Elaine Lalanne

Femtosecond Carrier Dynamics and Nonlinear Effects in Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are complicated unipolar semiconductor devices based on intersubband transitions and resonant tunneling. In this study, femtosecond mid-infrared (Mid-IR) pulses are employed to investigate the nature of carrier transport through the active and injector regions of a room temperature, pulse biased ultrastrong coupling design QCL. Despite the low average power (<;1 mW) of femtosecond Mid-IR pulses, the efficient coupling of these pulses into the QCL waveguide made the study of nonlinear effects in QCLs possible. Biased just below threshold, we observed ultrafast gain recovery within the first 200 fs mainly contributed by electrons resonant tunneling through a much thinner injector barrier than that of conventional designed QCLs, which overcomes the interface-roughness-induced detuning of resonant tunneling. Oscillation or overshooting within the first picosecond is caused by electron relaxation from continuum region excited by strong pump beam, as well as coherent electron tunneling transport from injector to active region. The former feature is supported by the observation of second harmonic generation (SHG) with emission of λ≈2.2 μm pulses and measured positive photoconductivity. The transport of electrons through the injector region contributes to a slower gain recovery. A much longer recovery (hundreds of picoseconds) can be explained as electrons are depleted from upper stages down to lower stages in real space.

Second harmonic generation in quantum cascade lasers pumped by femtosecond mid-infrared pulses

Second harmonic generation (SHG) pulses at λ ∼ 2.25 µm have been obtained from a 4.5 µm quantum cascade laser (QCL) when it is resonantly pumped by transverse magnetic polarized 120 fs, λ = 4.5 µm pulses through the QCL’s front facet at room temperature. The measured SHG spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHG with pump power is observed but saturates at higher pump powers. The linear to nonlinear power conversion efficiency is calculated (∼2 µW/W2) and compared with theoretical calculation. This experiment provides an alternative way of investigating the carrier dynamics in QCLs.

Treatment efficacy of laser photothermal therapy using gold nanorods

In vivo experiments are performed to induce temperature elevations in implanted prostatic tumours in mice using 0.1 ml commercially available gold nanorod solution injected into the tumour. Tumour shrinkage studies and histological analyses of tumour cell death are conducted, and the equivalent minutes at 43°C (EM43) for inducing tissue thermal damage are estimated based on temperature elevations during the treatment. It has been shown that the laser heating of 15 minutes in the tumour tissue containing gold nanorods is effective to cause irreversible thermal damage to the tumours, with a low laser irradiance on the tumour surface (1.6 W/cm2). The effectiveness of the heating protocol is demonstrated by tumour shrinkage to 7% of its original volume on the 25th day after the laser treatment and tumour necrosis events observed by histological analyses. The results are consistent with the EM43 distribution estimated by possible temperature elevations during the treatment.

MicroCT Imaging and In Vivo Temperature Elevations in Implanted Prostatic Tumors in Laser Photothermal Therapy Using Gold Nanorods

In this study, in vivo animal experiments are performed on implanted xenograph prostatic tumors in nude mice to investigate enhanced laser energy absorption in the tumors by an intratumoral injection of gold nanorod solutions. in vivo temperature mapping of the tumors during laser photothermal therapy has shown the feasibility of elevating tumor temperatures higher than 50 C using only 0.1ml nanorod solution and a low laser irradiance of 1.6W/cm 2 incident on the tumor surface. The temperature profile suggests that normal tumor tissue still absorbs some amount of the laser energy without nanorod presence; however, the injected nanorods ensure that almost all the laser energy is absorbed and confined to the targeted tumors. The inverse relationship between the temperature elevations and the tumor size implies a relatively uniform spreading of the nanorods to the entire tumor, which is also shown by microcomputed tomography (microCT) imaging analyses. The feasibility of detecting 250 OD gold nanorod solution injected to the tumors is demonstrated via a high resolution microCT imaging system. Compared to other nanostructures, the gold nanorods used in this study do not accumulate surrounding the injection site. The relatively uniform deposition of the nanorods in the tumors observed by the microCT scans can be helpful in future study in simplifying theoretical simulation of temperature elevations in tumors during laser photothermal therapy. [DOI: 10.1115/1.4007161]

Femtosecond measurements of near-infrared pulse induced mid-infrared transmission modulation of quantum cascade lasers

We temporally resolved the ultrafast mid-infrared transmission modulation of quantum cascade lasers (QCLs) using a near-infrared pump/mid-infrared probe technique at room temperature. Two different femtosecond wavelength pumps were used with photon energy above and below the quantum well (QW) bandgap. The shorter wavelength pump modulates the mid-infrared probe transmission through interband transition assisted mechanisms, resulting in a high transmission modulation depth and several nanoseconds recovery lifetime. In contrast, pumping with a photon energy below the QW bandgap induces a smaller transmission modulation depth but much faster (several picoseconds) recovery lifetime, attributed to intersubband transition assisted mechanisms. The latter ultrafast modulation (>60 GHz) could provide a potential way to realize fast QCL based free space optical communication.

