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W Ertmer

Publications and source records attributed to W Ertmer.

At least 19 recordsLinked to original sources
PubMed2003

Measurement of the spatial correlation function of phase fluctuating Bose-Einstein condensates.

We measure the intensity correlation function of two interfering spatially displaced copies of phase fluctuating Bose-Einstein condensates. It is shown that this corresponds to a measurement of the phase correlation properties of the initial condensate. Analogous to the method used in the stellar interferometer experiment of Hanbury Brown and Twiss, we use spatial intensity correlations to determine the phase coherence lengths of elongated condensates. We find good agreement with our prediction of the correlation function and confirm the expected coherence length.

Journal Article↗
PubMed2003

Quantum computing with spatially delocalized qubits.

We analyze the operation of quantum gates for neutral atoms with qubits that are delocalized in space, i.e., the computational basis states are defined by the presence of a neutral atom in the ground state of one out of two trapping potentials. The implementation of single-qubit gates as well as a controlled phase gate between two qubits is discussed and explicit calculations are presented for rubidium atoms in optical microtraps. Furthermore, we show how multiqubit highly entangled states can be created in this scheme.

Journal Article↗
PubMed2002

Interferometer-type structures for guided atoms.

We experimentally demonstrate interferometer-type guiding structures for neutral atoms based on dipole potentials created by microfabricated optical systems. As a central element we use an array of atom waveguides being formed by focusing a red-detuned laser beam with an array of cylindrical microlenses. Combining two of these arrays, we realize X-shaped beam splitters and more complex systems like the geometries for Mach-Zehnder and Michelson-type interferometers for atoms.

Journal Article↗
PubMed2002

Dynamics of dark solitons in elongated Bose-Einstein condensates.

We find two types of moving dark soliton textures in elongated condensates: nonstationary kinks and proper dark solitons. The latter have a flat notch region and we obtain the diagram of their dynamical stability. At finite temperatures the dynamically stable solitons decay due to the thermodynamic instability. We develop a theory of their dissipative dynamics and explain experimental data.

Journal Article↗
PubMed2002

Micro-optical realization of arrays of selectively addressable dipole traps: a scalable configuration for quantum computation with atomic qubits.

We experimentally demonstrate novel structures for the realization of registers of atomic qubits: We trap neutral atoms in one- and two-dimensional arrays of far-detuned dipole traps obtained by focusing a red-detuned laser beam with a microfabricated array of microlenses. We are able to selectively address individual trap sites due to their large lateral separation of 125 microm. We initialize and read out different internal states for the individual sites. We also create two interleaved sets of trap arrays with adjustable separation, as required for many proposed implementations of quantum gate operations.

Journal Article↗
PubMed2002

Bose-Einstein condensation in dilute atomic gases.

Bose-Einstein condensation is one of the most curious and fascinating phenomena in physics. It lies at the heart of such intriguing processes as superfluidity and superconductivity. However, in most cases, only a small part of the sample is Bose-condensed and strong interactions are present. A weakly interacting, pure Bose-Einstein condensate (BEC) has therefore been called the "holy grail of atomic physics". In 1995 this grail was found by producing almost pure BECs in dilute atomic gases. We review the experimental development that led to the realization of BEC in these systems and explain how BECs are now routinely produced in about 25 laboratories worldwide. The tremendous experimental progress of the past few years is outlined and a number of recent experiments show the current status of the field. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00114-001-0277-8.

Journal Article↗
PubMed2001

Observation of phase fluctuations in elongated Bose-Einstein condensates.

The occurrence of phase fluctuations due to thermal excitations in Bose-Einstein condensates (BECs) is studied for a variety of temperatures and trap geometries. We observe the statistical nature of the appearance of phase fluctuations and characterize the dependence of their average value on temperature, number of particles, and the trapping potential. We find pronounced phase fluctuations for condensates in very elongated traps in a broad temperature range. The results are of great importance for the realization of BEC in quasi-1D geometries, for matter wave interferometry with BECs, as well as for coherence properties of guided atom laser beams.

Journal Article↗
PubMed2001

Doppler cooling and trapping on forbidden transitions.

Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of (40)Ca atoms has been cooled and trapped to a temperature as low as 6 microK by operating a magnetooptical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method, more than 10% of precooled atoms from a standard magnetooptical trap have been transferred to the ultracold trap. Monte Carlo simulations of the cooling process are in good agreement with the experiments.

Journal Article↗
PubMed2001

[Optimizing laser parameters for intrastromal incision with ultra-short laser pulses].

