Gallium nitride’s (GaN) material properties of broadband transparency, high thermal conductivity, and wide-band gap make it a promising candidate for high-power frequency conversion devices. The strong internal polarization of GaN leads to large second-order nonlinearity, but conventional phase matching is prevented due to weak birefringence. To obtain efficient nonlinear optic frequency conversion, patterned inversion growth has been developed to induce quasiphase matching (QPM). We have fabricated and tested periodically oriented GaN (PO-GaN) devices to obtain QPM frequency conversion. This report discusses our recent measurements of second harmonic generation resonances for these devices.
Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho<sup>3+</sup> emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.<sup>1</sup>
A theoretical model for optical cooling is developed, which yields the overall efficiency of a single endpumped cooling system. This model includes the effects of background absorption and pump saturation, while in multi-level systems, the model accounts for the important energy transfer processes. Two-level efficiency is evaluated for the case of Yb:YAG and compared with a hypothetical three-level material with identical spectral properties. This model is readily modified to include more levels and different materials.
Laser Induced Breakdown Spectroscopy (LIBS) by self-channeled femtosecond pulses is characterized for detection
of energetic materials. Different polymers are spin coated on silicon wafers to provide a thin organic layer with
controllable thickness ranging from 500 nm to 1 μm. Spectral analysis of atomic and molecular carbon emission
shows CN molecular signal from samples that do not contain nitrogen. This can be explained by possible molecular
recombination between native atomic carbon and atmospheric nitrogen. As a consequence, caution must be
exercised when using spectral signatures based on CN emission for explosive detection by filament-induced LIBS.
This study makes a comparison of LIBS emission from molecular species in plasmas produced from organic
residues on a non-metallic substrate by both a 5 ns Nd:YAG laser (1064 nm) and a 40 fs Ti:Sapphire laser (800 nm)
in air and argon atmospheres. The organic samples analyzed had varying amounts of carbon, nitrogen, hydrogen,
and oxygen in their molecular structure. The characterization was based on the atomic carbon, hydrogen, nitrogen,
and oxygen lines as well as the diatomic species CN (B<sup>2</sup>Σ<sup>+</sup> - X<sup>2</sup>Σ<sup>+</sup>) and the C<sub>2</sub> (d<sup>3</sup>Π<sub>g</sub> - a<sup>3</sup>Π<sub>u</sub>). Principal Component
Analysis (PCA) was used to identify similarities of the organic analyte via the emission spectra. The corresponding
Receiver Operating Characteristics (ROC) curves show the limitations of the PCA model for the nanosecond regime
As an alternative to focusing nanosecond pulses for stand-off LIBS detection of energetic materials, we use
self-channeled femtosecond pulses from a Ti:Sapphire laser to produce filaments at 12 meters and create a plasma on
copper, graphite and polyisobutylene film. We show the possibilities of this Laser-Induced Breakdown Spectroscopy
configuration for thin organic sample detection on a surface at a distance.
In this work we present the status of our high repetition-rate/high power EUV source facility. The masslimited
target concept has demonstrated high conversion efficiencies (CE) previously, with precision solid
state lasers. Currently, experiments are in progress with high power high repetition-rate (3-4 kHz) Qswitched
laser modules. We present a new dedicated facility for the high power EUV source. Also, we
present a precision EUV energy-meter, which is developed for absolute EUV energy measurements.
Spectral measurements of the tin-doped droplet laser plasma are performed with a flat-field spectrometer
(FFS) with a back-illuminated CCD camera. We address the issue of maintaining the calibration of the
EUV optics during source operation at non-optimum intensity at high repetition-rate, which is required for
CE improvement studies. Here we present the unique metrology for measuring EUV energies under a
variety of irradiation conditions without degrading EUV optics, even at high repetition rates (multi-kHz).
Tin-doped droplet target has been integrated with several lasers including high power high repetition rate lasers
and demonstrated high conversion efficiencies for all the lasers. This implies the EUV source power is linearly
increasing as the laser frequency goes higher. The target exhibit very low out-of-band radiation and debris emission.
The drawback of increasing the repetition rate of the target and the laser will be limited. The total amount
of tin consumed for a EUVL source system is also small enough to be operated for a long term without large effort
for recycling of the target materials. We address and demonstrate in this paper the primary issues associated
with long-term high power EUV sources for high volume manufacturing (HVM) using tin-doped droplet target.
Laser interactions with bulk transparent media have long been investigated for material processing applications involving ablation and shock wave generation in both the nanosecond and femtosecond pulse width regimes<sup>1</sup>. Shock waves have been studied in fused silica and other optical glasses but previously have been characterized by the morphology of the concurrent ablation. We perform ablation at distances of 30 meters using the non-linear self-channeling effect. Using silicon wafers as targets because of their clearly defined ablation zones, we examine the effect that the filament has on the thin SiO<sub>2</sub> layer coating the wafer's surface. It is observed that the surface layer experiences a shock wave resulting from the explosive forces produced by the plasma. The use of several laser pulses in burst mode operation leads to the observation of multiple shock fronts in the material, and the possibility of shock wave addition for higher damage. Optical interferometry will be used to characterize the shock wave dynamics, using both traditional means of focusing in the near field and at 30 meters using propagating self-channeled femtosecond pulses. The novelty of using self-channeling laser pulses for shock wave generation has many implications for military applications. These experiments are to be performed in our secure test range using intensities of 10<sup>14</sup>W/cm<sup>2</sup> and higher incident on various transparent media. Interferometry is performed using a harmonic of the pump laser frequency. Experiments also include burst-mode operation, where a train of ultra-fast pulses, closely spaced in time, and novel new beam distributions, strike the sample.
Plasmas produced by laser-matter interactions are a known source of electromagnetic radiation. However, little has been done to systematically study the electromagnetic radiation emitted from laser produced plasmas. It is our intent to provide detailed time and frequency domain measurements of such emitted radiation. An ultra-fast femtosecond high intensity laser and a superheterodyne receiver are employed to study laser-matter interactions for various materials in the frequency range 1-40GHz.
The need for robust, versatile, and rapid analysis standoff detection systems has emerged in response to the increasing threat to homeland security. Laser Induced Breakdown Spectroscopy (LIBS) has emerged as a novel technique that not only resolves issues of versatility, and rapid analysis, but also allows detection in settings not currently possible with existing methods. Several studies have shown that femtosecond lasers may have advantages over nanosecond lasers for LIBS analysis in terms of SNR. Furthermore, since femtosecond pulses can travel through the atmosphere as a self-propagating transient waveguide, they may have advantages over conventional stand-off LIBS approaches<sup>1</sup>. Utilizing single and multiple femtosecond pulse laser regimes, we investigate the potential of femtosecond LIBS as a standoff detection technology. We examine the character of UV and visible LIBS from various targets of defense and homeland security interest created by channeled femtosecond laser beams over distances of 30m or more.