We compare terahertz-pulsed imaging (TPI) with transverse microradiography (TMR) and microindentation to measure remineralization of artificial caries lesions. Lesions are formed in bovine enamel using a solution of 0.1 M lactic acid/0.2% Carbopol C907 and 50% saturated with hydroxyapatite adjusted to pH 5.0. The 20-day experimental protocol consists of four 1 min treatment periods with dentifrices containing 10, 675, 1385, and 2700 ppm fluoride, a 4-h/day acid challenge, and, for the remaining time, specimens are stored in a 50:50 pooled human/artificial saliva mixture. Each specimen is imaged at the focal point of the terahertz beam (data-point spacing = 50 μm). The time-domain data are used to calculate the refractive index volume percent profile throughout the lesion, and the differences in the integrated areas between the baseline and post-treatment profiles are used to calculate ΔΔZ(THz). In addition, the change from baseline in both the lesion depth and the intensity of the reflected pulse from the air/enamel interface is determined. Statistically significant Pearson correlation coefficients are observed between TPI and TMR/microindentation (P < 0.05). We demonstrate that TPI has potential as a research tool for hard tissue imaging.
The aim of this study was to determine the ability of Terahertz Pulsed Imaging (TPI) to measure mineral changes in enamel lesions during de/remineralisation studies. A comparison was made between transverse microradiography (TMR) and microhardness measurements. Artificial lesions were formed in bovine enamel using a solution of 0.1 M lactic acid (pH 5.0) containing 0.2% Carbopol C907 and 50% saturated with hydroxyapatite. The 20 day experimental protocol consisted of four, one-minute treatment periods with dentifrices containing 10, 675, 1385 and 2700ppm fluoride, a 4 h/day acid challenge, and for the remaining time specimens were stored in a 50:50 pooled human / artificial saliva mixture. Terahertz images were generated by positioning the specimens at the focus of the beam and raster scanning the optics to collect the reflections from the air / enamel (AEI) and lesion / enamel (LEI) interface. Significant differences were observed in the intensity change from baseline of the AEI and LEI reflections upon treatment with the four dentifrices. A linear correlation was observed between ΔAEI vs ΔVHN (r2 = 0.997), ΔAEI vs ΔKHN (r2 =0.964), ΔII (ratio of LEI to AEI) vs ΔΔZ (r2 =0.875) and ΔLEI vs ΔΔZ (r2 =0.870). Statistically significant correlations (p<0.05 Pearson correlation coefficient) were also found between the TPI and microhardness / microradiography data. This study has demonstrated that TPI is a useful technology to measure in vitro (and possibly in situ) mineral changes in enamel and is sufficiently sensitive to discriminate between the levels of remineralization produced by the different dentifrices.
We report on the fabrication of THz quantum cascade lasers (QCLs) based on different optical waveguide
configurations and compare the thermal properties of THz devices fabricated with metal-metal optical waveguides based
on Au/Au or In/Au wafer bonding. In particular, we show how the careful choice of the metal sequence used for the
reactive bonding may lead to a considerable improvement of the device thermal performance. This information was
obtained from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in
continuous wave (cw). The experimental normalized thermal resistances (RL*), show that the use of Au/Au wafer
bonding optimizes the heat dissipation. An extensive comparison with a set of surface-plasmon based THz QCLs,
demonstrate that the use of metal-metal wafer bonding can allow cw operation at progressively higher temperatures.
Finally, we present the experimental results obtained on a bound-to-continuum QCLs (2.84 THz) emitting 77 mW peak
power at 4K, fabricated from an MBE wafers acquired by a commercial provider.
We report the operation of a 2 THz quantum cascade laser based on a GaAs/Al0.1Ga0.9As heterostructure. Lasing action takes place between an isolated subband and the upper state of a 14 meV wide miniband. In pulsed mode, with a 3.16mm long device, we report a threshold current density of 115 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device shows lasing action in continuous wave up to 47K, with a maximum power in excess of 15 mW at T = 4K.
In this work we demonstrate for the first time that terahertz (THz) quantum cascade lasers can be realised in a buried-waveguide geometry. In our prototype devices the optical mode is a surface plasmon bound at the interface between the top contact and the semiconductor, providing for both vertical and lateral confinement without the need to define a cavity ridge. Proton-implanted high-resistivity sections are used to define the current channel where electrons can be injected into the active region. This way the electrical and optical confinement can be controlled independently: the former is defined by the non-implanted regions and the latter by the width of the top contact metal strip. Compared to standard ridge waveguides this technique allowed for a narrowing of the device effective cross section without introducing additional losses and improving the thermal conductivity, resulting in an increase of the maximum operating temperature up to 77K in continuous wave at 2.9 THz. In addition, we present preliminary results from buried-waveguide THz quantum cascade lasers obtained by combining a double-metal waveguide geometry with proton implantation. Initial results are promising, yielding record low threshold currents of 19mA at 4.2K in both pulsed and continuous wave operation.
In this work we have investigated the operation of a recently demonstrated bound-to-continuum quantum cascade laser emitting at 2.9 THz under different active region doping densities. In addition, we have studied the injection efficiency as a function of the thickness of the Al0.15Ga0.85As barrier controlling the tunnel coupling between the superlattice miniband and the upper state of the laser transition. By tuning these two parameters threshold current densities as low as 52 A/cm2 and 83 A/cm2 were obtained at 5 K, corresponding to a reduction over the base design of 55% and 25% respectively. In both cases we attribute the improved threshold performance to a reduction of the parasitic leakage current. The decrease in threshold was always at the expense of a smaller laser dynamic operating range, which ultimately limited maximum operating temperatures in pulsed and continuous wave mode to 95 K and 80 K respectively.
We demonstrate the operation of a superlattice GaAs/AlGaAs quantum cascade laser emitting at λ = 103 μm. The maximum operating temperature is 95K in pulsed mode and 70K in continuous wave. At 4K, we measured a peak output power in the tens of mW range and a threshold current density of 110 A/cm2 (300 A/cm2 at 90K). We attribute this excellent performance to a low ratio between the lower and upper state lifetimes, as well as to a low leakage current. These characteristics are highlighted by a pronounced decrease of the differential resistance at threshold and by the fact that the slope efficiency remains constant up to approximately 70K. At any temperature, we observe an abrupt decrease of the output power at the breaking of miniband alignment, corresponding to a strong negative differential resistance feature in the current/voltage characteristics. Ultimately, this effect limits the operation of the device at high temperatures. By comparing this laser with a previous design, we will outline the importance of (i) having a diagonal rather than vertical laser transition in real space, and (ii) avoiding possible intersubband re-absorption of the emitted radiation.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.