R. Prazeres

Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor

We describe a new method of infrared microspectroscopy. It is intended for performing chemical mapping of various objects with subwavelength lateral resolution by using the infrared vibrational signature characterizing different molecular species. We use the photothermal expansion effect, detected by an atomic force microscope tip, probing the local transient deformation induced by an infrared pulsed laser tuned at a sample absorbing wavelength. We show that this new tool opens the way for measuring and identifying spectroscopic contrasts not accessible by far-field or near-field optical methods and with a subwavelength lateral resolution.

Sub-100 nm IR spectromicroscopy of living cells

We have performed IR spectromicroscopy of cells immersed in liquid water, with a lateral resolution better than 100 nm. Here, we use the motion of an atomic force microscope tip, probing the local transient deformation induced by an IR pulsed laser tuned at a sample absorbing wavelength. By Fourier analysis of the vibration of the cantilever tip, we can discriminate frequencies that are characteristic of the object, thus eliminating the influence of the water absorption. This opens the door of chemical imaging of living species in vivo, with spatial resolution of the order of the size of cell components.

Ultrasensitive spectroscopy of ionic reactive intermediates in the gas phase performed with the first coupling of an IR FEL with an FTICR-MS

First example of coupling a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR-MS) with an infrared Free Electron Laser (FEL) is presented. This experimental setup is ideally suited for the direct structural characterization of reactive polyatomic ions. Ultrasensitive measurements of the infrared vibrational spectrum of ionic reactive intermediate selectively prepared is allowed by the association of the high peak power of the FEL, its wide tunability, and the flexibility of FTICR-MS, where several mass selections and ion-molecule reactions can be combined. These possibilities are demonstrated in the case of Fe + complexes where two photofragmentation pathways compete. The resulting infrared spectrum is in excellent agreement, both with respect to the position and to the relative intensities of the infrared transitions, with predicted by ab initio electronic structure calculations. r 2003 Elsevier Science B.V. All rights reserved.

Saturation of intraband absorption and electron relaxation time in n-doped InAs/GaAs self-assembled quantum dots

We have observed the saturation of intraband absorption in InAs/GaAs self-assembled quantum dots. The investigated n-doped self-assembled quantum dots exhibit an intraband absorption within the conduction band, which is peaked at an 8 μm wavelength. The saturation of the intraband absorption is achieved with an infrared pump delivered by a pulsed free-electron laser. The saturation of the transition is observed for an intensity around ≈0.6 MW cm−2. The electron relaxation time under intraband excitation is measured by time-resolved pump–probe experiments. An electron relaxation time T1≈3 ps is reported.

Intersubband stimulated emission in GaAs/AlGaAs quantum wells: Pump-probe experiments using a two-color free-electron laser

Intersubband stimulated emission under optical pumping has been observed in the conduction band of GaAs–AlGaAs quantum wells. The asymmetric coupled quantum wells which exhibit three conduction bound levels are designed to exhibit population inversion under optical pumping. An optical excitation at λ=9.2 μm is used to bleach the absorption between the ground and second excited subband. The population inversion between excited subbands is pumped and probed on a picosecond time scale by a tunable two-color free-electron laser. The stimulated amplification is studied at low temperature in infrared waveguides as a function of the waveguide length and of the probe wavelength. A stimulated gain ≈80 cm−1 is measured at 12.5 μm in agreement with calculations.

Coherent harmonic generation in VUV with the optical klystron on the storage ring Super-ACO

Abstract The goal of this experiment is to explore the possibility of producing coherent light in the VUV spectral range using the nonlinear properties of the free electron laser gain that result when an electron beam is bunched by an external pulsed laser. The positive results, that have been obtained in 1987 on the old storage ring ACO, have lead us to install an experiment on Super-ACO, but with some modifications. This article presents the most recent results obtained on Super-ACO, where coherent photons have been produced at the 3rd (177 nm) and 5th (106 nm) harmonics of the incident laser. It presents also a brief summary of the practical interest and difficulties of this techniques.

Adsorbate vibrational spectroscopy by IR-visible sum-frequency generation using CLIO-FEL: CO from CH3OH electrochemical decomposition on Pt

The use of infrared-FEL allows adsorbate vibrational spectroscopy by IR-visible sum-frequency generation (SFG) beyond the frequency range accessible by benchtop IR lasers. Therefore, we synchronized a frequency doubled mode-locked YAG laser (532 nm) with the CLIO-FEL, which is tuneable from 2 to 17.5 μm. The feasibility is demonstrated by recording SFG-vibrational spectra near 5-μm of the CO adsorbate at the electrochemical interface Pt/HClO4 solution containing CH3OH. These spectra indicate the coexistence of linearly and doubly-bounded species. This work is a preliminary step towards adsorbate SFG-vibrational spectroscopy at longer wavelengths.

Midinfrared second-harmonic generation in p-type InAs/GaAs self-assembled quantum dots

Resonant second-harmonic generation is reported in InAs/GaAs self-assembled quantum dots. Frequency doubling is observed between confined states in the valence band of the quantum dots. The second-order nonlinear susceptibility is maximum at 168 meV (7.4 μm wavelength) and is observed for an in-plane polarized excitation. A value of χzxx(2) as large as 2×10−7 (m/V) is measured for one dot plane. A three-dimensional numerical calculation of the valence band states shows that the second-harmonic generation involves a resonant excitation between the h000 and h101 states and a state close to the continuum wetting layer states.

Optical performance of the CLIO infrared FEL

Abstract First laser oscillation on the CLIO infrared FEL was obtained in January 1992. This paper describes the layouts of the optical devices used for CLIO, and discusses the optical performances. This machine consists of an rf linear accelerator, described in a companion paper, providing a 30/70 MeV electron beam through a 48 period planar undulator ( K = 0 to 2). The optical cavity is 4.8 m long and uses broadband metal mirrors. The optical beam is extracted with an intracavity CaF 2 or ZnSe plate. Laser oscillation has been obtained thus far in the range of λ =2.5 to 15 μ m at accelerator energies of 32, 40 and 50 MeV. The average power of the laser is about 65 mW for the low duty cycle (6.25 Hz/32 ns) and up to 0.5 W for a duty cycle of 50 Hz/32 ns and should be 5–10 W at maximum repetition rate. The peak power extracted for 8 ps micropulses is 2.5 MW corresponding to 0.4% efficiency. Laser oscillation on the third harmonic of 10 μm has also been achieved (at λ = 3.3 μ m). Application experiments have already been done with CLIO infrared laser (companion paper on nonlinear absorption in InSb), showing the good reliability and overall quality of the laser. The programme now is to operate the accelerator at other energies so as to cover the rest of the designed wavelength range (2–20 μm).

Investigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells

We have investigated the mid-infrared intersubband stimulated emission under optical pumping in GaAs/AlGaAs coupled quantum wells. The quantum wells exhibit four levels bound in the conduction band. The energy between the ground and first excited subband is close to the optical phonon energy enabling population inversion. Intersubband stimulated gain between subbands E3 and E2 is observed around 14 μm wavelength by optically pumping the E1−E3 intersubband transition at 10 μm. The gain measurement is performed by time-resolved pump-probe experiments using a two-color picosecond free-electron laser. The dependence of the intersubband stimulated emission is analyzed as a function of the pump intensity, and the pump and probe wavelengths. We show that very large intersubband stimulated gain can be achieved at liquid nitrogen temperature in a 2 mm thick waveguide. The stimulated gain is resonant with the pump wavelength with a broadening ≈25 meV. The experimental results are explained with a nonperturbative ma...