Schematic of the setup. Pulses (Gaussian pulse duration , , and ) from a regeneratively amplified Ti:sapphire laser (not shown) drive two OPAs. The output frequency of both OPAs is computer controlled, and tunable from 3850 to . The pump pulse passes through an optical delay line with minimum time step of 10 fs. Both pump and probe beam pass through a chopper wheel that is synchronized to the laser output (see Fig. 2). Both pump and probe beam are focused on the same spot on the sample. Two InGaAs photodiodes are used to monitor the output variation of the OPAs as well as the reflected signal. The reflectivity from the sample is measured by a third InGaAs photodiode. Three boxcar averagers are used to hold the short output pulses of each detector for 1 ms, allowing simultaneous acquisition of separate pulse events of all three detector channels.
Schematic illustrating the alignment of the pump and probe beams (large red and small green circles) onto the chopper blade. The rotation of the chopper wheel is synchronized to the laser output. One full revolution of the chopper blade takes 8 ms, such that for each pulse event, pump and probe beams are blocked or unblocked in a different permutation. In one sequence of four consecutive laser pulses, both (a) excited reflectivity, (d) linear reflectivity, (b) pump background, and (c) detection background are collected.
Time traces of the boxcar output signals for probe monitor, pump monitor, and probe reflectance. The sample was a GaAs/AlAs distributed Bragg reflector, the experimental conditions were the same as in Ref. 16. The pump irradiance was and the probe frequency was . The switched reflectivity is roughly 10% lower than the unswitched reflectivity. Each datapoint in the plot corresponds to a single pulse event. Letters a, b, c, and d correspond to the chopper position during each event (see Fig. 2).
Reflectance signal vs probe monitor data for 1000 single pulse events of the data set shown in Fig. 3, displayed as a scatter plot. The 250 unpumped reflectivity datapoints (d) constitute a line, indicating that variations in monitor and reflectance signal are strongly correlated. The slope of the line is proportional to the reflectivity of the sample. The pumped datapoints (a) form a line with a reduced slope, due to the reflectivity decrease of about in the switched sample. Both background data sets (b) and (c) tend to the origin of the plot as it should in absence of offsets. Note that the small offset in the signals is automatically removed in the data processing routine.
Time resolved reflectivity measurement on bulk Si, pumped at , pulse energy , Gaussian pulse duration , , and peak irradiance (upper panel). The reflectivity of a probe with , , and Gaussian probe pulse duration decreases from 32% to 28%, corresponding to a calculated carrier density at the surface of the sample, using a Drude response (see right-hand scale). The time difference between 10% and 90% of the total change is , as indicated by the vertical dashed lines. The lower panel shows the irradiance ACF of the pump pulses. The FWHM of the ACF of 200 fs corresponds to a Gaussian pulse duration of .
Time resolved reflectivity of a switched double-side polished Si wafer. Unswitched reflectivity (open squares) and switched reflectivity (closed squares) are plotted over an extended range of probe delays compared to Fig. 5. Surprisingly, at a negative probe delay of 8.6 ps, a large step in the reflectivity from 38% to 32% appears. At zero probe delay the reflectivity decreases further from 32% to 28%. The time difference between the first and second step in reflectivity (indicated by dotted lines) is , which corresponds well to twice the optical thickness of the wafer . We identify three different probe delay regimes A, B, and C that are explained in the schematic plot in Fig. 7.
Snap shots of the reflected irradiance of a bulk Si wafer in the experiment in Fig. 6 at the arrival time of the pump. We consider three different probe delay positions, corresponding to the regions indicated by A, B, and C in Fig. 6. The intense pump pulse generates an inhomogeneous carrier plasma near the front face of the wafer indicated by the dark gray layer. This absorbing layer acts as an ultrafast shutter that blocks any internally reflected pulse that arrives at the front face after the pump pulse. At probe delay A, the pump arrives before the probe, and the switched reflectivity of the front face of the wafer is probed. At probe delay B, the pump pulse arrives in between reflection and , thus blocking pulses and . This reflection corresponds to the front face refection of the unswitched wafer . At probe delay C, the pump pulse arrives in between reflection and , thus only blocking . The total reflection is equal to .
(a) Broadband linear reflectivity spectrum in the (111) direction of a Si inverse opal, measured by combining the signal and idler range of our optical amplifier. Inset: high resolution scanning electron microscopy image of the Si inverse opal. The scale bar is . Image courtesy of Kalkman. (b) Differential reflectivity as a function of both probe frequency and probe delay. The pump frequency and peak irradiance were and on the red part, and and on the blue part of the spectrum. The probe delay was varied in small steps of on the blue edge and in steps of at the red edge. The probe wavelength was tuned from 1600 to 2100 nm in steps in the low frequency range, and from 1160 to 1600 nm in 5 nm steps in the high-frequency range. (c) Time resolved reflectivity of the spectral feature at , see arrow in (b). The time difference between 10% and 90% of the total change is , as indicated by the vertical dashed lines.
(a) Open aperture -scan measurement for a thick double-side polished GaP wafer. Pump parameters: , , , and . The curve represents the calculated transmission using a three-photon coefficient . (b). Three-photon coefficient for GaP were obtained at five wavelengths. The dashed vertical line indicates the three-photon absorption edge for GaP , the solid line serves to guide the eye. We observe that decreases as the pump frequency approaches .
Schematic of the -scan setup. Incoming laser beam from our OPA is split by a beam splitter (BS). Two InGaAs photodiodes are used to monitor the output variation of the OPA (D1) as well as the transmitted signal (D2). The detector signals are measured as a function of sample position .
Open aperture -scan measurement for a thick double-side polished Si wafer. Open circles: , , and . Closed squares: circles, , , and . The curves are calculated transmission using (dashed curve, ) and (solid curve, ).
Open aperture -scan measurement for a thick double-side polished Si wafer. Pump parameters: , , , and . The dashed curve represents the calculated transmission using .
Open aperture -scan measurement for a thick double-side polished GaAs wafer. Pump parameters: , , , and . The curve represents the calculated transmission using .
-scan measurement for a thick double-side polished GaAs wafer. Pump parameters: , , , and . The curve represents the calculated transmission using .
Two-photon absorption coefficients for Si and GaAs.
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