Due to its high charge carrier mobility, broadband light absorption, and ultrafast carrier dynamics, graphene is a promising material for the development of high-performance photodetectors. Graphene-based photodetectors have been demonstrated to date using monolayer graphene operating in conjunction with either metals or semiconductors. Most graphene devices are fabricated on doped Si substrates with SiO2 dielectric used for back gating. Here, we demonstrate photodetection in graphene field effect phototransistors fabricated on undoped semiconductor (SiC) substrates. The photodetection mechanism relies on the high sensitivity of the graphene conductivity to the local change in the electric field that can result from the photo-excited charge carriers produced in the back-gated semiconductor substrate. We also modeled the device and simulated its operation using the finite element method to validate the existence of the field-induced photoresponse mechanism and study its properties. Our graphene phototransistor possesses a room-temperature photoresponsivity as high as ~7.4 A/W, which is higher than the required photoresponsivity (1 A/W) in most practical applications. The light power-dependent photocurrent and photoresponsivity can be tuned by the source-drain bias voltage and back-gate voltage. Graphene phototransistors based on this simple and generic architecture can be fabricated by depositing graphene on a variety of undoped substrates, and are attractive for many applications in which photodetection or radiation detection is sought.
We exploit the dependence of the electrical conductivity of graphene on a local electric field, which can be abruptly
changed by charge carriers generated by ionizing radiation in an absorber material, to develop novel highperformance
radiation sensors for detection of photons and other kinds of ionizing radiation. This new detection
concept is implemented by configuring graphene as a field effect transistor (FET) on a radiation-absorbing undoped
semiconductor substrate and applying a gate voltage across the sensor to drift charge carriers created by incident
photons to the neighborhood of graphene, which gives rise to local electric field perturbations that change graphene
resistance. Promising results have been obtained with CVD graphene FETs fabricated on various semiconductor
substrates that have different bandgaps and stopping powers to address different application regimes. In particular,
graphene FETs made on SiC have exhibited a ~200% increase in graphene resistance at a gate voltage of 50 V when
exposed to room light at room temperature. Systematic studies have proven that the observed response is a field
A novel quantum phase amplification (QPA) technique is proposed to enhance the spatial resolution of passive
imaging systems, without the commensurate increase of the aperture size. The key component of this approach
is the production of phase-amplified light, a squeezed state of the electromagnetic field that can be generated in
phase-sensitive three-wave mixing (PSTWM). We have established the theoretical basis for QPA using PSTWM
and theoretically derived the conditions for the operation of the ubiquitous PSTWM realized by using standard
laser technology. In our approach sub-Rayleigh imaging can be explored to perform the super-resolution enhancement
of remotely sensed data. We applied the PSTWM to improve the distinguishability of two point sources
in several scenarios. In addition, detector segmentation was modeled to optimize the signal-to-noise ratio.
We present a study of the effects of electron-beam irradiation on the Raman spectra and electronic transport
properties of graphene and the operation of graphene field-effect transistors (GFET). Exposure to a 30 keV electronbeam
causes negative shifts in the charge-neutral point (CNP) of the GFET, interpreted as due to n-doping in the
graphene from the interaction of the energetic electron beam with the substrate. The electron beam is seen to also
decrease the carrier mobilities and minimum conductivity of the graphene, as well as increase the intensity of the
Raman D peak, all of which indicate defects generated in the graphene. We also study the relaxation of electronic
properties after irradiation. The findings are valuable for understanding the effects of radiation damage on graphene
and for the development of radiation-hard graphene-based electronics.
The average power and efficiency of processes that exhibit low interaction cross section and low optical loss can often be enhanced by recirculating the laser pulse in the cavity. Inverse Compton scattering of the photon pulse on an electron bunch, harmonic generation, and spectroscopy represent examples of such processes. Methods for laser recirculation that enhance the interaction efficiency have been proposed in the past, based on resonant cavity coupling, intracavity amplification, or electro-optical switching. Those methods exhibit limitations such as interferometric alignment accuracies, complexity, and nonlinear phase accumulation. A novel scheme for energetic short laser pulse recirculation, termed recirculation injection by nonlinear gating (RING), is described. RING is based on intracavity nonlinear frequency conversion for optical switching, does not exhibit interferometric alignment constraints, and is scalable to extreme peak power. Initial demonstration of the RING technique is presented at a 1-mJ level, with cavity enhancement factors exceeding 25 in a simple unstable resonator cavity. Applications of the RING technique in biomedical and other applications are outlined.
We report on the design and construction of the Texas Petawatt Laser. This research facility will consist of two, synchronized laser systems that will be used for a wide variety of high intensity laser and high energy density science experiments. The first laser is a novel, high energy (200 J), short pulse (150 fs) petawatt-class laser that is based on hybrid, broadband optical parametric chirped pulse amplification (OPCPA) and mixed silicate and phosphate Nd:glass amplification. The second laser will provide 500 J at 527 nm (>1 kJ @1053 nm) with pulse widths selectable from 2-20 ns. Design and construction began in early 2003 and is scheduled to complete in 2007. In this report we will briefly discuss some of the important applications of this system, present the design of the laser and review some of the technology used to achieve pulse durations approaching 100 fs. Currently, the facility has been renovated for laser construction. The oscillator and stretcher are operational with the first stage of gain measured at 2×106. Output energies of 500μJ have been achieved with good near field image quality. Delivery has been taken for Nova components that will compose the main amplifier chain of the laser system.
