Session W21: Optoelectronics & Photonics

Development and Test of a Travelling Wave Tube mm-wave Source

We report on the fabrication and test of a Traveling Wave Tube (TWT) amplifier designed for operation over a 40 GHz bandwidth centered on 220 GHz, and producing 50 W output power. The TWT amplifier uses a slow wave structure with staggered interdigitated vanes within a waveguide [1]. Each vane is 110 micron wide situated inside a 770 micron wide waveguide, and was directly machined into copper using a 100 micron wide end mill. This structure slows radiation down to group velocity of 8.16 x 10$^{7}$ ms$^{-1}$ where the velocity matches the speed of electrons from a 20 keV source. The TWT uses a sheet electron beam of 7:1 aspect ratio and 400 A/cm$^2$ charge density stabilized by a Brillouin flow magnetic field provided by an external permanent magnet. RF vacuum windows were designed and built using brazed diamond windows, providing less than 1 dB insertion loss across the full 40 GHz bandwidth. Solid state preamplifiers have been developed which provide 20 dB gain and 50 mW output power over the full bandwidth to the input of the TWT. \\[4pt] [1] Y-M. Shin {\&} L.R. Barnett, \textit{Appl.Phys. Lett. 2008,} 92 pp. 091501.   

Velocity-matching dispersion maps for zincblende and chalcopyrite terahertz sources

Pulsed terahertz radiation has been shown to be a useful diagnostic in fundamental and applied science. A common method for generating pulsed THz is by optical rectification. (110)-cut ZnGeP$_{2}$ was previously demonstrated as an efficient source of broadband THz radiation for near-infrared pump pulses [1], while other orientations have been modeled to show equal or greater efficiency [2]. Here we explore and compare phase-matching in ZnGeP$_{2}$ to that in other commonly used near-infrared THz sources including GaAs and GaP. We experimentally demonstrate that the three most efficient orientations provide distinct phase-matching configurations and thus distinct phase-matched near-infrared and THz frequencies. Our calculations also show that thin ($\sim$100 micron) crystals of ZGP may be promising sources for phase-matched and broadband THz emission out to 9 THz for 850 nm pump pulses. \\[4pt] [1] J. D. Rowley, J. K. Pierce, A. T. Brant, L. E. Halliburton, N. C. Giles, P. G. Schunemann, A. D. Bristow, Opt. Lett. 37, 788 (2012)\\[0pt] [2] J. D. Rowley, J. K. Wahlstrand, K. T. Zawailski, P. G. Schunemann, N. C. Giles, A. D. Bristow, Opt. Express 20, 16968 (2012)   

Modeling ultra-broadband terahertz waveguide emitters through difference frequency generation using coupled mode theory

We use a coupled mode theory that adequately incorporates both terahertz (THz) and infrared (IR) losses, to model and design ultra-broadband terahertz waveguide emitters (0.1-15 THz) based on difference frequency generation of femtosecond IR optical pulses. We apply the theory to generic, symmetric, five-layer, metal/cladding/core waveguides using transfer matrix theory. Our expressions for the conversion efficiency and output THz power spectrum depend on the pump power, pulse width, beam waists, laser repetition rate, material optical properties, and waveguide dimensions. Using this approach we design waveguides whose active cores are composed of a poled guest-host electro-optic polymer composite DAPC, comprised of DCDHF-6-V chromophores embedded in an amorphous polycarbonate matrix host. The resulting bandwidths are greater than 15 THz and we obtain high nonlinear conversion efficiencies up to $1.2\times 10^{-4}W^{-1}$. Our results reveal that a perfectly phase-matched structure is not necessarily the one with the highest conversion efficiency. The highest efficiency is obtained by balancing both the modal phase-matching and modal effective loss effects.   