BACKGROUND: With the assistance of ultrashort laser pulses (ca. 200 fs pulse duration) it is possible to perform precise incisions inside the corneal stroma with a width of a few microns. The advantage of ultrashort pulses is that the required energy of a few microjoules is more than an order of magnitude lower compared with longer pulse durations, i.e. ps or ns pulses. Therefore, the secondary effects, such as thermal and mechanical damage to the surrounding tissue and the amount of radiation reaching the retina, are reduced. This method of intrastromal photodisruption allows a very well defined deposition of energy within the laser focus inside the corneal stroma, accompanied by minimal collateral damage. METHODS: The possibilities of performing intrastromal cuts using fs-laser pulses at a wavelength of 780 nm and pulse durations of 200 fs were studied using a titanium-sapphire laser system. The treated tissue samples were analysed by light and scanning electron microscopy to determine incision quality, reproducibility and achievable accuracy. The mechanical side effects of fs-photodisruption inside the surrounding tissue were analysed by pressure measurements using pyroelectric transducers. CONCLUSION: The thermal and mechanical side-effects of this method are very low and comparable to the effects during excimer treatment. Therefore an application of ultrashort laser pulses in refractive surgery appears to be a feasible alternative.

Animals↗
PubMed2001

[Optoacoustic tissue alterations for optimizing laser cyclophotocoagulation. Transscleral detection of laser-induced optoacoustic pressure transients].

BACKGROUND: Considerable problems occur in transscleral laser cyclophotocoagulation concerning energy dosage. We investigated the feasibility of localizing the ciliary body by the detection of thermoelastic pressure transients and of supervising on-line the degree of tissue damage during treatment. METHOD: We used a specially designed handpiece to apply short pulsed laser radiation with low energy levels to enucleated bulbs of rabbits. With an adjusted pressure transducer we examined acoustical transients generated in the area of absorption of the ciliary muscle or the pigmented epithelial layer and measured axial resolution of the method at various distances to the corneoscleral limbus. RESULTS: We detected acoustic transients that allowed rough localization of the target area. A marked change in signal was recorded with increasing level of ciliary destruction. CONCLUSION: This procedure can serve as an essential tool in the on-line supervision of the coagulation process. The laser parameters can thus be adjusted optimally to the progress of the treatment.

Acoustics↗
PubMed2001

Pulsed photothermal radiometry as a method for investigating blood vessel-like structures.

Pulsed photothermal radiometry (PPTR) is known to be suitable for in vivo investigations of tissue optical properties. As a noncontact, nondestructive method it is a very attractive candidate for on-line dosimetry of laser treatments that rely on thermal laser-tissue interaction. In this article, we extend the one-dimensional (1D) analytical formalism that has widely been used to describe PPTR signals to a two-dimensional treatment of a simplified model of a blood vessel. This approach leads to quantitative description of a PPTR signal that, unlike in an 1D treatment, not only shows changes in time, but also varies in space. Using this approach, we are able to gain instructive understanding on how target characteristics of a blood vessel-like structure influence such a spatiotemporal PPTR signal. Likewise, the ability of extracting target features from those measurements is evaluated. Subsequently, we present experimental realization of the idealized model of a blood vessel as used in our theory. Comparison of actual PPTR measurements with theoretical predictions allow vessel localization laterally and in depth. Using our setup, we furthermore demonstrate the influence of flow inside the vessel on the measured signal.

Blood Vessels↗
PubMed2000

Wave packet echoes in the motion of trapped atoms.

We experimentally demonstrate and systematically study the stimulated revival (echo) of motional wave packet oscillations. For this purpose, we prepare wave packets in an optical lattice by nonadiabatically shifting the potential and stimulate their reoccurrence by a second shift after a variable time delay. This technique, analogous to spin echoes, enables one even in the presence of strong dephasing to determine the coherence time of the wave packets. We find that for strongly bound atoms it is comparable to the cooling time and much longer than the inverse of the photon scattering rate.

Journal Article↗
PubMed2000

Application of ultrashort laser pulses for intrastromal refractive surgery.

BACKGROUND: Recently, laser systems have become available which generate ultrashort laser pulses with a duration of 100-200 femtoseconds (fs). By generating micro-plasmas inside the corneal stroma with fs pulses, it is possible to achieve a cutting effect inside the tissue while leaving the anterior layers intact. The energy threshold to generate a micro-plasma with fs pulses is some orders of magnitude lower than it is for picosecond or nanosecond pulses. This results in a strong reduction of the thermal and mechanical damage of the surrounding tissue. METHODS: With a titanium:sapphire fs laser system, the cutting effect on corneal tissue from freshly enucleated porcine eye globes was investigated with different pulse energies. The irradiated samples were examined by light and electron microscopy. The laser-induced pressure transients and the laser-induced bubble formation were analysed with a broadband acoustic transducer and by flash photography. RESULTS: With fs laser pulses, the extent of thermal and mechanical damage of the adjacent tissue is in the order of 1 microm or below and therefore comparable with the tissue alterations after ArF excimer laser ablation. Using pulse energies of approximately 1-2 microJ and a spot diameter of 5-10 microm, intrastromal cuts can be performed very precisely in order to prepare corneal flaps and lenticules. CONCLUSION: Femtosecond photodisruption has the potential to become an attractive tool for intrastromal refractive surgery.

