EXPERIMENTAL TECHNIQUES IMPLEMENTED IN THE RESEARCH GROUP
a. Complex dielectric permittivity and dielectric relaxation
This technique enables access to the real and imaginary parts of the dielectric permittivity from 10 K to 350 K, and in the frequency range 2 Hz up to 2 MHz. Measurements can be made as a function of the ac measuring voltage (50 mV to 5 V) and of the an external voltage up to 40 V.
Moreover, dielectric relaxation can be carried out at fixed temperature by sweeping frequency (2 Hz to 2 MHz).
Experimental run and data acquisition is fully automatic.
Available equipment: Impedance meters and multimeters.
b. Thermal stimulated currents and polar relaxation
This technique is addressed to carry out measurements of currents that are originated by temperature changes. This is most attractive in those materials exhibiting electric polarization that changes with temperature. Some examples for this type of materials are: Multiferroics, magnetoelectrics, ferroelectrics, relaxors, electrets and micro- and nano-scale heterostructures of these materials. In this technique a constant temperature rate is fixed and the current measured using a short-circuit method.
Available equipment: Electrometers and multimeters.
c. Polarization inversion, and hysteresis cycles
This technique is used at fixed temperature and the signals are obtained from a modified Sawyer-Tower circuit. The electric polarization/electric field curves can be obtained. Materials showing non-linear behaviour are adequately studied with this technique, in particular those exhibiting ferroelectric ground states.
Available equipment: ac power supplies (frequency and amplitude as adjustable parameters and digital oscilloscopes.
d. Specific heat
The heat capacity is measured in an ARS Cryocooler, between 8 and 300 K, in a quasi-adiabatic fashion by means of an impulse heating technique. Experimental run and data acquisition is fully automatic.
e. Magnetic susceptibility
AC magnetic susceptibility (15K to 300K); external magnetic field from 0 to 300 G; frequency between 20Hz and 40000Hz. Acquisition of data are computer-controlled.
f. Temperature control and measurement
The temperature is changed and controlled using close-cycle cryostats (10 K – 300 K) and Linkam furnaces (80 K – 800 K). The temperature precision is ~0.1 K.
Measurements procedures and acquisition of data are computer-controlled.
g. Dielectric Spectrometer
The dielectric spectrometer system enables access to the real and imaginary parts of the dielectric permittivity and consists of a high resolution dielectric analyzer ALPHA. This equipment possesses a broadband dielectric converter with high input impedance on the entry and displays a resolution of ~1E-5. This system is also controlled by computer connected by a GPIB interface. To analyze the result one can resort to several software namely Windet, Winplot, and WinFLIT WINTEMP, also available at our facilities.
a. Carrier-envelope phase (CEP) stabilized Titanium: Sapphire ultrafast laser amplifier
Customized FEMTOPOWER compact PRO amplifier from Femtolasers GmbH. This system has dual CEP stabilized outputs, delivering: i) 2.5 nJ pulses with sub-7-fs duration at a repetition rate of 80 MHz (from the included Femtolasers Rainbow CEP laser oscillator); ii) up to 1 mJ pulses with sub-30-fs duration at a repetition rate of 1 kHz. This system is a key tool for advanced (high-resolution and CEP-dependent) ultrafast science, including ultrafast nonlinear optics, ultrafast laser-matter interaction and ultrafast pump-probe spectroscopy experiments.
b. Home-built Titanium:Sapphire ultrafast laser oscillator
This was the first few-cycle femtosecond laser in Portugal (built in 2000). It delivers 10 fs pulses at a repetition rate of 80 MHz and a central wavelength of 800 nm, with an average power of up to 600 mW. These pulses can be used in a variety of experiments, from nonlinear excitation and propagation to pump-probe studies. The emitted laser pulses can also be tuned over a broad spectral range (700-900 nm). By propagating the pulses in a short piece of photonics crystal fiber this system enables the generation of ultra-broadband coherent spectra (also known as super continua) spanning over the ultraviolet, visible and near-infrared regions, which has also enabled producing the shortest soliton-compressed pulses in the world (sub-5-fs).
c. Home-built hollow-fiber compressor for few-cycle pulse generation
This system takes advantage of the nonlinear propagation of amplified laser pulses in noble gas-filled hollow capillaries to produce unprecedentedly short laser pulses. Using a proprietary measurement and compression scheme (dispersion-scan) based on a set of ultra-broadband dispersion compensation mirrors, we have achieved the shortest pulses ever obtained from a single-channel pulse compressor: 3.0 fs. These pulses are already in the single-cycle regime and enable the direct observation of field-dependent phenomena in many materials.
d. Dispersion-scan system (d-scan)
Dispersion-scan (d-scan) is an innovative technique (patent pending) for the simultaneous measurement and compression of ultrashort laser pulses. D-scan (invented in our group, in collaboration with Lund University in Sweden) is more robust, much easier to implement and more performing than other conventional pulse measurement techniques. Our in-house design includes automated measurement/acquisition and pulse retrieval.
e. Sub-30-fs deep-ultraviolet pulse source at 266 nm
Ultrashort UV light is generated by non degenerate four-wave mixing of femtosecond pulses at 800 nm and their second-harmonic at 400 nm. These pulses can be used to directly excite and probe energetic levels in atoms and molecules at femtosecond time scales.
Self-organized nano structuring using template synthesis is a very promising and rapidly expanding field for the preparation of different nano structures and templates ranging from the micrometer to the 6 nm range. One appealing branch of self-organization is based on anodization methods, deeply interfacing with physical/chemistry. This technique is very versatile since the pore size, pore density and height can be readily controlled by the electrolyte species, anodizing temperature, voltage and time. The templates are well suited for the subsequent preparation of many ordered nano structures using different deposition methods.