Determination of the nonlinear refractive index of multimode silica fiber with a dual-line ultra-short pulse laser source by using the induced grating autocorrelation technique

We measured, within 6% accuracy, the nonlinear refractive index (n2) of 10 meter long multimode silica fiber of 17μm core diameter, using a modified induced grating autocorrelation technique (IGA). This measurement technique, based on time-delayed two beam coupling in a photorefractive crystal has been used to accurately measure n2 in short lengths of single mode fibers. For the first time to our knowledge, IGA is used to measure n2 of multimode fiber with a passively modelocked Nd:YVO4 laser operating with a dual-line near 1342 nm.

Investigation of giant Kerr nonlinearity in quantum cascade lasers using mid-infrared femtosecond pulses

We study the Kerr nonlinearity of quantum cascade lasers (QCLs) by coupling resonant and off-resonant mid-infrared (mid-IR) femtosecond (fs) pulses into an active QCL waveguide. We observe an increase in the spectral width of the transmitted fs pulses as the coupled mid-infrared (mid-IR) pulse power increases. This is explained by the self-phase modulation effect due to the large Kerr nonlinearity of QCL waveguides. We further confirm this effect by observing the intensity dependent far-field profile of the transmitted mid-IR pulses, showing the pulses undergo self-focusing as they propagate through the active QCL due to the intensity dependent refractive index. We experimentally estimate the nonlinear refractive index n2 of a QCL to be ∼8 × 10−9 cm2/W using the far-field beam profile of the transmitted pulses. The finite-difference time-domain simulations of QCL waveguides with Kerr nonlinearity incorporated show similar behavior to the experimental results.

Visualization and quantification of gold nanorods distribution in prostatic tumors using microct imaging

One uncertainty in use of gold nanorods for laser photothermal therapy is the non-uniform spreading of gold nanorods in tissue after either systemic delivery or intratumoral injections. High concentration of gold nanorods in certain areas influences the resulted optical absorption of the laser and thermal damage to tumors. This also provides challenges in designing optimal heating protocols via modeling thermal transport in laser photothermal therapy. For successful cancer treatment, the tissue should be heated with minimum thermal dosage to induce tumor cell damage, while minimizing overheating in the surrounding healthy tissues. Thus, one of the main challenges for reliable cancer therapy is to precisely control loading and distribution of gold nanorods in the tumour tissue. The critical mass transport processes are the distribution of gold nanorods after injection to the tumor and the redistribution of gold nanorods during laser treatment. Since tumors are opaque, nanostructure distribution in tissue is often studied either by theoretical modeling approaches1, or via dye enhanced imaging on superficial layers of tumors.2 It is important to find a technique which can directly visualize and analyze three-dimensional nanostructure distribution of tumors. Three-dimensional reconstructions of tumors with the ability to trace gold nanorod spreading have the potential for precise theoretical simulation of temperature fields. Previous studies showed that computer tomography (CT) scan is a promising technique to be utilized to characterize the distribution of intratumorally injected magnetic nanoparticles in tumors 3.© 2012 ASME

Thermal effect of gold nanorods in implanted prostatic tumors during laser photothermal therapy

In recent years, nanotechnologies have emerged as promising therapies due to their ability to deliver adequate thermal dosage to irregular and/or deep-seated tumors. Gold nanorods can be tuned to a specific laser wavelength and serve as strong laser energy absorbers. Due to the powerful optical absorption, the laser energy is concentrated in an area congregating by nanorods, and then the energy absorbed can be transferred to the surrounding tumor tissue by heat conduction.1–4 Currently, there are wide variation ranges of treatment protocols using photothermal therapy. A systematic approach is lacking to analyze temperature elevation history in tumors during heating to design an optimized combination of laser parameters to maximize thermal damage to tumors.Copyright

Treatment efficacy of laser photothermal therapy using gold nanorods: Tumor shrinkage study

Gold nanorods can be tuned to a specific laser wavelength and serve as strong laser energy absorbers. Due to the powerful optical absorption, the laser energy is concentrated in an area congregating by nanorods, and then the energy absorbed can be transferred to the surrounding tumor tissue by heat conduction.1–4 Previous studies have shown a wide range of heating parameters with or without temperature measurements. Our previous experiment4 has demonstrated that using only 0.1 cc gold nanorod solution can lead to tumor temperature higher than 50°C when the laser irradiance is only 2 W/cm2. Based on the measured temperature elevation and heating duration, thermal damage to the tumor is highly likely. However, some researchers raised the question whether temperature sensors used in those experimental studies are truly reflecting the temperatures in the tumors. The objective of this study is to measure quantitatively tumor shrinkage after laser irradiation to evaluate efficacy of laser photothermal therapy.Copyright