Time-resolved Optical Reflectivity Measurements Research Articles

Optical spectroscopy and ultrafast pump-probe study of a quasi-one-dimensional charge density wave in CuTe

CuTe is a two-dimensional (2D) layered material; yet it forms a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the $a$ axis in the $ab$ plane at high transition temperature ${T}_{\mathrm{CDW}}=335\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. However, the anisotropic properties of CuTe remain to be explored. Here, we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy studies to investigate the underlying CDW physics of CuTe. Polarized optical measurements clearly revealed that an energy gap gradually forms along the $a$ axis upon cooling, while optical evidence of a gap signature is absent along the $b$ axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurements revealed that the amplitude and relaxation time of photoexcited quasiparticles change dramatically across the CDW phase transition. Performing fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.

Signatures of Ultrafast Reversal of Excitonic Order in Ta_{2}NiSe_{5}.

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta_{2}NiSe_{5} and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta_{2}NiSe_{5} based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

Pulsed Growth of Vertically Aligned Nanotube Arrays with Variable Density

The density of vertically aligned carbon nanotube arrays is shown to vary significantly during normal growth by chemical vapor deposition and respond rapidly to changes in feedstock flux. Pulsing the feedstock gas to repeatedly stop and start nanotube growth is shown to induce density variations up to a factor of 1.6 within ca. 1-2 μm long layers, allowing the synthesis of new array architectures with distinct regions of controllable length and density variation. Z-Contrast scanning transmission electron microscopy of corresponding sections of the arrays is used to provide unambiguous measurements of these density variations. Time-resolved optical reflectivity measurements of the height and optical extinction coefficient of the growing arrays are shown to provide a real-time diagnostic of both array density and growth kinetics.

Kinetics of solid phase regrowth of self-ion-implanted amorphous SiC during low temperature furnace annealing

The solid phase regrowth (SPR) model for the amorphous SiC phase induced by self-ion-implantation is shown. This was derived from in situ time-resolved optical reflectivity (TROR) measurements, and the crystallinity evaluation method based on Raman spectrometry and Rutherford backscattering spectrometry (RBS). The roughness of an amorphous–recrystalline layer interface was monitored during a heat treatment. By the TROR measurements, it was proven that the roughness of moving interface increases during the SPR. Raman spectrometry measurements have confirmed that the polytype of the SPR layer is the same as that of the substrate. RBS channeling measurements have made it clear that the fraction of c-axis orientation in the SPR layer was about 13%.

Direct measurement of solid-phase epitaxial growth kinetics in GaAs by time-resolved reflectivity

Complete solid-phase epitaxial regrowth of ion-implanted layers in GaAs has been obtained in the temperature range 150–400 °C. Implantation of tellurium at an energy and dose slightly greater than the amorphization threshold was used to produce an amorphous layer in the near-surface region of the GaAs samples. Complete crystallization was achieved over the entire temperature range using a resistively heated sample holder and cw laser irradiation. In situ time-resolved optical reflectivity measurements were used to observe and measure the epitaxial growth rate of the process. It has been found that the solid-phase epitaxy process follows an activation law whose activation energy is 1.6 eV.

Investigation of the Melting Temperature of Amorphous Silicon

Time-resolved optical reflectivity measurements were used to investigate phase changes which occur in silicon during cw-laser heating. It is shown that amorphous silicon does not melt at a temperature significantly lower than the melting point of crystalline silicon (1693 K), contrary to recent reports which suggested a 300- to 500-K melting-point depression.

Evidence Against a Reduced Melting Temperature in Amorphous Silicon

ABSTRACTExperimental results are presented which indicate that amorphous silicon does not melt at a temperature significantly lower than the melting point of crystalline silicon (1693°K), contrary to recent reports which suggest a 300 to 500°K melting point depression. Time-resolved optical reflectivity measurements are used to determine the temperature and to investigate phase changes which occur in silicon during cw laser heating. It is shown that amorphous silicon films produced by arsenic implantation into Si(100) do not melt when heated to temperatures in excess of 1600°K. An alternate interpretation of previous work that is consistent with the present findings is proposed.

Dynamics of Nd : YAG laser annealing of silicon on sapphire

Epitaxial and polycrystalline silicon layers on sapphire have been annealed with Q-switch pulses from a Nd : YAG laser irradiated on the Si surface. Time-resolved optical reflectivity measurements have been performed. The annealing process is shown to be induced by melting and subsequent epitaxial regrowth. The best results were obtained if the whole Si layer was melted, thus allowing the (11̄02) oriented sapphire substrate to act as a seed for recrystallization. In this case commercially available Si-on-sapphire (SOS) wafers with additional Si implantation as well as polycrystalline layers of low-pressure chemical vapor deposited (LPCVD) Si on sapphire could be epitaxially regrown. The same material deposited on amorphous SiO2 did not show epitaxial regrowth, however an increase in size of randomly orientated grains from 60 nm to 1 μm could be achieved.