Inelastic Raman scattering: single and two-laser Raman thermometry
Single Laser Raman thermometry, 1LRT, is a contactless advanced technique for thermal conductivity determination based on the probe of the local temperature due to different physical mechanism. Accordingly, any aspect of phonons changing with temperature can be used to probe the thermal state of the system. These changes are reflected mainly in: the Stokes and anti-Stokes intensity components, the Raman peak position and linewidth (full width at half maximum, FWHM) associated with specific optical phonon modes, all of which vary with temperature.
In order to investigate the transport behaviour at nanoscale, we developed a novel and contactless technique: Two-Laser Raman Thermometry technique, 2LRT. This new technique is based on a two-laser approach to create and probe a thermal field in nanostructures. As is shown in Figure (left), while a heating laser with λ2 is used to produce a hotspot, a thermometer laser with λ1 measures the spatial distribution of the local temperature through the temperature dependent redshift of a Raman mode. One of the main advantages of this technique compared to other contactless steady-state methods, e.g. infrared thermometry, is its sub-micrometer spatial resolution, given by the spot size of the probe laser ~ 500 nm with a 100× high NA microscope objective. In addition, the spatial resolution can be also improved using, for example, Tip Enhanced Raman Spectroscopy, TERS, which could reach resolution better than 10 nm.
Inelastic Brillouin light scattering
The experimental technique to measure the transfer of energy and momentum between photons and phonons, is known generically as Inelastic Light Scattering (ILS), which allows a direct measurement of the energy or frequency of phonon, vibrational and rotational states of molecules, as well as plasmons, excitons and magnons. It is a non-contact, non-destructive method, with no pre- or post- processing required.
Historically, ILS by acoustic phonons has been known as Brillouin Light Scattering (BLS), while scattering from optical phonons, and vibrational and rotational states of molecules, has been known as Raman scattering. Due to the frequency shifts produced by the acoustic phonons are smaller than the optical phonons, the benchtop instruments to detect the inelastically scattered light are different. In practice, Brillouin scattering relies on the high resolution Fabry-Pérot interferometer (FPI) instead of a grating spectrometer typically used in Raman experiments.
The interaction of the quasiparticle with the photon causes a frequency down-shift (Stokes) or a frequency upshift (anti-Stokes) depending on whether the energy is given or absorbed by the photon. The typical frequency range accessed by BLS extends from 500 MHz to several hundred GHz, which is suitable to access acoustic phonons.
Asynchronous Optical Sampling: ASOPS
High-speed asynchronous optical sampling (ASOPS) is a superior ultrafast time-domain and THz spectroscopy technology with <60 fs time resolution. ASOPS method is based on traditional ultra-fast Pump-Probe techniques with the difference that use two pulsed lasers and there is no need for a mechanical delay stage. The two lasers are the Pump, utilized to excite the sample, and the Probe to measure its state of it at a certain time after the excitation. The Pump and Probe beams are separated in time gradually and the transmission/reflection of the Probe is collected. In this way, the triggered dynamics after the incidence of the Pump can be reproduced with a temporal resolution that depends on the delay between pump and probe beams and the width of the laser pulses, which are usually on the order of 10’s of femtoseconds. The advantage over the conventional Pump-Probe techniques is how the beams are delayed by each other. While in the conventional Pump-Probe setups, a translational stage is utilized to generate the delay between the Pump and Probe, in this case, the delays are generated due to a frequency difference between the two lasers. This is possible because different from the conventional P-P, two Ti:Sapphire lasers are employed instead of one.
With slightly detuned repetition rates, ASOPS uses this detuning between the pump and probe pulses to generate the delay between the beams. This detuning creates a monotonic temporal window that allows the dynamics of the system under study to be scanned without requiring any readjustments of the mechanical stage. The scan rate is determined by the difference in frequency, Δf_R, and the temporal window is given by the inverse of this difference, 1/Δf_R. For example, if the pump and probe repetition rates are 1 GHz and 0.999999 GHz, respectively, a temporal window of one nanosecond can be probed in 0.1 ms. The lasers are independent and can be tuned for a range between 750−850 nm.
The three-omega (3w) method is widely used to measure the thermal conductivity of solid materials. In our group, we designed a modified version of the common 3ω-method suitable for the measurement of liquids. As is shown in Figure, the liquid is placed on top of the 3w-heater which has been passivated with 200 nm of SiOx to avoid current leakage from the resistor to the conductive fluid. A 100 nm thick chromium-gold 3w-strip was patterned on a 0.5 mm thick quartz substrate by photolithography and electron beam physical vapour deposition (EBPVD). Then, a PDMS block is used to seal the circuit and as liquid container (well).
