitted by a Gaussian line of 18 nm FWHM. On the same figure, we plot the PL spectrum observed at high fluence. In the presence of ASE, the PL line g, , should narrow according to the following expression?where U, -1.15 X 10l6 cm2 is the peak gain cross section, R, -40 pm is the pump spot radius, and N -2.8 X loL9 cm-3 is the exciton density. The fitting of the PL band in Fig. 3 is obtained from Eq. (2). The good agreement between the fitting and the experimental data provides further evidence of ASE.In conclusion, we have shown that the line narrowing of the blue PL band present in m-LPPP at high excitation fluence is due to ASE, which is responsible for the fast exciton population depletion.The dynamics of photoexcited electrons in proton-bombarded InP1 are observed by ultrafast optical-pump-THz-probe spectroscopy. The transmission of 150-fs THz pulses is measured after excitation with 120-fs-visible laser pulses. The THz pulses are generated by optical rectification of 100-fs laser pulses in the highly nonlinear organic crystal DAST (4-dmethylamino-N-methylstilbazolium-tosylate) .2 The emitted THz pulses have a peak intensity at 7 THz with a spectral width of 6 THz and an average power of 100 nW.3The samples used in these experiments are 0.3-mm-thick semi-insulating (100) InP wafers that have been irradiated with 200-keV protons. These protons generate a broad damage profile exhibiting a maximum at a distance of 1.5 pm below the surface. The ion doses range from 1 X l O I 3 to 8 X IOI5 Ht/cmz. These samples are illuminated with 120-fs optical-laser pulses with an excitation energy of 52 meV above the bandgap and an average power between 200 and 800 mW focused to a spot with a diameter of 60 pm. Figure 1 shows the temporal evolution of the normalized, differential THz transmission ATIT after optical excitation for three different pump powers of 400,600, and 800 mW. The sample was bombardedwith an ion dose of 1 X lo1* Ht/cm2. Au the measured transmission curves show an initial decrease after optical excitation that is limited by our time resolution of -300 fs. This decrease is consistent with an increase in the free-carrier density, which leads to increased free-carrier THz absorption. The increase in the THz transmission after optical excitation is directly correlated to the lifetime of the photoexcited carriers in the conduction band. We assume a two-step process for the fast recombination of the conduction-band electrons into the valence band recombination of free carriers in...