The subject of the first part is the design, control, and characterization of the passively mode-locked CW dye laser capable of producing a stable continuous train of subpi cosecond pulses. In the second part, the photoconductive impulse response and excess carrier lifetime of semi-Snsulating CreGaAs are studied experimentally and analytically. It is shown to be consistent with longer carrier lifetimes in Cr:GaAs measured under steady state conditions with longer in 11 umination wavelengths.
Saturable Absorber
Nonlinear Ampiif ier
This is the principle beyond the passive mode-locked CW dye laser that has generated the shortest pulses to date (QJ 0.3 psec). Monitoring laser operation through pulse and bandwidth measurements is discussed in Chapter 4, while control of the system and its output characteristics are described in Chapter 5. Various configurations of passive mode-locked CW dye lasers have been demonstrated that emit pulses of subpicoseconds. f31,[6].
They are based on a combination of a slow saturable absorber (with recovery times up to a few nanoseconds) and a non-linear amplifier. The configuration described here was first proposed and demonstrated by lppen and Shank [J], but much of the understanding of its behavior was not available and had to be inferred here. Before then describing the dyes used in the system, the general properties of dyes are briefly discussed.
Finally, the laser configuration, which was used for the experiments discussed in the second part of this article.
The properties of the dyes used in the l a s e r are important for the proper design and construction of the system. Before describing the properties of the dyes used in our l a s e r , we will b r i e f l y discuss the properties of dyes in general. When a molecule is excited from So t to S1, for example by absorption of a photon, it thermalizes to the bottom of s1 in a few picoseconds [I l l.
This relaxation time can play an important role in synchronously pumped dye-locked modes, when the pump pulses last a few picoseconds.
SINGLET - STATES
TRIPLET STATES
The argon-ion beam l a s e r is focused into the j e t gain by one of the 10 cm radius internal curvature mirrors. The l a s e r system described in section i is capable of generating a continuous tr a i n of picosecond pulses such as those shown in Figure 5.1.3. Modifying the l a s e r , so t h a t i t will emit more powerful pulses, is described in the next two sections.
To lower the overall loss, and thus improve the Q factor of the cavity, the transmission of the output mirror should be low, ty p i c a l y 2% or l e s s . To increase the output power, highly coated mirrors and an acousto-optic modulator, made of a fused crystal and a transducer, deflect the beam away from the cavity by Bragg d i f r a t each time it is waved. When a cavity damper is operated in a passively blocked dye, the energy extraction process disturbs the steady state of the l a s e system.
The dumper should not be activated for more than twice the cavity time (enable to dump a single pulse at a time.) .. 3) The pulses should be stable, which allows the system to recover. The randomness inherent in the passively blocked color (see section 2.1) and the disturbance from the dumper in a number of pulses that are not perfectly periodic. 4) Dumper activation must be synchronized with the lid.
DUMPER
Fig. 2.6,4 The duration of the shortest activation pulse compared to the cavity round-trip time of the picosecond pulse. Note the remnants of the previous and subsequent pulses (510 ns from the main pulse). The pulse energy is several. The cavity of the passively locked CW dye laser with a dumper, shown in Figure 2.7.1, is examined here.
The discussion will be limited to the dimensions, radii of curvature of the mirrors, and folding angles of the actual cavity used in our l as. Accordingly, here we will study the cavity properties as a function of the length of only one of the arms chosen as the gain arm, while keeping all other cavity dimensions fixed. Let us now include the damper and the two colored j e t s in the cavity, a l l t e i r of the corresponding Brewster angles.
In order to highlight the astigmatic effect of Brewster windows on the stable region of the laser, the confocal parameter in the absorber arm of the parallel cavity, which shares a common axis with the rectangular cavity, is plotted in Figure 3.5.4. The influence on the effective range in which stable operation of the laser can be achieved is dramatized by the special behavior of the confocal parameter in the absorber arm. To increase the stable region of the cavity, we turn to the method of astigmatism compensation with oblique mirrors, described in chapter 3.4.
We set the dumper and absorber arms so that their spacing is equal to the sum of the appropriate radius of curvature of the final mirror and the focal length of the lens (or very close when the lens is replaced by a low-angle mirror).
