Carbon nanotubes (CNTs) have many uses in energy storage, electron emission, molecular electronics, and optoelectronics. Understanding their light-matter interactions is crucial to their development. Here, we study a film of single-walled CNTs with a thickness of 1.67 μm and a 2D orientational order parameter of 0.51, measured by polarized Raman spectroscopy. The film is expected to have a work function of about 5.1 eV. In this study, ~100-fs pulses with 1.5 (ℏω) and 3 eV (2ℏω) photon energy are used to pump the CNT film while observing its electron emission in vacuum. Ultrafast pulses produce nonlinear phenomena in enhanced field emission, as the CNTs absorb strongly enough that thermally excited carriers can tunnel through the potential barrier. Through curve fitting of the power dependence for each pump energy, we find that the light at ℏω is absorbed via 5-photon absorption, and the light at 2ℏω is absorbed via a combination of 2- and 3-photon absorption. Further study reveals a space-charge limited regime with low applied bias, a photoemission regime with moderate bias, and a laser-assisted field emission regime when the bias is high enough that the photon pump is no longer important. Cross-correlation pumping with the two colors simultaneously shows 4x enhancement of the emission, with a FWHM that suggests a lifetime of ~190 fs, similar to the dephasing time of electrons in CNTs. These studies help illuminate the properties of CNTs as a nonlinear optical material and go towards a more thorough understanding of their optoelectronic properties.
Acoustic wavelengths are about 5 orders of magnitude shorter than EM waves of the same frequency. This can be a critical advantage in the quest to further miniaturize devices that have dimensions linked to their characteristic resonance frequencies. Therefore, many novel multiferroic (MF) devices rely on the transduction of signals from EM radiation to surface and bulk acoustic waves, and yet acoustic wave interactions in complex MF heterostructures are poorly understood. Here, an all-optical interferometric imaging system is developed for two-dimensional scanning imaging of acoustic wave amplitudes with sub-angstrom surface displacement sensitivity and submicron spatial resolution. Measurement capability has been verified with a 50-MHz piezoelectric SAW bandpass filter, with the frequency spectrum showing similar behavior to the S21 transmission measured with a network analyzer, and with a linear dependence on the input power. Spatial dependence of the acoustic waves also behaves as expected, with the largest amplitude near the input electrode. Next, the system has been demonstrated to detect acoustic waves in MF heterostructure-based MEMS devices with frequencies up to 3 GHz. These detections pave the way for fast and reliable troubleshooting of new device designs involving acoustic waves, in which it may not be immediately clear to where or by what mechanism power is being dissipated.
Antiferromagnets are an important class of ordered spin systems, common in spintronic applications and providing a testbed for studying magnetism. Recently, the injection of magnons – coherent spin waves – has been explored by broadband terahertz pulses in antoferromagnets, such as MnO. Here, terahertz time-domain spectroscopy is used to detect magnon resonances in MnF2, which is a model antiferromagnet with uniaxial anisotropy and a Néel temperature of 67 K. Temperature dependence of a one-magnon resonances is examined from 5 K to 70 K. The center frequency of the one-magnon is recorded below the Néel temperature and fit to a Brillouin function. It is found that the degree of correlation between neighboring spins is j = 1.1. Namely, a weak correlation and appropriately modeled by mean-field theory befitting this simple system. From low temperature to room temperature, a two-magnon resonance is observed to broaden and strengthen as the temperature increases. Two-magnon modes arise due to zone-edge magnons being stimulated with -k and +k momenta and do not require magnetic ordering. Over this same temperature range, THz transients are used to monitor the time-of flight through the crystal, the refractive index, the internal energy and the heat capacity. Overall these quantities decrease with decreasing temperature, with behavior that falls into three regimes: a thermal dominated region above the Néel temperature, a magnetic regime below the Néel temperature; and a hyperfine interaction region at temperatures below 6 K. The latter is the first direct observation of the hyperfine interaction using terahertz time-domain spectroscopy.