The 3w method consists of applying an alternating current (AC) through a metal strip which is in direct contact with the sample, which is used as a heater and thermometer sensor. An AC signal with an angular frequency w (w = 2πf, with f being the frequency) flows through the strip generating heat which oscillates at 2w. The injected heat generates a temperature rise (ΔT) which will depend on the thermal properties of the sample. The ΔT is measured by a 3w-strip once its calibration is known. Then, the temperature oscillations is obtained by measuring the third harmonic component of the voltage (U3ω) across the resistor.
Acoustic and optical characterization: Fiber tapered optomechanic setup
During the last five years we have developed a state-of-the-art experimental set-up for characterizing optical and mechanical properties of optomechanical devices at room temperature. A tunable infrared laser covering the spectral range between 1460-1580nm is connected to a tapered fiber, which is fabricated by ourselves using a micro-oven and two piezomotors pulling in opposite senses. The long tail of the evanescent field and the relatively high spatial resolution (~5 mm x mm) of the tapered fiber locally excited the resonant optical modes of the OM photonic crystal. Once in resonance, the mechanical motion activated by the thermal Langevin force causes the transmitted intensity to be modulated around the static value. To check for the presence of RF modulation of the transmitted an InGaAs fast photoreceiver with a bandwidth of 12 GHz is used. The RF voltage is measured by a signal analyzer with a bandwidth of 13.5 GHz
Seebeck Coefficient & Electric conductivity measurement Unit: Linseis LSR-3
The thermal power, thermoelectric power, or Seebeck coefficient of a material measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. The thermal power has units of (V/K).
In recent years much interest has been shown in various methods of direct conversion of heat into electricity. Waste heat from hot engines and combustion systems could save billions of dollars if it could be captured and converted into electricity via thermoelectric devices For this measurements a Linseis LSR-3 has been used for thermoelectric characterization. Together with 3w-method a complete thermoelectric figure of merit can be obtained in our laboratories.
Scanning thermal microscopy (SThM)
Our Nanonics MV4000 SPM system is based on an open architecture, which allows flexible integration of different analytical tools.Figures left show the complete system which consists of several subsystems such as an integrated dual microscope, two scanning probe heads and a sample scanning stage. The dual probe configuration is shown in Fig. The MV400 it’s an SPM that depending on the probe can be used for atomic force imaging, near-field optical measurements optical (SNOM), thermal (SThM) measurements, nanochemical writing on a variety of structures and probe nanoindentation.
Frequency domain thermoreflectance
Thermal characterization of nanostructures often balances invasiveness and precision. In the P2N group, we use both electrical and optical, contact and contactless methods. Frequency domain thermoreflectance (FDTR), is a versatile all-optical contactless technique that allows to probe different thermal properties, such as in-plane and cross-plane thermal conductivity, thermal conductance, and interface thermal resistance of various supported or suspended nanostructures. This technique offers great prospects for characterizing thermal properties of 2D materials, for example.
A laser with modulated intensity (pump) will create a periodic thermal wave in the sample of interest. The reflectivity of the sample, probed at the same position by a second laser, will change as a function of the temperature, displaying a phase lag compared to the pump signal. This phase lag will be directly dependent on the thermal conductance and conductivity of the sample at the position of probing.
Free-standing Si films have been and remain an excellent example to study experimentally the effect of the reduction of the characteristic size on the phonon dispersion relation. A step further in geometrical complexity and, therefore, in increasing the control and manipulation of phonons is achieved by introducing periodicity in the medium to form phononic crystals.
We have developed the fabrication process of large-area, solid–air and solid–solid two-dimensional phononic crystals, directly on free-standing, single crystalline silicon membranes. The patterning of the membranes involved electron-beam lithography and reactive ion etching for holes or metal evaporation and lift-off for pillars.
We are constantly developing novel approaches and methods for nanoscale device-related research. Our group has pioneered research in nanofabrication (nanoimprint lithography (NIL), roll-to-roll NIL, nano-injection moulding, self-assembly and others) with the aim of demonstrating the feasibility of large area, high volume and low-cost nano-enabled products. We offer a unique set of nanoimprint lithography tools capable of imprinting micro/nano structures up to 300 mm diameter wafers onto various materials and substrates, giving added functionalities to surfaces (e.g. self-cleaning, anti-reflective, hydrophilic).
Our advanced manufacturing technology has been developed to the point where 3D and sub 50 nm patterning is feasible, with high throughput capabilities.
The added functionality allowed by introducing micro/nano structures onto surfaces will have a big impact on various industrial sectors. Sustainable production and products can be expected from our novel manufacturing approach, enhancing the performance of targeted products by improving their functionality and keeping production costs down.
In the P2N group we have established a new metrological device known as a diffractometer, which collects and analyses optical diffraction patterns from micro and nanopatterned surfaces. Combined with intensive in-house developed finite-difference frequency-domain (FDFD) modelling we are able to use this data to obtain fast (< 100 ms), non-invasive predictions of the critical dimensions of the diffractive structures. We then use this technique in-line with high-throughput fabrication techniques such as roll-to-roll UV nanoimprinting to perform live quality control of nanopatterned flexible films.