Intense Nanosecond-Pulsed Cavity-Dumped Laser Radiation at 1.04 THz

We report first results of intense far-infrared (FIR) nanosecond-pulsed laser radiation at 1.04 THz from a previously described\footnote{T.E. Wilson, \textit{Proc. Int. Conf. Lasers '91}, (STS Press, McLean, VA), 762-767 (1992), and references therein.} cavity-dumped, optically-pumped molecular gas laser. The gain medium, methyl fluoride, is pumped by the 9R20 line of a TEA CO$_2$ laser\footnote{Cornelius T. Gross et al., \textit{IEEE J. QE}, \textbf{QE-23}, 377-387 (1987).} with a pulse energy of 200 mJ. The THz laser pulses contain of 30 kW peak power in 5 nanosecond pulse widths at a pulse repetition rate of 10 Hz. The line width, measured by a scanning metal-mesh FIR Fabry-Perot interferometer, is 100 MHz. The novel THz laser is being used in experiments to resonantly excite coherent ns-pulsed 1.04 THz longitudinal acoustic phonons in silicon doping-superlattices.   

Tunable terahertz detectors on GaAs substrates

Despite considerable research activitities in terahertz science and technologies, there has not been much progress in terahertz detectors. At present, the sensitivity of room temperature detector does not exceed 10$^{-9}$ W/(Hz)$^{1/2}$ in terms of noise equivalent power. Also most detectors are not tunable, and their response time is slow. In order to make terahertz technology practical substantial improvements should be made on the detector. Earlier, throughout research collaboration with UCSB, we have demonstrated a terahertz detector based on a Metal-Semiconductor Field Effect Transitor (MESFET) technology, which enabled us to achieve a high speed, tunable terahertz detector. The detector was tunable over the frequency range from 0.1 THz to 1.4 THz with a sensitivity of 10$^{-8}$ W/(Hz)$^{1/2}$. Recently we have attempted to modify this earlier design in order to improve its sensitivity up to 10$^{-11}$ W/(Hz)$^{1/2}$ and the operating frequency range from 0.07 to 2.5 THz. We employed a GaAs/AlGaAs heterostructure substrate, and drastically modified the previous MESFET design.We will present our fabrication process and experimental results.   

Semiconductor-core optical fibers for terahertz waveguides

Waveguiding of terahertz (THz) radiation is important for imaging and communications applications. Simulations have been performed based on a fiber optic geometric waveguide with a poly-crystalline silicon core and silica cladding [1]. High-resistivity silicon has a flat dispersion over a 0.1 -- 3 THz range [2], making it viable for propagation of broadband picosecond pulses of THz radiation such as that produced by optical rectification [3]. Frequency-dependent mode indices are determined for 0.1 -- 0.3 mm diameter cores. The normalized frequency parameter V is also determined and a 140 micron core is selected as the low edge of diameters that can support a THz pulse. Finite-difference time-domain simulations are performed in two-dimensions to extract the propagation dynamics and the integrated intensity, from which transverse mode profiles and absorption lengths are extracted. It is found that for this core diameter the mode partially propagates in the cladding, such that the overall absorbance is only slightly less than in bulk polycrystalline silicon. [1] J. Ballato, T. Hawkins, P. Foy, R. Stolen, B. Kokuoz, M. Ellison, C. McMillen, J. Reppert, A. M. Rao, M. Daw, S. R. Sharma, R. Shori, O. Stafsudd, R. R. Rice, and D. R. Powers, Opt. Express 16, 18675-18683 (2008) [2] D. Grischkowsky, S{\o}ren Keiding, Martin van Exter, Ch. Fattinger, J. Opt. Soc. Am. B 7, 2006 (1990) [3] J. D. Rowley, J. K. Pierce, A. T. Brant, L. E. Halliburton, N. C. Giles, P. G. Schunemann, A. D. Bristow, Opt. Lett. 37, 788 (2012)   

Simulations of Brillouin Scattering in Optical Fibers

Brillouin scattering arises when a laser beam generates density variations in a medium via electrostriction. The density variations modulate the refractive index, resulting in a grating that Bragg scatters pump light into a Stokes beam. The Stokes wave is downshifted in frequency by the Doppler effect because the grating is moving at the speed of acoustic phonons. To conserve both energy and momentum, the Brillouin photons are backscattered. This back-reflected radiation is a major factor limiting the transmission of laser power in optical fibers for practical applications. It is mathematically described by a set of coupled partial differential equations. I will describe some of the known analytic solutions of these equations, as well as how to find numeric solutions using MATLAB.   

