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Persistent spectral hole-burning phenomena in CuCl nanocrystals

Shinji Okamoto

Published Dec 3, 2007• 0 citations

Abstract

We have studied persistent spectral hole-burning (PSHB) phenomena of cuprous halide (CuCl) nanocrystals.In particular, we concentrate on two aspects: Fundamental physics and applications of the PSHB phenomena.Stark effects by trapped carriers or photoionization were proposed as the mechanisms of the PSHB phenomena.However, the further study is needed for the better understanding of the mechanism.In the course of the study to this direction, we found correlations between the PSHB phenomena and photoluminescence spectral-changes of excitons bound to Cu+-ion vacancies in CuCl nanocrystals and of Cu+ dimers in a NaCl crystal in CuCl nanocrystals embedded in a NaCl crystal.The spectral-changes can be triggered by Cu+-ion displacements in CuCl nanocrystals and in a NaCl crystal.On the other hand, in CuCl nanocrystals embedded in alumino borosilicate glass, the PSHB phenomena are considered to be caused by photophysical mechanisms, e. g., photoinduced structural-change at nanocrystal-glass interface and/or glass.PSHB phenomena can be applied to not only optical memory but also the precise, highresolution spectroscopy for the nanocrystals with a certain size.We report the nanocrystalsize dependent acoustic phonons confined in CuCl nanocrystals by using the PSHB phenomena.The size-dependent energy-intervals between zero-phonon lines and phonon sidebands in the hole-burning spectra almost coincide with the calculated energies of confined acoustic phonons in nanometer-size elastic spheres.We conclude that the phonon sidebands are due to confined acoustic phonons in CuCl nanocrystals.The observed confined acoustic

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We have studied persistent spectral hole-burning (PSHB) phenomena of cuprous halide (CuCl) nanocrystals.In particular, we concentrate on two aspects: Fundamental physics and applications of the PSHB phenomena.Stark effects by trapped carriers or photoionization were proposed as the mechanisms of the PSHB phenomena.However, the further study is needed for the better understanding of the mechanism.In the course of the study to this direction, we found correlations between the PSHB phenomena and photoluminescence spectral-changes of excitons bound to Cu+-ion vacancies in CuCl nanocrystals and of Cu+ dimers in a NaCl crystal in CuCl nanocrystals embedded in a NaCl crystal.The spectral-changes can be triggered by Cu+-ion displacements in CuCl nanocrystals and in a NaCl crystal.On the other hand, in CuCl nanocrystals embedded in alumino borosilicate glass, the PSHB phenomena are considered to be caused by photophysical mechanisms, e. g., photoinduced structural-change at nanocrystal-glass interface and/or glass.PSHB phenomena can be applied to not only optical memory but also the precise, highresolution spectroscopy for the nanocrystals with a certain size.We report the nanocrystalsize dependent acoustic phonons confined in CuCl nanocrystals by using the PSHB phenomena.The size-dependent energy-intervals between zero-phonon lines and phonon sidebands in the hole-burning spectra almost coincide with the calculated energies of confined acoustic phonons in nanometer-size elastic spheres.We conclude that the phonon sidebands are due to confined acoustic phonons in CuCl nanocrystals.The observed confined acoustic

Persistent spectral-hole-burning (PSHB) phenomena have been observed in many kinds of semiconductor quantum dots (QDs). Luminescence hole burning and charged excitons have been observed in CuCl QDs in NaCl crystals. The observed PSHB phenomena are explained by the ionization of QDs. The energy of photoionized QDs is changed from the original energy and their new energies depend on the spatial arrangement of trapped carriers. Quantum confinement of carriers and the resulting strong Coulomb interaction between confined carriers and trapped carriers cause the energy change. PSHB has been widely applied to site-selective spectroscopy of QDs. Size-dependent excited exciton quantum states in CuCl quantum cubes, size-dependent excited electron-hole quantum states in CdTe QDs and renormalization of longitudinal optical phonons confined in CuCl QDs have been revealed by studying sideband holes. This also allowed the measurement of accumulated photon echo, which clarified the very sharp homogeneous spectral linewidth of QDs under weak excitation conditions.

So far, persistent spectral hole-burning (PSHB) phenomenon has been observed in a number of materials. They are molecules in glasses, crystals and polymers, color centers in alkali halide crystals, and rare earth and transition metal ions in glasses and crystals. They are molecular or ionic guests embedded in glass, crystal or polymer hosts. Basic requirements for the PSHB are summarized as follows: (1) The optical absorption of guests embedded in hosts must be broadened inhomogeneously. (2) There must exist more than one ground state configurations of the total system consisting of guests and hosts. (3) The relaxation among the ground states must be slower than the decay rate of any excited state. In fact, this set of requirements is satisfied in many kinds of molecular or ionic guests in hosts, such as glasses, crystals and polymers, at low temperatures. We can naturally understand that guests consisting of small number of atoms are greatly influenced by the surrounding hosts and that the energy levels of the guests are inhomogeneously broadened.

The persistent spectral hole-burning (PSHB) phenomenon was observed in semiconductor quantum dots, CdSe , CuCl , and CuBr , embedded in glass and crystal. In inhomogeneously broadened exciton absorption spectra of these dots, the narrow bleaching hole and its associated structure are made by the narrowband laser excitation and are conserved for more than several hours at 2 K after the laser irradiation. The observation of the PSHB phenomenon in these four kinds of samples shows the generality of the phenomenon in semiconductor quantum dots and requires the existence of more than one ground-state configurations of the total system consisting of quantum dots and surrounding matrix. It means that not only the size distribution but also these ground-state configurations give inhomoge-neous broadening to semiconductor quantum dots. Thermally annealing and light-induced hole-filling phenomena were observed. Hole-burning and hole-filling mechanisms are discussed. Quantum dots consisting of 103–104 atoms behave like molecules or ions in matrix to give the PSHB phenomenon.

Formation efficiencies of the persistent holes in various kinds of cuprous halides quantum dots, namely CuCl, CuBr and CuI quantum dots, embedded in glass or alkali halide crystals, were measured. The highest quantum efficiency of 0.097 was obtained at 2 K for CuCl quantum dots in glass. The persistent absorption decrease of quantum dots caused by a photon is larger than that of the most efficient ionic or molecular persistent hole-burning materials by 3 orders of magnitudes. The high sensitivity shows that the persistent hole-burning phenomenon in semiconductor quantum dots has potential for application in the multiple optical memory devices.

Persistent spectral hole burning (PSHB) is a new method in the laser spectroscopy of solids. This method is based on the existence of zero-phonon lines in the spectra of light-absorbing species, such as impurities (molecules, ions, etc.) or color centers in crystalline or amorphous materials. This chapter describes the electric field effects on persistent spectral holes and in particular special applications in electrooptics and photonics. It demonstrates that PSHB is useful not only for digital processes, such as data storage in the electric field domain, but also for analog processes concerning, in particular, the modulation and pulse forming of laser beams and hybrid optical bistability. The chapter also discusses the future prospects of the possible applications of PSHB and electric field effects. For future possible applications involving PSHB techniques in everyday appliances, it is important to provide room-temperature operation.