IFIMUP works in the fabrication process of nano porous membranes and nanotubes of Al, Ti, Hf and Fe oxides using different electrolytes. These allow obtaining ordered arrays of hexagonal pore structures with interpose distances and pore diameters ranging from 50 to 500 nm and 10 to 150 nm, respectively. IFIMUP also have the necessary facilities to fill these nano porous structures with perovskite manganite and silica nanotubes by sol-gel template methods, nano wires and segmented nano wires by electrodeposition and anti-dots by ion beam Deposition. Four applications of these membranes are ongoing:
1. Biotechnological 2. Plasmonics 3. Photoelectrochemical solar cells and Dye-synthetized cells 4. Spintronics - metal-insulator-metal memristors
Magneto-optical Kerr effect (MOKE) magnetometry is an indispensable, reliable, and one of the most widely used techniques for the characterization of nano structured magnetic materials. Information, such as the magnitude of coercive fields or anisotropy strengths, can be readily obtained from MOKE measurements. Our state-of-the-art vectorial MOKE magnetometer is an extremely versatile, accurate, and sensitivity unit with a low cost and comparatively simple setup. The unit includes focusing lenses and an automatized stepper motor stage for angular dependent measurements.
In our setup a HeNe Laser produces a green (543.5 nm) light beam with a power of 4 mW. This beam passes through a Glan-Thompson Calcite polarizer with an extinction ratio of 10−5 to get linear polarized light parallel to the plane of incidence (Ep). Subsequently, the light hits the sample at ~60º to its normal, to gain maximum reflection intensity due to the Fresnel equations. The sample is placed between the poles of an electromagnet, fixed on a bar which is mounted on a stepper motor stage, allowing the rotation of the sample and thus the automatization of angular dependent MOKE measurements. This enables to measure samples in a short time and without disturbing the measurement. The magnetic field (H) is applied in the plane of incidence and always parallel to the sample plane. The electromagnet is connected to an operational power supply (KEPCO BOP 100-4M) controlled by a signal generator (HP3245A), which enables to set the amplitude, frequency, and shape of H. To cool the electromagnet we use a small commercial fan. The stepper motor stage has an angular step precision of 5º and is fixed in a x-, y-, and z-translation stage (Thorlabs PT3/M; lateral resolution of 10 μm) for positioning the sample. It also allows sensing the magnetization in different regions of the sample. We can also include a pair of optical lenses (Melles Griot, 5 cm of focus length) to focus and collimate the laser beam before and after reflection, respectively. We are able to change the distance between the lenses and the sample and thus to vary the size of the laser spot down to 50 μm. This possibility allows us to study magnetic patterned samples with μm dimensions. The light reflected by the sample passes through a λ/2 wave plate and then a Wollaston prism being measured with two photodiodes. A homemade amplifier (up to 500×) was built to perform and amplify the difference and the sum of the two signals obtained from the two photodiodes. The Kerr and magnetic field signals are then displayed on an oscilloscope (Tektronix TDS 2024), which is connected to a computer via a GPIB interface.
For more details about our MOKE setup please see the reference below:
Versatile, high sensitivity, and automatized angular dependent vectorial Kerr magnetometer for the analysis of nanostructured materials, J. M. Teixeira, R. Lusche, J. Ventura, R. Fermento, F. Carpinteiro, J. P. Araujo, J. B. Sousa, Rev. Sci. Instrum. 82, 043902 (2011) (link)
Ion Beam Deposition of thin films. Both metallic and oxide films with sub-nm thicknesses can be fabricated in the Multifunctional Magnetic Materials and Nanostructures group using an Ion Beam Deposition system from Commonwealth Scientific. This unit is also equipped with a manual load-lock/oxidation chamber and allows for in-situ measurements to be performed. The main use of the unit is the fabrication of magnetic thin films and nano structures.
The group of Multifunctional Magnetic Materials and Nanostructures has an Ion Beam Deposition system from Commonwealth Scientific Corporation for thin film fabrication. This system possess two 3-cm diameter Kaufman dc ion sources, one for film deposition and the other for assisted deposition or ion-milling . In both cases, an Ar flow is used. For Ar ionization, a W filament, working as the cathode, is heated releasing thermionic electrons. The cathode current is set and measured at the terminals of thisW filament (and is typically 5.0–7.0 A); magnets placed around the anode confine the electron paths thus enhancing ionization rate. After the plasma is formed it is extracted only when a voltage is applied to Mo grids, placed at the exit of the ion sources. The two Mo grids are used in a focused configuration (with the focal point at the target). The inner grid prevents the erosion of the outer (acceleration) grid, where the voltage for beam extraction is applied.
Both ion sources are equipped with neutralizer filaments for deposition/milling of dielectric materials. The deposition ion source is placed 10 cm away from the target, which is tilted by 45º. The sputtered atoms hit the substrate that directly faces the target. A shutter surrounding the four target assembly prevents crossed contamination during the deposition process. Both target and substrate holders are water cooled, and the substrate holder can rotate during deposition to improve film thickness uniformity. A shutter, placed in front of the substrate holder prevents material deposition when a pre-cleaning procedure is in progress. The IBD chamber reaches a base pressure of less than 10^-7 Torr. This chamber is isolated from a load-lock that allows substrates to be manually transferred to and from the deposition chamber without breaking vacuum. The work pressure depends on the gas flow used for deposition, but is generally of 2 x 10^-4 Torr.