Metamaterial features for a pure dielectric fiber

We consider a solid cylindrical dielectric waveguide with an extremely thin coaxial cylindrical shell of higher refraction index inserted on it. We calculate the propagation parameters and the band structure of this fiber as function of the contrast index, and show that there exist propagating modes whose transverse distribution of amplitudes are both oscillating and evanescent. The oscillating modes exhibit the usual dispersion relation of a standard wave guide, whereas the evanescent modes gives rise to regions for which the group velocity almost vanishes and with propagation direction opposed to the Poynting vector, as seen in metamaterials.   

Measurements of Effective Schottky Barrier in Inverse Extraordinary Optoconductance Structures

Individually addressable optical sensors with dimensions as low as 250nm, fabricated from metal semiconductor hybrid structures (MSH) of AuTi-GaAs Schottky interfaces, display a transition from resistance decreasing with intensity in micron-scale sensors (Extraordinary Optoconductance, EOC) to resistance increasing with intensity in nano-scale sensors (Inverse Extraordinary Optoconductance I-EOC). I-EOC is attributed to a ballistic to diffusive crossover with the introduction of photo-induced carriers and gives rise to resistance changes of up to 9462{\%} in 250nm devices. We characterize the photo-dependence of the effective Schottky barrier in EOC/I-EOC structures by the open circuit voltage and reverse bias resistance. Under illumination by a 5 mW, 632.8 nm HeNe laser, the barrier is negligible and the Ti-GaAs interface becomes Ohmic. Comparing the behavior of two devices, one with leads exposed, another with leads covered by an opaque epoxy, the variation in Voc with the position of the laser can be attributed to a photovoltaic effect of the lead metal and bulk GaAs. The resistance is unaffected by the photovoltaic offset of the leads, as indicated by the radial symmetry of 2-D resistance maps obtained by rastering a laser across EOC/IEOC devices.   

Light Intensity Influence on the Effective Schottky Barrier Height in Extraordinary Optoconductance (EOC) Structures

Novel micro to nanoscale metal-semiconductor-hybrid (MSH) structures capable of room temperature light detection have been previously reported and classified as Extraordinary Optoconductance (EOC) devices. The devices are square stacked structures, with a Au-Ti shunt forming a Schottky-Interface with an n-doped Ga-As mesa. Resistance measurements were taken by a 4-point van-der Pauw method to remove contact and lead resistance and eliminate DC offsets. The device's resistance changes as light incident on the surface of the structure modifies the charge density within the body of the device. The change in charge density changes the effective Schottky Barrier height and shifts the measured 4 point resistance of the heterogeneous structure. We investigate the dependence of the effective Schottky Barrier height on the incident intensity of light by measuring the open circuit voltage under various intensities of optical perturbation at room temperature. The barrier height is negligible and the interface ohmic under HeNe laser 632.8 nm illumination at a power density of 636 mW/cm$^{\mathrm{2}}$, allowing the flow of current through the shunt. This device performance will be contrasted with that of an FET, where current does not propagate through the gate.   

Proposal for realization of one-way electromagnetic modes at the interface of two lossless metals

One-way electromagnetic waveguides are of special interest because of complete suppression of back-scattering by disorder. Such waveguides support a unique class of photonic modes that completely forbid propagation in the opposite direction. We show that a one-way electromagnetic waveguide can be realized at the interface of two dissimilar lossless metals in an external magnetic field parallel to the interface. Electromagnetic surface plasmon modes bound to the interface of the two metals and propagating parallel to it and normal to the direction of the external magnetic field, with the electric field polarized normal to the plane of the interface, support one-way electromagnetic propagation in a range of frequencies. Increasing the magnetic field increases the window of frequencies for one-way propagation. Adding damping reduces the range of frequencies. Details of the calculation and plots showing the dispersion relation will be presented.   

