Ladan Arissian

Investigation of carrier to envelope phase and repetition rate: fingerprints of mode-locked laser cavities

We use mode locked lasers in a non-conventional way, as a sensor to perform intracavity measurements. To understand this new technique of intracavity phase interferometry (IPI), one should take a detailed look at the characteristics of the frequency comb and its sensitivity to its parent cavity. The laser cavity provides a means to perform phase interferometry while outside the cavity one can only observe amplitude intereference. Many physical quantities such as nonlinear index, Earth rotation, magnetic field, Fresnel drag, etc are converted to phase. IPI is performed by designing laser cavities in which two pulses circulate independently, generating two pulse trains that can have a phase difference that will be converted to frequency. We also explore repetition rate spectroscopy in Rb87 by tailoring a laser wavelength, power and bandwidth. Coherent population trapping is observed when the laser repetition rate matches submultiples of hyperfine splitting.

Partitioning of the Linear Photon Momentum in Multiphoton Ionization

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 and 1400 nm in the intensity range of 10(14)-10(15)  W/cm2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.

Momentum space tomographic imaging of photoelectrons

We apply tomography, a general method for reconstructing 3-D distributions from multiple projections, to reconstruct the momentum distribution of electrons produced via strong field photoionization. The projections are obtained by rotating the electron distribution via the polarization of the ionizing laser beam and recording a momentum spectrum at each angle with a 2-D velocity map imaging spectrometer. For linearly polarized light the tomographic reconstruction agrees with the distribution obtained using an Abel inversion. Electron tomography, which can be applied to any polarization, will simplify the technology of electron imaging. The method can be directly generalized to other charged particles.

Precise in-situ measurement of laser pulse intensity using strong field ionization

Building on the work of Alnaser et al. [Phys. Rev. A 70, 023413 (2004)], we devise an improved method for an in-situ measurement of the peak intensity in a focused, femtosecond infrared laser pulse. The method is shown to be effective with both photoion and photoelectron imaging devices. The model used to fit the experimental data has no unphysical free parameters used in fitting. The accuracy of the fit is 4% and the overall accuracy of the measurement is 8%.

Multiple quantum wells for ring and linear lasers with long lifetime gain

Various solid state lasers such as Cr:LISAF, Yb:YAG, Nd:Vanadate, Ti:sapphire and Nd:YAG have in common a long lifetime of the laser level, which results in a tendency to Q-switching rather than pure mode-locking. These lasers are being used in a linear or ring cavity for intracavity sensing applications (displacements, rotation, electric and magnetic fields), and for applications in spectroscopy. The requirements for these applications are that the pulses be centered at a specific wavelength, and be of a specific pulse duration. Multiple Quantum Wells (MQW) typically used for ultrashort pulse generation have often a high defect concentration which causes losses incompatible with the large number of intracavity elements required by the applications. We have established for all these lasers a composition curve for the MQW, that enables one to tune to a specific wavelength. These saturable absorbers have excellent optical quality both in reflection and transmission. The approach to prevent Q-switching has generally been to use very low loss modulation (single quantum wells). With a large number of intracavity elements, a larger loss modulation is desirable, hence the use of multiple QW (4 to 100). We have successfully demonstrated stable continuous model-locked operation by using passive energy limiters in the cavity. Two-photon generated carriers induce lensing in the cavity, resulting in power dependent losses through an aperture in the cavity. We show that the attenuation is proportional to the square of pulse intensity, resulting in a steep energy limiter. We demonstrate theoretically and experimentally that the presence of two intracavity pulses required for sensor applications can be satisfied with multiple quantum wells appropriately positioned in the cavity. Examples of applications include rotation sensors (ring cavity) or acceleration sensors (linear cavity), magnetic filed sensor, displacement sensors.

Applications of Ultrafast Lasers

We discuss implementations of mode-locked ring lasers, their stabilization via passive optical cavities, and their applications to the development of ultrasensitive sensors.

The effect of propagation in air on the filament spectrum

Filamentation studies traditionally start from letting a beam focus in air. We present filament studies with control over the preparation propagation, in air or vacuum, using an aerodynamic window. The spectral content of the filament strongly depends on its preparation medium.

Stabilization of mode-locked trains, and dark resonance of two-photon lambda-level structures

Our ultimate objective is to design a combined frequency standard for optical as well as radio frequencies. A mode-locked laser provides frequency components that can be used as a ruler to measure any unknown optical source through direct beating. The frequency spacing of a pair of teeth of this comb is in itself a radio frequency reference. Fast control and correction for both the average frequency and the repetition rate of a mode-locked Ti : sapphire laser are achieved by locking the laser to a reference cavity of ultra-low expansion quartz with equal mode spacing. We measure an optical frequency with a mean square deviation of 700 Hz, instability limited by the radio-frequency sources used to count the repetition rate. As a reference standard to achieve absolute accuracy, we use the Λ transition 5S1/2 (F = 1) → 5D5/2 (F = 3) → 5S1/2 (F = 2) of rubidium. The theory for this coherent interaction shows that, with one mode resonant with the two-photon 5S1/2 (F = 1) → 5D5/2 (F = 3) transition, the fluorescence goes through a resonance for a change in repetition rate of less than 10 kHz. These results suggest that, by locking to the peak of that resonant feature, optical stability and absolute accuracy better than 1 kHz can easily be achieved.

Controlling the orbital angular momentum of high harmonic vortices

Optical vortices, which carry orbital angular momentum (OAM), can be flexibly produced and measured with infrared and visible light. Their application is an important research topic for super-resolution imaging, optical communications and quantum optics. However, only a few methods can produce OAM beams in the extreme ultraviolet (XUV) or X-ray, and controlling the OAM on these beams remains challenging. Here we apply wave mixing to a tabletop high-harmonic source, as proposed in our previous work, and control the topological charge (OAM value) of XUV beams. Our technique enables us to produce first-order OAM beams with the smallest possible central intensity null at XUV wavelengths. This work opens a route for carrier-injected laser machining and lithography, which may reach nanometre or even angstrom resolution. Such a light source is also ideal for space communications, both in the classical and quantum regimes.

Dramatic enhancement of supercontinuum generation in elliptically-polarized laser filaments

Broadband laser sources based on supercontinuum generation in femtosecond laser filamentation have enabled applications from stand-off sensing and spectroscopy to the generation and self-compression of high-energy few-cycle pulses. Filamentation relies on the dynamic balance between self-focusing and plasma defocusing – mediated by the Kerr nonlinearity and multiphoton or tunnel ionization, respectively. The filament properties, including the supercontinuum generation, are therefore highly sensitive to the properties of both the laser source and the propagation medium. Here, we report the anomalous spectral broadening of the supercontinuum for filamentation in molecular gases, which is observed for specific elliptical polarization states of the input laser pulse. The resulting spectrum is accompanied by a modification of the supercontinuum polarization state and a lengthening of the filament plasma column. Our experimental results and accompanying simulations suggest that rotational dynamics of diatomic molecules play an essential role in filamentation-induced supercontinuum generation, which can be controlled with polarization ellipticity.