Femtosecond Time Resolved Reflectivity of Optically Excited Silicon
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FEMTOSECOND TIME RESOLVED REFLECTIVITY OF OPTICALLY EXCITED SILICON R. YEN, C. V. SHANK AND C. HIRLIMANN Bell Telephone Laboratories, Holmdel, New Jersey, USA ABSTRACT We report measurements of the time resolved reflectivity of Silicon following excitation with an intense 90 femtosecond optical pulse. These measurements clearly time resolve the process of energy transfer from the optically excited electron-hole plasma to the crystal lattice and subsequent melting within the first few picoseconds. INTRODUCTION The reflectivity of Silicon following excitation with intense laser pulses has been the subject of extensive investigations.18][1,2] In previous work the dynamical processes of carrier cooling and heating of the crystal lattice have been too rapid to be resolved in time. In the experiments reported here we utilize recently developed femtosecond optical pulse techniques [3] to excite a dense electron-hole plasma in crystalline Silicon and have measured the induced reflectivity changes throughout the visible and near infrared as a function of time and excitation energy. The results clearly reveal the formation of an electron-hole plasma with a density greater than 1022 cm- 3 , the process of energy transfer to the crystal lattice, and subsequent melting. EXPERIMENTAL METHOD The experimental arrangement for performing these experiments is shown in Fig. 1. A 90 femtosecond pulse from an amplified CPM dye laser [4,5] is split into two pulses, forming a pump and a probe pulse. The probe pulse is focused into a cell containing D2 0 to generate a white light continuum pulse. The pump pulse is focused to about 150 microns in diameter. The continuum probe pulse is focused onto the sample and the central 10% of the probing area is imaged into a spectrograph with an optical multichannel analyzer vidicon array on the output slit. The measurements are performed at a 1OHz repetition rate. A two axis computer controlled stepper motor moves the Silicon wafer in a raster pattern so that each laser pulse sees a fresh region of the Si wafer.
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APERTURE IN IMAGE PLANE
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Fig. 1.
LIGHT PULSE
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Experimental set up.
SPECTROMETER 'TO OPTICAL MULTICHANNEL ANALYSER
Mat. Res.
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13 (1983) ®Elsevier Science Publishing Co.,
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14 RESULTS AND DISCUSSION The experimental results are shown in Fig. 2. The log of the reflectivity change is plotted as a function of time for different pump intensities. The pump wavelength was 620 nm and the reflectivity was probed at three wavelengths (a) 1000 nm, (b) 678 nm and (c) h40 nm. The pulse energy threshold, Eth, for 2 the formation of a clearly visible amorphous layer was O.lJ/cm . We propose to interpret these reflectivity spectra in terms of a simple model. At the earliest times following excitation, the reflectivity change is expected to be dominated by the electron-hole plasma. As the pumping intensity is increased the e-h plasma becomes so dense that significant energy is transferred to lattice phonons, and the crystalline Si structure becomes
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