ABSORBER ARM
The pulse resulting from incomplete mode locking can be considered a noise burst, and its coherence is determined by the peak width at t T = 0 of the corresponding SH collision. The difference in the behavior between these three cases of g l ( r ) and g 2 ( r ) serves as an indicator of the working state of the laser. It is preferred that the SHG measurement be performed with basic Gaussian beams, because the internal representation of the result is based on the assumption that the overlap of the two pulses depends only on the relative delay between them.
2 DELAY
PULSE SHAPE
SINGLE SINGLE LONG PULSE A PAIR OF
INCREASING PUMPING POWER
Part 11
The transmission is defined here as the ratio between the peak voltage shown on the oscilloscope and the applied DC voltage. Note, however, that in studying the photoconductive impulse response, the efferents were channeled to improve temporal resolution rather than to increase transmission. The "kink" shown in Figure 7.4.lb has been observed in the impulse response of the coupling devices, which are marked with ( x.
D PLANE
GROUND PLANE
COPPER BLOCK
If the recombination time is shorter than it might be in the case of Cr:GaAs, then trap-driven decay will begin long after most of the excess carbon density has already decayed in the manner of the non-trap material e. In this section, we compare our experimental photocarrier decay time with the electron lifetime in Cr:GaAs calculated with the forniules developed in Section 9.6 and measured in continuous i 11 urination at longer wavelengths. . i ) Calculation of the c a r r i e r lifetime with the recombination center formalism. Under the conditions of our experiment, the electron lifetime in Cr:GaAs due to recombination through the Cr center was shown in Section 9 .
I n (9.8.1) 0 n i is the electron capture cross section, vn i is the thermal velocity given by 1 4.5 xlO cm/sec at room temperature, and 7 Nt is the recombination center density. If (9.8.1) is assumed to be valid, several explanations are possible to accommodate the inconsistency with the observation: a) the cross section is higher, (b) the t r a p concentration is higher, (c) other impurity levels a s i s t . shorten the c a r i e r 1 ife time , (d) other recombination channels, rather than through recombination centers a r active.
- The Surface Effect on the Pleasured - Photocarrier Li f e t i me--An Analysis
- The Surface Effect on the Measured Photocarrier Lifetime--Ccccl~sion -
This ultritsfiort decay time also does not seem to agree with lifetime n~asurenrents under continuous i l - l urninatiorl ,:i t h wavelengths longer than 61013R, the a a v e l r n q t, ~ used in our experience. Tamnr 1551 showed that when the Kroriiq-Penney periodic square well potential is terminated on one side by a surface potential a1 b a r i e r , there would be discrete a1 lowered levels in the forbidden energy gap, corresponding to wave functions localized near the surface. Let the direction normal to the surface be denoted by x, then the carrier flux to the surface is given by Dp * a x for holes and Dn -a;- an for electrons, where D i is the diffusion coefficient and p and n a r e the hole and electron densities, respectively.
On the contrary, this field was responsible for the ambipolar diffusion, because it prevents a large separation of the positive and negative charge distributions, forcing them to move together. In our experiment, we measured the conductivity of a Cr:GaAs sample, which was determined by the total carrier density rather than by . Below the experimental injection levels p e, n e >> noo.po so that the ambipolar diffusion coefficient (9,9,18) can be approximated.
To summarize this section, we have shown that the ambipolar diffusion equation of the form (9.9.35) can be used to describe the temporal and spatial evolution of the photocarrier density involved in our experiment. In the next section, the solution of the transport equation will be given and the surface effect on our experimental results will be concluded 11 . In the previous section, we derived the transport equation that describes the excess evolution of carbon dioxide in our Cr:GaAs sample after picosecond pulse excitation.
In this section the solution of this equation will be given, as well as the surface effect on the observed photocarrier 1. These values of -E agree well with the carrier lifetime measured with CW illumination near the absorption edge (see section 9.8 ). In Chapter 7 an experiment was described in which the photoconductive impulse response of @r:GaAs was studied by irradiating the material with a continuous stream of picosecond pulses.
In this chapter, it has been shown that, contrary to the erroneous interpretations of previous authors, the chromium recombination centers in Cr:GaAs alone cannot be responsible for the observed short excess lifetime of carbon dioxide. This model explains the ul t r a f a s t decay of photocarriers in Cr:GaAs observed by us and others [3,4], after1 photocarrier excitation with a picosecond pulse near the surface. It agrees well with the c a r r i e r l i f e -time calculated from the Shockley-Read model and measured with CW radiation near the absorption edge.
