In this Rapid Communication we report the first time-resolved measurements of confined acoustic phonon modes in free-standing Si membranes excited by fs laser pulses. Pump-probe experiments using asynchronous optical sampling reveal the impulsive excitation of discrete acoustic modes up to the 19th harmonic order for membranes of two different thicknesses. The modulation of the membrane thickness is measured with fm resolution. The experimental results are compared with a theoretical model including the electronic deformation potential and thermal stress for the generation mechanism. The detection is modeled by the photoelastic effect and the thickness modulation of the membrane, which is shown to dominate the detection process. The lifetime of the acoustic modes is found to be at least a factor of 4 larger than that expected for bulk Si.Free-standing thin semiconductor membranes have a wide range of applications, for example, as key elements in nanomechanical systems, 1 sensors, 2 or optomechanical systems. 3 Thorough understanding of the mechanical and elastic properties in these structures is crucial for the design and engineering of the desired performance of these potential devices. 4 When the dimension of these structures is reduced to the order of magnitude of the phonon wavelength, the confinement of acoustic modes leads to a discretization of the acoustic spectrum. These effects have been studied via continuous wave light scattering techniques such as Brillouin and Raman scattering in supported and free-standing thin films. 5-9 Recently time-resolved experiments have contributed significantly to the understanding of phonon dynamics in nanoscale systems and nanoparticles. 4,[10][11][12][13] In this Rapid Communication we present results of femtosecond time-resolved pump-probe experiments performed on free-standing Si membranes with a thickness of a few hundred nanometers. A superposition of oscillations corresponding to frequencies of a fundamental mode and its higher odd harmonics up to the 19th order with lifetimes exceeding 1 ns is observed in the time domain. The results are successfully modeled using a combined elastic and electromagnetic model. It is shown that the detection process is dominated by the dynamic change in the membrane thickness. Our analysis demonstrates that free-standing Si membranes are a model system, which allows disentanglement of basic phonon-photon interaction processes.The pump-probe experiments were performed using highspeed asynchronous optical sampling ͑ASOPS͒ described in detail before. 14 Two femtosecond Ti:Sapphire oscillators are used to generate the pump and probe pulses with a duration of less than 100 fs. The repetition rates f rep of about 800MHz are stabilized in order to fix the difference of the repetition rates at ⌬f rep = 10 kHz. This offset allows for an automatic scan of the measurement window ͑f rep −1 = 1.2 ns͒ by the probe pulse in ⌬f rep −1 = 100 s without mechanical delay line. The time resolution achieved by this technique lies below 150 fs. Experiment...

We propose sub-harmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system's quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.In recent years, nanophononic and nanomechanical systems have emerged as intriguing subjects for studying mechanics, heat transfer and opto-mechanical coupling on a nanometer scale [1][2][3]. From a fundamental point of view, they provide a route to study mechanical excitations and their interactions with other elementary excitations [1,4]. ¿From an applied perspective they have opened a pathway for high sensitivity sensors in the zeptogram mass range and in the attonewton force range [5,6]. In established experimental methods these systems are driven electrically, magnetically, thermoelastically [7], via radiation pressure from continuous wave lasers [8], or via other optical non-radiation-pressurebased schemes [9,10]. The frequencies of typical systems investigated so far are in the megahertz to gigahertz frequency range [7]. The investigation of higher frequencies is strongly restricted by the driving and detection methods. Here, we report a new method for the investigation of a vibrational system by sub-harmonic resonant excitation with a high-repetition rate femtosecond laser. This excitation scheme can be regarded as tuning the separation of modes of the frequency comb of a femtosecond laser [11] to a commensurable of the frequency of the phononic system. By sweeping the comb spacing of the femtosecond laser, resonant impulsive excitation of the mechanical oscillator can be achieved, which allows the determination of its quality factor in the gigahertz to terahertz frequency range with femtometer sensitivity for the mechanical amplitude. We demonstrate the amplification of the fundamental eigenmode of a free-standing silicon membrane at 19 GHz by a factor of 20 compared to the off-resonant case and determine its quality factor.The dynamical properties of the free-standing silicon membranes were investigated by performing fs resolution pump-probe experiments using the recently developed high-speed asynchronous optical sampling (ASOPS) method [12,13]. This method is based on two asynchronously linked femtosecond Ti:sapphire ring lasers of repetition rate f R ∼ 1 GHz. One laser provides the pump beam and the second laser the probe beam. In this technique the time delay between pump-/probepulse pairs of the two pulse trains is realized through an actively stabilized 10 kHz repetition-rate-offset ∆f R between the tw...

Metallic point contacts (MPCs) with dimensions comparable to the Fermi wavelength of conduction electrons act as electronic waveguides and might operate as plasmon transmitters. Here we present a correlated study of optical and conductance response of MPCs under irradiation with laser light. For elucidating the role of surface plasmon polaritons (SPPs), we integrate line gratings into the leads that increase the SPP excitation efficiency. By analyzing spatial, polarization, and time dependence, we identify two SPP contributions that we attribute to transmitted and decaying SPPs, respectively. The results demonstrate the role of SPPs for optically controlling the transport in metallic nanostructures and are important for designing opto-nanoelectronic devices.

Acoustic frequency combs are optically excited and detected in silicon membranes covered with thin aluminum layers by femtosecond pump-probe spectroscopy. The various frequency combs consist of 11 up to 45 modes ranging in frequency from 10 up to 500 GHz. Evaluating the different modes of the combs allows us to quantify the dynamic properties of this two-layer system with great precision. Deviations of the frequencies of higher modes from a linear relation can be quantitatively understood. The time domain traces show clearly defined pulses which are detected in regular time intervals after each roundtrip in the acoustic cavity formed by the membrane and the metal film. By analyzing the individual reflected pulses and their evolution in time, damping times for the whole frequency range are determined. We analytically derive a deviation of the individual comb modes from integer values of the fundamental frequency which is corroborated by the experiments.

We report measurements of vibrational mode shapes of mechanical resonators made from ultrathin carbon nanomembranes (CNMs) with a thickness of approximately 1 nm. CNMs are prepared from electron irradiation induced cross-linking of aromatic self-assembled monolayers and the variation of membrane thickness and/or density can be achieved by varying the precursor molecule. Singleand triple-layer freestanding CNMs were made by transferring them onto Si substrates with square/ rectangular orifices. The vibration of the membrane was actuated by applying a sinusoidal voltage to a piezoelectric disk on which the sample was glued. The vibrational mode shapes were visualized with an imaging Mirau interferometer using a stroboscopic light source. Several mode shapes of a square membrane can be readily identified and their dynamic behavior can be well described by linear response theory of a membrane with negligible bending rigidity. By applying Fourier transformations to the time-dependent surface profiles, the dispersion relation of the transverse membrane waves can be obtained and its linear behavior verifies the membrane model. By comparing the dispersion relation to an analytical model, the static stress of the membranes was determined and found to be caused by the fabrication process. V C 2015 AIP Publishing LLC.