We measured first- and second-order Raman scattering in cubic and hexagonal boron nitride using excitation energies in the visible and in the UV. The nonresonant first-order Raman susceptibilities for cubic and hexagonal BN are 1 and $10\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{2}$, respectively. Raman scattering is thus very powerful in detecting the hexagonal phase in mixed thin boron nitride films. In cubic BN the constant Raman sucseptibility in the visible and the UV is due to its indirect band gap. For hexagonal BN a Raman enhancement is found at $5.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. It is well explained by the energy dependence of the dielectric function of hexagonal BN. The second-order spectrum of cubic boron nitride is in excellent agreement with first-principles calculations of the phonon density of states. In hexagonal BN the overbending of the LO phonon is $\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, five times larger than in graphite.

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Resonant Raman scattering in cubic and hexagonal boron nitride

Magnetic light scattering in low-dimensional quantum spin systems

Raman spectra of single (hexagonal wurtzite) $\mathrm{GaN}$ nanowires grown by vapor-liquid-solid synthesis were recorded. By investigating the polarization dependence of the Raman intensities of the observed bands as a function of the angle between the long axis of the nanowire and the electric vector of the incident (and scattered) light, one is able to distinguish between nanowires whose crystallographic $c$ axis corresponds to (${c}^{*}$ directed), or is substantially perpendicular to (${a}^{*}$ directed) the long axis of the nanowire---the two preferred directions of nanowire growth. For ${a}^{*}$-directed nanowires, polarization-dependent Raman intensity analysis can also determine, within tolerable limits, the orientation of the crystalline $c$ axis with respect to laboratory-fixed coordinates. Simultaneous transmission electron microscopy (TEM) analyses confirm the conclusions derived from the Raman measurements. The polarization-dependent Raman intensities could only be understood if one assumed a complex-valued Raman tensor. The detection of resonance enhancement together with the tendency of the nanowires to grow with a high concentration of defects and the observation of yellow luminescence in the photoluminescence spectrum may account for the complex-valued Raman tensor we observe for $\mathrm{GaN}$ even at sub-band-gap photon energies.

Polarized Raman scattering from single GaN nanowires

We report the systematic investigation of polarized Raman scattering on individual crystalline GaP nanowires (NWs) with diameters $40ldl600\text{ }\text{nm}$. At small diameter, $dl70\text{ }\text{nm}$, the NWs are found to act like a nearly perfect dipole antenna that leads to a polarized scattering intensity function $I(\ensuremath{\theta})\ensuremath{\sim}{\text{cos}}^{4}\text{ }\ensuremath{\theta}$ where $\ensuremath{\theta}$ is the angle between NW axis and the incident laser polarization. For larger diameter $(70ldl600\text{ }\text{nm})$ NWs, some other different polar patterns were found. A simple theoretical approach which takes into account the Raman tensor and photon confinement in NWs was used to understand the experimental data.

Polarized Raman scattering from single GaP nanowires

The fundamental parameters and limitations that determine the signal strength in tip-enhanced Raman scattering (TERS) are discussed. A semiquantitative analysis of the Raman signal expected in different experimental geometries and with different sample systems is presented, taking into account experimental parameters including Fresnel factor, numerical aperture of the illumination and collection optics, detection efficiency, and the Raman scattering cross section of the material. A top/side-on illumination geometry is essential for the study of nontransparent samples. It can yield the highest signal levels when strong tip–sample coupling using a metallic substrate provides large field enhancement. In contrast, axial/through-sample illumination is limited to transparent sample materials. Although conceptually simpler in experimental implementation and despite high numerical aperture signal collection efficiency, signals are generally weaker due to limited field enhancement. Crystalline solids with small Raman cross sections and dense molecular/biological systems with unavoidable far-field background provide the biggest challenge for TERS analysis yet at the same time hold the most exciting outstanding scientific questions TERS has the potential to answer.

Signal limitations in tip-enhanced Raman scattering: the challenge to become a routine analytical technique.

Absolute Raman scattering efficiencies for first-order light scattering of optical zone-center phonons were determined for cubic and hexagonal GaN. An improved method for the determination of scattering efficiencies by comparative measurement with a standard substance such as BaF2 was used to minimize the experimental errors substantially. © 1998 John Wiley & Sons, Ltd.

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Comparative determination of absolute Raman scattering efficiencies and application to GaN

Spin gap and spin-phonon interaction in CuGeO3

GuGeO, has recently gained a lot of interest as it is the first inorganic compound that undergoes a spin-Peierls transition (7\"SP = 14 K). We study the spin-Peierls state in this material in external magnetic fields by means of Raman spectroscopy. The experiments are carried out over a wide range of temperatures (2—50 K) and magnetic fields (0— 15 T). Spin-phonon interaction, which is intrinsic to the spin-Peierls state, manifests itself in form of a pronounced Fano-lineshape of the peak at 107 cm ~1 as well as several magnon-phonon coupled modes. We present a detailed analysis of the magnetic-field and temperature dependence of these excitations. Furthermore, we report details of a newly observed mode related to the incommensurate phase at high magnetic fields and low temperature (>12.5 at 2 K).

Magneto-Raman Study on CuGeO3*

http://www.ifkp.tu-berlin.de/fileadmin/i1/thomsen/publikationen/paper/191.pdf

S. Gronemeyer

Papers

Comparative determination of absolute Raman scattering efficiencies and application to GaN

Spin gap and spin-phonon interaction in CuGeO3

Magneto-Raman Study on CuGeO3*

Low-energy magnetic excitations in the dimerized and incommensurate phase of CuGeO3

Isotope effects in CuGeO3 studied by Raman spectroscopy