Pulse Shaping with Moire Volume Bragg Gratings

Optical pulses of various temporal profiles are required for many applications but their durations and shapes are available only in limited ranges for certain laser wavelengths. For generation of pulse durations around ten ps, we proposed to reflect short pulses from volume Bragg gratings (VBGs) with few millimeter thickness. In case of VBG reflection bandwidth much narrower than incident pulse spectral width the significant loss of power occurs but such approach can be acceptable if there are no other generation processes for required pulse duration. VBGs in photo-thermo-refractive glass developed in our group are characterized by wide transparency range, small absorption and high laser damage threshold. In comparison with fiber Bragg gratings VBGs have additional spatial transverse degrees of freedom which allow not only tuning the pulse carrier wavelength but also shaping of generated pulses. Recording of two gratings with slightly different periods in the same glass wafer provides VBG with moire fringe pattern. After skew cutting of specimen with thickness of moire semi-period the longitudinal modulation VBG profile will vary in transverse direction from sine semi-period to cosine one. Reflected pulse from VBG with apodized sine profile has temporal profile close to transform limited Gaussian one while pulse reflected from transverse part of moire VBG with cosine semi-period profile has zero dip in temporal profile. At intermediate position the flat-top pulse shape is achievable.   

Monolithic Single-Mode DFB Laser Array with Precise Wavelength Control for Optoelectronic Integration using an Equivalent Phase Shift Method

The integrated distributed feedback (DFB) laser array is a key component in photonic integrated circuits for wavelength-division multiplexing (WDM) system. However, it is difficult to precisely control the wavelength of individual lasers. When the rear facet of the laser is coated with a high-reflectivity mirror, a random phase change is introduced that shifts the lasing wavelength, making monolithic integration of a wavelength-controlled WDM array very difficult. To solve this problem, we propose a method to precisely control the lasing wavelength of DFB lasers over a wide range by introducing an equivalent phase shift in the cavity using sampled Bragg gratings, using wafer-scale optical lithography and requiring only coarse dimension control. The wavelength can be fine-tuned by applying different DC currents. It is shown that a WDM-DFB laser array with uniform wavelength spacing can be controlled accurately in this manner. Integrated arrays of single-mode DFB lasers for WDM systems can thus be fabricated in a low-cost manner without using low-throughput e-beam lithography, and is scalable for mass-manufacturing.   

Observation of Polarization Switching in Vertical-Cavity Surface-Emitting Lasers at Constant Injection Current

This study investigated the thermal characteristics of the polarization switching (PS) in vertical-cavity surface-emitting lasers (VCSELs) at constant injection current. The experiments were performed with a quasi-step function current experiment. A simplified temperature rate equation was used to simulate the experiment of the step function. The consistency of the experiments and simulations concludes that the thermal effect plays a major role in PS and PS's hysteresis. These results contribute to the understanding of the mechanism of VCSEL's polarization switching.   

Development of an image-analysis light-scattering technique

We describe the progress in developing a versatile image-analysis approach for a light-scattering experiment. Recent advances in image analysis algorithms, computational power, and CCD image capture has allowed for the complete digital recording of the scattering of coherent laser light by a wide variety of samples. This digital record can then yield both static and dynamic information about the scattering events. Our approach is described using a very simple and in-expensive experimental arrangement for liquid samples. Calibration experiments were performed on aqueous suspensions of latex spheres having 0.5 and 1.0 micrometer diameter for three concentrations of 2 X 10$^{-6}$, 1 X 10$^{-6}$, and 5 X 10$^{-7}$ {\%} w/w at room temperature. The resulting data span a wave-vector range of q $=$ 10$^{2}$ to 10$^{5}$ cm$^{-1}$ and time averages over 0.05 to 1200 sec. The static analysis yield particle sizes in good agreement with expectations and a simple dynamic analysis yields an estimate of the characteristic time scale of the particle dynamics. Further developments in image corrections (laser stability, vibration, curvature, etc.) as well as time auto-correlation analysis will also be discussed.