To enable high-energy petawatt laser operation we have developed the processing methods and tooling that produced both the world's largest multilayer dielectric reflection grating and the world's highest laser damage resistant gratings. We have successfully delivered the first ever 80 cm aperture multilayer dielectric grating to LLNL's Titan Intense Short Pulse Laser Facility. We report on the design, fabrication and characterization of multilayer dielectric diffraction gratings.
We are developing an all fiber laser system optimized for providing input pulses for short pulse (1-10ps), high energy (~1kJ) glass laser systems. Fiber lasers are ideal solutions for these systems as they are highly reliable and once constructed they can be operated with ease. Furthermore, they offer an additional benefit of significantly reduced footprint. In most labs containing equivalent bulk laser systems, the system occupies two 4’x8’ tables and would consist of 10's if not a 100 of optics which would need to be individually aligned and maintained. The design requirements for this application are very different those commonly seen in fiber lasers. High energy lasers often have low repetition rates (as low as one pulse every few hours) and thus high average power and efficiency are of little practical value. What is of high value is pulse energy, high signal to noise ratio (expressed as pre-pulse contrast), good beam quality, consistent output parameters and timing. Our system focuses on maximizing these parameters sometimes at the expense of efficient operation or average power. Our prototype system consists of a mode-locked fiber laser, a compressed pulse fiber amplifier, a “pulse cleaner”, a chirped fiber Bragg grating, pulse selectors, a transport fiber system and a large flattened mode fiber amplifier. In our talk we will review the system in detail and present theoretical and experimental studies of critical components. We will also present experimental results from the integrated system.
The next generation of high-energy petawatt (HEPW)-class lasers will utilize multilayer dielectric diffraction gratings for pulse compression due to their high efficiency and high damage threshold for picosecond pulses. We have developed a short-pulse damage test station for accurate determination of the damage threshold of the optics used on future HEPW lasers. The design and performance of the damage test laser source, based on a highly stable, high-beam-quality optical parametric chirped-pulse amplifier, is presented. Our short-pulse damage measurement methodology and results are discussed. The damage initiation is attributed to multiphoton-induced avalanche ionization, strongly dependent on the electric field enhancement in the grating groove structure and surface defects. Measurement results of the dependence of damage threshold on the pulse width, angular dependence of damage threshold of diffraction gratings, and an investigation of short-pulse conditioning effects are presented. We report record >4 J/cm2 right section surface damage thresholds obtained on multilayer dielectric diffraction gratings at 76.5° incidence angles for 10-ps pulses.
Continuous wave (CW) fiber laser systems with output powers in excess of 500 W with good beam quality have now been demonstrated, as have high energy, short pulse, fiber laser systems with output energies in excess of 1 mJ. Fiber laser systems are attractive for many applications because they offer the promise of high efficiency, compact, robust systems. We have investigated fiber lasers for a number of applications requiring high average power and/or pulse energy with good beam quality at a variety of wavelengths. This has led to the development of a number of custom and unique fiber lasers. These include a short pulse, large bandwidth Yb fiber laser for use as a front end for petawatt class laser systems and a narrow bandwidth 0.938 μm output Nd fiber laser in the > 10 W power range.
We have developed and demonstrated a large flattened mode (LFM) optical fiber, which raises the threshold for non-linear interactions in the fiber core by a factor of 2.5 over conventional large mode area fiber amplifiers. The LFM fiber works by incorporating a raised index ring around the outer edge of the fiber core, which serves to flatten the fundamental fiber mode from a Bessel function to a top hat function. This increases the effective area of the core intersected by the mode by a factor of 2.5 without increasing the physical size of the core. This is because the core is uniformly illuminated by the LFM mode rather than having most of the light confined to the center of the core. We present experimental and theoretical results relating to this fiber and its design.
Optical parametric chirped pulse amplification (OPCPA) is a scalable technology for ultrashort pulse amplification. Its major advantages include design simplicity, broad bandwidth, tunability, low B-integral, high contrast, and high beam quality. OPCPA is suitable both for scaling to high peak power as well as high average power. We describe the amplification of stretched 100 fs oscillator pulses in a three-stage OPCPA system pumped by a commercial, single- longitudinal-mode, Q-switched Nd:YAG laser. The stretched pulses were centered around 1054 nm with a FWHm bandwidth of 16.5 nm and had an energy of 0.5nJ. Using our OPCPA system, we obtained an amplified pulse energy of up to 31 mJ at a 10 Hz repetition rate. The overall conversion efficiency from pump to signal is 6%, which is the highest efficiency obtained with a commercial tabletop pump laser to date. The overall conversion efficiency is limited due to the finite temporal overlap of the seed (3 ns) with respect to the duration of the pump (8.5 ns). Within the temporal window of the seed pulse the pump to signal conversion efficiency exceeds 20%. Recompression of the amplified signal was demonstrated to 310 fs, limited by the aberrations initially present in the low energy seed imparted by the pulse stretcher. The maximum gain in our OPCPA system is 6x107, obtained through single passing of 40 mm of beta- barium borate. We present data on the beam quality obtained from our system (M2=1.1). This relatively simple system replaces a significantly more complex Ti:sapphire regenerative amplifier-based CPA system used in the front end of a high energy short pulse laser. Future improvement will include obtaining shorter amplified pulses and higher average power.