Animals↗
PubMed1998

A scanning and rotating slit arF excimer laser delivery system for refractive surgery.

PURPOSE: This study was designed to investigate the quality of a scanning and rotating slit delivery system of an ArF excimer laser (Nidek EC-5000). METHODS: The ablation patterns on polymethylmethacrylate (PMMA) wafers were examined by scanning electron microscopy. The influence of inhomogeneities in the beam profile was simulated on a computer and compared with a conventional large-area ablation system. The impairment of the ablation rate by radiation absorption of the ablation plume was measured as a function of the repetition rate and the application of a fixation ring. RESULTS: The scanning and rotating slit delivery system is tolerant of small-beam non-homogeneities. The ablation rate is sensitive to the dynamics of the ablation plume. CONCLUSIONS: Although the operating procedure takes less time with a large-area ablation system, a scanning and rotating delivery system has the advantage of reliable and homogeneous removal of corneal tissue.

Cornea↗
PubMed1994

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue.

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF-excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200-800 nm. The intensity of secondary radiation in the range of 200-315 nm (UVC and UVB) is approximately 20% of the total intensity at high laser fluences (> 2 J/cm2), and approximately 50% at moderate laser fluences (< 500 mJ/cm2); 10 muJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (< 1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (> 5 J/cm2) are required. As a consequence, laser-induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.

Animals↗
PubMed1993

Q-switched CTE:YAG (2.69 microns) laser ablation: basic investigations on soft (corneal) and hard (dental) tissues.

Ablative infrared lasers either show poor transmission in optical fibers (Er:YAG: 2.94 microns; ErCr:YSGG: 2.79 microns or are characterized by potential relevant thermal side effects (Ho:YAG: 2.1 microns). The CTE:YAG laser (Cr,Tm, Er doted YAG) emits radiation at a wavelength of 2.69 microns. Efficiently high optical fiber transmission is accomplished (attenuation: < 8db/m for Low-Hydroxy-Fused-Silica (LHFS): 0.3 ppm). Since the laser can easily be run in the Q-switch mode (pulse duration: 0.5-2.5 microseconds) thermal side effects of tissue interaction were expected to be low. Laser tissue interaction was studied on soft (porcine and human cornea), as well as on hard (human dental) tissue. Histological and micromorphological examinations were performed by light microscopy and scanning electron microscopy. It was found that ablation rates in corneal tissue increased from 5 to 90 microns/pulse with increasing laser fluences (5.5-20 J/cm2). Collateral thermal damage reached as far as 20 +/- 5 microns, and was higher (up to 50 microns) when craters where processed in the contact mode using LHFS-optical fibers. In comparison to soft tissue ablation, hard dental tissue ablation showed very little increase of ablation rate (1-3 microns/pulse) when higher fluences were applied. In dental tissue processing, the ablative effect was accompanied by a luminescence, indicating the presence of plasma. We conclude that the presented CTE:YAG laser can be considered as an effective tool for a variety of laser surgical applications where high power optical fiber delivery is required and where strong thermal side effects are not desired.

Animals↗
PubMed1993

Internal ablative sinostomy using a fiber delivered Q-switched CTE: YAG laser (2.69 microns).

Current trends of laser technology towards low-thermal photoablative pulsed mid-infrared lasers open new, more adequate approaches to experimental surgical procedures which have already been evaluated in the past. Transcorneal laser ablation of the trabecular meshwork (internal sinostomy) in human autopsy eyes was performed with a Q-switched CTE:YAG laser (wavelength: 2.69 microns, pulse width: 1 microsecond). Beam delivery was achieved with conventional optical quartz fibers (Low-hydroxy-fused-silica: 0.3 ppm, 50 cm length, 200 microns diameter). Light- and scanning-electron-microscopy were used for histological examination and micromorphological analysis. By applying two laser pulses (6 J/cm2) to the functional trabecular meshwork, a round sinostomy with a diameter corresponding to the diameter of the fiber-tip was achieved. It was possible to set several internal sinostomies into the chamber angle opposite to the entering paracentesis of the laser fiber-tip. Collateral thermal tissue alteration reached up to 50 microns, and since fiber-tip contact was maintained during laser application, thermal tissue alteration was also found around the opposite wall of Schlemm's canal. At higher energy fluences mechanical (disruptive vaporization) effects were significantly enhanced. It can be concluded, that low-thermal pulsed mid-infrared lasers are adequate instruments to perform transcorneal trabecular ablation (abinterno sinostomy). The laser used in this study (CTE:YAG) bears the advantage that its radiation can easily be delivered in conventional optical quartz fibers.

Fiber Optic Technology↗