A control system and apparatus for use with an ultra-fast laser is provided. In another aspect of the present invention, the apparatus includes a laser, pulse shaper, detection device and control system. A multiphoton intrapulse interference method is used to characterize the spectral phase of laser pulses and to compensate any distortions in an additional aspect of the present invention. In another aspect of the present invention, a system employs multiphoton intrapulse interference phase scan. Furthermore, another aspect of the present invention locates a pulse shaper and/or MIIPS unit between a laser oscillator and an output of a laser amplifier.

Control system and apparatus for use with ultra-fast laser

Using an optical tweezers apparatus, we demonstrate three-dimensional control of nanodiamonds in solution with simultaneous readout of ground-state electron-spin resonance (ESR) transitions in an ensemble of diamond nitrogen-vacancy color centers. Despite the motion and random orientation of nitrogen-vacancy centers suspended in the optical trap, we observe distinct peaks in the measured ESR spectra qualitatively similar to the same measurement in bulk. Accounting for the random dynamics, we model the ESR spectra observed in an externally applied magnetic field to enable dc magnetometry in solution. We estimate the dc magnetic field sensitivity based on variations in ESR line shapes to be approximately . This technique may provide a pathway for spin-based magnetic, electric, and thermal sensing in fluidic environments and biophysical systems inaccessible to existing scanning probe techniques.

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Electron spin resonance of nitrogen-vacancy centers in optically trapped nanodiamonds

We report on cell damage of single cells confined in continuous-wave (cw), near-infrared (NIR) multimode optical traps as a result of multiphoton absorption phenomena. Trapping beams at NIR wavelengths less than 800 nm are capable of damaging cells through a two-photon absorption process. Cell damage is more pronounced in multimode cw traps compared with single-frequency true cw NIR traps because of transient power enhancement by longitudinal mode beating. Partial mode locking in tunable cw Ti:sapphire lasers used as trapping beam sources can produce unstable subnanosecond pulses at certain wavelengths that amplify multiphoton absorption effects significantly. We recommend the use of single-frequency long-wavelength NIR trapping beams for optical micromanipulation of vital cells.

Cell damage in near-infrared multimode optical traps as a result of multiphoton absorption

Biomedical applications of nanodiamonds in imaging and therapy.

Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.

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Optical alignment and spinning of laser-trapped microscopic particles

We report the experimental results of our using irregularly shaped diamond microparticles as handles for laser tweezers. Because of their irregular optical shape, control of the rotation of diamond microparticles can easily be achieved in a gradient force optical trap by use of a fixed linearly polarized beam with a fundamental Gaussian mode. By changing the laser focal plane upon a diamond particle near the liquid surface or interfaces, one can fully manipulate both the direction and speed of the rotation. The ability to manipulate a diamond-particle-tagged biological specimen by optical tweezers is discussed. The application of these particles as handles for optical tweezers is demonstrated by optical manipulation of biological cells. Independent movement of linear translation and rotation, with controllable rotation directions and speeds, is successfully achieved.

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Cell manipulation by use of diamond microparticles as handles of optical tweezers

Triple optical autocorrelation of femtosecond optical pulses was realized simply with third-harmonic-generation technique. This optical technique provides complete knowledge of transient pulse intensity variation directly in time domain. Only analytic calculation is needed to obtain the pulse-shape from data without direction-of-time ambiguity. Combined with a spectral measurement and the Gerchberg–Saxton algorithm, except for pulses with complete temporal and spectral symmetry that will cause a twofold ambiguity, exact phase variation in time can also be retrieved through an iterative calculation with an O(n) complexity.

Triple-optical autocorrelation for direct optical pulse-shape measurement

Huge excitonic enhancement of two-photon absorption near the exciton-transition energy was observed in the GaN system. The peak value of the nonlinear absorption coefficient is at least 1500 cm/GW, corresponding to an enhancement factor of >100. The room temperature exciton dephasing time is also obtained based on a spectral analysis to be ∼150 fs in bulk GaN, which is close to the exciton ionization time.

Observation of huge nonlinear absorption enhancement near exciton resonance in GaN

Femtosecond transient transmission pump–probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100–250 femtoseconds was observed by the time-resolved pump–probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

A method for measuring an ultrashort optical pulse, in which third order autocorrelation of femtosecond (10 −15 second) optical pulses was realized based on third-harmonic-generation (THG). A THG signal with three fundamental frequency photon contributed from three different split pulses of the ultrashort optical pulse is first generated. The three split pulses have time delays τ 1 and τ 2 in between. Then, the intensity of the THG signal is detected while varying the time delays τ 1 and τ 2 between the split pulses to obtain a triple correlation of the ultrashort optical pulse. The triple correlation and its Fourier transform are used to obtain the magnitude |Ĩ(ν)| and the phase α(ν) of the ultrashort optical pulse intensity in the frequency domain, and the intensity of the ultrashort optical pulse in the time domain I(t) is determined using the magnitude |Ĩ(ν)| and the phase α(ν) by inverse Fourier Transform.

Yin-Chieh Huang

Papers

Cell manipulation by use of diamond microparticles as handles of optical tweezers

Triple-optical autocorrelation for direct optical pulse-shape measurement

Observation of huge nonlinear absorption enhancement near exciton resonance in GaN

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

Method and system for measuring an ultrashort optical pulse