Journal of Luminescence 30(1985) 243 247 North-Holland. Amsterdam
PROGRESS IN FEMTOSECOND MEASUREMENT TECHNIQUES C.V. SHANK .4 T&T Be/i Laboratories. Ho/mde/. New Jersey 07733, LISA
This paper describes the recent advances in femtosecond measurement techniques and the application of these methods to the study of highly excited semiconductors.
Rapid advances have taken place in the generation of optical pulses for making measurements in the femtosecond time domain. Dye lasers can now be made to produce optical pulses as short as 60 femtoseconds [I] and amplified to gigawatt optical intensities . Progress continues. Two years ago, optical pulses as short as 30 femtoseconds were reported , and most recently, 16 femtosecond optical pulses have been generated.  As we move into the femtosecond realm, a whole new range of problems becomes accessible. If we consider that a 30 femtosecond pulse corresponds in the frequency domain to 1000cm it is clear that we can study the properties of solids and liquids in a time less than a phonon collision. Experiments in this time domain present the possibility of exciting coherent or correlated states in matter that evolve to some statistical observable. The transient evolution of such processes provides a means of studying fundamental interactions. An important application of femtosecond techniques to the solid state physics has been the investigation of phase transitions induced by intense optical pulses on semiconductor surfaces. This problem has been the subject of intense interest and controversy. Time resolved reflectivity techniques performed with nanosecond  and picosecond  time resolution were able to establish that intense optical pulses modified the reflectivity of a Si surface to that of molten Si in a time too short to measure. Measurements of time resolved reflectivity using femtosecond optical pulses  clearly resolved the process of energy transfer from an optically excited electron hole plasma to the crystal lattice, followed by a subsequent phase change to a “melted state”. To further elucidate the melting process, a measure of the transition from order to disorder is useful. Since reflectivity is a scalar quantity, it is not possible to obtain information concerning crystalline order. However, the process of second-harmonic generation is governed by a tensor quantity that contains elements of crystal symmetry. For the case of the (111) surface of Si, the -
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Proqri’v.s in /emtoseeond ,nea,s’uremenl techniques
projection of the bulk symmetry onto the surface is three-fold symmetric. The second-harmonic radiation from this surface with p-polarized fundamental and second-harmonic radiation is given by I(2w)=k12(u’)[cos 30+ A(0)]2, where 0 is the angle of rotation about the surface normal. K is a constant and A(0) is the isotropic contribution to nonlinear polarization which is a function of the angle of incidence, 0. For this surface A(0) I resulting in a three-fold symmetry. Second-harmonic radiation from a weak 2.0 eV pulse was measured  following excitation with an intense 90 femtosecond optical pulse. The measured second-harmonic radiation is plotted as a function of crystal rotation and is plotted in polar form in fig. 1. Before excitation, a three-fold symmetric pattern is observed. By 240 femtoseconds the second harmonic crystal has become significantly disordered and by 3.0 ps an isotropic variation of the second harmonic with angle of rotation is observed indicating a loss of crystalline order and melting. At intensities significantly above the melting threshold, material is ejected from the crystalline surface leaving permanent damage. To investigate this process an imaging technique has been developed which can be used to produce images with femtosecond time resolution. Such images provide a detailed study of the physics of a highly excited semiconductor surface as a function of time, spatial position, and wavelength. The images were obtained using a variation of the pump and probe technique. An intense 80 fs pulse was used to excite a silicon surface. A weak probing pulse, generated using white light continuum generation.  was directed to the point of excitation at a fixed time delay. The specularly reflected probe radiation was imaged onto a screen or photographic film with a magnification of approximately tOO x Either still photographs or motion pictures could be taken by synchronizing the camera shutter with the 12 Hz repetition rate
-1.0 Ps ~~24o:s Jr ~
Fig. I. Polar plot second harmonic as a
: function of time and angle (/ for pump
energ~2.0 E 1
Proqress in /eozto second measurement
1 ig 2. Tinie—rcsol~edphotos of a silicon [Ill] surface fo1los~ing pholoexciiation h~ an 50 is optical pulse of (1.5 J cmi. Numbers indicate pump—probe optical dela~ in ps. Note the rapid appca ranec of the h ighR reflect’. C molten ~i Icon t(h) ih rough (dli. foIl ossed h~the eject ion and dissipation of es aporated material (dark central spo i fl (ci through liii).
C . V. .S’hank / Progress in femtoxccond measurement techniques
of the laser. The time delay between the pump and the probe was varied by a controllable optical path delay. The images of the excited silicon surface at time delays ranging from A= —0.5 to +600 ps. are shown in figs. 2(a) to (h). Here the pump fluence was approximately 5 times threshold for melting. Before arrival of the excitation pulse, only a uniformly illuminated region of the surface is seen (see fig. 2(a). After arrival of the excitation pulse, the reflectivity of the excited region is selectively increased because significant melting has occurred in this region (see fig. 2(b)—(d)). At At= +0.1 ps, this region is faint (see fig. 2(b)) for two reasons. First, melting is not complete by this time and second, there is a frequency sweep on the probe pulse because of dispersive optics in the path of the probe beam. The appearance of the highly reflective molten spot at later times (At> 1 .0 ps.) depends dramatically on the pump fluence. For fluence between the melting threshold and about 2.5 melting threshold, its appearance remains essentially unchanged from At= 1.0 ps out to At= 600 ps. This is consistent with earlier results  showing that for these fluence levels the reflectivity levels off after about 1.0 ps and remains unchanged until later than a nanosecond, when re-solidification beings. At higher fluence levels, a dark region begins to appear in the center of the molten spot at At = 5 to 10 ps, as shown in fig. 2(e). This central region continues to darken, becoming darkest between At=50 and 100 ps, although the edge remains bright (see fig. 2(f)). At still later times, the center of the dark spot begins to become transparent again (fig. 2(g)) and by At=600 ps, it has substantially dissipated (fig. 2(h)), except for a narrow dark ring at the outer edge of the original dark spot. We believe that this dark region originates from a gradually thickening cloud of material ejected from the hot molten silicon surface for as long as 100 ps following excitation. Maximum particle emission occurs in the hot, central portion of the molten silicon, while the edge remains cool enough that a substantial absorptive cloud never develops, thus explaining the bright outer ring of unobscured molten silicon which persists throughout the time of observation. Earlier studies have demonstrated substantial emission above melting threshold of both charged particles and neutral silicon atoms. In summary, this brief discussion reviews recent progress in femtosecond optical pulse techniques to make measurements of dynamic processes in solids on this extremely short time scale. These new tools give us a chance to peer into the as yet unexplored world of processes that take place in solids on an ultrafast time scale.
C. V. Shank Progress in fern fosecond measurement techniques
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