We demonstrate frequency domain THz anisotropy signature detection for protein crystal models using newly developed compact tunable narrow band THz sources based on Orientation Patterned Gallium Phosphide for turn-key spectroscopic systems.
We demonstrate tunable narrowband THz generation by optical rectification of a femtosecond pulse in Orientation Patterned Gallium Phosphide. Center frequencies of 0.9 - 3.8 THz with average power up to 15 μW were achieved using a 1.064 µm fiber laser for the pump laser. Biomolecular characterization for an early application of this system is also shown in this work by anisotropic spectroscopic signature detection of molecular crystals in the THz region.
We demonstrate video rate THz imaging in both reflection and transmission by frequency upconverting the THz image to the near-IR. In reflection, the ability to resolve images generated at different depths is shown. By mixing the THz pulses with a portion of the fiber laser pump (1064 nm) in a quasi-phase matched gallium arsenide crystal, distinct sidebands are observed at 1058 nm and 1070 nm, corresponding to sum and difference frequency generation of the pump pulse with the THz pulse. By using a polarizer and long pass filter, the strong pump light can be removed, leaving a nearly background free signal at 1070 nm. We have obtained video rate images with spatial resolution of 1mm and field of view ca. 20 mm in diameter without any post processing of the data.
We demonstrate video rate THz imaging by detecting a frequency upconverted signal with a CMOS camera. A fiber laser pumped, double resonant optical parametric oscillator generates THz pulses via difference frequency generation in a quasi-phasematched gallium arsenide (QPM-GaAs) crystal located inside the OPO cavity. The output produced THz pulses centered at 1.5 THz, with an average power up to 1 mW, a linewidth of <100 GHz, and peak power of >2 W. By mixing the THz pulses with a portion of the fiber laser pump (1064 nm) in a second QPM-GaAs crystal, distinct sidebands are observed at 1058 nm and 1070 nm, corresponding to sum and difference frequency generation of the pump pule with the THz pulse. By using a polarizer and long pass filter, the strong pump light can be removed, leaving a nearly background free signal at 1070 nm. For imaging, a Fourier imaging geometry is used, with the object illuminated by the THz beam located one focal length from the GaAs crystal. The spatial Fourier transform is upconverted with a large diameter pump beam, after which a second lens inverse transforms the upconverted spatial components, and the image is detected with a CMOS camera. We have obtained video rate images with spatial resolution of 1mm and field of view ca. 20 mm in diameter without any post processing of the data.
sonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via
difference frequency generation (DFG). A nearly degenerate optical parametric oscillator (OPO) was pumped by 6 ps
pulses at 1064 nm, producing signal and idler pulses with average total power in excess of 80 W. By placing a sample of
quasi-phasematched gallium arsenide (QPM-GaAs) at a focus of a ring cavity OPO, multicycle, narrowband THz
radiation was produced, with average powers in excess of 100 μW and peak powers exceeding 150 mW. The
dependence of the THz power on pump power shows no signs of saturation, so with higher power pump lasers, mW
levels of average THz should be obtainable.
KEYWORDS: Terahertz radiation, Waveguides, Extremely high frequency, Data conversion, Solid state electronics, Diodes, Sensors, Imaging spectroscopy, Free space, Frequency conversion
Solid state frequency multipliers extend the operating frequency range of Backward Wave Oscillators
(BWOs) to 2.6 THz, enabling continuously tunable, narrow linewidth THz sources across the 0.1-2.6 THz range.
Power conversion efficiency of frequency multipliers can be improved substantially by optimizing impedance
matching between millimeter wave BWOs and frequency multipliers. Performance of sub-millimeter wave BWOs
combined with frequency multipliers is limited by multi-mode output of these BWOs and lower power conversion
efficiency of solid state multipliers operating above 1 THz.
Ultrafast picosecond lasers provide the gentle cold ablation required to selectively remove a 400 nm metal film from
an unsupported ultra-thin polymer membrane without damaging the membrane substrate. Selected areas of the
metal film are completely removed in an ablative lift-off process enabled by a single laser pulse. No damage to the
polymer membrane is observed even for samples with the metal completely removed over a 50x50 mm area of the
membrane. The 400 nm thick metal films can be patterned into arbitrary forms with feature sizes as small as 10
micrometers, and even submicron features are realistically possible with a modification to the processing system.
