Francis R. Cruickshank

Pulsed source thermal lens. Part 1.—Theoretical analysis

The principle of operation of a pulsed source thermal lens is described and a practical system, based on a pulsed, CO2, line-tunable laser, is outlined. The time variation of the thermal lens is probed by a continuous He/Ne laser monitored on a photomultiplier tube behind a pinhole mounted on the beam axis in far field. This photomultiplier signal is fully analysed theoretically as a function of time and the physical properties of the sample gas. It is shown that this analysis enables calculation of the magnitude of the temperature at any point in the cell as a function of time. The risetime of the signal is related to the vibrational–translational relaxation time of the sample and the lifetime of the signal decay is related quantitatively to the thermal conductivity coefficient.

Pulsed-source thermal lens. Part 2.—Experimental tests of the theory

The theory of the thermal lens presented in Part 1 is tested by using the decay to calculate the thermal-conductivity coefficients of Ne, Ar, Kr and Xe (48.2, 18.48, 10.05 and 6.02 W m–1 K–1, respectively) in terms of one adjustable parameter, the pulsed laser beam radius, which is found to be close to the measured value for the c.w. beam. These values agree closely with the best values currently available. The treatment of the rise of the thermal-lens signal is tested by comparison of observed with a priori calculated variation of maximum probe-laser modulation as a function of added Ar pressure. The system is shown to be useful for TEM00 laser beam mode diameter measurement and calculation of the temporal variation of the spatial distribution of temperature in a laser-irradiated cell.

Transverse and longitudinal surface plasmon resonances of a hexagonal array of gold nanorods embedded in an alumina matrix

The specular reflectance from a hexagonal array of gold nanorods embedded in an alumina matrix supported on an aluminum substrate is reported. The rods were grown by electrodeposition of gold in an alumina template and were oriented with their long axis perpendicular to the film surface. Optical reflectance measurements performed with an incident light beam of S polarization only exhibited the transverse surface plasmon resonance whereas the measurements obtained with P polarization exhibited both transverse and longitudinal resonances. A model for the reflectance from a thin anisotropic film was developed and shown to be in agreement with the experimental data.

In-Situ Ellipsometry and SHG Measurements of the Growth of CdS layers on CdxHg1-xTe

In-situ measurements of ellipsometry and second harmonic generation (SHG) are carried out to examine the growth of thin CdS films on CdxHg1−xTe (CMT) from aqueous sulphide solutions. The change in the extinction coefficient (at λ = 632.8 nm) with film thickness indicates that impurities, notably β-HgS, become increasingly incorporated in the CdS layer. The low value for the refractive index (n = 1.73) on the other hand suggests that the film is porous. In this first reported monitoring by SHG of the growth of a non-centrosymmetric layer on another such layer, we have observed an increase of the SH response as the CdS layer was grown. From first principles of non-linear optics, this increase should be quadratic with the film thickness. However, the observed increase does not depart clearly from a linear increase owing to the very thin CdS films grown. Anisotropy measurements have been performed prior to the growth and after the growth of the thin film. A clear change of the pattern attributed to the CdS film is observed but the SH response does not arise solely from the CdS film.

Vibration–vibration energy-transfer kinetics monitored by thermal lens

The time development of the thermal-lens signal from CO2 at various pressures has been measured and interpreted. The waveforms obtained are found to be very sensitive measures of the rate constants for individual energy-transfer steps and comparison of theory with experiment has allowed the determination of rate constants for the processes CO2(001)→ CO2(030), k=(10.22 ± 0.7)× 10–15 molecule–1 cm3 s–1 and CO2(010)→ CO2(000), k= 7.9 × 10–15 molecule–1 cm3 s–1. The process CO2(001)→ CO2(040) is shown to be relatively insignificant and it is also shown that it is not necessary to postulate a specifically endothermic step in the mechanism to explain the transient cooling of pure CO2.

Second-harmonic generation in the characterization of surface effects in epitaxial layers

Second-harmonic (SH) rotational anisotropy measurements performed on epitaxial Cdx Hg1−x Te (CMT) layers grown on CdTe h111i B substrate exhibits an interference in the SH signal originating from the bulk and from the CMT surfaces. The threefold symmetry of the epilayer was shown to be sensitive to the nature of adsorbed species at the surface when in contact with an electrolyte solution despite the strong SH generation in the bulk of the layer. The modification of the SH response from a non-centrosymmetric semiconducting material to such an extent is unusual since bulk SH generation is considered as the dominant contributor in these instances. The case of CMT is, however, rather specific in that the observed SH signal originates from, at most, only the top 40 nm of the CMT epilayer. This important difference means that surface modification by means of electron donating or withdrawing groups will play a large role in the observed SHG signal.

Second harmonic generation by reflection from vicinal surfaces of epitaxial layers of cadmium mercury telluride

Reflection second harmonic generation is employed to examine the surfaces of epitaxial layers of CdxHg1-xTe (CMT) grown on vicinal GaAs (100) substrates. Different vicinal tilts, with respect to the GaAs substrates, are observed in the epitaxial CMT layers. We also report here on the measurement of the second-order nonlinear coefficient (d36) of CMT. Because CMT is strongly absorbing at the 1.06 µm wavelength, this measurement was performed by comparing the second harmonic intensity reflected from the CMT surface to that measured for a quartz sample in transmission. Directly comparable expressions for the reflected and transmitted second harmonic intensities are derived from which a value of d36 = 365±15 pm V-1 is obtained. This value is much larger than those reported for similar zinc-blende-type materials and is attributed to an electronic resonance enhancement.

Interference between second harmonic waves in an anodically grown cadmium sulphide thin film

Second harmonic generation has been employed in-situ to follow the growth of a thin anodic sulphide film on CdxHg1−xTe (CMT). The anisotropy patterns recorded for the bare CMT surface show that the sample is a vicinal {100} face. This is supported in particular by the observation of a non-vanishing pattern in the SIN–SOUT configuration. Also, the observation of a four-fold pattern in the case of the CdS film confirms that the film has retained the ccp structure of the underlying CMT substrate. The SH signal recorded during the growth of the CdS film follows an oscillatory pattern with film thickness. This has been attributed to the interference between the two dominating SH waves generated within the CdS film.

Thickness Dependence of the Anisotropy of Anodic Sulphide Films on CdxHg1—xTe Studied Using Surface Second Harmonic Generation

The growth of anodic sulphide films on vicinal Cd x Hg 1-x Te (CMT) surfaces has been investigated as a function of sulphide layer thickness by rotation anisotropy using second harmonic generation (SHG). The presence of thin CdS films on the CMT surface does not alter the fourfold pattern observed for the bare CMT surface. However, with increasing sulphide film thickness (>100 nm) and for growth at constant potential, a drastic loss in the reflected SH intensity occurs. This reduction has been attributed to the formation and incorporation of β-HgS in the layer at the more positive potentials. This compound absorbs the SH signal generated by the composite system of CMT/sulphide layer.

SECOND-HARMONIC GENERATION IN BIAXIAL CRYSTALS FOR A NON-DEPLETED FUNDAMENTAL GAUSSIAN LASER BEAM

The theory necessary for the optimization of second-harmonic (SHG) efficiency in biaxial crystals is developed and applied to the molecular crystal (-)2--(methylbenzylamino)-5-nitropyridine (MBANP). It is shown that, for the range of crystal lengths and beam walk-off angles available, the criteria for the selection of the parameters and for optimization of the second-harmonic signal are the same in biaxial and uniaxial crystals. The maximal possible SHG figure of merit, , is found to be for a 50 mm long crystal.