The skin depth of aluminium in the THz regime is significantly shorter than the 400 nm metal thickness, so thicker
metal films that are significantly more difficult to machine are not beneficial. As an example, thin-film wire grid
polarizers for the THz regime are demonstrated. The thin-film polarizers are much easier and faster to fabricate than
polarizers made by winding free-standing wires around a frame and their performance is very comparable. The thin-film
polarizers also have the added benefit of a significantly higher potential for functionality deeper in to the THz
spectrum due to their capacity for smaller feature sizes. More intricate patterns, such as meshes, can also be made to
create THz bandpass filters. This method can be extended to cold ablation processing of multilayer films fabricated
on thin polymer substrates for applications such as plastic electronics, displays and solar cells.
Resonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via
frequency down conversion. Efficient THz wave generation was demonstrated at 2.8 THz previously by difference
frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical
parametric oscillator (OPO). A new, simplified approach to resonantly-enhanced THz-wave generation in periodic GaAs,
featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-II-phase-matched
PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond 1064-nm fiber laser
as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the desired THz output
frequency is presented here. Intra-cavity THz generation was performed by 2.1 μm anti-reflection coated stacks of
optically contacted GaAs wafers (OC-GaAs) and diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion
placed in the second OPO focal plane. Using 6.6 W of average pump power, narrowband output in the range 1.4 - 3 THz
was produced with more than 130 microwatts of average power at 1.5 THz. By optimizing the OPO PPLN crystal length
and spectral characteristics of the fiber pump laser and OPO the demonstrated approach can be extended to generate 1-10
mW of THz output in a compact setup.
The operating frequency range of millimeter wave Backward Wave Oscillators (BWOs) are extended to 2.2
THz using solid state frequency multipliers, enabling continuously tunable, narrow linewidth THz sources across the 0.1-2.2 THz range. Power conversion efficiency of frequency multipliers was improved substantially by optimizing
impedance matching between the BWOs, multipliers, and free space. Impedance matching extracts more THz power out
of BWOs coupled to frequency multipliers, than is emitted from BWOs radiating into free space.
KEYWORDS: Terahertz radiation, Data conversion, Free space, Waveguides, Sensors, Extremely high frequency, Diodes, Imaging spectroscopy, Frequency conversion, Electron beams
Solid state frequency multipliers extend the operating frequency range of millimeter wave Backward
Wave Oscillators (BWOs) to 2.2 THz, enabling continuously tunable, narrow linewidth THz sources across the 0.1-
2.2 THz range. Power conversion efficiency of frequency multipliers can be improved substantially by optimizing
impedance matching between the BWOs and multipliers. This impedance matching helps to extract more THz
power out of BWOs coupled to frequency multipliers, than is emitted from BWOs radiating into free space.
Substantial improvement in the efficiency of photonic THz-wave generation via frequency downconversion results from
resonant cavity enhancement. Previously, efficient THz wave generation was demonstrated at 2.8 THz by difference
frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical
parametric oscillator (OPO). We present a new, simplified approach to resonantly-enhanced THz-wave generation in
periodic GaAs, featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-IIphase-
matched PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond
1064-nm fiber laser as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the
desired THz output frequency. 2.1 μm anti-reflection coated stacks of optically contacted GaAs wafers (OC-GaAs) and
diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion were placed in the second OPO focal plane for intracavity
THz generation. Narrowband output in the range 1.4 - 3 THz was produced with more than 130 microwatts of
average power at 1.5 THz using 6.6 W of average pump power. The demonstrated approach can be extended to generate
1-10 mW of THz output in a compact setup by optimizing the OPO PPLN crystal length and optimizing spectral
characteristics of the fiber pump laser and OPO.
We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium
arsenide (GaAs). We used three types of micro-structured GaAs to quasi-phase-match the interaction: optically
contacted, orientation-patterned, and diffusion-bonded GaAs. The room-temperature GaAs was placed in an optical
parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO
signal and idler waves. 250-GHz-bandwidth radiation was generated with frequencies spanning 0.4-3.5 THz. We
measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant
oscillator (DRO) configuration, the efficiency increased by 21 times over the singly resonant oscillator (SRO)
performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW.
Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because
of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch
between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient.
Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses.
Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched
GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate
picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.
Narrow-band, multi-cycle terahertz (THz) pulses have been generated in the pre-engineered domain structure
of periodically-poled lithium niobate (PPLN) crystals. The mechanism for THz generation is quasi-phase-matching
(QPM) optical rectification. Recently, THz generation of high conversion efficiency in a new material,
QPM GaAs, were demonstrated using mid-IR femtosecond pulses. GaAs has several advantages for QPM
THz wave generation, as compared to PPLN. First, it is highly transparent at THz frequencies (absorption
coefficient below 1.5 THz < 1 cm-1). Second, the mismatch between the optical group velocity and THz
phase velocity is much smaller: the corresponding group (ng) and refractive (n) indices are ng=3.431 at 2&mgr;m
and n=3.61 at 1 THz. In this work, we report on generation of THz wave packets in three different types of
QPM GaAs, combined with their coherent detection using two-color THz time-domain spectroscopy. The QPM
GaAs structures are optically-contacted GaAs, diffusion-bonded GaAs, and all-epitaxially-grown orientation patterned
GaAs. The QPM optical rectification in GaAs is a nonresonant mechanism, as opposed to widely used
photoconductive antenna technique in GaAs, where THz radiation is produced via ultrafast charge transport
caused by photoexcitation with femtosecond laser pulses of the near-IR range. In order to avoid linear and
two-photon absorption in GaAs, we use 2&mgr;m femtosecond pulses to generate THz pulses. We measure the THz
waveforms via electro-optic sampling in ZnTe using 0.8&mgr;m probe pulses. The corresponding power spectra are
also measured by a THz Michelson interferometer. Frequency tunability in the range 0.8-3 THz is achieved with
several structure periods.
We report a new highly efficient source of frequency-tunable (0.5-3.5 THz) narrow-bandwidth terahertz wave packets with up to 1 mW average power, based on parametric down-conversion in quasi-phase-matched GaAs. Different lasers were employed as a pump source, including femtosecond OPA/DFG system (wavelength range 2-4μm), Tm-fiber femtosecond laser (wavelength ~2μm), and near-degenerate synchronously-pumped picosecond OPO system with extra- and intracavity THz generation. We prove experimentally that the optical-to-terahertz conversion efficiency is fluence-dependent, with the scaling factor being the same for femtosecond (optical rectification) and picosecond (difference frequency generation) pump pulses, with optical-to-terahertz conversion efficiency on the order of 0.1% per μJ.
We demonstrate an efficient room temperature source of narrow-bandwidth terahertz (THz) radiation using femtosecond pump pulses and periodic GaAs structure as a nonlinear material. In the past, several THz generation schemes exploited optical rectification in nonlinear crystals using femtosecond laser technology. Most of them generated single-cycle THz-pulses with broad bandwidth, using nonlinear crystals shorter than the phase-matching coherence length. Recently a novel technique to generate multi-cycle THz-pulses in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals has been demonstrated. Quasi-phase matching (QPM) structures such as PPLN consist of a periodic system of domains of inverted crystal orientation. The sign of second order nonlinear polarization generated by femtosecond pulses is inverted at domain boundaries. If domain length is comparable with coherence length, QPM between THz-wave and nonlinear polarization extends the
interaction length between THz and optical pulses. In the present
work, using periodic GaAs structures we have achieved exceptionally high photon as well as energy conversion efficiency: 3% and 0.07% respectively. We have examined two different types of periodic QPM GaAs samples: diffusion-bonded GaAs wafers and all-epitaxially-grown orientation-patterned GaAs crystals with 3-10 mm thicknesses. The incident optical pulse energy was in the micro-Joule range and pulse duration was ~100 fsec. We measured spectral properties of THz radiation using Michelson interferometer and a bolometer. Narrow-bandwidth (~100GHz) THz output, tunable between 1 and 3 THz, was achieved. THz frequency was tuned either by tuning the light source wavelength between 2 and 4.4 microns, or by selecting GaAs samples with different QPM periods. Our theoretical analysis, based on known GaAs dispersion properties, shows good agreement between the measured and predicted THz frequencies.
We review recent results form two types of small molecule organic light emitting devices (OLEDs). For flat panel display applications, we have developed a novel OLED pixel in which the R, G and B emission layers are vertically stacked to provide a simple fabrication process, minimum pixel size, and maximum fill factor. In separate experiments, we have worked towards achieving electrically- pumped organic lasers by demonstrating low-threshold lasing in an optically-pumped thin film double heterostructure consisting entirely of organic semiconducting materials.
Spectroscopic work is reviewed which focuses on the microscopic mechanism of gain in ZnSe-based quantum well (QW) lasers, under optical and electrical injection, respectively. Excitonic processes are rather distinct at cryogenic temperatures in the strongly quasi-2 dimensional case of a ZnCdSe QW. More strikingly, recent studies on the room temperature diode lasers show that electron-hole pairwise Coulomb correlations remain relevant in this case as well.
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